Building a DIY Dynamic DC Load

 VB Dynamic DC Load

Construction and Build Instructions

Here is a list of items and instructions that will help you to build a Dynamic DC load instrument. 

The design process for the instrument is documented on this Blog here:
https://www.paulvdiyblogs.net/2024/04/build-diy-dc-dynamic-load-instrument.html

The BOM's, Gerbers, Firmware and other required files can be downloaded from the GitHub.|
https://github.com/paulvee/Dynamic-DC-Load

I also added the instrument details as a Shared Project on the PCBWay site so you can order the boards directly from there:
https://www.pcbway.com/project/shareproject/DIY_Dynamic_DC_Load_8643ddca.html
https://www.pcbway.com/project/shareproject/DIY_Dynamic_DC_Load_Face_PLate_12899b9c.html
https://www.pcbway.com/project/shareproject/DiY_Dynamic_DC_Load_Back_PLate_cda4ffcf.html

Bud, the designer of the instruments core, started his own Hackaday project to document the design here: https://hackaday.io/project/195380-dynamic-electronic-load
I took all his stuff from there and copied it in the design process Blog, so it's all in one place.

Parts & Components

I will show a few parts and components in this description, but all the main PCB parts are listed in a BOM with their order numbers, and the additional off-board parts are in a separate BOM, both are available on the Github site.

The Printed Circuit Boards

The Printed Circuit Boards (PCB's) are available through PCBWay, who are sponsoring my activities by providing the PCB's free of charge. I have been using their service for several years now and the boards are of excellent quality, every time.

The main board can be ordered here:
https://www.pcbway.com/project/shareproject/DIY_Dynamic_DC_Load_8643ddca.html

To create a professionally looking instrument, and to aid with the construction, I have also created a front panel and a back panel PCB. Both also available through PCBWay.

The back plate can be ordered here:
https://www.pcbway.com/project/shareproject/DiY_Dynamic_DC_Load_Back_PLate_cda4ffcf.html

The front Plate can be ordered here:
https://www.pcbway.com/project/shareproject/DIY_Dynamic_DC_Load_Face_PLate_12899b9c.html

The back of the front panel has 4 solder pads that can be used to glue to the OLED display into position, so it will be positioned in the middle of the square cut-out.

 

Stuffing the PCB

Adding components to the PCB should be a straightforward job, except maybe for soldering the minute packages for the ADC, the reference, the two analogue switches and the MCP6V51 Opamps.

I highly recommend you use a socket for the ESP32. To that extend, make sure you order the VROOM 4MB DEVKIT V1 module without the headers installed. (the one below has the blocking diode installed, details below) Note the pin layout and make sure you get one that has the same layout!

 I use the following headers and sockets for the ESP32, I buy them by the strip.

 

To add the components to the PCB, my recommendation is to start with all the passive components like resistors, capacitors and then the active transistors, MOSFET’s (not the main NFET’s M1, M2), diodes and the voltage regulators U9 and U12. Do not install the relay and the snubber circuit yet. What I then would do is to connect a Lab Power Supply with a setting of 12V and 50mA to the DC input of the board. If it powers-up without getting into current limitation, you will probably not have any shorted rails to ground, and can proceed to test the +12, +9 and +5 supply voltages. If they check-out, you can add the ICL7660 and test the -5V rail. After that, you can add the REF5040 and check the presence of  4.096V. If that also checks-out, you can add the trimmers, connector strips, DAC, ADC, MOSFET switches (U14, U15), all the Opamps and finally, the main relay and snubber circuit. Basically, everything except placing the ESP32 module in its socket, and soldering the main NFET’s (M1, M2) and the LM35.

 

ESP32 variations

The recommended ESP32 board is the VROOM 4MB DEVKIT V1. However, I have found that there are slight variations or differences for two components on various sources for this board. One is the capacitor that should ensure a reset after a download of new firmware. On one of my boards, this capacitor was there, but not well soldered to the chassis part of the USB-mini. On another one, there was no diode to block the incoming 5V through the USB, and the VIN supplied to pin 1 of the board. Having no diode but a 0R resistor means that the VIN supply will back-feed 5V into the USB port of your PC. Not recommended!

To accommodate these two issues, we put provisions on the PCB to help fix that.

Verify that the ESP32 module has a 1uF 0402 capacitor most likely connected to the solder pad of the EN button. It may be soldered at a 45-degree angle, see the left blue arrow. Also make sure that there is a diode installed and not a 0 Ohm 0402 resistor (marked 000) as you see here on the board, see the middle blue arrow below. The picture of the ESP32 further up does have a diode, which can be easily identified.

If the capacitor is not installed, you need to install C44 on the PCB. If there is a 0 Ohm resistor on the board, you don't need to modify the ESP32 module but need to remove or not install R53 and install D10 on the main PCB. See the schematic below.

 

The Back Panel

At this moment, it will help to mount the fan on the back panel, so you can use it to double check the space between that fan and the heatsink/fan combination that will be mounted on the bottom shell later.

The back panel fan will be mounted using the flexible silicon mounts that come with the fan.

To help the installation by pulling on these thingies, it is helpful to first counter sink the 4 holes in the PCB on either side so the silicon will more easily slide into position when you pull them in.

Note the position of the fan on the back panel, and where the wires need to come out.

 

Modifying the enclosure

The bottom shell of TEKO AUS33.5 enclosure needs to be modified so we can mount a fan on the bottom to suck fresh air into the heatsink fins inside the enclosure. The fan mounted on the back panel will suck the hot air away from the heatsink to the outside of the enclosure.

