18V/3A HY1503 PSU - weird CC/CV behavior while charging battery cells - Page 1
Forum section: electronics / repair

author: iMo
author_rank: Super Contributor
timestamp: September 12, 2024, 02:57:52 pm
content: Hi, I've been coping with following crazy issue - I modded an older HY1503 classic far east PSU I got as a gift - I set it to 18V (was 15) and added a several ohm shunt in series with the existing 0.3ohm, with a switch to select 200mA or 3A CC range. When shorting the leads or messing with potentiometers under a set load the current limit seems to work.Now the issue:When I've been charging say 16V (an example) cell with set 1A (and set 16V without the load) the current is perfectly stable and then it starts to increase slowly (like ~5 minutes of rise) when the voltage approaches the set 16V.Like from 1A to 1.25A, and not stopping even it crosses the set 16V.When I move the voltage pot a liiitle bit down the current jumps to zero. I've spent some time to observe the behavior but cannot find the clue.My current understanding is when the battery voltage is approaching the set voltage the current shall start to decrease to zero (with let say plus minus some 10-30mV of a hysteresis, my guess).PS: I do not think it is a thermal stuff because it runs for 2 hours stable current 1A into the battery, and the last minutes around the set voltage the current starts to rise..PPS: for example below charging 3S 18650 from 11.5V, PSU set to 12.62V and 0.99A.

author: iMo
author_rank: Super Contributor
timestamp: September 16, 2024, 04:46:00 pm
content: After spending a whole day with the issue, while measuring every node on the pcb around the critical point of the rising current, I found out the touching the pin 5 of the 741 (CV control) with the voltmeter's probe caused an immediate jump of the current to zero (when in the region of rising current).So I removed the C10 (a small 100p ceramic cap) and the stuff started to behave normally, it steadily drops the current into the battery to zero at exact set voltage.The C10 there is wired from the 741's output pin 6 to pin 5 (one input of the offset nulling). That is an interesting wiring, perhaps they put C10 there in order to speed up the CV transition?? Perhaps experts here know more??PS: Or, perhaps they used to use a different opamp in past with an "overcompensation" at the pin5 (like the LT1028 has got)??

author: floobydust
author_rank: Super Contributor
timestamp: September 16, 2024, 08:56:33 pm
content: I find these PSU circuits are copied and some designs go back to the 1970's. Like the ubiquitous 0-30V 2mA-3A kit from china.The ICs get updated, changes made by people who don't really understand the circuits. I think they make bad changes, get a stability problem and then fart around trying to stop it oscillating in production.Mastech had the same cap C10 in their HY3020, and on the voltage (error-amp) op-amp at pin 5.Pin 5 is +ve feedback I thought on a 741 but not for a 709. It must be an archaeological artifact.

author: iMo
author_rank: Super Contributor
timestamp: September 16, 2024, 09:58:31 pm
content: Yep, all those PSUs share common concept and schematics inclusive the same values..The 741s there have the 100pF capacitor at pin5 which is offset zeroing input in 741 (it is internally freq compensated), not the freq compensation like in 748 or others. In LT1028 it is overcompensation pin for an external capacitor.The 748 has pins 1 and 8 for the freq compensation as well as the LM301.The LM709 has got pin 1 and 8 for "input freq compensation" and the pin 5 for "output freq compensation", however..The concept of those PSUs is not bad, imho, I've put the schematics into LTspice and it oscillates in sim with the 741 and 324 opamp models I have handy.The real stuff does not oscillate under normal situation, but the 100pF feedback needs some investigation as it showed problems with slow input signal.In the simulation I even tried the 741 made of transistors I found in Examples, there is the pin 5 and pin 1 for offset setting, but it worked the same. I will try at least with a simple 741 sim to see how the 100pF impacts the 741 speed..

author: xavier60
author_rank: Super Contributor
timestamp: September 17, 2024, 03:21:13 am
content: Try a higher value for R21.

author: iMo
author_rank: Super Contributor
timestamp: September 17, 2024, 07:49:14 am
content: Here is the mod2 schematics - the red are replaced parts (3 resistors),2N3055KD503, C10 100pF removed, and the green circled are highly suspicious parts as the simulation shows oscillations with the values as in the schematics.In sim replacing the C1 with 10n and C11 with 10n shows mostly no oscillation, but not an any voltage/current.I also tried the sim with two anti-parallel schottky diodes instead of C1 and it is less prone to oscillation.

