Help with diy bench psu circuit. - Page 1
Forum section: electronics / projects

author: gabri.peic
author_rank: Regular Contributor
timestamp: January 27, 2022, 11:40:12 am
content: So i got my hands on a pretty beafy toroidal transformer for free. Size is 11cm x 4.5cm and i think the power is around 200va since it says 200 on the sticker, it has 2 identical secondary windings and each one is 26v ac unloaded.Anyways lets get to the point.I want to build myself dual channel power supply with it so i can tets amplifiers and things that need two power rails.So ideally i need something that can take around 36v dc input and provide adjustable output preferably with variable current limit.My plan is to have two adjustable 30v outputs that i can put in series for dual rail or more voltage. Or in parallel for more current.If anyone knows of any good way to do this pleas let me know.Note: I know this will require huge heatsink with forced air cooling.

author: m3vuv
author_rank: Super Contributor
timestamp: January 28, 2022, 04:20:39 pm
content: something with 2x lm723 chips maybe,check out the datasheets!

author: amosborne
author_rank: Contributor
timestamp: January 29, 2022, 05:57:29 am
content: This is an awesome but deceptively advanced project depending where you take it.Is this linear or switching?Step up or step down?How will you load share? Passive droop or master/slave?How tight and fast is the regulation?Constant current mode?User controllable OVP/OCP?How much current are we talking?I would probably look around at commercial supplies and find a set of specs you like. Then try and hit those specs with your design. Saying you want 36V or whatever is the tip of the iceberg.I’m jealous of your transformer.

author: ledtester
author_rank: Super Contributor
timestamp: January 29, 2022, 06:56:08 am
content: There's been a lot of interest in using transformers with the Riden RD60xx power supplies, e.g.:https://www.eevblog.com/forum/testgear/ruideng-riden-rd6006-dc-power-supply/msg3881924/#msg3881924Look in that thread for an alternative firmware provided by user Unisoft which adds a lot of features and enhancements, e.g.:https://www.eevblog.com/forum/testgear/ruideng-riden-rd6006-dc-power-supply/msg3941395/#msg3941395

author: gabri.peic
author_rank: Regular Contributor
timestamp: January 29, 2022, 10:45:26 am
content: Basically i would like to build something based on a linear regulation with constant current/constant voltage mode, range would preferably be 0-30v and 0-4A,I would like to have two isolated channels since i have two secondary windings. And a switch that switches modes between independent tracking, parallel tracking or series tracking.Independent tracking would give me two independent power supply outputs, Parallel tracking would allow me to hook them in parallel and get more current, And series tracking would be used to either get more voltage or center tapped supply.I was planning on something based on this video:https://youtu.be/_CFIovMkRygObviously I would have to adjust it for my current and i would have to build two but would this be able to operate in series/parallel configuration.Thanks.

author: ledtester
author_rank: Super Contributor
timestamp: January 29, 2022, 05:08:07 pm
content: If you've never built a power supply before I would start simple - for instance, one using an LM317 (might have to use a LM317HV with your transformer) with pass transistors to get more current. This will allow you to pick suitable pass transistors + heat sink and you can carry this over to a more complex design. Here's an example design:https://electronics.stackexchange.com/questions/356610/lm317hv-npn-pass-transistorAnother thing I would suggest is to initially use an analog meter to display current. Most of the digital voltage/current readout modules you find on aliexpress/ebay/etc. perform low-side current sensing which isn't ideal for a power supply.There have been several linear power supply design threads on this forum. Going through my bookmarks here are some of them:-https://www.eevblog.com/forum/beginners/lab-power-supply-221926/-https://www.eevblog.com/forum/projects/diy-bench-power-supply-psl-3604/-https://www.eevblog.com/forum/projects/diy-bench-power-supply-psl-24012/-https://www.eevblog.com/forum/projects/dual-channel-15v-500ma-power-supply/msg2924452/#msg2924452-https://www.eevblog.com/forum/projects/0-30v-0-10a-power-supply-projects/-https://www.eevblog.com/forum/projects/how-this-linear-psu-circuit-works-and-why-it-doesnt/-https://www.eevblog.com/forum/beginners/building-the-elektor-1982-power-supply-what-pcb/msg3444578/#msg3444578-http://diyfan.blogspot.com/2013/03/adjustable-lab-power-supply-take-two.html-https://www.eevblog.com/forum/projects/2x-lm723-max-safe-voltage-for-this-circuit/The longest thread:-https://www.eevblog.com/forum/beginners/lm324-power-supply-with-variable-voltage-and-current/925/-https://www.eevblog.com/forum/beginners/lm723-small-power-supply-psu-as-a-beginner/-https://www.eevblog.com/forum/metrology/linear-power-supply-with-ultra-low-noise/msg3934431/-https://www.eevblog.com/forum/beginners/critique-my-power-supply-design/msg3938147/There's also the infamous 30V/3A cheap power supply kit you can get from ebay/aliexpress and the like for which you'll want to read these threads and blog posts:-https://www.eevblog.com/forum/beginners/diy-0-30v-0-3a-again/-https://www.paulvdiyblogs.net/2015/05/tuning-030v-dc-with-03a-psu-diy-kit.html

