Trying to improve an old generic chinese PSU - Page 1
Forum section: electronics / projects

author: ElectronSurf
author_rank: Regular Contributor
timestamp: September 26, 2024, 08:54:42 pm
content: I have a Dazheng PS-305d PSU for almost 12 years now, over the years of abuse it has never failed except the POTs that I had to replace once. the only modification I did was adding a temperature controller circuit for the fan because it was too loud.A few days ago one of the potentiomers failed again, so I opened it to replace the potentiometer but this time I was really disgusted by the crappy old analog board with 741 opamp and TL431 reference voltage, I could replace the reference voltage with something better like LM4040 but there was no substitute for the op amp that I could find.The potentiometers are REALLY unstable they drift a lot and takes a long time to stabilize, I also don't like the coarse and fine idea, I want to have a range switch instead.Also the output capacitor is too big, the sparks on short circuit are fun tho...What I plan to do:1. Redesigning the analog board from scratch with faster opamp and better Vref.2. Replacing one of the voltage and one of the current adjustment pots with a switch to switch between 30V/5A and 10V/1A.3. Replacing the potentiometers with anything better than what it has! single turn.4. Adding a switch in the middle (it has the room but no switch was added) to set the PSU output on/off and while it's off also show the set current.5. Adding a temperature controlled fan circuitry.I'm not going to touch the V/A meter, it's good enough.I have not designed the voltage and current set front panel board...ps. In my schematic the op amp supply is upside down!

author: daisizhou
author_rank: Super Contributor
timestamp: September 26, 2024, 10:55:13 pm
content: It seems that the transformer in the middle is a reduced version. Can the current actually reach 5A?Potentiometer You need to choose the type of potentiometer shown in the figure below,It is a metal resistance wire wound type.It turns out that the type is a carbon film potentiometer, which has a short service life.

author: ElectronSurf
author_rank: Regular Contributor
timestamp: September 27, 2024, 11:12:22 am
content: Quote from: daisizhou on September 26, 2024, 10:55:13 pmYeah it's a rip off of a rip off, there's room on the main board and also on the heatsink for a second transistor to be placed in parallel and share the heat and they even omitted that...I've tested the transformer up to 6A, the output voltage after rectifier dropped to ~32V. unloaded voltage was ~39V.I'm living in the middle east, I don't have access to the free market so my options are limited.I think these are still being carbon POTs maybe better than what was there -link.

author: ElectronSurf
author_rank: Regular Contributor
timestamp: September 28, 2024, 03:08:02 pm
content: After readingthis articleI decided to change the reference voltage to lm336, the 2.5V version is quite low noise.Added a buffer to the voltage reference, I can now switch between 1, 1/2 and 1/4 of output voltage. I'm still worried OP07 would add some noise to the circuit though.Had to add more gain to the main transistor to lower the amount of current NE5532 is going to sink, at +/-15V it's going to get almost hot.Plus some minor changes, can I get some feedback please?

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 05, 2024, 01:20:21 pm
content: So after fiddling around I ended up with 5V reference, buffered to 10V and can switched to back 5V to get half the range (15V/2.5A). the RC circuit should remove any noise down to ~15HZ.Added a diode to the output, so no back-feeding when charging stuff.Plus some minor changes here and there.Tidied up the schematic and added some comments.Designed the PCB and lined up the "hot" parts next to each other to be screwed to the heatsink. separated the control section (op amps and voltage reference) to sit vertically on the power board, this way I can fit things in the tight space that the PSU have and I also can shield it from heat and noise if needed.I'm planning to CNC a 4mm copper plate as the heatsink, I'm not sure yet...Ordered the parts, probably arrive tomorrow.I'll update this topic later with more progress.

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 07, 2024, 08:11:26 pm
content: I finished building the PCBs andpartiallytested the functions, there's only one minor error in CC/CV indicator that I have to fix, more on that in later posts.Today I went and asked for the price of a 0.4x12x12cm copper plate for the heatsink and it was too expensiveSo we go back to the good old aluminum, bought a 3mm thick plate for cheap and I'm going to CNC that, attached the heatsink design. I'm not sure if the pattern is good enough, any idea?The hot parts are going to sit upside down on top of the heatsink (marked with rectangular).