A number of holes need to be drilled in the bottom shell to get enough airflow to the heatsink. I used Excel to create a raster for the 6mm holes. The picture shows an earlier position of the fan, I later moved it further down and added a number of extra holes.

To feed plenty of fresh air from the bottom of the enclosure to the fan, we need to mount 4 feet that are about 1cm in height to lift the instrument from the shelf or desk to create the space.

The bottom fan needs to be mounted in the middle of the enclosure, and the mounting holes 25mm away from the back edge of the bottom shell, to make room for the fan that is mounted on the back panel. Before you drill the holes, use the back panel with the fan mounted on it to double check the space you need. The minimum space between the two fans would be 2mm.

 

The bottom fan and heatsink

The bottom fan will be mounted on the shell by 4 30mm long M3 screws. Because of the plastic, and the two hole positions in the ventilation area, I recommend you put M3 or M4 rings on the bottom and on the inside of the plastic shell. Use a normal (flat) M3 nut to tighten the screws to the enclosure while making sure that the fan will easily fit. After the fan is positioned on the 4 mounting screws, use an isolated M4 ring on top of the fan before you mount the brackets holding the heatsink.

The heatsink will be mounted about 6mm above the bottom fan by two strips of a 90 degree angle 15x15mm piece of aluminum. These strips can be found in your local hardware store and probably come in a length of 1mtr. Use a metal hacksaw to cut two pieces with a length of 90mm, which is the same as the size of the fan. Drill two holes of 3mm 15mm from each end, that will hold the self-tapping screws mounting the bracket to the heatsink.

I used very short 3mm self-tapping screws to mount the angle pieces to the heatsink. I also drilled the hole through the second flange of the heatsink, to make room for the tip of the screw, so it does not bend the thin flanges.

Drill two more holes in the other flank that are 81 mm apart, so the screws holding the fan to the bottom of the enclosure will go through these holes. Make sure that these holes are spaced such that when the strips are mounted on the heatsink, the holes of the fan will pass through the 4 30mm M3 screws.

Because the heatsink can get very hot, I used 4 3mm insulating rings that are also used to mount TO220 type devices. They also serve to electrically isolate the screws from the potential of the heatsink, that can be up to 100V DC.

You’ll need a fifth one to isolate the LM35 from the heatsink later. These rings should fit the holes of the brackets, if not, drill the holes a bit larger to 3.5-4.0mm to create some wriggle room. I then used a Teflon ring on top of the insulators and used self-locking M3 nuts to loosely secure the heatsink, so you don’t need to tighten the nuts too much and put too much pressure on the plastic frame of the fan. You want to minimize the transfer of the mechanical vibration from the fan to the enclosure. It should be a loose fit. 

The thermal paste you see in the pictures is because I took my instrument apart to make these instructions and the pictures after I finished my build.

Mounting the Main NFET’s and the LM35

Before you mount the heatsink on top of the fan, you need to create three M3 tapped holes in the heatsink to mount the two NFET’s and the LM35 temperature sensor. If you don’t have a good quality and sharp M3 tapping set and the right size drill (2.5mm), you can also use 3mm self-tapping screws. In that case it is even more important that you counter-sink these holes well, so the aluminum material does not come-up and push the devices up from the heatsink when you fasten the screws. If you use the M3 taps, lubricate the drill and the taps well and clean the taps and lubricate them regularly while you are tapping, to make sure that the threads are not damaged. The NFET’s and the LM35 will need to be tightened to the heatsink well, to get the maximum heat transfer, so the threats need to be well made.

I created M3 threaded holes for the two NFET’s 23mm from the back-end of the heatsink, 33.5mm apart, and about 23mm from the edge of the heatsink.

The LM35 goes in the middle of the heatsink and 27mm from the back. Countersink the holes and make sure everything is flat. Use a fine file if needed.

 

The other 4 holes you see are from previous mounting positions and are no longer used.

The front of the main PCB will be supported by two extenders towards the front of the enclosure. I used two 30mm long M3 extenders screwed together. You can use any length parts, as long as they are 60mm in total. The height can be adjusted when the PCB is mounted. The position of the holes in the bottom enclosure for these supports is best determined when the PCB with the NFET’s and the LM35 is mounted on the heatsink.

As you can see from the picture below, the bottom fan is positioned such that the wires come out at the left-hand side. The open side of the fan must be facing the bottom of the enclosure so it sucks the air into the enclosure. 

Because the 4 wires for the fans have a shrink-wrap tube, they are too sturdy to my liking, so I took the tubing off.

The NFET’s and the LM35 will be mounted flat on the heatsink. The main PCB needs to be mounted as high as possible away from the heatsink, to minimize heat transfer. With earlier prototypes, I found that there is a surprising amount of heat transferred through the NFET leads to the PCB. Keeping the legs long helps.

The challenge is to move the body of the NFET's as far away from the PCB as possible, to again reduce the heat transfer by way of the leads, and the heatsink heating the bottom of the PCB. We also need to create space away from the PCB to mount the two smaller TO220 heat sinks on top of the NFET's without them touching the PCB. The place where you bend the leads of the NFET's upwards is critical to accomplish this. I bend the leads of the NFET's after about 6mm from the case with 90 degrees (not too sharp!) upwards. 

 

You can position the three devices loosely with a screw using the M3 holes you created earlier, and then position the PCB on top of them, with the leads protruding through the holes in the PCB. At this moment, you can use the two mounting extenders on the other end of the PCB so it is about horizontal, or use something else to keep the PCB horizontal and stable. I also used two spacers of 8mm to support the PCB near the NFET’s so you can position the PCB horizontally in order to solder the leads, and the right amount away from the PCB. Make sure that the PCB is positioned high enough such that the NFET leads only just protrude 1-2mm through the PCB holes to allow for proper soldering. The LM35 leads are a bit longer, so that’s OK. I suggest you solder only the middle leads at this moment. Be careful not to drop the other end of the PCB and bend the leads.