author: gabri.peic
author_rank: Regular Contributor
timestamp: January 30, 2022, 11:50:37 am
content: Thanks for all the tips.But i still have some questions.1 can i put the two separate channels in seriesor parallel if i keep them floating?2 is the circuit from the video good or not ?3 is there some design already out there that would fit my needs ?Also i tested the transformer with a dummy load.I loaded it up to around 4 amps per coil and voltage dropped from 26v no load to 24v fully loaded. It was pulling around 196w from the socket and didnt get hot at all. Just slightly warm. So i feel pretty comfortable pulling 4A from it. But will 24v ( around 33 Vdc after rectification ) be enough to get 30v regulated output or will i have to drop my requirements a few volts down ?

author: tunk
author_rank: Super Contributor
timestamp: January 30, 2022, 01:01:17 pm
content: I think a rule of thumb is to use ~2000uF/A smoothing capacitor(s)and that this will give you ~4V ripple, in your case 29-33V.

author: gabri.peic
author_rank: Regular Contributor
timestamp: January 30, 2022, 06:45:01 pm
content: Hmmm what about building two of these.http://electronics-diy.com/30v-4a-adjustable-bench-power-supply.php

author: ledtester
author_rank: Super Contributor
timestamp: January 30, 2022, 07:02:26 pm
content: Quote from: gabri.peic on January 30, 2022, 06:45:01 pmThat schematic looks like it is based on the same design as the "30V-3A" supply I mentioned above:Quote from: ledtester on January 29, 2022, 05:08:07 pmThere are a lot of caveats with that design which you will want to be informed about so I'd read that thread and blog post.

author: gabri.peic
author_rank: Regular Contributor
timestamp: January 30, 2022, 07:34:18 pm
content: Quote from: ledtester on January 30, 2022, 07:02:26 pmI looked at the links you posted. I still didn't read all of them but i did take a look at them.Some of the designs seem really interesting and are definitely informative but they don't fit my needs.The 3A 30V one was the one i was interested in but then i found this 4A version.I will do more reading as soon as i can.

author: JohanH
author_rank: Frequent Contributor
timestamp: January 30, 2022, 09:24:35 pm
content: Quote from: gabri.peic on January 30, 2022, 07:34:18 pm3A or 4A doesn't matter. It's the same design. And even more, it is claiming 4A with a single 2N3055 transistor. I wouldn't do that. Two or three will be fine. But you still need good cooling. This design originates from an article in Practical Electronics, 10-1978, page 1070:https://worldradiohistory.com/UK/Practical-Electronics/70s/Practical-Electronics-1978-10.pdfNot sure if it can be recommended today, other than for historical reasons. There are simpler and easier to build circuits today.

author: gabri.peic
author_rank: Regular Contributor
timestamp: January 30, 2022, 09:54:38 pm
content: OK i did some more reading. As far as i can see the 0-30v,0-3A circuit is not the best but it should work. I am thinking of building version modified by Audioguru. I am probably going to add some things like fan speed control.If there is something better pleas let me know.Also any tips on pcb layout would be great.I am also thinking of adding relays to the outputs so i can connect or disconnect the load with a switch so i can avoid possible spikes when powering off the power supply.

author: gabri.peic
author_rank: Regular Contributor
timestamp: January 30, 2022, 09:56:04 pm
content: Quote from: jukk on January 30, 2022, 09:24:35 pmCan you recommend some better circuit?

author: JohanH
author_rank: Frequent Contributor
timestamp: January 31, 2022, 07:00:03 am
content: Quote from: gabri.peic on January 30, 2022, 09:56:04 pmThe one I could recommend for less experienced designers is something based on the LM723 regulator (as also mentioned by others). It is an old circuit, but it is cheap and there are things to learn about it. If you search in this forum, you can find several designs. As usual, there will be lots of opinions, one way or the other. The most important thing is to read component datasheets and look at reference designs. A quick googling turns up good information like this:https://electronicprojectsforfun.wordpress.com/power-supplies/a-collection-of-proper-design-practices-using-the-lm723-ic-regulator/If you read this, you can see there are a lot of drawbacks with this regulator, too.

