Diy Bench supply questions - Page 1
Forum section: electronics / beginners

author: iampoor
author_rank: Frequent Contributor
timestamp: October 04, 2015, 05:12:31 am
content: HiTo make a long story short, I would like to build a bench supply with 4 0-25v current limited 1-1.5amp outputs.Each supply will have a +, -, and GND terminal. GND will be tied to the chassis an earthed. If I use a seperate transformer for each supply, and they are fully isolated (while being in the same chassis) I should be able to tie the + of one supply to the - of another supply and use these are bipolar +- regulated supplies, correct? I have only used dual tracking supplies for these applications in the past, but I got a goo hookup on transformers, and would like the most flexibility possible....In the spirirt of simplicity an to use commonly available parts, I am planning on using this schematic. It is straight from the On-Semi datasheet. If anyone has any links or simple designs with vastly better performance, I wouldnt mind trying them out however. I do auio work, so clean supplies is very important to me.However, the transistors used as (current sources I believe?) are not currently avaliable. What makes these special? What should I be looking for in a replacement? I am not sure what characteristics a transistor used as a current source should have in this application. And I dont really want each transistor to cost more then the transformers I got!Thanks!EDIT: Check post #8, now considering new circuits

author: bitseeker
author_rank: Super Contributor
timestamp: October 04, 2015, 05:33:01 am
content: Regarding chaining the outputs of two supplies in series to get positive and negative voltages, yes that's fine. You can either use a separate transformer for each channel or a center-tapped one for both. For a four-channel supply, you can get one big transformer with separate taps for each channel.

author: Ian.M
author_rank: Super Contributor
timestamp: October 04, 2015, 05:37:41 am
content: I discussed replacing them with NPN bipolar transistors here:https://www.eevblog.com/forum/beginners/lm317-variable-power-supply-question/msg687356/#msg687356It needs a little redesign, but you should be able to get superior performance from 'jellybean' parts.

author: singapol
author_rank: Frequent Contributor
timestamp: October 04, 2015, 06:03:27 am
content: Someone asked a similar question here:Explaining the function of current source.https://www.eevblog.com/forum/beginners/puzzling-jfet-in-power-supply-circuit/It's your responsibility to check the specs if they match or not.http://english.electronica-pt.com/components-cross-reference?ref=2N5640http://english.electronica-pt.com/components-cross-reference?ref=2N3822

author: Kleinstein
author_rank: Super Contributor
timestamp: October 04, 2015, 07:19:05 pm
content: The proposed circuit is not really good: the dropout voltage is rather high - two of the LM317 in series loose at least 5 V plus up to 1.2 V at the resistor. Also Power disipation at the LM317 will be a problem - they can't dissiate much power and will thus go into thermal shutdown under certain conditions. This circuit is more one of the could work but not practical.I would look for a different circuit. Options are a circuit based on LM723 or Ref and OPs (e.g. TL431 + LM358/LM324) driving an external darlington transistor like TIP140. The only problem ist finding a working solution - there are 100s of such circuits in the web, but most of them have some mistakes / weak points, and are thus not allways better then the solution above. A good circuit does not have to be complicated. Staying at 25 V and 1-1.5 A makes things relatively easy.

author: iampoor
author_rank: Frequent Contributor
timestamp: October 05, 2015, 08:13:13 am
content: Quote from: bitseeker on October 04, 2015, 05:33:01 amSweet. I got a great eal on some 24v 2.4amp transformers, so I will probably just stick with them.Quote from: Ian.M on October 04, 2015, 05:37:41 amThanks. This was pretty easy to follow. I think I "get it"!Quote from: singapol on October 04, 2015, 06:03:27 amThanks!Quote from: Kleinstein on October 04, 2015, 07:19:05 pmI have also seen this circuit with lm350's...probably runs better at higher currents.Finding a suitable circuit is always the issue (for me). Thats a large reason why I was happy finding this circuit, but it seems to have almost a few too many shortcomings.... I was look at those parts and see if I can come up with something....there are too many crappy power supplies on the internet! Or worse yet, decent "bench" supply schematics without current limiting! It seems to me like the majority of linear PSU schematics online are ~30yrs old!

author: Kleinstein
author_rank: Super Contributor
timestamp: October 05, 2015, 06:33:11 pm
content: Its no big difference whether a LM317 of LM350 is used. The problems are about the same. In many respects this circuit is not better than many of the poor suggetstions for a power supply you find in the web. The good is that the LM317 is protected from excessive temperature - the bad is the extra 2-3 V lost from the current limiter. Its just a bad idea to have two separate power divices for current and voltage regulation. The second point is that voltage regulators like the LM317 are designed to drive a resonable well behaved load rather fast. In a lab supply you want unconditional stablility is more important than very fast regulation.The other would be finding a resonable plan and possibly adjust it to the 24 V transformer and 1.5 A max current.The good thing is that 25 V max voltage is just what many OPs can deliver. About 1.5-2 A is about what one can do with a single BJT like 2N3055 or TIP140. There is a parallel thread on a simple supply circuit that might work resonably (though not very good), especially if you can live with a 25 V max voltage.

