Comparing Linear Adjustable Power Supplies (and kits)?  - Page 1
Forum section: electronics / beginners

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: February 18, 2023, 07:18:40 pm
content: Looking into getting an adjustable linear power supply, still undecided if I want to go for a premade kit (ex.https://www.aliexpress.com/item/1005003371792766.htmlorhttps://www.aliexpress.com/item/1005004154574280.html) and I've noticed that some adjustable linear power supply designs use relays... But how exactly does this work?I understand that dropping 30v DC to 1v 3a would waste A LOT of power in a linear design (if my understanding is correct, ~90W?), and *I think* the relays click in to provide lower voltages (ex. 15v, 10v) so less power is wasted, but wouldn't there be a period where the power is briefly stopped or where two voltages are on the same rail, when the relays are clicking in? Is this not problematic at all?Bonus points if anyone knows any design flaws/disadvantages to the linked Aliexpress linear kits.

author: Grandchuck
author_rank: Super Contributor
timestamp: February 18, 2023, 08:50:01 pm
content: I have two of the less expensive power supplies that you linked. I used fan cooling for the pass transistor. I added a 10-turn voltage vernier pot for fine adjustment and a cheap digital panel meter for voltage and current. They work well, considering the low cost. However, I bought mine elsewhere and paid a little more ... don't know if that matters?

author: Kim Christensen
author_rank: Super Contributor
timestamp: February 18, 2023, 09:19:07 pm
content: Quote from: LooseJunkHater on February 18, 2023, 07:18:40 pmThe 0-30V 0-5A supply you linked from Ali does exactly that (See snippet below). The way they've wired the relays ensures that "multiple voltages" are never on the same rail (Transformer taps cannot be shorted out). There will be a bit of a delay as they switch, but the filter capacitors should hold the voltage long enough so that the regulator doesn't drop out (Output voltage dipping below it's set point)About the only flaw might be small voltage spikes/dips on the output due to the imperfect transient response of the regulator circuitry. I'd definitely test it with a scope under different loads to see how bad it was before using it to power some sensitive circuitry. (Or just adjust the voltage before connecting the load)Note: Those Ali kits don't come with a transformer, which will add quite a bit to the final build cost. And it also comes with this uninspiring disclaimer:Quote

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: February 18, 2023, 11:21:50 pm
content: Quote from: Kim Christensen on February 18, 2023, 09:19:07 pmI'm aware neither design comes with a transformer, I've got a bin full of them with varying voltages and current capabilities. However thank you for pointing out that quote from the $30 power supply as that's quite concerning... Makes me wonder whether that fault is well-known on the web and whether anyone has designed a fix for it.Quote from: Kim Christensen on February 18, 2023, 09:19:07 pmTaking a look at the schematic of the $30 power supply it looks like it has quite a significant amount of output capacitance (1300uf?) which I'm not the biggest fan about (maybe this is a normal amount of capacitance? Feedback welcome) as it seems quite high based off of what I've seen from more professional setups, plus the $5 power supply only has 10uf on the output.

author: Kim Christensen
author_rank: Super Contributor
timestamp: February 19, 2023, 12:05:14 am
content: Quote from: LooseJunkHater on February 18, 2023, 11:21:50 pmThe filter capacitors I was talking about are the ones right after the bridge rectifier. As long as they are large enough for the current draw, they should be able to supply enough power for the time it takes for a relay to switch between it's NO and NC contacts.Yes, I don't like big caps on the outputs of bench power supplies either. Especially those with adjustable current limiting since it means a longer surge current beyond the CC set point when the supply switches from Constant Voltage mode (CV) to Constant Current mode (CC).

author: Jwillis
author_rank: Super Contributor
timestamp: February 19, 2023, 03:10:59 pm
content: For the sake of simplification I'll refer to each as the Red or Green one . I have both.I won't delve into the red one much because it has been discussed to death on this forum . You can do a search in the beginners section and find the draw backs and solutions for that one. The biggest issue is the excessive voltage a the TL081 Op amps . They burn out. This (after careful investigation) is do to the inrush current at the peak voltage of the AC supply This causes a voltage spike across the unit which promptly destroys the TL081 op amps.The green one is really not that bad but don't expect the 5 amps without adequate heat sinking .The relays are for tap switching as you've probably guessed . This ensures the the SOA of the 3055 power transistors are not exceeded . The taps are 0V 15V 23V 35V I believe ( haven't got the schematic in front of me right now).The idea is to keep the Voltage difference (Vce) between the collector and emitter as low as possible . Vce X I = power dissipation of the transistors . So the first tap is 15V , so after rectification its 15 x 1.414 = 21.21 V . So if you have 1V at the emitter and 21V at the collector that's a difference of 20V . 20V X 5 A is 100W . If the emitter voltage is 19V the Vce is 2V . 2V X 5A = 10W. So as you can can see when the emitter voltage is low you get the highest power dissipation on the transistors . So you will need really good heat sinking. I think I changed the power transistors as well.Now after a set out put voltage the tap switches up . Since the emitter voltage is already say around 21 V and the next tap after rectification will be around 32V . The Vce is now around 11V . These are ball park figures so it will depend on how you calibrate the tap switching .So as you see the tap switching keeps the Vce within the SOA of the transistors.Again these are ball park figuresThe higher voltages at the transformer is to compensate for the Transformers internal regulation . As more current is drawn from the transformer the voltage will begin to sag. So you have to have a higher voltage than your maximum output requirements to maintain a stable voltage .If you would like I can show you how I set mine up tomorrow .Regards

