Transformer Selection for Dual Linear Power Supply - Page 1
Forum section: electronics / projects

author: Dumpsterholic
author_rank: Regular Contributor
timestamp: August 06, 2023, 09:33:41 pm
content: Greetings all.From a YouTube channel called TheStuffMade, I've downloaded the attached schematic for a dual-rail power linear supply. For an output of +/- DC 0 to 20V, the designer recommends either a center-tapped transformer that can deliver 18-24V per side, or two 18-24V transformers wired in series to accomplish the same goal. I have amassed almost all the parts, including some shiny new PCB's, but I don't have the proper transformers. What I do have are three salvaged center-tapped XFMR's which gave me these AC voltage readings when measured with a cheap DMM : 38-0-38, 42-0-42, and 48-0-48.Based on my research. the forward voltage drop of a 1N4004 diode is around one volt, and the rectified DC voltage will equal VAC X 0.707. So, using the first XFMR, I should expect to get a DC output from the bridge rectifier of ((38-2) X 0.707), or about 25.5VDC per side, right? The LM317 and LM 337 datasheets tell me that these regulators can handle maximum inputs of 40V each, so in theory I can use any of the three XFMRs.BUT, if I only draw, say +/- 12V from the supply, those regulators will have to drop a significant voltage, and that will mean they will generate a lot of heat, right? So, my question is, assuming I use large heat sinks equipped with individual fans, and build an enclosure with plenty of space for ventilation, will my supply operate smoothly? If not, what options do I have, apart from buying the XFMRs specified by the designer?Any comments, criticisms (or even crude jokes about my mental health) will be most appreciated.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 06, 2023, 09:39:48 pm
content: If you have transformers and a meter, just connect a diode and suitable voltage rated cap (eg 100V) and measure what you get ?

author: TimFox
author_rank: Super Contributor
timestamp: August 06, 2023, 09:45:51 pm
content: The 38-0-38 V transformer will give very roughly 38 x 21/2V on both sides, not 19 x 21/2.The peak voltage is 1.414 x the RMS voltage, not RMS / 1.414.

author: mariush
author_rank: Super Contributor
timestamp: August 06, 2023, 10:00:16 pm
content: First of all, don't be so specific with the diodes... there's no point getting 1n4004 when 1n4007 will be more common and work just as well, the difference is just the maximum voltage they can support.I would also suggest replacing the diodes right after the transformer with higher current ones, like for example 3A ones, like 1n5408 or SB3100. They'll be a bit thicker so they'll dissipate heat easier.Don't forget that LM3xx regulators can only handle up to around 37v DC, depends on the regulator.Look at the output transformer as if there's no center tap - your 38-0-38 transformer will be 76v AC on the secondary ... rectified will be Vdc peak = sqrt(2) x Vac - 2 x voltage drop on diodes = 1.414 x 76 - ~2v = 105v DC , so each side will get around 50vI would suggest getting a transformer with 4 secondary windings, or use two transformers each with two secondary windings, and switch between using one winding or two depending on desired output voltage.For example, have one transformer with 2 15v secondary windings, and use only the first winding as long as the output voltage is 0...12v DC , as with rectification and smoothing capacitor the dc voltage will be at least a few volts above 12v, allowing for 1.5-2v drop on the regulator.If you want to go above 12v, you can switch to the total secondary winding.This will result in lower dissipation on the voltage regulators - with very good heatsinks with maybe fans, those linear regulators can do around 15-20w of heat dissipation ... which can happen if your input voltage is much higher than output voltage ... for example, 24v DC input, 5v output at 1.0A = (24 - 5 ) x 1A = 19 watts but using only one secondary winding : (15v - 5v ) x 1A = 10 watts.

author: ledtester
author_rank: Super Contributor
timestamp: August 06, 2023, 11:36:30 pm
content: I would suggest building the circuit and use whatever transformer(s) you can find. Even a power supply that is only capable of +/- 12V and a couple hundred mA is very useful. Modern op-amps are very power efficient and can operate at very low voltages.Finding the optimal transformer can take some time and it's something you can shoehorn in later so I wouldn't let it block the project.

author: Dumpsterholic
author_rank: Regular Contributor
timestamp: August 07, 2023, 03:52:48 am
content: I had a suspicion that I'd got the math all wrong... D'oh! Glad I posted this query.As ledtester pointed out, a +/- 12V@300mA supply would be a useful piece of equipment. In my junk box I have two identical 12V XFMRs which are pretty massive, meaning they can probably source a lot more than 300mA of current. Based on the math kindly provided by mariush, a 12-0-12V unit, after rectification, should be capable of delivering close to 16VDC per side to the regulators. Assuming these will drop another 2V each, that gives me about +/- 14V max output, which is probably all I really need.Problem solved! Many thanks to those of you who took the time to respond.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 07, 2023, 07:40:22 am
content: The circuit shown has 2 regulators in series and would thus loose more than 2 V - more like 2 x 2 V. So the maximum output votlage would be a bit lower.On the other side the diodes drop tends to be a bit less than 1 V and there is only 1 diode in the path per side. With low load the transformer voltage is a little higher than nominal. So one may get a little more raw DC voltage than 16 V. With significant load there is however also voltage lost to the ripple. Under significant load one may only get some 10-12 V.Just for information: the transformer current rating should be higher by about a factor of 1.6 than the used DC current. This due to the power factor from the more pulsed current.One could well try the x 12 V transformers, especially if one already has them. Otherwise the choice would be more like 15 or 18 V.

