Updated schematic for a popular DIY linear bench power supply - Page 1
Forum section: electronics / projects

author: Aramp
author_rank: Contributor
timestamp: February 02, 2024, 09:06:12 am
content: Hi everybody,I have recently purchased a popular power supply DIY kit on Aliexpress. It's available on eBay, Aliexpress and Amazon. If you google something like "0-30V 0-5A Adjustable Power Supply Constant Voltage And Current Diy", you'll find all the links. The attached ZIP file contains the image of the empty board, so that you can easily identify it.The power supply is rather decent and could serve well on the bench. However, there are two problems with it.The first problem is that it may not work, because the linear regulators used are not supplemented with dumping capacitors as per the datasheet, and will most probably start oscillating. And the second, more general problem is that the schematic supplied with them is for an old version of the power supply, so if something does not work, it's very hard to debug the problem. That's exactly what happened to me: it didn't work, and figuring out the reason was a headache because the schematic was inconsistent with the board.So I went on to draw the updated one, and that's where I figured the problem with the capacitors, and as you may guess, it was in the part that was changed. In doing so, I have also found the original Chinese copy in PDF format, as the one on the sites is translated into English with automatic translation and is converted into a low-resolution raster image, so it's sometimes difficult to discern. The alignment instructions that are provided are also poorly translated, so I had to redo the translation and correct it for proper English.In the attachment, there is a drawing of the new version of the schematic, with all the newly replaced components, as well as the original high-quality PDF drawing with proper translation from Chinese that comes with the boards when you purchase them. Please note that the C20 and C21 capacitors are not supplied, and there are no places on the board for them. As I mentioned above, they are needed to stop the regulators from oscillating. I have soldered them to the legs of regulators before soldering them into their places. I have tried to retain the labeling of the parts close to the one found in the original schematic and on the silk print.The ZIP file also contains a proper translation of adjustment instructions. The adjustment is actually rather simple.UPD Sep 20, 2024: thanks to @enut11 for pointing out the typo with diode D2 polarity, fixed and updated.

author: moffy
author_rank: Super Contributor
timestamp: February 02, 2024, 09:39:12 am
content: Just a quick note the 3DF20 in the redrawn schematic is shown as PNP instead of NPN.

author: Aramp
author_rank: Contributor
timestamp: February 02, 2024, 09:49:04 am
content: Quote from: moffy on February 02, 2024, 09:39:12 amThank you! I have fixed that.

author: p.larner
author_rank: Frequent Contributor
timestamp: February 02, 2024, 12:05:01 pm
content: I built the same a while ago,that used two transfo's,do you just use one?,there is a yt vid of a fella building this,also his website has the schematics.

author: Aramp
author_rank: Contributor
timestamp: February 02, 2024, 12:38:35 pm
content: I've used one transformer, happened to have one scrapped from an old transistor amplifier. They are often sold cheaper than a separate transformer. Tried to find vids on youtube when run into the problem, found a couple, none had my problem and was unable to find the schematic. So had to draw the schematic myself. It also helped to understand how it works, which is a good thing.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 02, 2024, 02:15:12 pm
content: As drawn for the new version the voltage feedback is from the wrong side of the ampmeter. The regulator will not compensate for the drop at the ampmeter.

author: dietert1
author_rank: Super Contributor
timestamp: February 02, 2024, 10:25:49 pm
content: I found the kit at mercadolivre.com.br for 149 R$ (about 30 €). That version appears to have the 7812 and 7912 voltage regulators for the opamps. The main problem with this kit is the difficulty to get a proper mains transformer. Then the meters are missing and the enclosure.Considering that i got OWON P4305 supplies for 140 €, i think that kit only makes sense for learning purposes. The OWON has a toroidal transformer and it includes digital control of its settings and readings of actual current and voltage via RS232. Its main disadvantage is the missing analog (fast) overcurrent limiter.Regards, Dieter

author: Aramp
author_rank: Contributor
timestamp: February 03, 2024, 07:06:59 am
content: Quote from: Kleinstein on February 02, 2024, 02:15:12 pmYou are definitely right. Overall, there are several changes, some are good, some are not that good. Judging from the original Chinese forum where this was first published, the guy seemed to be focused on turning the new design into a business of selling the kits, hence he didn't publish the schematic and didn't disclose what exactly he has changed.

