Dual channel 0-15V 0-500mA power supply - Page 1
Forum section: electronics / projects

author: kallek
author_rank: Regular Contributor
timestamp: February 17, 2020, 09:41:38 pm
content: Hello everyone!This is my first post in the Forum!I want to share you my power supply project progress. I have been using basic Lm317 power supply which i built four years ago, but i want capability to limit current when testing new circuits.Specs:- Two independent channels, 0-15V, 0-500mA- Linear, TIP122 darlington series pass transistors- Passive cooling, heatsink on back panel- 40VA 2x15VAC toroidal transformer that I had already- Zener references- Lm324 OpAmp for sensing voltage and current, and driving CV/CC leds- Low side current sensing, seven 10 ohm 0,6W resistors in parallel- Analog panel meters for voltage and current readings, two per channel- Switch to short output to set current limit- Load switch- Single turn wirewound potentiometersI have already etched and assembled the circuit board, and it looks like working fine. Schematic is not ready yet, but I will share it when I have finished it. Transformers voltage is slightly low, but not too bad, no large ripple on 15V at 500mA with 3300uf smoothing capacitors. I intend to use toggle switch as load switch. I know it have that contact bouncing effect, but is it too bad practice if it does not overshoot? Next step is get sheet metal for case, and wait panel meters to arrive.Thank you for advises and comments!

author: David Hess
author_rank: Super Contributor
timestamp: February 18, 2020, 05:44:50 am
content: Quote from: kallek on February 17, 2020, 09:41:38 pmWere you going to make the channels floating from each other? This had the advantage of allowing them to be combined in parallel for higher current, series for higher voltage, or series for a bipolar tracking.Quote10 watts is in the range where integrated regulators which provide built-in thermal protection could be used as pass elements.QuoteI would not get hung up on using a quad operational amplifier. Some of the better designs take advantage of separate amplifiers with different supplies for each output or the same output.QuoteWhy require low side current sensing?QuoteI prefer good quality single turn potentiometers for coarse and fine control but both methods have their virtues.

author: kallek
author_rank: Regular Contributor
timestamp: February 18, 2020, 07:38:48 am
content: Thanks David Hess!Quote from: David Hess on February 18, 2020, 05:44:50 amYes, I will use them just floating or connect to ground binding post. I assume that this is possible with two separate windings? That is true.QuoteI have to think that.QuoteI have these on hand and layout was easy to make. With high enough value current sense resistor accuracy is about 2mA and current can be set down to under 20mA. I choose low side sensing because then diff. amplifier is not deeded.QuoteIt could be nicer... Good quality pots are not cheap so I try with single pots with big knobs.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 18, 2020, 01:46:27 pm
content: The LM317 can be used as a kind of protected transistor at the low power. However it may cause a little more voltage lost - so it could be a problem in this example with a relatively low transformer voltage.I like the choice of low current and voltage for a beginner's project. so less of the magic smoke to escape.With separate windings one could use the 2 supplies in series / floating. However the maximum permissible voltage between the 2 parts may be limited.

author: MarkF
author_rank: Super Contributor
timestamp: February 18, 2020, 09:23:33 pm
content: Pictures broken...

author: Zero999
author_rank: Super Contributor
timestamp: February 18, 2020, 09:55:54 pm
content: Quote from: MarkF on February 18, 2020, 09:23:33 pmYes, the original poster should attach them here, rather than some image hosting site.

author: kallek
author_rank: Regular Contributor
timestamp: February 18, 2020, 10:29:07 pm
content: Thanks, tried to fix them.

author: MarkF
author_rank: Super Contributor
timestamp: February 18, 2020, 10:40:16 pm
content: Quote from: kallek on February 18, 2020, 10:29:07 pmThey're okay now.

author: exe
author_rank: Unknown
timestamp: February 19, 2020, 01:09:19 pm
content: Nice build! Can we see the schematics, please?

author: David Hess
author_rank: Super Contributor
timestamp: February 20, 2020, 06:00:30 pm
content: Quote from: kallek on February 18, 2020, 07:38:48 amNothing requires high side current sensing to use a difference amplifier; the error amplifier for controlling the current can operate at the high side directly if the control voltage is referenced to the high side which is trivial when poteniometers are used. A simple example is shown below.