author: xavier60
author_rank: Super Contributor
timestamp: September 17, 2024, 08:03:11 am
content: For CV loop compensation, there should be a resistor in series with C9, start with 10K. C1 removed and a resistor between pin 2 and G, 10K again.C10 should be whatever makes the opamp unity gain stable but may not be needed.Edit: C9 might need to be higher to start with.The LM741 is unity gain stable anyway. Remove C10.

author: iMo
author_rank: Super Contributor
timestamp: September 17, 2024, 11:19:29 am
content: While messing on the pcb I smoked the CC opamp U2B (the pot input pin5 stays at -5V and output at -11V), fortunately there are 2 spare opapms in the 324 package w/ pins hanging free, I rewired it and it works again. Cap there is a 120pF now.CV - putting a 10k resistor into the pin2 is difficult, as the pin2 is grounded into a larger copper, so I have to desolder the 741.. I will leave it as is for a while..Btw. the pass transistor here is the Tesla's KD503 (2N5303) a bit beefier than the 2N3055.PS: in the sim the C11 has a big impact on the stability as well. It works in the sim better with something smaller like 10n.

author: iMo
author_rank: Super Contributor
timestamp: September 17, 2024, 12:23:54 pm
content: Some repair shots

author: iMo
author_rank: Super Contributor
timestamp: September 17, 2024, 04:34:16 pm
content: I've managed to cut the pin2 of the 741 off its thermal pad to gnd, so the current schematics looks like below.The 741's feedback is 330pF in series with 1k, and 10k from the pin2 to ground.The C1* 100n in the original schematics is not populated (at least I have not found it on the pcb).The only outstanding capacitor to potentially mess with is the C11 1u (in the original schematics, C4 in the ltspice below) as it influences the loop somehow.

author: iMo
author_rank: Super Contributor
timestamp: September 18, 2024, 08:20:31 am
content: FYI - I've tried to do a stability analysis - the FRA in LTspice24. I've been using 2 different transistors as I have not imported the old libraries yet (I added AB's lib only). Also a green led instead the 2.4V zener.The phase margin of CV is from 60 to 100deg for lowest to highest output voltages with 10ohm load (and CC not acting) and C11 (the C4 in the sim) 10n to 1u.

author: xavier60
author_rank: Super Contributor
timestamp: September 18, 2024, 08:37:12 am
content: Load transient tests are meaningful to me. I don't think R19 and C2 are proportioned properly. C2 should be made larger or R19 should be made larger. Actually, making C2 large only affects the settling time, not high frequency stability and transient response.

author: iMo
author_rank: Super Contributor
timestamp: September 18, 2024, 09:03:51 am
content: Quote from: xavier60 on September 18, 2024, 08:37:12 amYep, I can see in the sim C2 affects the settling with pulsing load.What do you mean by the "..C2 should be made larger or R19 should be made larger.." ?Could you point me to a formula or an approach how to design/balance it?PS: I know it has been discussed for years already here, but perhaps you have the info handy..

author: xavier60
author_rank: Super Contributor
timestamp: September 18, 2024, 09:13:04 am
content: Quote from: iMo on September 18, 2024, 09:03:51 amI dont understand the theory too well, just experience from experimenting. The value of C2 seems too small for R19 being 1K. Make C2 0.1uF. Then confirm that the CV loop is stable but likely with slow transient response. Increase R19 to improve the transient response while maintaining good stability. Then decrease C2 to improve settling time without causing instability.

author: iMo
author_rank: Super Contributor
timestamp: September 18, 2024, 09:45:13 am
content: What I remember from somewhere the RC should introduce a zero well below gain BW of the opamp.Like GBW=1MHz with the 741. The DC gain is set to 1 by the 10k/10k (so I had to use R=10k let say, with my 1k the gain will be 0.1 when talking inverting amp). The transistors have got none gain there.The capacitor is calculated from f_zero=1/(2*pi*R*C), where the f_zero should be say 10% of the gain BW, for example. Thus 100kHz for example. So the C = 1/(6.3*100k*10k)= aprox 150pF.Or something like that?

author: xavier60
author_rank: Super Contributor
timestamp: September 18, 2024, 10:06:04 am
content: That sounds right. An earlier "HY1503modded.JPG" shows 10K for R19 and 1nF for C2. What does the load transient test look like?