author: Aramp
author_rank: Contributor
timestamp: February 03, 2024, 07:12:43 am
content: Quote from: dietert1 on February 02, 2024, 10:25:49 pmDieter, agree with you, unless there is a transformer laying around that you want to utilise, which was exactly my case. The second reason that made me opt for DIY is the sheer weight of linear power supplies. You see, there are parts of the world where shipping gets very expensive with weight, and I live in that kind of country. And finally, unless very restricted in place, one wants to have at least a couple of power supplies anyway.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 03, 2024, 07:19:32 am
content: It is a bit problematic of having a kit that needs an extra, relatively special transformer. Without a schematics it would be a bit tricky to adjust to a slightly different transformer (e.g. lower power or different voltage). The principle circuit is quite flexible and the same PCB could be used with different voltage ranges with more minimal changes in the parts.

author: Aramp
author_rank: Contributor
timestamp: February 03, 2024, 08:21:36 am
content: Quote from: Kleinstein on February 03, 2024, 07:19:32 amI have used a typical transformer that was widely used in many power amplifier designes. Opamp based amplifiers normally need bipolar source, and often the rails for low-power opamps and high power push-pull stages are separated. For the purpose of this board you would need a low-current bipolar winding for 15V and then a high-power winding that outputs voltage that depends on the output power of the amplifier. I was lucky enough that the transformer I've used had additional taps in the high-power secondary. However, in most cases it is realtively easy to get out the iron and add taps, or just wind a layer over the existing coil even without removing the iron to get the additional lower voltages. I have done that several times.

author: p.larner
author_rank: Frequent Contributor
timestamp: February 03, 2024, 10:43:08 am
content: i have a big transfo with about 10 or so taps ranging from 3v to 50v can i use taps from that for the cct control as well as the main power?,or will doing that cause a short?,i have a small 18v transfo but no room to fit it,using just the main big transfo would be better if poss for both the main power and control,is that dooable?.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 03, 2024, 11:03:24 am
content: The type of circuit needs the supply for the control (7812 and 7912 in the circuit) and the main power for the output isolated from each other. So one needs at least 2 separate secondary windings. If really needed one could reduce the +-12 V to a lower value. 12 V may be handy for the relays. The control part only needs relatively low power (e.g. 2 W) - it is mainly the relays and if used a digital display part.Unless one finds a suitable transformer using 2 transformer is a real option. An alternative maybe using a torroidal main transformer and add an extra secondary - here it could be easier with a lower voltage and lower voltage relays.

author: enut11
author_rank: Super Contributor
timestamp: April 07, 2024, 10:03:09 am
content: Hi@ArampThanks for posting this power supply kit. After a frustrating time with another Chinese kit (30v/3A, red pcb), I am looking forward to a better experience and this kit seems to fit the bill.I only need about 1A-2A so could do with just one output transistor and one relay. I will think about it when the kit arrives.BTW, I found a small error on your re-drawn circuit diagram. You have the emitters of Q2 and Q4 joined. Q2 emitter only goes to R23 (1K)EDIT: and the bases of the output transistors.Maybe let out a puff of smoke if the error was also on the actual PCBenut11

author: Aramp
author_rank: Contributor
timestamp: April 08, 2024, 08:30:26 am
content: Hi enut11Quote from: enut11 on April 07, 2024, 10:03:09 amI've also got the red kit, and I am playing with it right now. The one I have posted here is clearly better because of automated voltage switching and separate balanced power for the opamps independent of the main AC supply. It requires a more sophisticated transformer, though.As to the "red" kit, I am thinking of adding the same switching circuit to it. Otherwise, it's a very clever design. BTW, its original schematic comes from the Oct 1978 issue of "Practical Electronics" journal, which can be easily found online. At the time, opamps were expensive, and the author did amazing work minimizing their number. And that's why the Chinese like the schematic: they also want everything cheap. However, while the author tried to save dozens of dollars in the 1978 currency value, today, our Chinese friends are saving cents. The problem I have now with the "red" kit is that I cannot get more than 23 Volts from it, no matter how much the input AC voltage is. It seems to be the result of the driver transistor being fake and not providing enough current/amplification or having a wrong bias. Will figure it out.Quote from: enut11 on April 07, 2024, 10:03:09 amI have just double-checked the schematic that I have posted and found nothing wrong there. The emitter of Q2 is not connected to the emitter of Q4. It goes to the bases of both Q4 and Q5 (they basically are connected in parallel for double the power output). Q2 works as an emitter follower, providing enough power to drive the powerful transistors out of the outputs of U1a and U1b, which control current and voltage respectively. Without it, the opamps may have a hard time providing enough base current for two transistors.