author: kallek
author_rank: Regular Contributor
timestamp: February 20, 2020, 07:15:38 pm
content: Thank you. Actually, I used that ti circuit to design my own. I just put current sense resistor below negative terminal, because my opamp can go to negative rail but not to positive. I will show my schematic when I have drawn it.

author: Zero999
author_rank: Super Contributor
timestamp: February 20, 2020, 08:50:07 pm
content: Quote from: David Hess on February 20, 2020, 06:00:30 pmThe LM101A is now expensive and hard to get hold of. Use the LM201A instead.What's going on with the diode across C8? I thought polarised capacitors can withstand short reverse voltage transients, so I doubt it's to protect against that.

author: David Hess
author_rank: Super Contributor
timestamp: February 21, 2020, 01:59:13 am
content: Quote from: Zero999 on February 20, 2020, 08:50:07 pmI just use that schematic as an example. The unique thing about the LM301A at the time was that its input common mode range includes the positive supply which made it almost uniquely useful in high side sensing applications. A lot of old JFET parts would also work. Today many rail-to-rail input parts meet that requirement or a slightly higher supply voltage for the operational amplifier could be used.Incidentally, there is some advantage to putting the current sense resistor directly adjacent to the output capacitor although this could be done on the low or high side. If AC feedback is taken before the current sense resistor, then the increased ESR of the output capacitor as seen from the error amplifier provides extra phase lead to increase stability. Here R15 provides the same function but at the cost of increasing the high frequency output impedance. The old Tektronix PS503 type power supplies do this with the current sense resistor on the high side but without the AC feedback part. Some modern low dropout integrated regulators which can use ceramic capacitors do this internally.QuoteQ2 could potentially sink 2 amps of current when pulling down a large capacitive load into the negative supply which is unlikely to be rated for that so the diode is required to avoid damaging C8. Q2 and associated parts could be left out if that function is not required or undesirable.

author: exe
author_rank: Unknown
timestamp: February 21, 2020, 01:33:03 pm
content: Quote from: David Hess on February 21, 2020, 01:59:13 amInteresting, so a capacitor can be damage by too large current through it? I had this question is a long time ago, all I could find was that average current through the cap should be within its rating, or it may overheat. But no information that, say, shorting leads of a charged capacitor can damage it. Or charging it too quickly.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 21, 2020, 02:56:48 pm
content: The problem is more that the negative supply may not be able to provide 2 A of current. So if the supply than goes into sinking mode this could bring the negative supply to positive values and thus reverse the capacitors. This would be an unlikely scenario and many electrolytic caps do survive a little reverse voltage for a short time. The diode would just limit that voltage.In simulations I found ot tricky with the shunt at the collector side of the power transistors. This kind of slows down the transistor and thus can help with instability. There is nothing really bad with a low side shunt, especially with an OP that works at the negative rail.

author: kallek
author_rank: Regular Contributor
timestamp: February 21, 2020, 06:49:38 pm
content: Update to this project. Two days ago I got four of these nice Kyoritsu panel meters.Yesterday we bought 1mm and 1,5mm thick sheet metal for metal project box from local metal store. Today I started cutting and drilling.

author: kallek
author_rank: Regular Contributor
timestamp: February 27, 2020, 01:03:28 pm
content: Here is the schematic. I might have made mistakes, so please give your suggestions.EDIT: R3 should be 4.7k, not 470.

author: xavier60
author_rank: Super Contributor
timestamp: February 28, 2020, 04:23:36 am
content: Make provision for compensation components for the CC op-amp.A series resistor feeding pin 6, also a speed up diode across this resistor.A series resistor and capacitor from pin 7 to pin 6.The high value of R7 will cause some extra CC op-amp input offset.It might be better with D8 across the B-E junction.

