What can I use a 13.8 V 20 A linear supply for? - Page 1
Forum section: electronics / projects

author: ohren
author_rank: Contributor
timestamp: January 19, 2025, 04:50:52 am
content: Someone apparently didn't want their 13.8 V 20 A power supply, so I picked it up in true EEVBlog fashion. Only, I don't know what to do with it. Are there any fun applications for it, or is it rightfully just needlessly heavy e-waste?I've come to understand they're used to run two-way radios meant to run on car batteries, but I'm not really into that. I more enjoy repairing things, but I don't see 13.8 V being particularly useful there...With this in mind, are there any interesting things I can do with it?- Schematics available (attached)- No current limiting (except the fuse)- Super heavy

author: MathWizard
author_rank: Super Contributor
timestamp: January 19, 2025, 05:42:43 am
content: The circuit is pretty small, so no big loss if you didn't use all of it, but I wonder what the transformer is capable of, if you didn't use the center tap ? In my hobby use, I haven't needed high currents for anything really. Maybe I would for some chemistry experiments, or some huge BJT homebrew computer/MCU.I have a few transformers from stereo's, that I've used or can use in PSU projects. Most of them didn't really have the windings or voltages I wanted, and required more transformers.So far I made 1 decent PSU, from an old all BJT design. Next I want to make one with op-amp's, and I bought some uA723's ages ago, I'll have to use them soon.

author: the Chris
author_rank: Regular Contributor
timestamp: January 19, 2025, 06:11:52 am
content: I have a similar unit now for over fifteen years and it is still unused. I originally got it for testing car hifi amplifiers outside the car but due to a new car, the whole car hifi thing came to an end and I never picked that hobby up again but rather restrict myself to home theatre stuff.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 19, 2025, 06:22:49 am
content: Quote from: ohren on January 19, 2025, 04:50:52 amThat's enough to run Peltier coolers/heaters, and you can easily add a regulator block to make a variable supply, for general electronics.Even 1A is useful there.

author: ledtester
author_rank: Super Contributor
timestamp: January 19, 2025, 06:35:20 am
content: Quote from: ohren on January 19, 2025, 04:50:52 amMost of these kinds of supplies have current limiting configured at their stated max amp rating. Usually it is set up as foldback limiting.

author: iMo
author_rank: Super Contributor
timestamp: January 19, 2025, 07:01:36 am
content: Ham radio transceivers are using those supplies for example (up to 100-150W into antenna with that PSU).Ham radio operators would be pretty thankful (me too) to have such a PSU handy (no noise compared to the modern switchers).

author: Gertjan
author_rank: Frequent Contributor
timestamp: January 19, 2025, 07:53:21 am
content: Quote from: ohren on January 19, 2025, 04:50:52 amThere is current limiting. It is even adjustable.Look at the schematics you provided. Current limiting is set with RP1

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 19, 2025, 12:14:47 pm
content: Quote from: iMo on January 19, 2025, 07:01:36 amWouldn't a high current switcher in parallel with a lower current linear make more sense? The switcher would only be on with high current draw.

author: thephil
author_rank: Regular Contributor
timestamp: January 19, 2025, 12:38:31 pm
content: A few applications I can think of:Electrolytic rust removalPowering automotive things for testingHack it into a battery charger

author: coromonadalix
author_rank: Super Contributor
timestamp: January 19, 2025, 01:14:03 pm
content: many pyramid psu are similar and othersthis circuit could be modded to go down to 1.2v not zero, lm723 based psu's unless they have a negative voltage on the pot sectioncurrent limiter is very basic, it can be tweaked too enhanced etc ....the problem is the xformer design, in some ways you could get twice the output voltage half the current, but the lm723 is supplied by a second xformer tap, called floating voltageyou would need to provide an small independant xformer who would give the same output voltagemeaning an almost complete redesign ...if it works fine, resell it and buy yourself an good linear one variable to better voltages and current ?