Once you are happy and double checked the position of the PCB and the three devices, you can mark the position of the front supports and then remove the three screws holding the three devices and remove the two extra 8mm standoffs.

You can now finish the mounting of the two front supports by drilling the holes. After you have secured the bottom portions to support to the enclosure, you can roughly adjust their total height. 

Before you finally mount everything again, apply heat paste to the NFET positions on the heatsink and use an insulated silicon pad with an insulator ring for the LM35. Loosely tighten the 3 screws again and use an Ohmmeter to make sure that the LM35 tab is insulated from the heatsink, because the NFET’s are not, and their pads will connect to the DUT voltage of up to more than 100V to the heatsink.

When everything is in position, you can tighten the screw for the LM35 and again check that the tab is still isolated. Unscrew the two NFET’s screws again and now add the extra TO220 heatsinks on top of them, using plenty of heat transfer paste.

Make sure that the heatsinks do not touch the PCB. You can now carefully tighten the screws for the NFET’s and solder all the leads.

After that, you can do a final adjustment of the two front supports so that the PCB is horizontally mounted inside the enclosure. When that is done, re-heat the middle pins of the three devices, to let the stress out, and then solder the other pins. Use plenty of solder for the NFET leads, they will get stressed by the developing heat (up to 100 degrees C).

 

Preparing for a first power-on

After you have finished the above steps, we can start to power-up the board again and verify some of the vital signs.

You only need to add the 12V DC power to the board, but I recommend you use a Lab supply initially with a current setting of 100mA. If there is no large current, indicating a short or another issue, you can now switch to the 12V DC wall wart.

Do not install the ESP32 in its socket at this moment.

 

Verifying the Power Rails

You can now verify all the power rails:

·         DC Input 12-15V at J4

·         +12-15V at pin 1 U9 (LM7809)

·         +9V at pin at pin 3 of U9

·         +5V at pin 3 of U12 (LM7805)

·         -5V at left side of C31 or pin 5 of U5 (ICL7660)

·         +4.096V on either side of R60

When that is OK, you can go to the next step, but first disconnect the 12V DC input again.

Adjusting the OLED display position on the front panel

For this step we need to install the ESP32 module on the PCB and load the ESP32_OLED_Test_V1.ino firmware.

Do not apply the main 12V at this moment, it is not needed for this step.

The OLED module connects to J6 on the PCB. The module comes with flying leads that each have their own connector. I recommend you change these single connectors to a 7-pin connector so you can’t make mistakes with the order, and it’s a lot easier to remove and install.

You can connect your PC to either the mini-USB of the ESP32 module, or start to use the mini-USB to USB-C adapter. When you have made the connection, the red power LED on the ESP32 module should light-up and your PC probably will give a signal that a new device is connected.

I’m assuming that you already installed the Arduino IDE. If not, you need to do that now. There are plenty of instructions on the internet to do that successfully. I was using IDE V2.3.2 and now the just available V2.3.3. The Arduino IDE is needed to load the firmware for the ESP32 module. Locate the ESP32_OLED_Test_V1.ino on the GitHub and download it to your PC. Save it in a new directory called “VB Dynamic DC Load” in the Arduino directory on your PC, it is most likely located in your Documents directory.

After you saved it there, use the Arduino IDE to locate the file and load it into the IDE.

After you connect the USB from the PC to the ESP32 module, you now need to prepare the Arduino IDE for the ESP32 processor setup. There are many tutorials on how to prepare the IDE for ESP boards. Do that first.

After that installation, in Tools select Board and search for the DOIT ESP32 DEVKIT V1 in the esp32 branch. That’s the ESP module we use, and the firmware will most likely only work with that module.

Under Tools, select the connected COM port.

Compile and download the firmware and see if it gives you any errors.

If there are, you need to figure out what the problem is. There are too many possibilities to help you from here.

Install the connector from the OLED display to J6 on the main PCB, and position the module so you can see the screen and the white connector on the back is on the right-hand side of the display. When done, you can now connect the main 12V power again, because it is needed to power the OLED. You need to press the EN button on the ESP32 module to force a reboot after you supplied the 12V. When successful, you should first see a splash screen with the version number for a few seconds and then see a white square on the OLED screen because all pixels are lit. Congratulations if you got this far!

The text of the splash screen will show the orientation of the OLED display. You could change the orientation of the display in this test code and repeat that later in the firmware, but if you keep to the instructions, and have the white connector on the right-hand side of the OLED module, you should be good to go.

You can now proceed to mount the OLED display in position on the Front Panel.

 

Mounting the OLED display on the Front Panel

To prepare for the mounting of the OLED display in position on the back of the Front Panel, we need to change the mounting hardware that came with the display.

During all these operations, keep the screen protector foil on, or apply it again. It will prevent you from scratching or smutching the glass. Use gloves when appropriate.

Remove the 4 M2 screws and standoffs. Keep the 4 screws, but we will not need the standoffs. You need 8 extra M2 nuts and 4 M2 washers to prepare to mount the contraption on the font panel.

Add an M2 nut to the M2 screws. Stick them through the hole of the board, and add an M2 washer and then another M2 nut. Tighten loosely. The M2 washer is needed to create some extra space between the glass of the OLED and the Front Panel, so don’t skip it. You may otherwise break the OLED display. Do this for all the 4 holes of the OLED board.