author: xavier60
author_rank: Super Contributor
timestamp: February 28, 2020, 08:03:55 am
content: Actually I bread-boarded the CC loop and found that compensation isn't really needed. There is a short burst of ringing when the load is applied due to lead inductance. I don't expect this to be a practical problem. I would still put a resistor in series with pin 6 just for input protection.

author: kallek
author_rank: Regular Contributor
timestamp: February 28, 2020, 08:09:08 am
content: Thank you xavier60, some good points!I am not very experienced in electronics yet, and not very good at english, so there are couple of things i have to clear.I somehow understand the purpose of resistor feeding pin 6, but what that diode across it do? How I should bias it?I put D8 there to prevent Q1 turning on if pin 7 can not go complitely down. But is it something to do with switching speed? Did you mean reverse biased diode across B-E junction?Because I made the board before understanding to ask help, I will now experience with it and modify it if necessery. I will use your tips to do things better in the future. I really appreciate you sharing your knowledge!Edit. Thank you xavier60 for your time! So you only recommend resistor or resistor and diode both?Kalle

author: xavier60
author_rank: Super Contributor
timestamp: February 28, 2020, 08:31:44 am
content: Quote from: kallek on February 28, 2020, 08:09:08 amThe diode across the resistor to pin 6 isn't needed now.While you were posting I did another experiment. I set the unreg rail to 20V and put a 2.2K resistor from unreg to the output of the CC op-amp. The op-amp's output was only able to pull down to 1.2V with about 9mA load. Add to this the 0.7V drop of D9 increases to about 1.9V, so D8 is needed where you have put it and may not lose enough voltage to allow Q1 to turn off fully.My idea was for the op-amps to be able to pull some current out of the Base of Q1 to help it to turn off more quickly when the power supply is suddenly unloaded. A diode anti-parallel with D8 should help.Have you tried bread-boarding the circuit so that you can further investigate these issues?

author: kallek
author_rank: Regular Contributor
timestamp: February 28, 2020, 08:53:53 am
content: That confirms what I have experienced. Did you have that 2,2k pull-down resistor too? If I remember correct, I measured something like 0,7V from pin 7. I have modified circuit since last bread-boarding, but I will do it and further investigate it.Kalle

author: xavier60
author_rank: Super Contributor
timestamp: February 28, 2020, 09:18:48 am
content: I didn't have the 2.2K to ground. And I'm using an LM358 which might be a bit different.Using a 1A schottky for D9 will also help.

author: Alti
author_rank: Frequent Contributor
timestamp: February 28, 2020, 09:52:43 am
content: LM324 does not have input current bias compensation so you have to make sure input impedance of both input nodes is equal. The LM324A has 0.1R at one node and 470k at the other.As reference voltage, you use zener diode. Get a TL431. A single one will do.Do not connect voltage feedback before ammeter but after it.I'd put a fast 0.5A fuse and a varistor just after feedback tap.

author: xavier60
author_rank: Super Contributor
timestamp: February 28, 2020, 10:42:39 am
content: Another surprise. Pulsing the output with 1A load caused a very small 50mV overshoot with 100uF output capacitance and a D44H BJT.It still would be a good idea to add the anti-parallel diode in case the BJT leakage rises too much with temperature.

author: xavier60
author_rank: Super Contributor
timestamp: February 28, 2020, 11:59:19 am
content: Because the bias current for the CV reference flows though the CS resistors, expect some interaction between unreg voltage and CC set point, Also some output ripple in CC mode. It might not be bad enough to worry about.

author: xavier60
author_rank: Super Contributor
timestamp: February 28, 2020, 01:31:07 pm
content: I have just realised the TIP122 is a Darlington. It should have no trouble turning off because of the built in B-E resistors. Leave the circuit as it is.

author: David Hess
author_rank: Super Contributor
timestamp: February 28, 2020, 08:37:15 pm
content: Quote from: exe on February 21, 2020, 01:33:03 pmIn this case it is because the large current could potentially apply reverse voltage across the polarized capacitor. The operational amplifier might also object to having its supply pin raised significantly above ground.Limiting the current would solve the problem but then discharging a large output capacitance to ground would take longer.