author: radiolistener
author_rank: Super Contributor
timestamp: January 19, 2025, 08:41:41 pm
content: Usually such PSU are used to power radio transceiver, they consume pretty high current up to 20-40 Amps at 13.8V and needs to be linear in order to minimize noises.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: January 19, 2025, 09:54:42 pm
content: The previous posters are correct. A 13.8 volt supply is the common radio power supply. 20 amp is plenty for most Ham radios since they are usually 100 W RF output. CB radios also use this sort of PS. I think it would be worth about $100 in the US at a radio shop.Radio users usually prefer Linear PS. The new Switching PS are much better with a cleaner output and less RFI from the internals compared to the old switchers but many people still stay away from them for their radios.

author: johnboxall
author_rank: Unknown
timestamp: January 20, 2025, 12:04:48 am
content: Quote from: NiHaoMike on January 19, 2025, 12:14:47 pmNo. When in amateur radio mode, we mentally go back forty years and just want linears.Don't care about power usage at home.

author: coromonadalix
author_rank: Super Contributor
timestamp: January 20, 2025, 11:22:00 am
content: i would say trials and errors with smps, some are very clean and some crap are not, you have the frequency harmonics ....i know a friend ham radio guy who played with smps, meanwell ones and crappy ones, the meanwell where almost perfect, he said they where noisy on a radio range, but silent on others ?twisted red / black wires with ferrites on the cables helped a lot .. ? the meanwell are around 200khz switching on the smps controller ic,it was a closed frame model with an thermally controller fan 12vdc at 40 45 ampsmileage may vary as they said ? but linears for the same specs are huge as you may guess ... pyramid 51 amp is now the one he uses ? biiig

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: January 20, 2025, 01:49:12 pm
content: The SMPS that are made for specific Ham radios do not interfere with those Ham radios they are made for. In other words, a SMPS that comes with a particular radio has its possible spurious interference at freq that do not affect the radio very much, if at all. This includes the various different IF freq used by the Ham radios, or any other radio that the SMPS was made for.Linear PS have very little interference and are desired by radio users because they can be partnered with just about any radio. This is a gross, general statement and there are exceptions.However newer SMPS are getting better and better, with less interference at any freq, but are more complicated to design and therefore more complicated to fix. Also repair people are not yet used to repairing SMPS yet. I know that I have limited ability to repair SMPS. Linears are, in general, easy to repair and much more simple in design.I have seen SMPS made for cell towers that have very bad spectral pollution for radio use but they work just fine in cell applications, and appear to be very well made. These are not usually 13.8 volt PS though.Edit: If you do not plan on using the supply possibly take it to a Ham Flea Market and trade it for something. It is a valuable piece of equipment. The heavier the better.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: January 20, 2025, 02:32:33 pm
content: if i'm not mistake, the schematic in the first posting seems to lack a connection between the wiper of RP2 and pin 4 of the LM723.cheers,rob :-)

author: madires
author_rank: Super Contributor
timestamp: January 20, 2025, 02:51:56 pm
content: Like DMMs, you never can have enough. For example, I use one as a central 12V-ish PSU to power several T&M devices and bench lights. A linear PSU is ideal for this, much better than a power strip full with SMPSU wallwarts.