author: iMo
author_rank: Super Contributor
timestamp: September 18, 2024, 10:40:45 am
content: I do not have the HW pulser here - but here are the sims.With C4 1uF it makes no difference, with C4 10nF the difference is visible.(C11 in the original schematics).With 1k 330p I get less swings than with the 10k 1n..

author: xavier60
author_rank: Super Contributor
timestamp: September 18, 2024, 10:54:50 am
content: That seems to show the transitioning from CC to CV sometime after switch-on.Cam you simulate a load transition test?

author: iMo
author_rank: Super Contributor
timestamp: September 18, 2024, 10:55:31 am
content: ..and with 100pF only it is the best..CC - Yep, CC is set to aprox 0.7A and the load is 1A/0.1A pulsing..Load transition - what exactly you want to see?

author: xavier60
author_rank: Super Contributor
timestamp: September 18, 2024, 11:02:06 am
content: Yes, it does look that way.

author: iMo
author_rank: Super Contributor
timestamp: September 18, 2024, 11:12:53 am
content: And set Vout=13V, CC 500mA, pulses 0.1/1A..Mind the cco nad cvo colors are messed up against the previous sims.. grrh..

author: xavier60
author_rank: Super Contributor
timestamp: September 18, 2024, 11:17:52 am
content: Don't allow it to go into CC for load transient response test.

author: iMo
author_rank: Super Contributor
timestamp: September 18, 2024, 11:22:21 am
content: Could you define the test, plz..

author: xavier60
author_rank: Super Contributor
timestamp: September 18, 2024, 11:26:39 am
content: Quote from: iMo on September 18, 2024, 11:22:21 amThe same 1A pulse load but with the CC set to something higher like 2A.

author: iMo
author_rank: Super Contributor
timestamp: September 18, 2024, 11:36:19 am
content: CC set to 2.5A, 12.85V, pulses 0.1/1A..Now, how to design the RC??

author: xavier60
author_rank: Super Contributor
timestamp: September 18, 2024, 11:45:40 am
content: That looks good with the 100pF. I'm curious to see it with 1K and 1nF.

author: iMo
author_rank: Super Contributor
timestamp: September 18, 2024, 12:00:39 pm
content: 1k and 1nF..I am going to wire up a load pulser.. I have to find here the design (the "PSU Shorter") I made years back..PS: added with an OP37 in CV to compare..

author: xavier60
author_rank: Super Contributor
timestamp: September 18, 2024, 12:17:16 pm
content: Maybe C2 needs to be larger. I know from a lot of experimenting that I have never been able to get good performance from Hi-Z current sourcing LDO designs like Harrison with type 1 compensation, just the capacitor with no series resistance.

author: iMo
author_rank: Super Contributor
timestamp: September 18, 2024, 02:11:58 pm
content: .. and the original 100pF and 100pF in pin5 and pin 2 grounded and 100ohm to the shunt, with 741 transistorized modelThe C11 (C4 in sim) is 10nF instead of the original 1u.

author: xavier60
author_rank: Super Contributor
timestamp: September 18, 2024, 02:18:21 pm
content: Does the real PSU have a problem with its CV stability?

author: iMo
author_rank: Super Contributor
timestamp: September 18, 2024, 02:23:58 pm
content: The problem was with slow charging batteries - see above details. Removing 100pF from 741's pin 5 to its output cured the issue fully.No idea on the stability when pulsing the load, it needs some equipment to create, then I will look at it with o'scope.It needs more tests on the HW, anyhow...

author: iMo
author_rank: Super Contributor
timestamp: September 18, 2024, 06:06:02 pm
content: The story continues.. Another issue I have not identified till today. While manually switching a 10ohm load into/from the post (playing with settings and watching some nodes on the pcb) I found out sometimes the output voltage drops by aprox 0.9V after inserting the banana into the post. That was not caused by CC setting but a random process.So again I tracked the stuff and again it stops at the 741. The noninverting input sometimes set itself to a 0.1xx volt. There is the C11 1u (in orig schematics) at the other side of the 7k5 resistor so I thought it is perhaps a faulty capacitor, after removing it happened still the same.There is the C1* 100n between 741's inputs in the schematics not populated on the pcb, therefore I tried to put there something and after several xx nF experiments I finished with a C3=330pF (C3 in the LTSspice) which was enough and which helped.I also replaced the 1uF C11 with a 22nF ceramic (C4 in the LTspice).The latest schematics of the affected part below.I do not know - perhaps is the 741 somehow faulty or it is a clone..too much issues around it to my taste..What still needs to be understand better is the RC in the feedback of the 741 opamp..