You can now position the OLED board to the back of the Front Panel and check the fit. It should slide on the 4 nuts, not the glass.

Apply the 12V main power again and do a reset of the ESP32 (press EN) to verify the orientation of the text from the splash screen and then see if you can position the white rectangle in the center of the square of the front panel. You can now peel the protective cover of the glass, because that may be impossible later on. I strongly suggest you use gloves and avoid any finger prints or worse, glue getting on the glass.

Before you do the next steps, I suggest you practice the next moves once or twice to make sure you can position the OLED properly without tangling the wires or moving the OLED display that you just glued in position. If you are not happy using Superglue, then use normal glue that you can still move a bit, but you then should tape the two together and let it dry for a few hours.

With the OLED module powered and showing the white square, hold the OLED board with the display side up in one hand, with the white connector on the back to the right-hand side and add a drop of (Super)glue on each of the 4 nuts that will connect to the solder pads on the back of the Front Panel.

With your other hand, pick-up the front panel and position and hold it just above the OLED screen with the silkscreen text from the front panel up and make sure the white rectangle square is in the middle of the square hole in the front panel. When satisfied, slowly lower the front panel and press the two together and let the (Super)glue dry. After a few seconds with Superglue, you can carefully turn the Front Panel together with the OLED face down so the OLED stays in position and you can let it dry a bit more.

If you used normal glue, you then need to secure the OLED in position with sticky tape so it will not move while you let it dry for several hours.

Turn off the power and disconnect the USB connection to the ESP32.

Wait with the next steps until you are sure the OLED display glue is dry!

 

Loading the Dynamic DC Load firmware

The firmware for the Dynamic Load is located on the GitHub site and needs to be downloaded before you can load it into the Arduino IDE. The firmware is needed to operate the instrument, and at power-up will make sure everything starts in a known and safe state. 

The firmware is in a zip file with the name of the version number. You need to unzip the zip file and put the 6 files in a new directory that must have the same name as the zip file, and create that directory in the ../Documents/Arduino/VB Dynamic DC Load directory you created earlier. 

The firmware consists of 6 different files that the IDE will stitch together to create the firmware during the compile, link and then download process.

Load the file into the IDE and compile & download it to the ESP32 module. You don’t need to turn on the 12V just yet. Just make sure it finishes without errors. The first time can take a several minutes so be patient and just look at the bottom screen of the IDE to see what is going on.

 

First firmware turn-on

Do not supply the 12V yet and also do connect the two fans to the PCB at this moment yet. Position the front panel with the OLED connected to the main board such that you can easily see the display. The front panel does not need to be mounted to the enclosure yet, but you may want to do that.

When there are no compile or download errors, the firmware is loaded and started, and with the red power LED on the ESP32 module already on, the blue LED now starts to flash. If that is the case, then congratulations are in place again. Well done!

With the USB power applied, and when you now supply the 12V main power, the ESP32 is not getting a reset. You need to do a manual reset (press the EN button) again so the firmware is restarted, the OLED now becomes operational and after the splash screen with the version number, you should see the following start screen.

Great if you are this far! Don't worry about the "negative" or "invalid" warning in red for the voltage. That is because nothing is connected yet and the inputs are floating.  The temperature reading for the heatsink in the lower right corner should be OK though.

Connecting the fans

Turn off the 12V and remove the USB connector. If you still use the Lab Supply, raise the current to 350mA. You can now connect the two fans and observe the starting or boot sequence when you turn the power on.  

 

Boot sequence

When you apply the 12V main power, the red power LED on the ESP32 module will switch on, and both fans will start to run at full speed. After about 2 seconds, the splash screen appears and the blue LED on the ESP32 module will start to flash. The flashing is an indication for the execution of the firmware main loop. The splash screen makes place for the initial screen display and the fans will be turned off. That actually completes the boot sequence. The instrument is now ready to make measurements and accept commands.

If you got this far again, great!

 

The front panel

It is now time to finish the front panel installation, and connect the remaining leads and parts to make the unit fully functional.

In the picture below you can see my wiring on the back of the front panel.

To connect to the rotary encoder, I used a ready-made and assembled wire-harness that I purchased in different pin sizes. Here I’m using a 5-pin version. On the rotary encoder itself, I bridged the middle of the 3-pin connector which is GND to one of the switch pins. The VCC pin of the connector is not used. It is there in case you have a rotary encoder with a debounce circuit that needs power. The firmware works well enough with the rotary encoder without hardware decoding support.

The main power leads for the DUT are heavy duty and I used so called car connectors to make it easy to mount them. Make sure they can handle 10A.

It’s a little hard to see, but there is a 10nF capacitor directly soldered on the Current Monitor BNC, to take care of a nasty glitch caused by the induction of the cables.

For the main DUT and sense inputs, I used these 4mm Banana binding posts. They have the correct 19mm distance.

The wiring of the sense switch is quite simple. I connected leads between the main Banana terminals and the sense terminals with solder lugs and used two wires to go to J3 on the PCB.

The power switch is in between the plus lead coming from the DC jack on the back panel and the positive connection of J4 on the main board. The ground lead from the DC jack goes directly to J4.

I used 2-pin wire harnesses for the Current Monitor and the Transient Input connectors, and used these BNC connectors:

The main power switch and the USB adapter are not showing in the picture.

To make sure you don’t pull out the USB-micro to USB-C adapter from the ESP32 socket, I positioned the adapter with everything mounted, and used a tie-wrap flush to the front panel to secure it. To make sure it stays in position, I used some glue to fix the tie-wrap into position.

