How good/bad does this 30V-3A PSU look ? - Page 1
Forum section: electronics / projects

author: MathWizard
author_rank: Super Contributor
timestamp: June 06, 2025, 11:22:10 pm
content: I got an old INSTEK PC-3030 2x 30V/3A, and 5V/3A PSU. It might be from the 1990's, I couldn't find the exact model/schematic, so I made one of the whole thing. It's been opened up before though, the case was worked on for sure, but I can't say I noticed any difference to the solder joints. I have to put a couple of parts back in it, before I test it out this weekend.It has coil meters, LM336 2.5V references, one is closer to 2.6V. The +/-24V rails are unregulated, the -15V rail is barely regulated, the +15V rails has an LM7815 at least. There's hardly any extra caps anywhere, like around the op-amps rails, or reference pins. The op-amps are old LM301A (I think the date codes are 1975), and uA741. The volts and amps are set with old, common potentiometers.Of the 1/4W resistors, it uses about 4 types, a few are 1%, the rest are 5% tolerance, but some of them probably have lower temperature drift or something. I checked all the electrolytic caps, and they seem pretty good. It also uses a few 555's as comparators to help with series and parallel modes, controlling some relays.The 5V output uses a uA723 circuit.So far I've worked out how most of the DC aspects work, so it will be easy to repair.Here's the main section of a 30V/3A channel, I think I put back all stock LTSpice parts, but it should work about the same. I added some -15mVoffset like 1 of the uA741's must have with a 100R to the negative rail. I don't think I left out any caps either. Like the PSU, the local supply rail GND's, are tied to the +Output.The easy things I could upgrade with parts on hand, are adding more bypass caps, making +/-24V rails with LM317/339, and also the -15V rails. Maybe replace the LM336 2.5V ref's. I could use modern op-amps like COS77 or 277 , they are like OP07 and better, and have the same pinouts and adj-pins as the uA741 and LM301A. Also adding some fine adjust pot's to the front panel, at least for the voltage. W/o those, there's probably not much point to change the op-amps.Here's a somewhat compacted picture, but it's still hard to read I guess.The 1N4148 that was right on the output of the current regulator uA741, that's a mistake, it's extra.

author: MathWizard
author_rank: Super Contributor
timestamp: June 09, 2025, 06:06:23 am
content: Well it's a pretty basic design, but it does well enough so far. I put it back together, and had to fix a broken connector and trace for the one of the Volts meters to work. For low currents, like 10V at 100mA, it's pretty stable, over a few hours, on a DMM anyways. It looks good on a scope too, but towards the end I saw some funny noise in the hundreds of kHz, IDK what that was or where is came from, surely it's not my new scope or probes.I'll have to test it at some higher currents, and series/parallel, but at low power, ripple is not a concern anywhere. The pots seem good enough, although I will add some fine control pots for sure, then it will be easy to set mV levels. Right now 10mV increments are pretty easy to set.This PSU has 1 HV rail per channel, so say 40V each for the main channels, and then it has a main +/-35V rail for each channel, that was labelled 24V on the PCB. There's also 1 other unused low-voltage winding available. It has relays that choose different taps too. I'd leave the main power and relay board alone.I didn't find any temperature sensing circuits, I'll have to add some, and also a small fan off some rail. This PSU has a large heatsink sticking off the back of the case.But I'd make I'd like to make a better, more accurate and stable, control and regulation board, running off "+/- 24V" and +/-15V derived from those rails. So without going to digital, what are some more advanced designs I should look into, what are the best old analog PSU's with lots of op-amps?.I have lots of nice op-amps, I also have new uA723's.

author: MariuszD
author_rank: Regular Contributor
timestamp: June 10, 2025, 10:22:04 am
content: A similar block diagram can be found in the HP Journal from 1962.http://www.hparchive.com/Journals/HPJ-1962-07.pdfIn school, I repaired power supplies and became familiar with many similar designs. I also designed a similar power supply using OP07 and LM385. It had Kelvin connections and excellent line and load regulation. Output resistance was 15uOhm. Its impulse response was poor because I didn't have much knowledge about it at the time. OP07 is precise but slow.Make sure to measure the parameters before deciding to make modifications. Measure the contribution of specific elements to the imperfections you want to improve. You can measure how the reference voltage changes with temperature variations. You can measure input and output voltages of the amplifiers .Newer amplifiers have a smaller offset, but it doesn't matter with the power supply. Newer amplifiers have higher bandwidth but bandwidth of U3 is limited by external components. Newer amplifiers have higher gain it can make better regulation.