author: Dumpsterholic
author_rank: Regular Contributor
timestamp: August 07, 2023, 04:54:10 pm
content: QuoteNice catch, Kleinstein, and good supplementary information. I've been sitting on this project for awhile, mostly out of laziness (building a case is a lot of work!) but also because of lingering doubts about the XFMR ratings. As it happens, I just dug out a newish 12-0-12 unit that actually has a readable legend, specifying that it is rated for 4A. It is small, lightweight and compact, so I will use it and see how it performs. In future, if I can scrounge some 15 or 18V XFMRs, swapping them into the circuit will be easy.

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 07, 2023, 06:27:09 pm
content: Quote from: Dumpsterholic on August 07, 2023, 04:54:10 pmHave a look atthis summaryabout the basic formulas for different rectifier configurations.

author: Dumpsterholic
author_rank: Regular Contributor
timestamp: August 08, 2023, 03:38:05 pm
content: Thanks, Kim!This single-page document summarizes a great deal of useful information in a manner that even a "slow learner" like me can understand. I have duly downloaded a copy for future research.

author: Dumpsterholic
author_rank: Regular Contributor
timestamp: August 08, 2023, 04:10:57 pm
content: Quote from: mariush on August 06, 2023, 10:00:16 pmWow, maruish, have you been snooping in my junkbox? Turns out I have just that sort of transformer, whose purpose was unclear up until now. IIRC, it came out of a discarded UPS. I tore it apart and recognized a power XFMR, but after removing it was a bit mystified by its multiple windings. It appears to have both multiple primary AND multiple secondary windings, so a simple Ohmmeter check gave me puzzling results. After a little more probing, I realized that by connecting mains AC to two (primary?) wires, I could get two completely isolated outputs of 15-0-15 each. WTH? Now I'm beginning to suspect why a device might have been made.Some time ago, I watched a YouTube video which demonstrated a power supply using such a XFMR and some switches (and/or relays?) to bring one of the output windings in or out of the circuit, depending on the kind of load that was being driven. This made no sense to me at the time, but now I think I'm beginning to understand. Since power dissipation necessarily involves heat transfer, there can be situations when it makes sense to draw more or less current out of the same XFMR, and one way to do this is by using the multiple secondary windings to efficiently match the driven load.Thanks for the suggestion.

author: mariush
author_rank: Super Contributor
timestamp: August 08, 2023, 05:44:29 pm
content: Just be careful, some UPS transformers may not exactly be very well made, I mean not designed to run for long periods of time... they may assume you're gonna have rare power failures and that most of the time the batteries will not be fully discharged, so they're thinking the transformer's not gonna work more than a few hours each time.On a UPS, the multiple primary windings may be used in case of over voltage or under voltage for example you have 90v - 110v - 130v ... if the mains voltage is coming too high, or too low, it switches the winding to keep the secondary where it should be.It makes sense to be 15v AC because the batteries are charged with around 13.8v so rectify, smooth out with a small capacitor, use transistor / mosfet or linear regulator to top the built in battery.

author: Dumpsterholic
author_rank: Regular Contributor
timestamp: August 09, 2023, 05:53:21 pm
content: Gotcha. Consumer-grade UPS's can't be considered good sources for continuous-use transformers. I'll go with the 12-0-12 unit which has unambiguous markings and is rated for a very respectable 4A output. It also has two separate primary windings (two black leads and two reds); a series connection allows it to be used with a 220V input, while a parallel connection accommodates 110V, which is what I have at home. Until I get a couple of parts that are on order, I won't be ready for hookup and testing anyway. I now understand that the rectifier, filter capacitors and voltage regulators will all have an impact on my final DC output.If I end up with a serviceable dual-rail linear PSU that gives me both voltage and current limiting, with ~+/- 0-12VDC @~0-500mA output, I'll consider myself very fortunate. And since I ended up buying the minimum order of (5) PCB's, at some future point I can always build another supply with whatever XFMRs I have laying around. I salvaged two identical UPS's, so I have two identical mysterious "octopus" XFMR's to experiment with. And I already know that the metal-recycling place I go to does pay $$ for XFMRs, working or not....