author: kallek
author_rank: Regular Contributor
timestamp: February 28, 2020, 09:32:32 pm
content: Some update. I painted the case last Saturday and I was in school this week. So paint was allowed to dry well.Today I started the most exciting part, putting it all together!Small toggle switch on the left is momentary switch for setting current and the right small one is normal toggle switch to switch load off and on.

author: exe
author_rank: Unknown
timestamp: March 01, 2020, 08:43:51 am
content: Well done!How did you make the enclosure?

author: kallek
author_rank: Regular Contributor
timestamp: March 01, 2020, 02:03:20 pm
content: Thank you!First step was cutting top cover and bottom side with angle grinder. Then I marked and drilled all holes. For big holes I drilled eight small holes around the hole and then filed it to round. It was very hard and time consuming process.After that I bended pieces with my homemade metal brake. Then I welded small flat-bars to both sides where I drilled and tapped threads to screw in top cover. If one want something similar and do not have required tools, some local school or workshop will do something like this for very little amount of money.Today I did some wiring. I want to make it nicely, so potential repairs in future will be much easier.For last picture I just put everything in place to see what finished power supply will look like.I am not sure how I should do marking to front panel, or let it without markings.

author: ggchab
author_rank: Frequent Contributor
timestamp: March 01, 2020, 03:52:00 pm
content: Very nice work

author: kallek
author_rank: Regular Contributor
timestamp: March 11, 2020, 10:29:28 am
content: This morning I made some oscilloscope test to this power supply:

author: exe
author_rank: Unknown
timestamp: March 11, 2020, 10:34:07 am
content: I think it's not a ripple, the psu is oscillating at freq ~1MHz, imho. So, more compensation is needed.Edit: and power-on glitch is an inductive kick or something, I have similar on my power supply. So, for my next psu I'll add a soft-start.

author: kallek
author_rank: Regular Contributor
timestamp: March 11, 2020, 10:43:56 am
content: I think you are right. But should I be worried about it although it is only 5mV oscillation?

author: exe
author_rank: Unknown
timestamp: March 11, 2020, 11:08:41 am
content: Only you can decide if this is a problem or not. Also, I don't know how it was measured. May be it can become worse under different condition. Like, a capacitive or inductive load may make it worse.I'd say the whole point of a linear *bench* psu is to have a clean and stable output. I treat it as i a piece of test gear that I can trust. So, if I, say, troubleshoot an amplifier, and I see an oscillating output, I want to be sure it's not because of oscillation injected from a power rail. So, yes, I'd call this a problem. And it seems to be easy to fix.

author: xavier60
author_rank: Super Contributor
timestamp: March 11, 2020, 11:09:05 am
content: Try increasing C5 to something like 100pF.

author: kallek
author_rank: Regular Contributor
timestamp: March 11, 2020, 11:16:00 am
content: Thank you exe and xavier60. I will try to put 100pf cap parallel with c5 and lets see what happen!

author: kallek
author_rank: Regular Contributor
timestamp: March 11, 2020, 11:59:52 am
content: I tested 22pf and 100pf parallel C5 and no effects at all. Problem seems to be only in CC-mode. that picture with 5mV oscillation was taken output shorted. Now I connected 1 ohm resistor and adjusted voltage so that current was just below 500mA. Results:

author: xavier60
author_rank: Super Contributor
timestamp: March 11, 2020, 12:07:35 pm
content: Try adding a compensating capacitor to the CC op-amp.On second thoughts, a compensating capacitor won't do much unless there is a 10K resistor in series with pin 6

author: kallek
author_rank: Regular Contributor
timestamp: March 11, 2020, 12:18:24 pm
content: I will try it, but at the moment I have no time to do another board. For me the current limit is mostly for protecting things and testing components, so I think it is ok to me.

author: xavier60
author_rank: Super Contributor
timestamp: March 11, 2020, 12:21:13 pm
content: It doesn't seem like a serious problem.

author: kallek
author_rank: Regular Contributor
timestamp: March 25, 2020, 04:03:29 pm
content: I finally got power supply finished and took some photos. It is now sitting next to my old power supply and soldering station.