author: gupa
author_rank: Newbie
timestamp: September 18, 2024, 06:47:21 pm
content: Hello, I'm ignorant but I'm learning, somewhere I saw that with four ICs, unconnected operational amplifiers are connected to the ground so that they don't make a mess. And I see that the plus pin goes from the LM324 to the problem IC. So maybe I don't drink too much water.

author: iMo
author_rank: Super Contributor
timestamp: September 18, 2024, 09:18:58 pm
content: Yep, the 324 in this PSU is using only 2 opamps, the other 2 are free, not connected to anything on the pcb.That is actually greatas it saved me replacing the entire 324 as I smoked one of the opamps inside while experimenting, so I took the spare one and it works now. But sure, the not used opamps in the package have to be handled accordingly.All the issues I had here are around the LM741 which is a single opamp in DIL8 so no spares handyI have the chips in my junkbox somewhere, but I am too lazy to disassemble the entire PSU in order to replace the chips, so I've been trying to debug it under a "minimal effort" methodology..

author: xavier60
author_rank: Super Contributor
timestamp: September 18, 2024, 09:43:37 pm
content: If it's because of oscillation, try changing the 2N3055 B-E resistor to 100 ohms as well as trying to find the best compensation.I'll do some experimenting here as I get time.

author: xavier60
author_rank: Super Contributor
timestamp: September 19, 2024, 03:17:45 am
content: I wasn't having much fun at all until I realized that I shouldn't have omitted the 10 ohm Base stopper resistor.For the compensation, a 2.2K and 150pF gave fast stable response, 3us. Just a 470pF gave a slower stable response, 10us.As I discovered, it's possible to have oscillations in localized areas other than the whole loop.

author: xavier60
author_rank: Super Contributor
timestamp: September 19, 2024, 07:19:24 am
content: 470pF and 2.2K improves tolerance to capacitive loads.First is a load transient test with a 470uF POSCAP directly across the output.Then with a 0.1 ohm in series with the POSCAP.The trace is inverted as I'm looking at the negative output WRT +Ground.

author: iMo
author_rank: Super Contributor
timestamp: September 19, 2024, 07:49:48 am
content: I disconnected the 330p at 741's inputs and wired 100ohm in parallel to the 1k at BE of the 2N3055.It did not help with jumping voltage at switching ~10ohm load on/off.I returned the 330p between the 741's inputs and the jumping voltage is gone.PS: as you are well equipped - could you try the 100pF between pin5 and pin6 of the 741 as in the original - how it affects stuff, plz??PPS: below the transient at the posts when I switch in a resistor, 15V/1.9A... and the 741's output, the oscillation might come from a longer lead.

author: xavier60
author_rank: Super Contributor
timestamp: September 19, 2024, 09:25:29 am
content: I'm using an LM358. Even a 1000pF across the input only caused a small amount of ringing.With a 470pF and 2.2K, mine settles in 10us.That high frequency ringing could be coming from the wiring to your shorter.

author: xavier60
author_rank: Super Contributor
timestamp: September 19, 2024, 09:31:21 am
content: Your LM741 might be faulty if it's very old. I have seen old opamps randomly develop large input offsets.

author: xavier60
author_rank: Super Contributor
timestamp: September 19, 2024, 09:58:49 am
content: Quote from: xavier60 on September 19, 2024, 09:25:29 amThe test isn't valid as I had omitted R6. With R6 fitted, ringing becomes noticeable with 470pF across the inputs, so not a problem anyway.This capacitor is likely to prevent RF getting into the opamp, rather than compensation related.Edit: I can't see any useful purpose of putting a capacitor between pins 5 and 6 on an LM741.

author: iMo
author_rank: Super Contributor
timestamp: September 19, 2024, 08:24:17 pm
content: Spent a half day with the CC. When it engages the output of the 324 oscillates 0.5Vpp at aprox 2MHz.I have not found a way how to calm it down..PS: talking real hw..

author: xavier60
author_rank: Super Contributor
timestamp: September 19, 2024, 09:38:26 pm
content: CC loops are usually well behaved. What compensation values are you using and what has been tried?