author: gabri.peic
author_rank: Regular Contributor
timestamp: January 31, 2022, 01:12:55 pm
content: Quote from: jukk on January 31, 2022, 07:00:03 amThanks i will take a look at that too and hopefully learn something.

author: gabri.peic
author_rank: Regular Contributor
timestamp: February 01, 2022, 07:22:38 pm
content: Ok i did some research.I am going to go either with modified kit orElektor 1982 design.What do you think of Elektor 1982 design ?Only thing is i would need either a separate power transformer or wind some low power windings on my transformer for that 2x12v that is needed for the Elektor design.What about the pcb? Is there some design already out there?

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: February 02, 2022, 12:00:17 am
content: Quote from: gabri.peic on February 01, 2022, 07:22:38 pmThe Elektor 1982 is not a bad circuit. I did build one once and sold it for good money.You should think right now about some options, like switching ranges for the instruments, sense int/ext, etc.. also, if you get an separate transformer for the auxiliary supply, have another voltage on it for control and instrument lights (if analogue) or separate panelmeter supplies. Do not use those crappy panelmeters that claim to work as well without one!With a separate transformer, you could use a relay or SSR for the main transformer and switch it only on if the auxiliary voltage(s) is up and good. Also, the circuit lacks thermal safeties, which you could also wire into this circuit.Place at least 2 thermal switches between the transistors and maybe one with the rectifier and 1..2 on the main transformer. With really simple circuits like a small additional relay you could also have an indicator for overtemperature. It is not among the most efficient ones as it lacks a pre-regulator, but that makes it easy to understand and modify.Use really good, precision, inductance free resistors in the power circuit (emitter res. too). Be aware that you can easily switch the A-meter range, but not the current adjustment. In that case you would have to use a circuit which does not interrupt the current shunt (ring-shunt switch).The thread discussing this PS circuit here on the forum sems to have covered some of those options, although in a more electronic implementation. Some other improvements as well.Not the worst choice.

author: gabri.peic
author_rank: Regular Contributor
timestamp: February 02, 2022, 11:28:21 am
content: Ok, now what about the pcb. Is there some design already available or should i design it similar to the original old design.I saw something mentiomed above in one of the suggested links. It is supposed to have fan controller and thermal protection.https://www.eevblog.com/forum/beginners/building-the-elektor-1982-power-supply-what-pcb/So how does that design look ?Also will there be problems if two of these are connected in series pr parallel. This project is getting expensive and i dont want to blow something up.

author: coromonadalix
author_rank: Super Contributor
timestamp: February 02, 2022, 11:47:30 am
content: Well if things get complicated, and way too expensive just buy two identicals models ... to put them is serial or in parallelDesigning your own power supply can be tricky and dangerous if you put sensitive devices on it ... and can get pretty expensive over the time, you want this option / that option etc ...You will find my name in a few threads, i did design some psus around the famous lm723, but in the end there was always a missing cv-cc crossover function that i would needEnded up buying already made reputable brands for the peace of mind, my 2 centsFor a parallel operation, the supply (ies) must have isolation between the input voltage (ac side) and the output, and check if the output ground (negative) is common or not with the case chassis ground, sometimes you need physical isolation between them Or you have a metal shorting bar on the terminals you can remove (or should be removable ??)You normally need two identicals models to work with in serial or parallel, for a good / equal load share between themIF NOT you will be limited from the weakest of the two in current capacity

author: gabri.peic
author_rank: Regular Contributor
timestamp: February 02, 2022, 11:52:34 am
content: Also i am calculating the prices and i think i am for now going to go with something simpler without constant current. I just cant go with something so expensive and involved for my first power supply project.But i will come back to this design in the future since it is really interesting.So i would need something to regulate the voltage. I don't strictly have to go to 0v but it has to be able to be put in series or parallel.And maybe adding resetable ocp would be a good idea.Note:i already have small lab supply that has current limit so this big one not having it is not a big deal for now.

author: coromonadalix
author_rank: Super Contributor
timestamp: February 02, 2022, 11:55:37 am
content: check the velleman k7200 psu you have explanations and schematics ... even some pcb filesIt is a simple circuit, not very pricey, built around common parts .... should help cut the costs downCheck the manual section for the schematic

author: gabri.peic
author_rank: Regular Contributor
timestamp: February 02, 2022, 12:27:43 pm
content: So if i go with just variable voltage supply would something like lm338 (one per channel) work ?

