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Sunday, October 2, 2016

Building a Bench Tracking Dual Voltage Supply

For some of my experiments and tinkering, mostly with op-amps, I wanted to have another power supply that would give me a precise dual-tracking complimentary voltage, 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 at 10mV levels.
5. Accurate display of output Voltages with less than 0.01% error.
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 my other supplies and DC Load.
10. Maximum current between 80-100mA 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.

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. Note that I made an error by naming the LM337 a 377.

Let's just start with the positive voltage supply, and dive right in, the rest follows. The output voltage is regulated and set by IC7, an LM317AHVT, which is the high voltage version. To get a regulated 30V at the output, I need to supply several volts more. When the transformer is not loaded much, the voltage can get to levels that are too high for the standard LM317, which is why I use the "H" 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 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 adjust voltage 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 at about -1.3V. That's close enough. If you want to quickly remove the output level of the supply, you need a way to remove the output voltage. I used a switch (S3) across the Volt Adjust potmeter, to do that. And that pretty much covers the positive voltage setting.

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.

Let's switch our attention to the current limiting section, and we'll use the positive supply again. IC4, yet another LM317 is used as the current limiting device. The current limiting is depending on the voltage of 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 1K. D11 and D12 limit that range to 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.

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 is again a mirror copy of the positive portion.

OK, let's move our attention to the dual voltage tracking circuit. I used a simple method with two precisely matched 10K resistors to create a virtual ground level at the midpoint. After testing the result, I found that I still needed an adjustment trimmer R30. The level at the wiper goes to the inverting input of an op-amp, IC8, and that compares that with the true ground. The op-amp will make sure that it's output is driven such that the 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. 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/C18 and C23/C24 to filter unwanted noise. I kept C23/C24 as low as possible to protect them from dumping their load into my precious DUT circuit. D15, D14 and D5 are an insurance for my stupidity, if there are capacitors 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 I may accidentally try to dump into the supply.

The supporting team is made up of transformer TR1 to supply the main voltage of the supply. I have used R1, R2 and R3, which are PTC's to add a level of protection for over currents. They are self-healing.

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 to high when there is no load.

To minimize the development of heat and use normal regulators for the +/- 8volt supplies, I used a separate transformer. 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 last element is the voltage display. I found a module that has a real DMM "inside", is very accurate and works up to 33V.   Voltmeter
(if the links is broken, search for "LED 5 digit DC 0-33.000V Digital 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 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 then reverses this for the negative supply (positive input is now ground, and the input ground is now the minus output supply. Simple and effective.

There is one caveat with a tracking supply like this one. The negative supply tracks the positive one. 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 will need to find a way to make the positive supply follow the negative supply.

After I finished building the supply, have been using it for a few months 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. Hmm... Maybe this is a hint to the other design that I have been contemplating for a while and will have even more accurate voltage and current setting? In the mean-time, this works for me.

Here is a picture of the main circuit board in an earlier stage, when I was still using the 1.25 references (the SMD parts), 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.

I didn't make pictures of the enclosure yet, but I will when I'm back home and also update the picture below.

Sorry for the bad focus, I can do better!


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