author: ohren
author_rank: Contributor
timestamp: January 20, 2025, 10:28:56 pm
content: Quote from: MathWizard on January 19, 2025, 05:42:43 amThat's it! I've wanted to do electrolysis rust removal for years. I have a full-size rusty anvil and some cast iron. Except I'd have to Google the amperage of that since the manual explicitly says not to use it for applications that draw above 20 A...Quote from: PCB.Wiz on January 19, 2025, 06:22:49 amThat crossed my mind, and since you say it's easy it definitely sounds interesting. It would be both useful and instructive, although I'd need to find a guide or primer on the subject. (In my head I'm not currently a person who designs or modifies circuits, but merely repair them.)Quote from: iMo on January 19, 2025, 07:01:36 amThat was my first thought too, after discovering that the model was discontinued and replaced by its SMPS counterpart — surely HAM operators are a bit fussy about that (well Paul of Mr Carlson's Lab seems to go to great lengths to only use DC in his LED radio mods at least).I don't see the whole radio appeal currently — not that I know much about it.Quote from: Gertjan on January 19, 2025, 07:53:21 amI see. I don't know where I got that from then — I was sure I had read it somewhere. Maybe the store listing for it simply didn't mention it, while the listing for its replacement did.This would've been posted in the beginners section except for that that didn't seem the best fit for asking about project ideas. In other words, my pattern recognition isn't good enough to spot this in the schematics, so thanks for taking a look.Looking at it now and... ok, it spells it out in plain text. Haha....I wonder why the manual is so adamant I should keep track of max current draw.Quote from: thephil on January 19, 2025, 12:38:31 pmYou beat me to it.I started drafting this reply but new posts just kept piling up 🙂Quote from: coromonadalix on January 19, 2025, 01:14:03 pmYeah maybe. I currently don't have a good way of testing it that I know of. Would be a bit embarrassing to sell it before that.Quote from: madires on January 20, 2025, 02:51:56 pmThis is interesting. Why is linear ideal and better for this? I mean compared to a similarly central SMPS?I've not done much testing more than confirming correct output voltage under no load and determined it currently draws 9 W doing nothing, with a power factor of 37%. Centralized 12 V appeals to me a great deal, but I'm not sure if 9 W standby does.

author: silly sausage
author_rank: Regular Contributor
timestamp: January 21, 2025, 08:14:49 am
content: mod it for better current control and turn it into a lab/bench psu, That would power anything you threw at it !.

author: madires
author_rank: Super Contributor
timestamp: January 21, 2025, 02:07:42 pm
content: Quote from: ohren on January 20, 2025, 10:28:56 pmLess EMI and noise. But the drawback is lower efficiency, of course.Quote from: ohren on January 20, 2025, 10:28:56 pmDoesn't it have a power switch?

author: BILLPOD
author_rank: Frequent Contributor
timestamp: January 22, 2025, 07:22:58 pm
content: Good Morning Ohren, Hook your OSU to one of these modules:https://youtu.be/0qjLx_HsKUQ

author: floobydust
author_rank: Super Contributor
timestamp: January 22, 2025, 07:48:30 pm
content: TheRidenRD6006 looks good but there's not much input voltage for it? I measure 22VDC at light load as my Samlex 13.8V PSU has. For charging say lead-acid batteries... how much headroom the RD6006 requires? There is the smaller RK6006 as well.You could change over to a full-wave bridge rectifier (instead of the original center-tapped config), upgrade (voltage) on the filter caps and a fan, then it's looking pretty useful.These 13.8V linear power supplies are cheap, made in Korea or china and also over-rated. The 20A is intermittent (TX) use I guess.It's 30 minute max. at rated load then a cool down for 10-120 minutes at reduced load down from 20A to 8A.I have a dumpster dive one, an inch from the garbage. The electrolytic caps all were bad and leaked. Replaced them and a few years later, I've never needed it, no use for a big heavy linear PSU taking up so much space.

author: radiolistener
author_rank: Super Contributor
timestamp: January 24, 2025, 04:09:05 pm
content: Quote from: ohren on January 20, 2025, 10:28:56 pmLinear one has no interference which is very critical for sensitive equipment such as radio receivers and precise measurements.The drawback of linear PSU is that it has less efficiency than SMPS. So, it depends on your need. If you prefer clean output, then you're need linear one. But if clean output is not required, but efficiency is your goal, then SMPS is your choice.For lab PSU efficiency doesn't matter, but clean output is often required. This is why linear PSU is preffered if you're looking for lab PSU.There is also hybrid PSU, which uses high quality SMPS for rough voltage regulation and then high quality filters and linear regulator for more precise regulation. With high quality design it allows to keep low EMI interference and good efficiency. But any SMPS produces EMI noise, so if its very critical for you, then your choice is linear PSU only.