author: daisizhou
author_rank: Super Contributor
timestamp: October 08, 2024, 12:24:29 am
content: I don't know what you actually experienced.This allows you to replace the 741 op amp and TL431 reference voltage。1、I would like to know in what aspects the performance can be improved by replacing the op amp and voltage reference?2、It seems that the components that need to be cooled by your radiator are the rectifier bridge and MOS tube.And you choose to CNC process in the middle of aluminum or copper heat sink, is your purpose for faster heat dissipation?3. It looks like you replaced the original analog board with two circuit boards designed by you, and they are modular. Is this really an improvement? At least the installation looks worse than the original solution.I am also using a similar power supply. Usually, this kind of power supply adjusts the voltage or current potentiometer, and the potentiometers used are all low-quality. You can repair it by simply replacing the potentiometer with a better one.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 08, 2024, 04:08:48 am
content: Quote from: ElectronSurf on October 07, 2024, 08:11:26 pmHmmm...Thermal mass is good, but a thin plate at 90' to the fan flow is really the worst use of fan airflow.I guess you're constrained by wanting to keep the original case and major parts placements.A tunnel is far better, even if your space dictates a somewhat short one ?You might fit an 80x80x100 tunnel if the fan shifted to outside the box (and the big caps flipped to the other side of the transformer) ?or the second one I think can come 80x80x50mm, for a lower price. 3rd and 4th ones are 100mm and 130mm

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 08, 2024, 07:43:46 am
content: Quote from: daisizhou on October 08, 2024, 12:24:29 am1- Better op amp = higher slew rate = faster responce = hopefully lower noise | better voltage reference = better temperature coefficient | lower noise with higher reference voltage.2- The current heatsink is (2.5mm thick) has pre-drilled holes which I have no use for, it has no cut out for air flow, mine is 3mm thick with the directed air flow. it's not the best but considering the constrains it will definitely perform better than what it already had.3- How is it worse!? You have to keep in mind the the original board was on top of a power transformer, the emissions from PT will induce noise to the analog circuitry, where 2.5V voltage reference gets amplified an order of magnitude to get 30V, my plan was to move the board away from transformer.So you see I didn't want to make it modular, as I mentioned in an earlier post of mine I can now easily shield the control board with copper band very easily. I'm also planning to add a "flux band" to the transformer to further reduce the radiated emissions.Yeah, potentiometer faulty, replace it. transistor burned, replace it. too much noise? add bulky capacitors. problems solved!The problem with the "fine / coarse" on this PSUs is that the variations with the fine pot is not linear, at higher voltages you get some range and at lower voltages you get another. what I did was adding a range switch, now a single pot can change the selected range very accurately.

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 08, 2024, 07:47:19 am
content: Quote from: PCB.Wiz on October 08, 2024, 04:08:48 amI'd love to have a wind tunnel in my PSU but there's VERY little space to work with. if it was just a transistor it could fit but I have to fit the PCB too...

author: daisizhou
author_rank: Super Contributor
timestamp: October 08, 2024, 10:11:04 am
content: This is a picture of the inside of my power supply, including the fan temperature control

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 08, 2024, 11:25:44 am
content: Quote from: daisizhou on October 08, 2024, 10:11:04 amWell if you take a look at my very first post you will see that my version of this PSU is VERY different from yours, mine is also older. same principal but different build.

author: daisizhou
author_rank: Super Contributor
timestamp: October 08, 2024, 03:12:10 pm
content: I didn't find any voltage offset in actual use，Since I bought it I have adjusted 3 potentiometers because the power supply cannot display 0.1v or sometimes when adjusted below 0.5v, the alarm sound will sound.There is a slight deviation between the displayed voltage and the actual voltage.So I adjusted the 3 potentiometers,Since then, it has been used normally.I know that the TL431 is not very accurate when used as a reference, but usually this kind of power supply does not require very precise voltage accuracy.I think as long as it can do the general job, it is perfect. Excessive pursuit of extreme performance is not a wise choice.

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 08, 2024, 04:39:21 pm
content: Quote from: daisizhou on October 08, 2024, 03:12:10 pmHow is this extreme? it's like what teenagers build out of boredom or what hobbyist built in the 90s!Please keep your wise eastern advices to yourself!

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 09, 2024, 12:25:08 am
content: Quote from: ElectronSurf on October 07, 2024, 08:11:26 pmThose slots may remove too much material ?Looking at the other vendor model in #10, they have slots, but I'd say the top 6 slots are wrong, and should have been horizontal, to give maximum material towards the hot TO3 parts.They've isolated the To3 there and force the heat to flow around the ends of the slots !If you are going to CNC this, what about radial slots roughly matching the fan circle, so you have maximum material conducting heat flow from each slot edge back to the hot devices ?There are some interesting LED cooler heatsinks, designed for 80mm fans, that could bolt onto your flat plate with heat paste to boost cooling ?eg ones like this

author: daisizhou
author_rank: Super Contributor
timestamp: October 09, 2024, 12:37:55 am
content: I mean you replaced the TL431 with the LM336, but there was no noise reduction or any other improvement as you said.I don't know what instrument you used to measure such a small noise.At least I can hardly see the noise you mentioned using a regular digital multimeter or digital oscilloscope.The power supply voltage display is also normal, and the voltage fluctuation in cold or hot state is less than 0.1v, so I don't understand the real meaning of your improvement