 

With everything now installed, you should have a fully working instrument that should look like this:

 

 

 

You are now ready to move on to the Calibration & Verification section.

Calibration & Verification

Remove the top cover of the enclosure. You need access to the trimmers and test points.

Initial step

Make sure you have the latest firmware version installed that is available on the GitHub. 

To start the full procedure again after an earlier procedure, you need to revert some of the calibration factors in the firmware back to the original value. These constants are located in the calibration section in tab ESP32_V4_xx.ino of the firmware in the Arduino IDE starting at around line 54.

·         double const shuntVcalib = 1.0; // optional

·         double const dutVcalib = 1.0;

·         double const cvCalFactor = 1.0;

 Set these values to 1.0 if not already and recompile and load the firmware.

Calibrate the OLED voltage display

1. Set a Power Supply to 2.50V, or use a voltage reference, connect it to the main terminals of the Dynamic Load (DL). Use a DMM to verify the actual voltage.

2. Set the remote sense switch to off.

3. Make sure the NFET’s are OFF so there is no current flowing.

4. Note the OLED display for Voltage and adjust RV1 on the PCB so the OLED shows the same voltage or as close as you can get it. (due to the averaging and the resolution, take your time)

5. Use a Power Supply to raise the voltage to just below 100V or as high as your supply supports (stay below 105V), and verify using a DMM.

6. The difference between the DMM measurement and what the OLED shows should be less than +/-0.5%. If it is more, you can activate the optional calibration factor in the firmware to apply a correction.

7. Calculate the difference with the DMM and the OLED voltage and use that as a factor below.

·         double const dutVcalib = 1.0;

The calibration constants are located in the calibration section in tab ESP32_V_4_xx.ino of the firmware starting around line 54.

8. Recompile the firmware to apply the calibration, and run this calibration section again to verify.

 

Verify the sense switch operation

1. Set the Power Supply to 30V and a current of at least 1.5A. Connect sense leads from the DL sense inputs to the Power Supply. Directly connect them at the Power Supply connections.

2. Switch the sense switch to on and verify that the DUT voltage is the same in both on and off settings.

3. Turn the DL input ON with a short press of the decoder button, and check that there is no current flowing yet. Adjust the rotary encoder of the DL load such there is at least 1A flowing as a load.

4. Alternate the sense switch in both positions to show the effect of the external sense input that will compensate the voltage reading for higher currents flowing through the main leads, that will cause a loss and therefore a voltage drop. If you switch to higher currents, the effect will even be larger.

 

Current calibration for the DAC

1. Set the Power Supply for a voltage of 10.0V

2. Connect a DMM to the DAC test point and a GND test point on the main PCB and use the rotary encoder to get a reading closest to 400mV on the DMM - let the system warm up for a few minutes and readjust if needed.

3. Adjust RV2 on the main PCB to get a current of 1.000A on the power supply or measure the current with a DMM. Note that the current on the OLED display itself will not be accurate yet, it still needs a calibration.

 

Verify the DAC-ADC and the Power Section accuracy

1. Measure the voltage at the DUT_I test point on the main PCB -- it should be between 396 and 404mV which is an accuracy of +/- 1%.

2. Store the error in DUT_Current.

·         double const DUTCurrent = 400.00 - your reading in mV; (it is not used however)

 

Calibrate the OLED display to show the correct current value

1. Make sure that the shuntVcalib constant in the firmware has a value of 1.0.

2. If not, set it to 1.0 and re-compile and load the firmware with that factor.

3. In the CC mode, with the load ON, set the current with the encoder such that the Power Supply shows a current of 3.00A, or use a DMM in current mode to measure the current.

4. Note the current value on the OLED display and divide the Power Supply or DMM reading by that of the OLED value.

5. Enter that result (it will be around 2.5) in calibration constant shuntVcalib and re-compile the firmware to activate it.

6. Verify that the OLED current now is virtually the same as the Power Supply or DMM current at different load values.

 

Calibrate the CV mode tripping voltage

1. Make sure that the calibration constant cvCalFactor in the firmware has a value of 1.0.

2. If not, set it to 1.0 and re-compile and load the firmware with that factor.

2. Use a Power Supply and apply a voltage of 50.00V, set the maximum supply current to 100mA. (When the CV mode is tripping, a Lab Power Supply will go into the CC mode, and will apply the full current to keep the voltage the same. We want the current to be low, to keep the temperature low during this calibration)

3. Activate the CV mode, the Set voltage will be automatically set to 10% higher or 55.00V. Turn the DL to ON. There should be no current flowing, if there is, raise the set voltage until there is no current flowing.

4. Slowly dial down the Set voltage with the rotary encoder and take note of the Set value when the tripping point happens. This is when current starts to flow. You will have to raise the Set voltage about 0.5V higher in order to turn the current flow back off. This is the normal hysteresis activity.

5. Calculate the deviation factor of the DUT voltage and the tripping point voltage and apply that to the calibration constant cvCalFactor and recompile and load the firmware to make it effective. As an example, with a tripping voltage of 52.50V, the deviation is 52.50/50.00=1.050000 and that is the calibration factor that needs to be stored in the firmware.

7. After recompilation and loading, verify that the cut-in voltage is now very close. (it is unlikely you will get it exactly the same, but it should be within a few percent)

8. Try the CV mode now with a Power Supply setting of 2.00V and verify that the tripping point is very close. You will have to dial-back the rotary encoder from 50V down to 2V, which, even with fast dialing, will take some time. You can also cycle through the modes by long-pressing the rotary encoder, until you are back at the CV mode, in which case the Set voltage is automatically set to 2V + 10%.