author: enut11
author_rank: Super Contributor
timestamp: April 09, 2024, 07:11:47 am
content: @ArampThis is what I found in your updated circuit. Emitters of Q2 and Q4 are connected??

author: enut11
author_rank: Super Contributor
timestamp: April 09, 2024, 07:17:02 am
content: @ArampThis is probably not the thread to discuss 'red kit' problems but I found the output is a little lower than the opamp positive rail.In my case I had main filter cap at 34v but was powering the opamps from a 24v regulator resulting in a max output of around 22v volt.enut11

author: Aramp
author_rank: Contributor
timestamp: April 09, 2024, 02:56:31 pm
content: Enut11,Indeed, you are right! There is a dot that should not be there that skipped my attention. Fixed and updated the ZIP file.As to the "red kit," indeed, that's offtopic, but the problem I have is probably related to the opamp not delivering enough output voltage for some reason at 25 volts region. Even when the rail-to-rail voltage is close to 36 volts, and the non-inverting input has a higher potential than the inverting one, the output does not get higher than 30 volts in relation to -5.2 volts negative rail (which is some 25 volts from the ground). I am investigating the issue further. It could be an out-of-spec opamp; they probably get them cheap from factories.

author: enut11
author_rank: Super Contributor
timestamp: April 10, 2024, 01:55:24 am
content: @ArampSuggest we continue discussion of 'red PS kit' on another thread or revive this one:https://www.eevblog.com/forum/projects/hobbyist-current-limited-power-supplykit/msg4676974/#msg4676974Post #3 leads on to the red kit mods.

author: enut11
author_rank: Super Contributor
timestamp: April 13, 2024, 09:14:07 pm
content: The 'green PCB' 30v/5A kit arrived and I spent yesterday populating the very crowded the board. Only one problem so far, a component with no legs! Fortunately I had a spare.EDIT: be aware that some of the cheapest kits do not have output transistors.

author: enut11
author_rank: Super Contributor
timestamp: April 13, 2024, 09:19:18 pm
content: Suggest you save pictures of the unpopulated PCB to assist with any future troubleshooting

author: enut11
author_rank: Super Contributor
timestamp: April 13, 2024, 11:40:35 pm
content: First run with the 'green' power supply.Cold start low voltage with no load. Output is rising and never seems to settle down even after 50 min. Vertical scale is 16mV. This is not good enough for my needs.The voltage reference is a TL431 IC configured for 2.5v output. It sits between 2 voltage regulators, not ideal even though they are not running hot. The 431 could be a fake chip.I will have a look at fitting a better Vref...

author: enut11
author_rank: Super Contributor
timestamp: April 14, 2024, 01:40:26 am
content: Found a problem with@Aramp's wiring for the voltage pot. To be fair, it was also wrong on the PCB.As shown, the voltage pot presented a variable load on the TL431 reference. This explains why the PS never settles down at low voltages - see above.Fixed by reversing the wiper and hot leads to the voltage pot.

author: enut11
author_rank: Super Contributor
timestamp: April 14, 2024, 03:38:07 am
content: A repeat of the above test (low volts, no load) shows stability is reached in less than 2 min so the TL431 reference chip is ok and working well.Having reversed the hot and wiper leads of the Vpot to the PCB now provides a constant load to the Vref line.Next step is to fit a larger heat-sink to the output transistor and to do some load tests.

