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Sunday, October 2, 2016

Building a Bench Tracking Dual Voltage Supply

Dual Tracking +/- 30V @ 100mA Power Supply

While I was building this supply, I added some functionality but did not update the front panel. That's why you see the pen markings to indicate the switch position for the DMM display measuring the positive supply or the negative supply, and that the Tracking switch puts the positive supply in the Master role. The "S" means separate supply adjustments.

The PWR switch is not a main power switch, but removes the voltages from the output to protect the DUT. It also allows you to set the voltages of the supply without having to take the leads away from the DUT. The switch should have "on" and "off" labels.

My Design Goals

For some of my experiments and tinkering, mostly with op-amps, I wanted to have an additional power supply that would give me a precise dual-tracking complimentary voltages, up to +/- 30V.

Here are a few design goals I set for myself:

1. True 0 to +/- 30V.
2. Non tracking mode to set two different voltages for the negative and the positive supplies.
3. Precise dual tracking within 1% or better.
4. Precise Voltage level setting with 1mV accuracy
5. Accurate display of output Voltages with less than 0.01% error. (no need fro an additional DMM)
6. Pretty good current limiting setting with a visual indicator. (not at a precise exact value, but good  enough because I don't want to blow-up an expensive device.)
7. Pretty good constant current/voltage operation.
8. Low noise and stability without going to extremes.
9. Small package, using the same housing as several of my other supplies and DC Load.
10. Maximum current of at least 100mA for each supply separately.
11. Using components like voltage display and transformers to be used with a drastically different design. (just in case I wanted something complete different)
12. Some protection against blowing things up and doing stupid things myself.

Using standard Regulators

For a while, I was contemplating a simple tracking LM317/337 supply, and I looked around of what designs where out there on the Web. There were surprisingly few, actually, and none fitted my bill.
Eventually, I started to piece some things together myself, but by the time I added the bells and whistles I wanted, things were getting complicated quickly.  Rather than scrapping the whole idea, I continued as a learning experience to see how far I could get this to work. In the back of my mind however, I always considered starting all over with a more traditional supply design, so I made sure most of the more expensive components could be re-used.

Here is the circuit diagram of the complete supply. Looks pretty wild when you look at it initially, but when I'll go through the building blocks it's actually not that bad. 

Let's just start with a partial diagram of the positive supply, and dive right in. 

Voltage Regulation

The output voltage is regulated and set by IC7, an LM317AHVT, which is the high voltage version of the LM317 regulator. To get a regulated 30V at the output, I need to supply several volts more. When the transformer is not loaded much, the input voltage can get to levels that are too high for the standard LM317, which is why I use the "HV" (High Voltage) version.

R28 is used in combination with R27, the 10 Turn potmeter to set the output level. R28 also makes sure that there is some minimum current flowing to keep the regulation in check. That only works well with higher output voltages, so I use a J-FET, Q5, used here as a constant current source, to ensure that the LM317 always sees an 8mA or higher current. The J-FET needs a few volt to work with, and I decided to give it -8V, because I can use that voltage level in other places as well.

The voltage adjustment setting is stabilized with C17, but that means that you also need D17, to protect the LM from the C17 discharge levels going the wrong way. To make sure that I can regulate down to 0V, I have to overcome the reference voltage of the LM317, which is 1.25V. Initially, I used a -1.25V voltage reference to create that counter-balance, but I was not too happy with how that worked. D25 and D26 in combination with the -8V will do the same and actually clamp the negative supply at the Source of Q5 to about -1.3V. That's close enough. 

Current Regulation

Let's switch our attention to the current regulation/limiting section. IC4, yet another LM317 is used as the current limiting device. The current limiting is depending on the voltage over the current shunt resistor, R12. The 12 Ohm value will limit the current to a maximum of 104mA. To make that current start from 0mA, I used the same circuit around D13, Q2 and the negative supply of -8V to do that. The variable current limiting settings are accomplished with a normal 1 turn potmeter R17, in combination with R16, to make the potmeter effective over the complete range of at least 100mA. D11 and D12 limit that range to about 1.3V, and that creates a pretty accurate way of setting the current limit. Q2, another J-FET, also functions here as a constant current source of about 8 mA, keeping IC4 into regulation at all times.

Current Limit Indicator

To get an indication of the entering into the Current Limiting or Constant Current mode, I used the circuit with Q3 and a red LED. Q3 measures the voltage drop over the LM317, and if it goes over a certain level (> 0.6V, when the limiting gets tripped), the LED with be turned on. Simple but effective.

The Negative Supply

The negative supply is a virtual mirror image of the positive supply. If you now look at the equivalent circuit on the negative side, around IC6, an LM377T, you'll see exactly the same circuit, with the Tracking Switch S2 in the position shown. Because IC6, the LM377, does not come in a high voltage version, I had to use another LM377 (IC3) as a pre-regulator to limit the voltage going in to IC6. IC3 limits the maximum voltage of about -40V to a -36V level which is safe for the 377 and provides plenty regulation head-room. Using another LM377 may look like an overkill, but the 5 components (The 377, a protection diode, two resistors and a capacitor) costs are really minimal. Yes I could have used D14 to go across all three LM377's, but that's the way the circuit developed.