author: iMo
author_rank: Super Contributor
timestamp: September 19, 2024, 11:03:19 pm
content: The aprox 2MHz signal at the psu's output is below 40mVpp with active CC, and the CC basically works.There is a clean signal when CC is set off. Oscillation is visible in entire active CC region.The original is 120pF in the fb.Tried blocking the inputs, various RC combinations in the feedback (C from 330 to 6n8, R 0/1k/4k7/10k), 22 ohm resistor at the opamp's output, messing with wiring layout, etc. The amplitude at the output of the opamp decreases perhaps by 30% with larger values of caps. The third and fourth opamp in the package is clean, supplies clean. The oscillation is of course visible everywhere with low amplitude as it is boosted by the transistors.PS: the 1k or 100ohm in the BE of the 2N3055 has no impact on the amplitude.

author: xavier60
author_rank: Super Contributor
timestamp: September 20, 2024, 12:43:21 am
content: That's puzzling. The CC opamp is configured as a Miller Integrator which is a low pass filter. With say 1nF and 10K, the attenuation would make a 2MHz signal barely visible if at all at the output of the opamp.

author: xavier60
author_rank: Super Contributor
timestamp: September 20, 2024, 01:39:00 am
content: I added CC to my mock-up. Works fine with 10K for the compensation resistor and even with no capacitor.I by chance noticed that the 470pF across the CV opamp inputs was causing a 200KHz oscillation at higher currents. No problems with 120pF.

author: iMo
author_rank: Super Contributor
timestamp: September 20, 2024, 05:59:30 am
content: This is 324's CC output with 120pF feedback (in mid of the CC range).CV wired as in the above schematics.When I disconnect its output from the 1N4007 output gets clean.

author: xavier60
author_rank: Super Contributor
timestamp: September 20, 2024, 06:20:19 am
content: Try the 6n8 and 10K. The attenuation will be about 1000:1 at 2MHz. The only possible reason I can think off so far is that somehow there is too much inductance in series with the CS resistors.

author: xavier60
author_rank: Super Contributor
timestamp: September 20, 2024, 06:51:08 am
content: I tried putting 200mV p-p of 2MHz through my output stage. Most of it gets blocked by the 10 ohm resistor at the Base of the 2N3055. I see about 10mV at the output.What are you loading with?

author: iMo
author_rank: Super Contributor
timestamp: September 20, 2024, 07:02:51 am
content: The 10k ser 6n8 increases the amplitude almost twice (500mVpp).The load is a 10W ww (a white ceramic brick) resistor on aprox 80cm lab wires.The only thing which lowers the amplitude significantly (10x) is when I put my finger at the 324 output..So there is a HF coupling somewhere.My o'scope is isolated via a trafo, but connecting its ground with psu's earth does not help.The probe is a Tek probe, 10x, grounded to the ground of the schematics.The tracks around the opamp were cut, the 3 red wires are in+ in- and out of the CC opamp. Rearanging them does not help.

author: xavier60
author_rank: Super Contributor
timestamp: September 20, 2024, 07:21:36 am
content: Maybe I'm seeing it differently. The compensation RC should be wired the same as in the original schematic. R21 from the 2N3055's Emitter to the Inverting input. C7 from the CC opamp's output to the Inverting input.

author: iMo
author_rank: Super Contributor
timestamp: September 20, 2024, 07:55:47 am
content: Ok, with 10k in series with the 1k5 sense resistor and 6n8 ceramics or 10n ceramics (3mm long leads directly wired to in- and out of the opamp) is the amplitude at the CC opamp's output 600mVpp and period 600ns (10n ceramic).Touching the opamp's output with my finger decreases the amplitude to 400mVpp.I boosted the ground pcb from the probe point to the point near the 741's ground with a thick short wire with none change.

author: xavier60
author_rank: Super Contributor
timestamp: September 20, 2024, 08:11:58 am
content: With 10K and 10n, it's unlikely that the 1.7MHz is coming through the opamp in any normal way. What signal level is there on both the opamp's inputs?Try touching parts of the circuit to see if anywhere changes the frequency.

author: iMo
author_rank: Super Contributor
timestamp: September 20, 2024, 08:20:37 am
content: The signal at the in- is the ~same as the output (10n fb cap), in+ is clean.Touching the CC's in- or output decreases ampl to 400mVpp and period increases to 800ns.Other 324 pins incl 741's pins do not show any relevant impact.Transistors - the touching of the base of the first transistor - the same change as with in-/out of the CC opamp, other points around the transistors - no relevant impact.