CV mode with a regulated (VC/CC) power supply

Note that with a Lab Power Supply as a DUT, the CV mode is not very functional due to the rapid switch of the power supply from CV to CC. To better test the CV mode and get more familiar with it, you can use a battery, or an (unregulated) supply that does not have current fold-back or current limiting/regulation. A transformer with a (bridge) rectifier and an electrolyte filter capacitor will much better show the CV mode regulation in operation. 

The Battery Mode PC software

The Battery Mode application that communicates with the Dynamic Load and sets-up the instrument and does the graphing is available on the GitHub site. It's a single executable that you can install on your PC wherever you like, and you can run it from there.

I may add more later... Stay tuned...

Designing a DIY DC Dynamic Load Instrument

 

The Design Process of Building a DIY DC Dynamic Load

The conclusion of a very long time investigating the building of a combined AC-DC Dynamic Load (DL) led me to the realization that this is not really possible. At least not without making drastic compromises about accuracy and precision. The AC measurements do not need to be very precise and are, in my case, only occasionally needed, but DC measurements typically need to have the highest precision, stability and accuracy.

The attempts for this combined AC-DC DL are described in another Blog post. There is a lot of information there, so get some fresh Java and have a look. https://www.paulvdiyblogs.net/2022/08/dynamic-acdc-load-cc-cv-cw-batt.html

The above picture shows a stage in the development with the new DC DL prototype as the basis.

To help design this new version of the instrument, I asked for the help of my friend Bud. He is a real designer, and we've done a number of projects together, like the high current RPi UPS, the VBA Curve Tracer and the Differential Probe all described on this Blog in different posts.

Bud started a Blog post on Hackaday mostly for the hardware design aspects that can be found here: Dynamic Electronic Load | Hackaday.io I have added that same information in a section below here, so it's all together in one place.

At this moment, we have finished the design of the new version and the final board V5.1 has been produced by my sponsor PCBWay.

I'm still working very hard on the software side, because this instrument will do some things a bit different, compared to many other DIY DL designs.

I'll explain that later in more details but here are some pictures to hopefully wet your appetite from the previous prototype  V5:

The initial target specifications:

Input voltage: 1..100VDC.

Reverse polarity protection to -100V and a 10A fuse.

DUT is disconnected by a relays for invalid inputs like reverse polarity.

Maximum current of 10A @ 40V

Maximum power 180W @25 degrees ambient temperature

Volt Accuracy: 0.003%

Current Accuracy: 0.004%

Power input: 12VDC Wall-wart 0.5A with reverse polarity protection to -24V and PTC fuse

CC & CV modes with real-time operation, CP, CR and Battery Mode supported in software.

Pulse/transient mode supported by an external Function Generator. 5V=10A

Current monitoring with a DSO. 1V=10A

GUI: 128x128 OLED 1.5' color display and a rotary encoder with a dual button functions.

Two temperature controlled fans.

Overall dimensions: 21cm long, 18cm wide and a height of 118cm.

Weight: approx. 1110 grams

Testing the V4 prototype

This earlier version for the AC-DC attempt is still using the Arduino Nano as the processor. It also had provisions for an on-board transformer, but that got too hot so I took it off.

The Nano is not fast enough to process the CP and CR modes, which are regulated in software. At this time, I was planning on implementing the CV mode in software as well, but that did not work at all with this prototype. While looking for a faster processor, I eventually settled on the ESP32. It has all that we need, even though we won't be using the BLE and WIFI capabilities.

This was the first time I really started to work with the ESP32 so I started to experiment with it. This processor is not only much faster than the Arduino Nano, it has a dual core and it has a native RTOS. (real-time operating system) I was hoping that these major features would overcome the Nano limitations.

In the picture below you can see how I tested the ESP32 in-place of the Nano. I simply took the Nano out of it's socket, and used jumper leads to connect to the ESP32.

The next step was to test the ADC that we were planning to use, the 16-bit ADC1115 instead of the on-board 8-bit ADC from the Nano. The on-board ADC for the ESP32 has too many issues, so we can't use it with this application. The ADC1115 has double the resolution, the down side is that the communication with the chip through i2c is very slow.

In the above picture you can see that I wired-up the ADS1115 ADC on a break-out board. With this prototype, I still used the 16-bit PWM functionality, but we were already planning to use a hardware 16-bit DAC.

So after checking and verifying the changes, this resulted in a new prototype, that we called V5.

Testing the V5 prototype

Version 5 incorporated the ESP32, the 16-bit ADS1115 ADC, the REF5040 4.096 Voltage reference for the DAC, the 16-bit DAC8571, test hardware for the CV mode and Bud's vastly improved circuitry to drive the MOSFET's.

Unfortunately, we discovered a few layout issues that we needed to fix by jumper wires and Manhattan style additions. We also found that the ADS1115 was happy with 3V3 i2c voltage levels, but the DAC, that we could not test earlier because of its tiny package, did not. That called for two bi-directional 3V3 to 5V level convertors, and I used a 4-channel circuit board connecting it Manhattan style to test that. To be able to increase the i2c clock speed, I changed the 10K pull-ups to 2K2.

We then figured out a slightly different circuit for the CV mode, eliminating an extra DAC that we were planning to use. Using that earlier method would actually be cumbersome to drive with the planned user interface so the new method uses two CMOS switches to automatically change the configuration to use the DAC for the CC mode and also for the CV mode.

Unfortunately, we also found out that the heatsink I was planning to use, that would fit nicely in the enclosure I also wanted to use, could not handle the heat transfer. Not even close. Not only did things got really hot, we were far away from the specifications! I measured the NFET package temperature at well over 160 degrees C, and the heatsink temperature rose to almost 100 degrees, and that at a mere 90W for only a few minutes. Bummer!