author: Aramp
author_rank: Contributor
timestamp: April 14, 2024, 04:02:53 am
content: enun11,You are right; that's how the original schematic and the PCB were designed. Interestingly, the pot for the current limiting circuit is placed properly, while the current drift is much less important. I'll change my device as well, as I have experienced some voltage drift and did not think there could be a design flaw. Now, I will probably leave the schematic as is. Otherwise, people will be perplexed by the difference between the PCB and the schematic. I will only add a link to this forum in the doc inside the zip file so that people who download the file from now on will be able to find this discussion.BTW, I have noticed the efficiency of current limiting in this power supply compared to the "red" one. Shortening the output is the prescribed way to set the current limit for this circuit. There are no sparks, nothing. The current is immediately dropped when the output is shorted, and you can control it with the pot. With the "red" one, you get some serious sparks due to a significant spike in current. I have seen many people replacing the output transistor with a powerful one (myself included) and placing it on a good heatsink before even trying shorting. Others tried and blew almost all active components.

author: Aramp
author_rank: Contributor
timestamp: April 14, 2024, 04:32:26 am
content: The other design flaw of the "green" power supply, as was mentioned by Kleinstein above, is that the amperemeter is placed after the voltage probe connection point. Thus, the drop in it will not be compensated. This means that the effective output resistance is increasing, suggesting voltage drift as the load grows. The original schematic had the amperemeter in series with the shunts to eliminate that problem; however, the standard cheap digital ammeters will not work that way as they are expected to be connected in series with the negative line. On the other hand, the shunts on these ammeters have resistance in the range of 0.01 ohms, which is lower than the resistance of most wires one may use to connect to the power supply.If the output resistance still matters, the best way would be to have a custom-designed indicator that can be placed in the positive line or use an old-school mechanical meter measuring the voltage across R11.

author: enut11
author_rank: Super Contributor
timestamp: April 14, 2024, 06:09:53 am
content: Hi ArampThe original circuit is confusing as both voltage and current pot connections are shown as 2-pin instead of 3.My suggestion is to correct it on your updated circuit as there are no instructions with the kit on how to wire the pots to the plugs anyway. Better to present it the right way.As I was only using a 2-tap, ie 3 wire transformer, I had to disable the second relay by cutting a track going to the coil otherwise the circuit switches to a non-existing winding and shuts down the power supply momentarily only to repeat the whole process.I have just finished load testing my PS and all is well.This PS is missing a Constant Voltage (CV) indicator. I believe this may be as simple as inserting an LED in the U1B pin 7 line either in series with or instead of D17.enut11

author: enut11
author_rank: Super Contributor
timestamp: April 14, 2024, 06:26:07 am
content: Quote from: Aramp on April 14, 2024, 04:32:26 amNot a problem for me as I have decided to use analog meters anyway. The cheap digital meters inject noise into the PS output.R11 won't work for this mod as there is almost no current flowing through it. The ammeter shunt would have to be after the emitter resistors (R24 and R25).In my case it will be simple as I need 2A output so only one output transistor (Q4). The analog ammeter will be wired across it's emitter resistor( R25).enut11

author: magic
author_rank: Super Contributor
timestamp: April 14, 2024, 06:57:07 am
content: Isn't it this kit?https://www.eevblog.com/forum/beginners/shitty-chinky-psu-kit/The thread is mostly a few pages of spoonfeeding an utterly incompetent OP, but I recall that there was some important discussion there, including fixing potential problems with the design.

author: enut11
author_rank: Super Contributor
timestamp: April 14, 2024, 07:21:54 am
content: Thanks@magic. I just did not think to search for"shitty chinky psu kit"!I have not had any serious problems yet but will go through that thread.enut11

author: enut11
author_rank: Super Contributor
timestamp: April 14, 2024, 07:30:16 am
content: The Constant Voltage (CV) LED mod was easier than expected. The indicator was wired in series with D17 to pin 7 of U1B.Now new indicator is lit in CV mode but is extinguish if CC mode is invoked.EDIT: I no longer recommend this mod due to the possibility of LED failure. See Reply #38 for a better solution.