author: xavier60
author_rank: Super Contributor
timestamp: September 20, 2024, 08:28:33 am
content: Quote from: iMo on September 20, 2024, 08:20:37 amThat should mean that the signal is being imposed directly onto the output somehow or directly onto the Inverting input rather than via R21.I can't imagine how.Edit: try removing D12 and carefully testing CC mode.

author: xavier60
author_rank: Super Contributor
timestamp: September 20, 2024, 08:33:47 am
content: Quote from: iMo on September 20, 2024, 08:20:37 amHave you tried another opamp?

author: iMo
author_rank: Super Contributor
timestamp: September 20, 2024, 08:45:31 am
content: With the D12 disconnected the amplitude at the CC opamp's output is 800mVpp period 750ns.The replacing the 324 will be the last stepNope, I will do it now, I put there a socket..

author: iMo
author_rank: Super Contributor
timestamp: September 20, 2024, 10:39:28 am
content: So - I've desoldered the 324, it is TI's LM324.Soldered in the gold plated socket. I have not found the 324 in my junkbox, so now using TL084 (TI).CV - as in my schematics.CC - 100pF in the fb (orig one from top side) and 1k5 to shunt sense, added a 100nF decouplingThe circuit behaves differently:1. with CC engaged no oscillation, noise below 20mVpp at CC's opamp output2. with CC disengaged and loaded (2.2A, 18.7V):a. when PSU switched on there is 30-40mVpp 270ns period oscillation at CC's output, it slowly decreases to 20mVpp, sometimes disappears (noise below 10mV at CC's opamp output) and appears, it disappears when touching output of the 741 or the feedback of the 741b. when the load is reinserted (2.2A 18.7V) the oscillation starts again at 30-40mVpp, then within aprox 15secs its amplitude goes down and disappears (noise below 10mV at CC's output), but still sensitive..3. after 40minutes when the board is already hot the CC's opamp output shows 20mVpp 150ns period oscillation CC engaged or disengaged..The TL084 gets pretty hot (+12/-12V).A really mysterious stuff, indeedI have here a different 084 and a TLC274, I will also try to find a 324 somewhere as well..

author: xavier60
author_rank: Super Contributor
timestamp: September 20, 2024, 11:54:34 am
content: Keep in mind that the opamp for the inactive mode should have its output at maximum + voltage, about 11V and not capable of producing any output signal.

author: iMo
author_rank: Super Contributor
timestamp: September 20, 2024, 12:15:01 pm
content: It might come from the 741 via the diode's capacitance (the 084's output is not zero impedance), even though those frequencies are rather odd for that circuit. Later on I will continue (perhaps I would socket the 741 too) - it is enough for today.. Thanks!PS: next step - to handle the inputs of the two not used opamps in the package, perhaps they mess up..

author: iMo
author_rank: Super Contributor
timestamp: September 20, 2024, 06:02:14 pm
content: So, I have wired together the in- with its out at both spare opamps in the TL084, and their in+ to ground (the ground in the schematics).The CV and CC wiring remains as the latest above.The load is the ww resistor and it gives 2.27A at 18.7V (the max PSU voltage here) with CC disengaged.After power up the PSU I can see aprox 10mVpp 300ns period oscillation (probe 1x, AC, BW limit off) at the CC opamp's output, which is slowly dropping down (within 5-10secs) into the noise of my Rigol 1062CA (perhaps 1-2mVpp). That is with the CC disengaged. With engaged CC I see clean output (<2mVpp noise).The output of the 741 is clean.In the lower portion of spectra I can see 5mVpp saw tooth ripple w/ 10ms period at the CC opamp's output, and 7.5mVpp at the CV opamp's output. The ripple increases a bit (30mV) when going to CC's zero current output.The above oscillation is superimposed on this ripple.The 10mVpp 300ns oscillation at the CC opamp's output appears always when doing abrupt changes, it slowly disappears into the background noise after couple of seconds.So I will leave it now and will have a look at it tomorrow, what is new there as it lives its own life, it seems

author: iMo
author_rank: Super Contributor
timestamp: September 21, 2024, 08:17:18 am
content: Some shots from this morning..Immediately after the cold start the oscillation (2.3A/18.7V CC disengaged) was as below. It lasted for about 30-40min without disappearing. Afterwards after re-inserting the load the effect of slow disappearing started again.The 741 CV output is clean - see below the ripple.With CC engaged I do not see oscillation.The CC output is DC 11.56V with disengaged.I replaced the original 100p capacitor in the CC fb with 120p C0G, but it did not help. Cleaned with IPA couple of times.The oscillation is visible at the output posts as well.Probe Tek 1x, BW off, AC, no oscillation or ripple seen with probe's gnd and input put on the same node at PSU.It seems the rising edge of the ripple starts the oscillation burst (it reminds me on my experiments with superregeneration), the oscillation somehow depends on the temperature, the effect of disappearing with 5-10sec time constant leads to some thermal effect or leakage or??