The obvious solution is to use a different heatsink with a significantly larger surface. 

Unfortunately, larger heatsinks are very hard to find, unless you go for the typical PC CPU coolers. The ones that are available literally tower above the NFET, and it's not easy to use them for two NFET's or find an enclosure for this construction. 

We found two heatsinks that looked promising and I ordered both of them. Luckily, the most promising arrived early, so we could start to test with it. This is the one on the left. It has the largest amount of contact surface. 

The one on the right relies on a fan at the end (or even both ends) to blow the air through. Unfortunately, that calls for a small diameter fan and they have difficulty getting enough air displacement, unless they rotate faster, making a lot more of a whining noise. Which is another reason why it is not my favorite.

The one on the bottom is the one I had been using. I purchased a number of them for other projects many years ago. I did an extensive search, but I could not find anybody offering them anymore. 

The different heatsink called for drastically different construction because the length of the leads to the NFET's and the critical components on the PCB are very critical. We did not want to split the NFET's and give them each their own heatsink, or mount them on either side of the heatsink.

This is why we ended-up with the contraption below to just try it out. It did not need many PCB changes, so was relatively easy to setup and test. The original fan is now moved from the top to the bottom, getting its air from the bottom of the enclosure. The fan is blowing directly to the fins of the heatsink. We will need to use a second fan to suck the hot air out of the enclosure, which has some ventilation slots.

 

The first test already showed that we could now easily handle 90W for quite some time, which was not possible with the previous setup. The enclosure we selected later required us to rotate the heatsink 90 degrees horizontally, so a fan mounted on the back panel can suck the hot air out.

A longer-term test at 150W showed that we were on the right track. Everything stayed at manageable temperatures.

At this moment, let me explain what you see on the above OLED display.

On the top, you see the actual DUT (Device Under Test) voltage in blue and below it, the actual DUT current value in green.

Below that is a line that shows the mode (CC = Constant Current) the NFET's are driving the DUT (ON)  and the actually calculated DUT power in Watt. The display is in red because the power approaches the maximum value and is there to warn the user.

Underneath the horizontal line is the parameter you set with the rotary encoder. The mA suffix changes with every mode. The last line shows the actual 16-bit DAC value (while testing) and the heatsink temperature in degrees Celsius. The color is orange to warn the user that the temperature is above 60, it turns to red above 90 degrees.

The colors are used to warn the user for conditions like over temperature (90), over-power (150), low (<1V) or negative DUT (wrong polarity), Voltage too high (>100V), over current (>10A), etc.

The discrepancies in the display values for set/actual current are caused by the fact that I for this test I didn't use the sense inputs yet so the drop in resistance due to the high power and long leads is not accounted for. Yet.

The User Interface to drive the instrument is kept as simple as possible, and I let the software do most of the setup underneath the hood.

The rotary encoder that you see mounted on the PCB is there so I can more easily test the setup with this prototype. It will later be placed on the front panel. In the CC mode, as you see above, you set the current by turning the knob. The knob currently has two speeds. If you turn it fast, the resolution goes to 10x so you get to your destination rather quickly. Short pressing the rotary encode button switches the DUT output to on and off, by turning off the drive to the NFET's. A long press switches to the next mode. You simply cycle through the modes in a loop, so from CC to CV, to CP, to CR and from the BT mode back to CC. When you switch to another mode, the output is automatically disconnected, and the instrument is prepared for the next measurement. The mA suffix you see in the Set line, changes with the modes, so from mA, to V, to W, to R and in the BT mode back to mA. There is no separate transient or "pulse" mode, because this feature can be used in all modes. That may not be very practical in all modes but the pulse functionality is as versatile as your function generator supports. (amplitude, frequency, pulse form, pulse width, rise/fall times and offset)

On the front panel, in addition to the rotary encoder and the OLED display, there will be two toggle switches, one for selecting the sense input terminals, and one to select power. There are two BNC connectors, one for the transient/pulse input and one for the DUT current output to a DSO. There will be an USB-C connector to the ESP32 that is required in the Battery Mode and also allows for updating of the firmware. And finally, two 4mm Banana connectors for the power connections and two 4mm Banana connectors for the sense inputs. Simple and efficient. And no, there is no keypad. It is not needed. I dislike keypads so did my best to design around it.

A suitable enclosure

The heatsink has been tested and verified that we can now easily get to 150W, but that is still outside of the enclosure. Because of the height of the contraption, we also had to find another enclosure that would accommodate this construction. Unfortunately, there are not many compact enclosures that answered our requirements.

We prefer a plastic enclosure, because the heatsink is connected to the DUT through the non-isolated NFET's and that can get up to 100V DC.

We also need to modify the enclosure to get enough air in and out. That's a lot easier to do with a plastic enclosure. And we want to be able to make a PCB-based front and back panel.

The height of at least 9cm dictates that you will get (instrument) enclosures that are very wide, and not very deep. They are unsuitable for our requirements, but it's up to the other makers to select what they want.

The enclosure we selected is from the company TEKO. They have many enclosure types, but we selected the AUS series that is pictured below. The top one in the picture below is the one I already used for several of my designs, and was planning to use. With the new hardware construction, it is not deep and high enough. The middle one is wider, but has the same height. We settled on the bottom one, the  TEKO AUS33.5.  TEKO is a German manufacturer but with a little Google-action, you can find them all over the world, and there are suppliers that send them all over the place. I use www.tme.eu a lot for my purchases, and they carry these enclosures.