author: Aramp
author_rank: Contributor
timestamp: April 14, 2024, 07:43:57 am
content: Quote from: enut11 on April 14, 2024, 06:26:07 amOf course, you are right, as the opamp has nanoamp current at its inputs. If two transistors are used, the good compromise would probably be to put the ammeter, as in the original version of the circuit, after the junction of two shunt resistors, and connect the voltage regulation circuit after that point. I have used a digital voltmeter for this one. I measured the level of digital noise with an oscilloscope. It's clearly there, but it seems to be below 100 uV. However, am I planning analog indicators for the other one, as I also need a silent PS for radio equipment that has no noise.

author: Aramp
author_rank: Contributor
timestamp: April 14, 2024, 07:58:37 am
content: enut11,One question about your mod: is it safe from wiper disconnecting? Will the 1M resistance in parallel suffice to protect from the opamp misbehaving? Did you test that?

author: magic
author_rank: Super Contributor
timestamp: April 14, 2024, 08:16:09 am
content: Wiper disconnection is one of the issues I remember ranting about in that old thread.I think the other one was what happens when voltage on input pins of the opamps goes below their negative supply under fault conditions. LM324/358mayget phase reversal if this is allowed to happen.

author: enut11
author_rank: Super Contributor
timestamp: April 14, 2024, 08:17:14 am
content: Hi@Aramp. No I did not. Also found another thread from@magicthat highlights the danger of LED failure.A safer option suggested by @xavier60 in that thread:"D17 could be replaced with an LED also but LEDs can and do go open circuit.A safer option is an LED with series dropper resistor from the +15V control rail to the output of the CV opamp, pin 7"In our case that would be the +12v rail. This is the mod that I will be going with.enut11

author: enut11
author_rank: Super Contributor
timestamp: April 14, 2024, 07:06:28 pm
content: Quote from: magic on April 14, 2024, 08:16:09 am@magic, would you tell me more on this please.

author: magic
author_rank: Super Contributor
timestamp: April 14, 2024, 07:36:01 pm
content: 1. The wiper of a pot may lose contact with the track. Whatever PSU you are building, it's a good idea to ensure that this doesn't result in the output jumping to maximum. LM358 has input bias current flowing out of the pins, so never leave IN+ floating under any circumstances. R13 on your schematic should prevent problems in such case, make sure it is installed. The original diagram in your post above (unlike the red lines drawn over it) is safe even without R13.2. Under some circumstances, if the voltage on either input pin of LM358 goes ~0.5V below the negative rail, the output may (or may not) jump to the positive rail. The details are a bit murky, but several manufacturers caution against such operation. Make sure negative voltage is never applied there. Here I think this could happen if output voltage of the PSU is still hight due to charge stored in output caps but the CV pot is quickly turned down to zero.

author: enut11
author_rank: Super Contributor
timestamp: April 14, 2024, 08:35:03 pm
content: @magicI was about to change the 7912 for a 7905 on the control circuit. This would not be a good idea given your explanation above?enut11EDIT: I did not go ahead with this mod as it is unnecessary.

author: enut11
author_rank: Super Contributor
timestamp: April 17, 2024, 04:27:45 am
content: I have completed the 'Green PCB' kit often labelled as 35v/5A or 30v/5A purchased from Aliexpress.I used a digital panel meter, despite my previous comments but added a disable switch if needed.The plastic Jiffy control box (200x60x110mm) contains everything except the main power transformer which sits in its own box connected by 3 wires the the control box.My need was for only 1A so I eliminated one output transistor and one 0.22R emitter resistor.The transformer I used had 17v and 11v windings so I also eliminated one of the switching relays.I modified the circuit in 2 other ways, one I consider very important:1) I added a constant voltage LED indicator as per@xavier60- nice to have. I no longer recommend the mod in Reply #30.2) The voltage pot was re-wired by swapping the hot and wiper leads.This significantly improves stability at lower voltage outputs.The voltage pot wiper now feeds into the opamp ( U1B pin 5) as per normal and no longer adversely affects the Vref line from TL431 at low output voltages.enut11

author: enut11
author_rank: Super Contributor
timestamp: April 17, 2024, 04:40:22 am
content: Noise measurements at 10v were made with 70mA and 500mA loads. They were 40uV pp and 70uV pp respectively - good figures given the low cost of this kit (<$30AUD).The 500mA recording is shown below. The Y scale spans 1mV.Thank you @Aramp for your up-to-date schematic. Much appreciated.enut11