author: MathWizard
author_rank: Super Contributor
timestamp: June 10, 2025, 04:48:18 pm
content: Yeah it's not much more modern than a BJT PSU w/ discrete diff-pairs that I built, but that one does pretty good too, for something so basic.I'm reading through an old 1970 H.P. DC-Power Supply Handbook, and besides SCR protections and pre-regulators, so far there's not much that seems missing from this INSTEK.I need to calculate some small signal parameters, and figure out the BW too. In LTSpice, I can get the voltage op-amp to oscillate, but not much of it showed up on the O/P. I haven't tried those settings on the real thing yet.Series and parallel operation seems to be working ok.But for fun, and the challenge, I want to try building some new PCB's for this, maybe even make it capable of sinking current too.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 10, 2025, 05:50:27 pm
content: The circuit looks like the basic floating regulator circuit. The stability can depend on the ESR of the capacitor on the output. Some ESR here can help and improve the stability - in the simulation one may miss that part. The current part looks like there is no extra compensation FB present and it uses the full BW of the OP-amp. With a faster / different amplifier one may have to add extra RC feedback there too.When looking for a moder replacement OP-amp, I would consider the OPA202 - about as precise as the OP07, but a bit faster, lower power and cheaper. However no DIP - so only a thing for a new PCB.The auxiliary supply does not need to be high. Some +-5 V would likely be enough. An extra high voltage adds delay in the cross over between CC and CV mode. Depending on the supply the ref. voltage may need to be adjusted.For an old circuit it is a bit unusual to have the voltage adjustment at the reference side - this is already the better choice than adjusting the FB divider.

author: MathWizard
author_rank: Super Contributor
timestamp: June 10, 2025, 11:14:32 pm
content: The ESR of the 100u output caps was ~0.25R, I'll have to find some more documents about the LM301A and uA741, and their compensation and offset/balance pins and clamping. I'm not sure what the O/P transfer function should look like yet anyways. It's an amplifier, but too much gain and the wrong phase can make for an oscillator.I've done 30V-3A tests for a few minutes, but so far only a few seconds with the OP shorted. I'll have to do some standard tests with it, like going from CV to CC.I do already have 2MHz COS77 and 10MHz COS277, they have a lot tighter specs then what's in there now.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: June 16, 2025, 07:59:06 pm
content: FWIW, I use the larger version from the same series - Instek PC-6030 (2x 60V 3A + 5V 3A) for over 25 years as the main bench PSU in my home lab and it works great for me from the time I've bought it new from Farnell. I never had to open it and it did all I've asked it to do very nicely .CheersAlex

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 16, 2025, 08:20:17 pm
content: The best way to get some confidence in this thing is to do some tests.How does it behave during power up?Any overshoot when the output is turned on?How does it behave when the current suddenly changes? (Use a 555 blinker, a N-channel FET and a power resistor).

author: MathWizard
author_rank: Super Contributor
timestamp: June 17, 2025, 06:37:26 am
content: I haven't done many tests yet, I tried some load connect and disconnect tests, I guess it looked ok, small voltage ringing that went away fast. I'll have to do more tests and compare it side-by-side to my Siglent PSU. In the pic below, it's AC coupled, and that was disconnecting 10ohms, from something like 20V, with alligator clips.Notice all the noise on the trace in that pic, it's around -80dBV and 350kHz, I was worried that was the PSU, but it was there with the PSU unplugged, and the nearby lights turned off.The frequency domain pic was in current limiting mode, and that looks pretty clean to me.I made an AC model of the BJT block, with all the Cpi,Cu's, and most of the bulk resistances, and solved an 8x8 matrix and got B and D of the ABCD parameter's, for the case when the AC output is shorted. And the Bode plots match pretty good with the BJT's in LTSpice.But I haven't factored out the exact zero's and poles, it would be a monster polynomial. I'll have to try something, or try it numerically.But I treated the high voltage supply as an ideal Vsource, so now I don't want to go measuring the inductances of the transformer, not tonight anyways, and trying to learn and calculate everything for the transformer and diodes.So then say the HV output idles at 43V , and is ~40V with a load 3A load, so say the Rseries was 1 ohm. Then there's a 4.7k and 6.8mF cap in parallel with all that.For small signal analysis, pretending the transformer is just a Vsource, what series inductance should I use? Does the inductance value change much with frequency ?So what should I add to this AC output model ? (Rsc1,2 are actually 250m, not 25)

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 17, 2025, 04:51:12 pm
content: The thread below has a simple schematic for a 555 / resistor based load tester, screenshots, and improvement modifications for a Korad power supply. Just modifying two tiny capacitors has a quite huge improvement.https://www.eevblog.com/forum/testgear/korad-ka3005d-dynamic-load-ripple-noise-tests-and-modifications/msg5764803/#msg5764803I also thought there was a thread here with some "standardized" tests for load testing, so people can do the test at home and upload them, with the intention to have a fairly decent comparison of how different power supplies behave, but I could not find that thread.

author: MathWizard
author_rank: Super Contributor
timestamp: June 18, 2025, 12:49:27 am
content: Yeah I'll have to try something like that. I don't have a current probe, so I'd use some current sense resistor like they do.I partially built a DC E-load before, but the heatsink can't handle much, maybe 40W. As I get better with op-amp's and MOSFET's, I'll get a proper version going someday.