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Friday, August 26, 2022

Dynamic AC/DC Load

This will be a description of my process to build a new Dynamic Load.

Below is the latest version of the prototype.



I already have a Dynamic Load, described here (https://www.paulvdiyblogs.net/2015/08/dynamic-dc-power-load.html). This instrument served me well while testing Power Supplies. It is a rather simple analog Constant Current mode Dynamic Load, but also has a pulsed output possibility and an offset capability so you can set the minimum and maximum current while pulsing.

For the new Dynamic Load, I wanted to add more modes like Constant Voltage, Constant Power (wattage), Constant Resistance and a Battery Discharge option. This will mean that it will have to be a digitally controlled instrument.

There are many designs that accomplish this already, but I could only find one that implemented an AC input, next to the usual DC input. The AC capability is useful for applying a load to non-DC inputs, like an AC sine-wave coming from transformers so you can profile them, and also other waveforms like triangle or pulsed like PWM signals.

The first challenge:

The first challenge is to see if I can combine a DC input with an AC input.

Adding an AC input goes hand-in-hand with a True RMS measurement capability which is an additional challenge for me. To top this off, I also want to use the new Raspberry Pi Pico board. It has a lot of interesting features and also has a dual core. This may be required to calculate the True RMS voltages if I decide to do that in software. Dedicated chips that can do that are pretty expensive.

I have not decided on a set of specifications yet. My initial requirements for the design phase will be if I can get up to 400VAC at say 1A. A lower voltage could be 60-80V at 3A and 30V at 5A. I don't need a larger current sink, this should do for my own use.

The unit should have a "pulsed" mode to test transients, and as I mentioned above, a battery discharge feature. I'm not interested to build a PC controlled unit, that's beyond my capabilities, but I could possibly create an interface that is compatible with existing PC-controlled software applications.

The user interface itself will be very simple. I envision a rotary encoder for the settings and menus, with maybe only one toggle switch or push-button to turn the load to the DUT on and off. I will try to use the rotary encoder push button to enter values and select menus. No key-pad! The display will most likely be a small 128x128 OLED display, most likely in color because the price difference between color and monochrome is very small these days.

The enclosure should be a small as possible, because the available real-estate in my lab is very mimimal.

So there you have it.


Standing on the shoulders of others

I have been collecting various designs from others and looked at the way they did things. Several stand out, like the multi-part Youtube series from John Sculley:
https://www.youtube.com/playlist?list=PLUMG8JNssPPzbr4LydbTcBrhoPlemu5Dt

Then the contributions from Jay_Diddy_B on the EEVBLOG. 
https://www.eevblog.com/forum/projects/dynamic-electronic-load-project/

I also studied the tear-down and schematics of the Array 37XX Dynamic loads done by another frequent user Kerry Wong, that can be found here:
http://www.kerrywong.com/2018/11/05/teardown-of-an-array-3711a-300w-dc-electronic-load/

Here is another one: 
https://www.instructables.com/Arduino-Programmable-Constant-Current-Power-Resist/

As I mentioned earlier, I could only find one DIY design that features a true AC/DC capability. That project from Rainer Schuster can be found here:
https://www.elektormagazine.com/labs/electronic-load-for-dc-and-ac (there are two versions of diagrams available, you really have to search for the latest one)

Here is a Youtube video of the instrument with an elaborate explanation of the AC mode of operation and background theory.
https://www.youtube.com/watch?v=YXUa-xOSNLE



Monday, June 20, 2022

The VBA Curve Tracer

 The VBA Curve Tracer


Following is a description of the final version for the VBA Curve Tracer instrument.
 
The name VBA is derived from the surnames of the three main contributors:
  • Paul Versteeg
  • Bud Bennett
  • Mark Allie

The VBA CT is the culmination of a project that spans several years, and started with my design of the first version, which was actually a working prototype. This is described in the post with also a detailed Theory of Operation. https://www.paulvdiyblogs.net/2017/12/building-curve-tracer.html

There is also a description of the second generation, based on the first prototype. This is a fully functional CT that has been build by a few, but has some problems and shortcomings that we're addressing in V3.
https://www.paulvdiyblogs.net/2021/03/building-curve-tracer-v2.html

The following post shows some of the stumbles and falls in addition to discoveries and improvements made during the design process of the Version 3, we now call the VBA Curve Tracer.
https://www.paulvdiyblogs.net/2021/03/building-curve-tracer-version-3.html

Finally, there is also a blog post with a list of measurements we can make with the VBA Curve Tracer.
https://www.paulvdiyblogs.net/2021/11/making-meassurements-with-v3-curve.html

 
All the files (schematics, 2D and 3D board layout pictures, BOM's, build instructions, calibration & verification and the Gerber files) that are needed to build this instrument are added to a dedicated public Github project:
 
Note  
We will not publish the native Altium formats, only PDF or PNG files to preserve our ability to control the quality and protect ourselves from unauthorized manufacturing. Contact us if you have plans to do so.

Note also that we may upload newer files when we see the need to update schematics or documents. We will overwrite older version, so there will be no version control. You can use the dates to see if changes were made. 

We have now uploaded the latest files to the Github project, and incorporated the latest schematics in this Blog.

Ordering PCB's
We have asked two board houses if they would consider allowing the ordering of 1 set of the 4 boards in one order. Unfortunately, at this moment, both JLCPCB and PCBWAY cannot handle this order type.
You need to order the boards in sets of 5. You could provide or sell the other boards to other users, something we currently will not do ourselves.
In case you will have extra boards available, you can use the following blog to offer that possibility to other users (this blog is not suitable-please keep it clean):



PCB Assembly
PCBWAY can also populate one or more of the Main and Front boards for you. The few parts on the Front panel do not warrant this due to the high setup costs. The minimum that JLCPCB will produce is two. 

Mark has worked with PCBWAY to let them manufacture and populate 10 Main and 10 Front boards for him with only the SMD parts that they normally carry (so no specials).
That required changes to our BOM's to make that more straighforward. We also needed to produce Centroid files for each side with the part coordinates for the pick&place machines. The required information has been added to the Github project. 


"Design is easy, production is hell"
At this moment, we're finding out what this means. While going through the process of having PCBWAY produce the boards, we have found several issues with our BOM's due to part end-of-life, shortages, prices and some hard to get resistor values. Part availabilty and end-of-life situations will continue to effect this project. At this moment we do not update the BOM's with that information. Please search for applicable replacements yourself.

The sets of 10 boards that Mark ordered will be used to build 10 Curve Trace instruments by the students of his university. The instruments will eventually be used in the university labs. We will report the findings here. 

Ordering parts/shortages:
We have found that the main capacitor C6 of 2200uF/250V is no longer in stock. Even the 4-pin alternative is now out of stock. It seems that it will take until 2023 before stock levels will replenish. As an alternative, you can use a 1500uF/250V version like the  380LX152M250A052 that Mark will be using for his 10 instruments. This capacitor has a specification for the ripple current of 4A @ 120Hz, which should be good enough. This is added to the BOM.


Because the ordering process with parts can be picky, you can reference the two orders for the two sets of 10 each for the Main board and the Front board as they were ordered by Mark and manufactured by PCBWay:
Main board order number for PCB and parts: T-U517W12659A
Front board order number for PCB and parts: T-U519W12659A


Stay tuned...