So with this version of the prototype, I have been very busy with the testing, implementing and again testing the changes we needed/wanted to make, and all the while further developing the software.

Final Version 5.1 PCB

Based on the now fully working V5 prototype, we have redesigned the board for the new heatsink configuration and made several other changes and corrections. The PCB has been produced again by my sponsor PCBWAY. They financially sponsor this project by producing the boards and sending them to me for free. The turn-around is mostly within a week, and the quality is excellent.

The small TO220 heatsinks you see on top of the NFET's help to disperse their heat better, because they are in the forced airflow. Instead of one 4-pin fan, we now have two connectors (the white ones top left) so the first fan sucks fresh air to the heatsink and the second fan that sucks the hot air out of the enclosure. Both fans are temperature PWM speed regulated by the sensor on the heatsink.

We also switched to another OLED display source, because the one I purchased many years ago was no longer available. We selected a good quality and commonly available version but that has a different pin-out so we fixed that on the PCB as well.

Here is a quick sneak preview of the completed instrument . Note that I switched to even larger TO220 heatsinks on top of the NFET's to get rid of even more heat now that these fins are in the airflow of the second fan.

Here is an overview of the construction.

The unit has been stress tested with a 180W 15 minute test while using an IR-camera to keep an eye on the temperatures. I believe we can support 200W, but I don't have a large enough current source to test it out. 

Above is a picture of the completed instrument. The sheet metal front panel that came with the enclosure was used to experiment and try out the connections. It will eventually be replaced with a PCB that will have the silkscreen indicating the functions and connections.

Top left is the 12V DC power switch. The larger empty hole to the lower right of the power switch was a mistake. The power switch was in that position earlier, but it was too close to the ESP32 board so I had to moved it away.

The black oval next to it is the USB-C to USB-micro adapter providing access to the ESP32. The USB connection to a PC is required in the Battery Mode, and can also be used to debug the firmware or upload newer versions.

The four empty holes around the OLED display stem from an earlier attempt to fasten it to the front panel. At this moment, I use sticky tape to mount it. I have an idea that I will try when I design a PCB as the front panel that will not show any holes or screws.

To the right with the large knob is the rotary encoder that controls the setup of the instrument and the measurements.

Lower left is the pulse/transient input BNC connector. It can connect to a Function Generator. The input voltage is 5V for a DUT current of 10A.

Below it are the two 4mm Banana jack sense input connectors.

To the right is the sense toggle switch, switching the voltage measurement to the sense inputs or to the main inputs. Using the sense inputs is really needed with high currents, due to the voltage drop over the main DUT leads.

To the right are the main 4mm Banana jack DUT power connections.

Slightly above that is the DUT current monitor BNC connector. The output is 1V for 10A of the DUT current.

The back panel has the second 90mm fan mounted and also the 12V DC input connector.

Battery Test Mode

The Battery Mode application that runs on a PC, communicates with the Dynamic Load, sets-up the instrument and does the graphing during the run is available on the GitHub site. It's a single executable that you can install on your PC wherever you like, and you can run it from there.

There is a separate Blog post that has more information about the project here: 
https://www.paulvdiyblogs.net/2019/03/a-pretty-universal-battery-cell-tester.html

My project was based on the work from John Lowen and his details can be found here:

http://www.vwlowen.co.uk/arduino/battery-tester/battery-tester.htm

I modified the original code that was running on the Arduino, made it work in the ESP32 environment of the DL and integrated it into the main menu structure.

Below is an example of the Battery Test mode. I'm using a Lithium 3.7V 14500 cell. The cell is several years old and had not been used. The specification is 650mAh and I tested it with a discharge current of 2500mA. The cut-off voltage was set to 3V.

These values are entered into the Battery Test PC program below on the bottom left hand side.

The test is started from this program, and it sends the parameters to the Dynamic Load and starts the measurement. 

During the measurement, the display is refreshed with the parameters coming from the Dynamic Load, such as the voltage, the current, the time and the calculated mAh value, shown on the bottom right. The graph is dynamically updated during the measurement.

As soon as the Dynamic Load measures that the cell voltage has dropped below the cutoff voltage of 3V, the measurement stops and removes the load. 

Just above the green Test Ended message, the Controller Message shows a Serial Time-out, instead of the Cutoff Voltage, because it was waiting for another measurement that took too long to send the data. The DL terminates the waiting loop and restarts the reception of the setup parameters or a mode change automatically.

This was a test of an EFEST 14500 IMR cell of 650mAh. These IMR cells can deliver up to 9.75A for short periods, or up to 6.5A continuous. Bud and I used these cells in the Raspberry Pi 3 UPS, to supply enough voltage to let the RPi ride out a power glitch or let it power down safely. I had not used these cells for many years and just charged them again for a few times to run this test. 

With just over 550mAh, the test shows that the cell is no longer meeting the specifications of 650mAh, but is still usable.  

In order to test at higher currents, you really need to use the sense inputs, which I did. I also used a metal battery holder, plastic ones would melt with these high currents.

 

Note that I faked the Rated Capacity of the cell in mAh from 650mAh to to 2000, to overcome the calculated  time limitation in the software (rated mAh/current), which would terminate the measurement prematurely. You can save the details including the graph, and when you do, you can enter more information about the cell and also add the weight of the cell, which is a good give-away indication for poorly "specified" or plane fake cells.

When the measurement is terminated or ended, you can restart the measurement in the PC software and test another cell. Long-pressing the rotary encoder switch on the DL brings you back to the CC mode. The original hardware that was intended for this PC program would also re-start the Arduino Nano controller. We don't need to do that, the DL software automatically takes care of all that.