author: Kleinstein
author_rank: Super Contributor
timestamp: February 19, 2023, 03:22:06 pm
content: For some reson the links don't work for me (I know Ali has wiered cookies that sometimes send you to strange langish or similar).The one common version with the TL081 gettting too much voltage just does not work with higher supply, higher voltage. With a lower voltage transformer and thus some 20 V max it is reasonable OK.The is still a bug in the PCB causing quite some ripple in CC mode.In most cases the current is way to high for the power transistors. Maybe half is more reasonable.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: February 19, 2023, 10:35:41 pm
content: Quote from: Jwillis on February 19, 2023, 03:10:59 pmI actually tried to search for posts about both of the products and couldn't find any? Do you maybe have a link? I am probably using the wrong search terms though.

author: Jwillis
author_rank: Super Contributor
timestamp: February 20, 2023, 12:28:40 am
content: Quote from: LooseJunkHater on February 19, 2023, 10:35:41 pmAll the fine folks helped here helped me with a problem I had on this linkhttps://www.eevblog.com/forum/beginners/diy-0-30v-0-3a-again/You can search DIY 0-30V 0-3A power supply and you should be able to eventually find the original post where Audio Guru redesigns that kit among other posts.There is also loads of information on the internet concerning the design.There is also the original DIY design of the one sold herehttps://www.qsl.net/z33t/dc_0-30v_0-3A_eng.htmland herehttps://www.paulvdiyblogs.net/2015/05/tuning-030v-dc-with-03a-psu-diy-kit.html. There are numerous other sites to vast to post all of them.I did at one point find the original design from around 1995 I believe. But the PDF required translation from Czechoslovakian. At least I I think it was Czechoslovakian. Not very good with other languagesThat design included voltage regulation on the positive side for the opamps which isn't included in the DIY kit. Which really isn't that much of a problem . I'll see if I still have the design kicking around . No guaranteeIn the end I found it to be much easier to find a drop in op amp that can handle a higher supply voltage and using multiple parallel power transistors. The LT1637https://www.analog.com/media/en/technical-documentation/data-sheets/1637fd.pdfwould probably work. Ill give that try today if I can dig the kit out of the junk heap.I am working on one based on that design with tap switching that is capable of over 800W with absolutely no ripple at the output. Also running the control circuit on a separate supply from the Power transistors significantly reduced the transients caused by the tap switching . Biggest problem is getting a custom transformer made.AnTekhttps://www.antekinc.com/did custom winding for me .But that probably won't do you any good since the shipping might be prohibitive. And if you mess up like I did with the tap voltages then it can get really costly.Personally I didn't care for that DIY kit. I found the one with tap switching was a much better kit. Providing you use different power transistors with a broader SOA than what is supplied.The two TIP3055 they provide get really hot with out a big heat sink and forced air. The instructions provided can be somewhat confusing but not impossible to understand.The one issue I had was that it doesn't switch taps well or at all when under load. But that is most likely a a calibration issue on the tap circuit that I over looked. Also finding an appropriate transformer could be problematic.If you have any other questions I would be happy to help out.Best regards and good luck.I did find the original design see PDF below. Sorry it was 1996 not 1995.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 20, 2023, 08:06:18 am
content: Tap switching can be a bit tricky at high power. When switching between the taps there there is quite some current spike from charging the filter capacitors and thus possible contact wear. A poor contact at the load or the so called file test (scratch a contact over a file) can stress the contacts a lot, causing possible sticking contacts. Larger transformers get increasingly lower impedance so that the current pulse gets worse even faster than the power going up. Larger relays also tend to be slower causing more gap and the need for even more filter capacitor. So for the high power range a more contineous pre-regulation (SMPS or SCR / phase angle baded) may be the better choice.The first DIY lab supply should not be high power - more like a smaller one (e.g. 20 V 1 A) to limit the amount of maging smoke.The think the simple PSU desin is quite a bit older than the Chech article. The oldest version I remember was from an US magazin in the early 1970s, using LM741 OP-amps, that at least can work with a little higher supply.

author: Jwillis
author_rank: Super Contributor
timestamp: February 20, 2023, 10:31:31 am
content: Quote from: Kleinstein on February 20, 2023, 08:06:18 amYour right About it being low power for sure . I did a small experiment and swapped out the TL081's for the LT1637's and it works at the 33 rectified unloaded voltage without further modification. Unfortunately the ripple at 30V and 1A is pretty high at over 2V peak to peak. That's just terrible .With a 60VA transformer the voltage drop is quite high even at 1A load . Then again 60Va is kind of light for the application.The 4700uF filter cap is not near enough either.. At lower output voltages it's not to bad but still not great. The kit is fine as a play around project but I wouldn't use it as a bench tool.I just did a quick test and at 30V 1A and the basic results are below.

author: JohanH
author_rank: Frequent Contributor
timestamp: February 20, 2023, 10:37:05 am
content: Quote from: Jwillis on February 20, 2023, 12:28:40 amNo it's much older. The design originates from an article in Practical Electronics, 10-1978, page 1070:https://worldradiohistory.com/UK/Practical-Electronics/70s/Practical-Electronics-1978-10.pdf(page 40 in the pdf).