Output Voltage Removal

In order to quickly remove the output level of the supply, I use a switch (S3) across both the Volt Adjustment potmeters, to do that. Eliminating the voltage over the potmeters will force the LM317/337 outputs to zero volt. And that pretty much covers the voltage regulation parts.

Dual Voltage Tracking

OK, let's move our attention to the dual voltage tracking circuit. I used a simple method with two precisely matched 10K resistors (R29 and R31) to create a virtual ground level at the midpoint. After testing the result, I found that I still needed an adjustment trimmer R30. The virtual ground or mid-point level at the wiper goes to the inverting input of op-amp IC8, and that compares that input with the true ground. There is nu current flowing so R10, the 4K7 resistor will not cause a voltage drop. The op-amp will make sure that it's output is driven such that the two inputs are equal. The output goes to the Tracking On/Off switch, and when that is flipped, it actually takes over from the potmeter setting of the negative supply, making that a Slave of the positive supply, the Master. The negative supply will now follow (track) the output level of the positive supply, also when the positive supply goes into current limiting. I have selected the TLE2141 op-amp for this job, because it can handle the supply voltages of -36V plus +8V = 44V.

The positive and negative outputs have C22/C23 and C18/C24 to filter unwanted noise. I kept C23/C24 as low as possible to protect the dumping of their capacity into my precious DUT circuit. This is a significant and often overlooked factor of most power supplies. 

Some Protection

D15 for the positive supply and D14 and D5 for the negative supply are there to protect the regulators in case the output voltage is higher than the set voltage. These diodes then dump the over-voltage into the main reservoir capacitors C6 and C5. This situation can happen when there are capacitors or batteries in the DUT that want to dump their charge back into the supply. They are protection for the LM317/337 devices. D18 and D19 are protection for reverse voltages that may accidentally try to dump into the supply.

The Supporting Cast

The supporting team is made up of transformer TR1 to supply the main voltage of the supply. Initially, I used R2 which is a PTC to add a level of protection for over currents. They are self-healing. After using the supply for a while, I took it out of circuit because it interrupted too early and I didn't have other values.

The main supplies are rectified with a full bridge filtered by reservoirs C6/C5 and C12/C9 to remove high frequency noise. Both R7 and R6 make sure that the reservoirs are emptied relatively quickly, so no voltages are present for very long when the mains is switched off. They will also put a minimum load on the transformer to protect for voltages that may become too high when there is no load supplied to the DUT.

To minimize the development of heat and use normal TO-92 regulators for the +/- 8volt supplies, I used a separate transformer with 9-0-9VAC at 80mA. These print transformers are relatively inexpensive and small, and the +/- 8V supplies are now independent of any voltage swings on the main supply. The filter circuits around IC1 and IC2 are text book stuff.

The Voltage Display

The last element is the voltage display. I found a module that has a real DMM "inside", is very accurate and works up to 33VSearch for: 0.36" 5-digit DC 0-33.000V Digital LED Voltage Meter

These displays typically generate a lot of switching noise that you really don't want to have injected into the power supply rails. At the same time, I wanted to use this voltmeter to measure the positive supply as well as the negative supply. Unfortunately, these meters only handle positive voltages. In order to switch the volt meter from one output to the other, the power for the meter needed to be floating from the main power. So, I needed a third transformer with 9VAC, to isolate the power rails and I could then do the switching with S1. S1 applies the positive output voltage to the plus input and the ground to the minus input, and reverses this for the negative supply (positive input is now ground, and the input ground is now the minus output supply. Simple and effective at only the cost of a little transformer.

Tracking Mode Side-Effect

There is one caveat with a tracking supply like this one. The negative supply (Slave) tracks the positive one (the Master). If the current limiting for the positive supply kicks in, the negative supply will follow. However, when the current limit for the negative supply kicks in, the positive supply will stay at it's set level, creating an unbalanced output situation. I have not figured out a way to solve that.

Real-Life Experiences

After I finished building the supply, have been using it for a few years now, and I'm very happy with it. The voltage level shown on the display is very accurate, it really acts like a good DMM, and so is the tracking accuracy which is well below 0.1%. During my experimenting, I find myself grabbing this supply more and more, even though I sometimes find the output dropping because I pull too much current from it. 

Below is a picture of the main circuit board in an earlier stage, when I was still using the 1.25V references (the SMD parts on the carriers), and without the current limit indicators. It has been modified quite a bit since then.

All parts within the dotted rectangles on the circuit diagram are mounted on the metal back-panel of the enclosure. The last addition, the third transformer for the display is mounted on the top half of the enclosure because I didn't have the room on the circuit board.

Sorry for the bad focus, but you get the idea.


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