author: Ian.M
author_rank: Super Contributor
timestamp: October 05, 2015, 07:01:25 pm
content: OTOH its cheap and easy to build (as long as you have a big enough salvaged heatsink)!I'd assemble it on veroboard and use it till you've got a better design worked out + parts and PCB ordered.

author: iampoor
author_rank: Frequent Contributor
timestamp: October 08, 2015, 02:55:38 pm
content: Quote from: Kleinstein on October 05, 2015, 06:33:11 pmOkay that makes sense. Im assuming Using 2 devices for separate current an voltage regulation is a bad idea because:A. Higher dropout voltage? (Without compensation)B. Higher voltage drop/needs higher power supplyC. Higher power supply dissipationAre there any other things that could be added to this list?Ive spent the last few days attempting to study power supply circuits, and Ive come to the understanding that the LM317/LM350 is probably not a great option for power supplies with a variable current limit.By "well behaved" you mean a load with a constant impedance, well defined current limits, and not potentially being shorted every 10 seconds right?At this point, I would rather get a circuit that will work well...I dont plan on building this supply for another month. I have narrowed it own to a few options that seem pretty decent, but I could use some direction. I have been trying to find circuits that are not 40 years old, an will not explode if the output are shorted!How do these solutions look? Out of the many many circuits on the lab, these appear the best to me...but I could use some advice as my knowledge on the finer technical points is very limited.Circuit #1http://www.eleccircuit.com/variable-regulator-0-30v-5a-by-lm723ca31402n3055/ProsA. SimpleB. Commonly used pots, and "fine" voltage control.C. Seems relatively modern (Okay...maybe this isnt really a pro O0)Cons:A. Ca3140 is expensiveB. Not sure how this would act when shorted?C. Not sure what transistors Q3/Q4 are. Bc series?http://www.eleccircuit.com/0-30v0-2a-adjustable-voltage-and-current-regulator/Pro'sA. Output protection!B. No modification necessary to achieve right voltage/current outputCons:A. Hard to find 3k potB. 24-0-24 transformer needed? Seems like alot of voltage drop...I would need to get new transformers for my projectCircuit 3http://kakopa.com/APR3005/index.htmlPro's:A. Seems like a proper "lab" power supplyB. Short circuit protectionCon'sA. Multitap transformer (Not sure where I can fin a cheap source for these!)B. ComplexAny advice? Im open to trying any circuit out, these just seemed the best in my (limited) experience. I would really appreciate some advice or a finger pointing in the right direction.Thanks guys as always!

author: Paul Price
author_rank: Super Contributor
timestamp: October 08, 2015, 04:12:38 pm
content: Looking at the schematics and specs of the power supplies you have listed, i get it that all of them seem to be poorly designed.Try to do more research, you could start with finding schematics for Agilent/HP bench or lab supplies, definitely look at the all-so-many other DIY solutions posted on this forum.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 08, 2015, 04:21:31 pm
content: I agree, all three solutions are not good. Especially the second one seems so miss fast current limiting.The first one might be a starting point, though unusal in using the LM723 as a floating regulator and also with at least to small mistakes in the plan. The CA3140 indicates this as a rather old circuit - modern replacements would be possible.I would look for another starting point, not necessary with a LM723, though there are also working circuit with this chip, thougg usually only with rough current regulation. Many more modern circuits use OPs and a separate reference chip / zener.

author: iampoor
author_rank: Frequent Contributor
timestamp: October 17, 2015, 04:26:19 am
content: Man, Ive been having a really hard time finding a good starting point...I have been studying some Hp/Lambda supplies an I would almost rather just buy one...most of the older units are pretty complex discrete designs.Any good recommended books on the subject?Im trying to not get overwhelmed!

author: dadler
author_rank: Unknown
timestamp: October 17, 2015, 05:01:16 am
content: Quote from: iampoor on October 17, 2015, 04:26:19 amCheck out this video series by Peter Oakes:https://www.youtube.com/playlist?list=PL_atu5RtEPi4aNzoMtZ5_S6ruhFR98T_pHe explains and demos a few different approaches that may work for you.

author: iampoor
author_rank: Frequent Contributor
timestamp: November 04, 2015, 07:51:00 pm
content: Quote from: dadler on October 17, 2015, 05:01:16 amJust finished the whole series.That was exactly what I was looking for. Im glad his design uses modern components and isnt 30-40 years old.

author: Mephitus
author_rank: Regular Contributor
timestamp: November 04, 2015, 10:00:52 pm
content: Quote from: dadler on October 17, 2015, 05:01:16 amThanks! This is exactly what I needed too!