author: Kleinstein
author_rank: Super Contributor
timestamp: February 02, 2022, 04:34:08 pm
content: Quote from: gabri.peic on January 30, 2022, 07:34:18 pmWith a 200 VA transformer and 2 x 24 V AC one would get a little over 4 A AC current. Because of the power factor (1/1.6...1/1.the maximum DC current after a simple rectifier with filter is more like 2 to 2.5 A. So a higher current would start to overload the transformer already. So a circuit meant for 3 A is not bad, when reducing the current a little. More than some 2.2 A would be for 1 channel at a time or short time use.One may still allow a slightly higher current limit if one has an eye on the transformer temperature.With some 33 V to start with, a single 2N3055 is good for some 2 A with a good heat sink, maybe a fan. More gets tricky with the SOA and cooling. As the transitors are no longer that expensive I would prefer a 2nd parallel transitors to allow a somewhat smaller heat sink.

author: gabri.peic
author_rank: Regular Contributor
timestamp: February 02, 2022, 05:44:11 pm
content: If i go with just voltage regulation without constant current. What would be the best approach for that. I am having trouble finding parts and not to mention prices...I dont want to spend so much right away...So just variable voltage regulation would be enough for now.

author: gabri.peic
author_rank: Regular Contributor
timestamp: February 02, 2022, 06:48:01 pm
content: Would this be alright for just variable voltage?https://www.electronics-lab.com/project/5a-adjustable-regulated-power-supply/It would obviously need larger heatsink and i would not use more then around 3A but does the circuit look good for just variable voltage?

author: JohanH
author_rank: Frequent Contributor
timestamp: February 02, 2022, 07:05:13 pm
content: If you want the cheapest possible this is probably it. Go ahead and build it. Once you have the chassis, meters, connectors and switches in place, it isn't hard to replace the PCB with something more complicated in the future. I would use a larger heat sink mounted to the chassis and possibly use a chassis mount rectifier. The hardest part is probably finding a heat sink that fits your needs. Plan the chassis so that you could add a fan if needed. Then you could easily change the PCB to something else in the future. But that's just my opinion.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 02, 2022, 07:20:08 pm
content: The LM338 is likely a relatively expensive solution, but a simple circuit. Because of power limits the regulator likely has a fold back current limiting. So if little voltage is droped the current can be high (like 5 A), but if more voltage is droped the current limit would be lower (like 1.5 or 2 A). This helps to limit the worst case power loss and is still OK to power a circuit running at a relatively high voltage (e.g. < 10 V drop).There are a few threads about lab supply circuits - some of them are OK, e.g. one based around LM324 OPs.

author: ledtester
author_rank: Super Contributor
timestamp: February 02, 2022, 07:24:13 pm
content: Basically it looks ok, but I'd compare it with the reference designs in the datasheet:https://www.ti.com/lit/ds/symlink/lm338.pdfYou might want to add the protection diodes in Figure 18.Check the ** note in Figure 23 for the value of R2 in the schematic (120R vs 220R).The LM338 also has a min. load current (< 10mA). You might consider adding some sort of permanent load on the output to satisfy this.In general the TI datasheet has got a lot of good bits of advice especially with regard to component selection and layout.

author: gabri.peic
author_rank: Regular Contributor
timestamp: February 02, 2022, 07:34:53 pm
content: Ok,thanks. I will think about all the options and choose one.

author: JohanH
author_rank: Frequent Contributor
timestamp: February 02, 2022, 08:07:20 pm
content: Quote from: ledtester on February 02, 2022, 07:24:13 pmI have used a power-on LED (3mm LED and resistor) with some other regulator to satisfy its minimum load current. Then its not just wasted in a dumb resistor

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: February 02, 2022, 08:24:43 pm
content: I can only warn you that the LM338 can be cooked faster than its thermal protection can act when not mounted on a really good and generous heatsink. Then any retry will waste more money than one of the simpler PS with current limiting would cost you. If using a LM338, my tip would be the circuit with the additional OPA and current limit.

author: gabri.peic
author_rank: Regular Contributor
timestamp: February 02, 2022, 09:25:02 pm
content: Thats what worries me. That i will just waste money.The "30V 3A kit" with modification by audioguru seems like the cheapest somewhat decent circuit.

author: gabri.peic
author_rank: Regular Contributor
timestamp: February 03, 2022, 06:05:14 pm
content: So before i start spending money. Will the circuit attached below work alright or not?I would build the elektor circuit but it is a bit more expensive.The circuit attached is the chinese kit modified by audioguru.