author: Aramp
author_rank: Contributor
timestamp: April 17, 2024, 06:02:29 am
content: @enut11,Great job! The device looks and performs very well. I'll make both mods and add a switch for the display. They are indeed very useful.From my experience, the use of the PS is satisfactory. It sits on the shelf on my bench and just works. Mine uses a heavy transformer and produces all the 5A without problems, and I have never seen the fan turning. I have forced it by heating the thermistor during the test. I have installed a multi-turn pot for voltage regulation to make it less touchy and more precise and to avoid very fast swings that may potentially lead to the device misbehaving.

author: enut11
author_rank: Super Contributor
timestamp: April 17, 2024, 07:23:27 am
content: @ArampAgreed, 10T pots are worth installing. When I was fiddling with my kit I tried an open-circuit pot wiper to U1B and the supply shut down. Good to know that the 1M resistor from pin 5 to ground works.I have ordered a fan as I am using a small heatsink.enut11

author: Aramp
author_rank: Contributor
timestamp: April 17, 2024, 10:45:23 pm
content: BTW, I have just modeled in LTspice the transformer taps switching circuit to help analyze its workings and found out that it is very cleverly designed. If you look at the schematic, there is a D7-R26 in series going from the collector of the transistor T1 (and R27 and D8 from T2) that introduces hysteresis. The values of R26 and R27 are different to compensate for the higher threshold voltage for Stage 2, so the hysteresis value remains roughly the same for both stages. The idea is that whenever the voltage at the non-inverting input of the opamp becomes larger than the threshold defined by the pot RV3, the opamp opens the transistor, and the collector is grounded. This immediately drops the voltage on the inverting input even further to secure the switching and avoid oscillations. Then, when the output voltage is regulated downwards, it should achieve a lower value for the opamp to switch back, and then the same effect happens in the opposite direction. C17 is there because whenever the switching happens with high load connected, the current going through R7-R8 may fluctuate, so it makes sure the voltage at non-inverting inputs is not jumping around. The Zener diode D11 is needed to provide a reasonable voltage on the base of the transistor.

author: xavier60
author_rank: Super Contributor
timestamp: April 18, 2024, 01:07:30 am
content: Quote from: enut11 on April 17, 2024, 07:23:27 amBecause of the way wire-wound multi-turn Pots can fail, the PSU's output can go high, regardless of how it's wired. This is due to the wire breaking rather than loss of wiper contact.I have found the cheap 3590S style Pots on ebay and Aliexpress to be very unreliable.In 3 years, I have not had any WXD3-13-2W style Pots fail. But they are huge.There is a shorter 5 turn variant.

author: dietert1
author_rank: Super Contributor
timestamp: April 18, 2024, 05:45:14 am
content: It's fairly easy to implement a configurable voltage safety limit.Just get a rotary switch with let's say 6 positions and wire 5 different zeners to it. Connect that between ground and the control signal at the base of Q2. A 6.8 V zener will result in about 5.5 V maximum output voltage. Each safety limit will be about 1.3 V less than the nominal zener voltage. One could implement 5 V, 9 V, 12 V, 15 V, 18 V limits and the 6th switch position remains for "no safety limit".Regards, Dieter

author: xavier60
author_rank: Super Contributor
timestamp: April 18, 2024, 06:13:44 am
content: With my build of the design from the other large thread, I implemented Pot failure protection by monitoring for a rapid increase in wiper voltage exceeding 300mV. This idea can only be used with designs where the Pot voltage divides the reference voltage.

author: enut11
author_rank: Super Contributor
timestamp: April 18, 2024, 10:44:14 am
content: Quote from: xavier60 on April 18, 2024, 01:07:30 am@xavier60I accept that multi-turn pots can be unreliable and it happened in one of my previous projects. However, in this case, I deliberately disconnected the wiper going to opamp pin 5 on my PS and the output went to zero. This can only be due to the grounded 1Mohm resistor and appears to be fail-safe.enut11