author: floobydust
author_rank: Super Contributor
timestamp: January 24, 2025, 07:48:23 pm
content: I think that is a myth nowadays. I find there is endless RF pollution from wall warts, USB chargers, PC's with glass cases etc. that a quiet linear power supply is not much use in a ham shack. The airwaves are dirty.Modern offerings of 13.8V 20A PSU's are all SMPS and for radios they use double filtering - which works fine.

author: radiolistener
author_rank: Super Contributor
timestamp: January 25, 2025, 06:04:02 pm
content: Quote from: floobydust on January 24, 2025, 07:48:23 pmNo, this is very important point which needs to be carefully taken into account when you choose PSU.Quote from: floobydust on January 24, 2025, 07:48:23 pmSMPS for radio is cheap low end, it is cheap lightweight and efficient. It has EMI noise, but many peoples can accept it. I never seen SMPS which don't produce noticeable noise for radio. If any kind of EMI noise is not acceptable for you, then SMPS is not suitable for you. In this case you have just two options: linear PSU or battery.For example, I know ham radio amateur who using high quality SMPS with good filters to power their radio. He just got used to it. One day he got hold of a linear power supply for a while and tried to power his radio with it, after which he was very surprised that the noise was significantly reduced and he began to hear weak stations that he had not heard before because of the noise and thought that the problem was in the antenna.Another area which is very sensitive to PSU noise is laser spectrometry. It is even more critical than for radio, and sometimes it even require to not use mains at all, because noise can leak from mains through PSU filters. In this case low frequency sine transformers are used to get mains AC voltage from battery with no using switching mode conversions (just powerful low distortion 50 Hz sine generator and step-up transformer). It allows to power sensitive equipment and isolate it from noise leakage from mains.This is because modern mains line connected to many SMPS which has noise leakage, so even if you use good linear PSU, noise still can leak from mains to your equipment.By the way, you are right about the noise. During the power outage in our city, I had the opportunity to compare the noise level when there is no power in the city and when there is. The difference in the noise floor on short waves was about 60-80 dB. It's just crazy how much power is in EMI noise in modern city environment...And by the way, when power outage was happened, I was able to listen FM stations from nearby city (about 100-200 km away) and I notice how much noise is produced with modern powerbanks. When I power something from powerbank it just worked like strong RF jammer for FM stations... When there is no power outage, I even don't see power bank noise, because it is flooded with city environment noise

author: Siwastaja
author_rank: Super Contributor
timestamp: January 25, 2025, 07:01:31 pm
content: While it's certainly possible to have a switch-mode supply totally clean enough to power ham radios, I totally understand the desire to use something which is old, well-known and simple and kind ofprovennot to cause the kind of interference thatmany(especially poorly designed) switchers do cause. And efficiency is not so important to many. The energy spent in radios is mostly dissipated as heat anyway (some of course goes out through the antenna). Even if you add 30-40% more heat to it, it isn't significant. And a lot of folks prefer large and heavy instruments on their desks. Switch-mode supply gets really appealing when you are on-the-road but then you would use batteries anyway.

author: floobydust
author_rank: Super Contributor
timestamp: January 25, 2025, 08:09:46 pm
content: What I saw with these linear power supplies is they got in the "race to the bottom".Commonly made in Korea or china or Hong Kong, no safety approvals, over-rated components and then add in the cheap garbage electrolytic capacitors. Their quality is pretty low.Same circuit, sometimes small variations. Most don't have a LM723, it's a few transistors. It's widely copied and scaled up from 3A to 50A even just my adding pass transistors etc. Sold under Samlex, Alan, PowerTech, Jaycar, Pyramid private labels as well.Look at these Samlex pics- the power transformer windings aren't even tape wrapped, they are loose BUZZZZZZZZ. Caps took the piss. Heatsink so small and they get super hot even at 1/2 rated output. Let's push the 2N3055's to their limit.It has EMI actually, from the rectifier diode switching hash and they don't have any disc caps to deal with RF except one at the output binding posts.I see manufacturers changed over from linear to SMPS, not sure you can even buy these today due to the their weight. I guess it's not the 1980's.Samlex Americaonly offering SMPS now.