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 09, 2024, 02:27:06 am
content: Quote from: daisizhou on October 09, 2024, 12:37:55 amThe LM336 has better temperature change specs, and it also defines a ppm/1kHr drift.The classic '431' has also changed and some newer models have improved tempco from the original, and you can get them down to ~0.25%

author: daisizhou
author_rank: Super Contributor
timestamp: October 09, 2024, 02:46:18 am
content: ThanksI understand that LM336 is better than TL431 in terms of performance.I mean that in the actual circuit, there is no intuitive feeling, such as the voltage is more stable or the voltage change is more precise.Because this power transformer and transistor determine the maximum current carrying capacity。This power supply is usually used to test circuits or repair circuit boards.

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 09, 2024, 07:23:12 am
content: Quote from: PCB.Wiz on October 09, 2024, 12:25:08 amI had 4 options; laser, CNC, wire cut (EDM) and water jet.The laser was melting the aluminum, IDK maybe the guy couldn't configure his machine. some said it would hurt their machine (WHAT!) and can't do it.Couldn't find any water jet cutting machine and EDM was waaaay too expensive.So I had to do it with CNC and they charge by the hour, ~20$/H. my piece could take anywhere from 4 to 6 hour with that 2mm maze like cut outs that I designedI had to simplify the design to reduce cost and the end result cost me ~10$.So I had to resist the urge to make something decent and go with "good enough", I'm not paying the price of several crappy chinese PSU for a single heatsink.

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 10, 2024, 09:57:39 pm
content: Wrapped the transformer with a flux band and did the heatsink CNC cut.The schematic have some minor changes and CV/CC bug fixed, hopefully this is the last revision.Tomorrow I'll wire things and cross my finger.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: October 11, 2024, 04:16:10 am
content: if r1 r2 RC filter , better move it to input LMincrease current for lm336 ; if you wish 10v ref , you can add 2 in sequenceDarlington 30W + 140W - look strange with q1 hfe 50 passing 5a you need 100 ma ; do q1 2 in parallel as in theory it on edge of safety quadrantyou may look at opamp input resistors to limit curr. from diff. voltage1n4148 - maybe something better for currentin overall it will oscillateand dond bother about noise and 5532 thingi as everything will skewed by diff grounds and pcb traces

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 11, 2024, 07:23:21 am
content: Quote from: GigaJoe on October 11, 2024, 04:16:10 amMove it to input of LM? what?Quote from: GigaJoe on October 11, 2024, 04:16:10 amThat will defeat the purpose of range switch.Quote from: GigaJoe on October 11, 2024, 04:16:10 amYes usually you need two for 5A but The maximum power on Q1 is ~50W, there's a tap switch in place. So I didn't bother.Quote from: GigaJoe on October 11, 2024, 04:16:10 amElaborate!Quote from: GigaJoe on October 11, 2024, 04:16:10 amSure Joe, dond worry

author: GigaJoe
author_rank: Frequent Contributor
timestamp: October 12, 2024, 06:34:12 pm
content: r1 r2 to input not output , as RC filter , no make sense to output, as you need to filter anything that going inhard to tell what "range" sticker means as it only sticker on schematic ...to200 - usually large drive capacitance , not recommended1N4148 - low signal diodes, if it fail , guess what happens, better use more robust ?you will have a diff. signal on voltage opamp, when current triggered, that curr need a limit on inputwith Darlington up to 20000 opamp slew to 8 , no feedback , and 22pf compensation --- what can go wrong , right ....oh .. 0.1 uf that on your schematic to feed opamp , on PCB layout should be move to opamp package as close as possible shorting positive negative ground feedhave fun

author: iMo
author_rank: Super Contributor
timestamp: October 12, 2024, 06:55:17 pm
content: I'm not sure the R12 is on the proper place, I would put him directly into the collector (and w/ a lower value). Also the transformer's tap switching is quite tricky, imho.. X fingers the tap switching will work..