author: xavier60
author_rank: Super Contributor
timestamp: September 21, 2024, 08:57:28 am
content: Post a schematic of how everything is. The LM741 has been replaced?Also, a load transient test might show if there is loop instability.

author: iMo
author_rank: Super Contributor
timestamp: September 21, 2024, 09:40:59 am
content: Below the mod v5 schematics.There is one thing which may cause the instability - the 200mA/3A switch is mounted on the front panel, shorting the 4R7 for 3A (aprox). The leads are thick, but aprox 15cm (!!) long coming to the shunt on the pcb. That may cause some HF feedback.. I will doublecheck that later on..Ideally I planned to place a relay near the shunt with the 4R7, but there is small space for it and of course the laziness..

author: xavier60
author_rank: Super Contributor
timestamp: September 21, 2024, 09:49:41 am
content: If you plan to do the load transient test, go ahead.If not, remove the 330pF from across the 741's inputs. Put a 1nF capacitor or higher between its output and Inverting input.The 10K between the Inverting input and GND must be left in place.

author: iMo
author_rank: Super Contributor
timestamp: September 21, 2024, 09:59:08 am
content: The transient test will come next week, I need some low L higher power resistors for the load at the shorter and some rugged construction for it as well..

author: xavier60
author_rank: Super Contributor
timestamp: September 21, 2024, 10:14:08 am
content: I did more experimenting. The 120pF that I though was ok to put across the inputs, isn't.And it has just occurred to me that even with a large capacitor between output and Inverting input of the CV opamp, the response can never be purely Integral. The AC or proportional gain can never be lower than one.

author: iMo
author_rank: Super Contributor
timestamp: September 21, 2024, 10:41:49 am
content: Here is the CC and CV out with only 1n ceramic in the CV fb only (330p at 741's inputs removed).10k in 741 in- is there.

author: xavier60
author_rank: Super Contributor
timestamp: September 21, 2024, 10:53:49 am
content: Maybe it's oscillation in the output stage. If so, the signal should be a bit stronger on the anode of D12.

author: iMo
author_rank: Super Contributor
timestamp: September 21, 2024, 11:10:37 am
content: I've removed the rather long leads (my really bad idea) from the shunt (the 0.33ohm placed in rear part of the box) to my new switch on the front panel (creating a loop antenna) and all oscillations are gone.. (with 1n only in the 741 feedback). What is left are the couple of mVpp 100Hz ripples at the CV and CC opamp's outputs.I will make the transient test next weeks.Also I have to solve the 200mA/3A switching as well as I need the lower CC range as well..Thanks!PS: ok.. when not having an oscilloscope handy I would consider my idea with the switch pretty

author: iMo
author_rank: Super Contributor
timestamp: September 23, 2024, 06:05:00 pm
content: Here is the "PSU Pulser" (aka "PSU Shorter" in past)..Rather hot at 18.7V and 1.7A..40ms active, 300ms off. The load 68ohm/6 active, 330ohm off.Rising edge on the mosfet's drain 200ns, falling edge 60ns at 18.7V/1.7A.So we may finetune the PSU's CC/CV now..

author: iMo
author_rank: Super Contributor
timestamp: September 26, 2024, 10:54:55 am
content: After a final exercises with ltspice and PSU pulser/shorter below the o'scope pictures when pulsing with 1.65A (set 18.7V max voltage, CC off).The 470uF original output capacitor replaced with 47uF one..The pos/neg peaks at the PSU's posts are less than 100ns wide (when zooming in)..

author: iMo
author_rank: Super Contributor
timestamp: September 26, 2024, 10:55:32 am
content: ..and the final schematics.. case closed..

author: xavier60
author_rank: Super Contributor
timestamp: September 26, 2024, 12:15:25 pm
content: The narrow spike is likely due to the way the DSO probe is connected to the PSU.The real response is 4us, a little fast for that design resulting in the slight ringing. It will be fine.