author: radiolistener
author_rank: Super Contributor
timestamp: January 25, 2025, 09:10:56 pm
content: Quote from: Siwastaja on January 25, 2025, 07:01:31 pmI'm sure for 100% this is totally impossible due to physics limitations at least at adequate price, size and weight. Linear one always win in this comparison. You can create a high-quality switching power supply whose noise levels are satisfactory by shifting the spectrum of its main power to unused frequencies. However, even with such a power supply, you will still have switching noise across all frequency bands. The only question is whether you will consider their presence acceptable or not.It is much more easy to not create noise, than trying to eliminate it. Adding even super cutting-edge high quality filter cannot help to filter noise completely, because there are a lot of paths for noise leakage. It's like adding an infection to water and then trying to purify it. Adding is easy, purifying is hard, and completely purifying it at a reasonable cost is almost impossible. Thus, an SMPS with low noise will be several orders of magnitude more complex, expensive, more heavy and larger in size (due to filtering and shielding) than a linear power supply, while still noticeably losing to the linear one in terms of noise levels.This is why there is no SMPS in the world that does not produce EMI noise detectable by a good radio receiver. You can easily see noise from any SMPS with a good receiver. I didn't seen SMPS which noise is not visible on a receiver with pretty mediocre sensitivity, not to mention professional receivers whose sensitivity is comparable to the level of thermal noise.

author: JoanBS
author_rank: Contributor
timestamp: March 14, 2025, 01:20:57 am
content: Quote from: floobydust on January 22, 2025, 07:48:30 pmI totally disagree. I have a power supply with similar characteristics, 13.8V and 30A, which I built about 40 years ago to power an amateur radio station. It has a large 500VA toroidal transformer and a simple circuit with only discrete transistors (no integrated regulator). It has current limiting (internally adjustable), and can withstand a short circuit indefinitely with 0 amps of consumption. The power section consists of a 2N3055 driving five 2N3771s in parallel, with static cooling and also forced air cooling, above a certain temperature.I still use it every day, now in my workshop. I modified it not long ago and I can vary its voltage between 7.5 and 18V, and it's my first resource among a dozen other power supplies, to power all kinds of devices with power within its range. Its stability is unwavering, and it has never let me down in these 40 years. It's quiet, keeps as warm as it needs to be, consumes more power than an equivalent SMPS, and also weighs considerably more, but I hope it will be with me for many more years to come.Switching power supplies and true amateur radio are simply incompatible.

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 14, 2025, 01:52:52 am
content: Quote from: radiolistener on January 25, 2025, 09:10:56 pmSmall linear in parallel with a large switcher would be the cheapest overall and just as low noise as a linear during receive.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: March 14, 2025, 02:25:45 pm
content: While I agree with radiolistener, there are Ham radios that come with their own PS that work quite well. Of course the spurious signals of the PS are outside of any critical frequency and the specific PS for specific Ham radios are typically very good for switchers. Also the radios themselves have amazing noise filters that probably were not as necessary in the past.If you wish to see the radio waves given off by a switcher, put an antenna near it and attach the antenna to your spectrum analyzer.Of course there are also all sorts of spurious signals running loose in today's radio environment. From computers, wifi and ethernet and all the wall worts etc....so the additional noise from another switcher may not be significant ( if they are good switchers), although I have had a few switchers that made radio listening very troublesome. They also can grossly affect any RF measurements if you are doing any RF analysis while trying to fix something.