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 12, 2024, 10:00:43 pm
content: Today I was the victim of fake op amp; the current didn't go all the way down to zero, I was scratching my head for a few good hours to diagnose the problem, after checking EVERYTHING I replaced the op amp with an LM358 just to see what happens and BOOM it workedSo I went out today and bought some original NE5532 and now it's doing the job.About an hour ago I set the voltage to 2.007V and as of now the voltage drifted only +1mV (my 4000 digit crappy DMM says so), so considering so many factors that can contribute to the voltage drift it's pretty good I guess. honestly I didn't expected the potentiometers to perform so well.Gods willing tomorrow I'll take the PSU to my friend's shop to test the ripple, response, transitions etc. I don't have a scope :'(The 7805 regulator on the VA meter was getting quite warm so I added an small heatsink for that, had to do some voltage divider modification on the ammeter section too. other than that didn't do any modification on the VA meter, yet...Quote from: GigaJoe on October 12, 2024, 06:34:12 pmWhy do you save words and don't finish the sentences?R1 and R2 are there to filter the regulators noise to some degree.The range goes to a resistor and to GNDREF to form a voltage divider to change the buffer output voltage which results in a different range.4148s are only seeing 1mA, I don't think they fail...Didn't understand the rest, thanks for the help I guess!Quote from: iMo on October 12, 2024, 06:55:17 pmThere was no R12 in the schematic if you take a look at rev1.3, I just added that to protect the transistor if something went wrong and intentionally before R13 to reduce the LEDs current.The LEDs are still too bright, damn new LEDs lit up with just a few hundred micro amps. maybe I increase R13 to 15K or something!

author: Vovk_Z
author_rank: Super Contributor
timestamp: October 13, 2024, 09:24:27 am
content: Quote from: ElectronSurf on September 28, 2024, 03:08:02 pmI'm not sure a voltage reference is a weakest point here. Have you seen a whole noise and pulsations spectrum? Are you sure a noise is a main problem but not pulsations (50-hz harmonics)? A good sound card and an oscilloscope can be enough to investigate this.My experience says a PCB design is a typical main source of mains harmonics, and only after this other noise sources are meaningful. (I don't know much about this exact PCB).

author: daisizhou
author_rank: Super Contributor
timestamp: October 13, 2024, 09:47:06 am
content: Quote from: Vovk_Z on October 13, 2024, 09:24:27 amJust like you think, I feel the same way.But the author seems confident，I still don’t know how much actual optimization effect the improvements described by the author can have in realityWhat I can't understand is that he actually thinks that step adjustment is more useful than the current coarse voltage adjustment and fine voltage adjustment.He meant to set 3 switches, switch 1 is closed, the range is 0-10v, and then the voltage is adjusted. Switch 2 is closed, the range is 10v-20v, and then the voltage is adjusted again. Switch 3 is closed, the range is 20v-30v, and then the voltage is adjusted again.Maybe he prefers this method of operation.

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 13, 2024, 05:14:21 pm
content: Quote from: daisizhou on October 13, 2024, 09:47:06 amI also don't understand why china have a social credit system but you don't see me nagging about it!The range works like this: 0-5V | 0-15V | 0-30V.This topic is like a project log, please don't post nonsense.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: October 13, 2024, 06:53:20 pm
content: changing schematics like that , its orange to rotten orange ; artificially focused on imaginary noise problem with a flawed schematic - i don't see any problem there at allmost useful redesign would dial trackable to ±15-17 V (as much as transformer allow it) 1-1.5A max with fine tuning CC from up to few mA.

author: daisizhou
author_rank: Super Contributor
timestamp: October 14, 2024, 01:27:07 am
content: I'm just giving a friendly reminder,No malice.If you still insist, then I can only wish you good luck.I also believe that you get what you pay for, and it has nothing to do with the country where it is produced.It has nothing to do with reputation.The choice is yours. It is not reasonable to blame the country of production after choosing a cheap and low-quality power supply.

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 17, 2024, 09:52:11 am
content: Quote from: ElectronSurf on September 26, 2024, 08:54:42 pm1. Done.2. Done, The ranges are: 5V/0.8A - 15V/2.5A - 30V/5A.3. Done.4. Done, Left switch does this.5. Done.I have achieved my goals, Soon I'll post the noise and response results, hopefully...

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 20, 2024, 12:55:38 pm
content: Here are the test results, note that the noise ground is ~25mV.I could make it even smoother but didn't want to sacrifice performance.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: October 30, 2024, 01:42:45 am
content: seems oscillation at few Mhzif you think 40 mV p-p - low noise - give me a break)

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 30, 2024, 06:49:31 pm
content: Quote from: GigaJoe on October 30, 2024, 01:42:45 amQuote from: ElectronSurf on October 20, 2024, 12:55:38 pm