High current low voltage power supply - Page 1
Forum section: electronics / beginners

author: anvoice
author_rank: Frequent Contributor
timestamp: February 25, 2020, 09:10:05 am
content: Hi, I'm trying to generate a maximally large magnetic field in a loop of wire, and for that need a low voltage, high current power supply. Something along the lines of 50-100A, at a few volts. At the moment, all I have is a 30V, 10A variable PSU, which can deliver the power but not the Amps required for the application. I would also like to do this as cheaply as possible. The question therefore is: what's my best bet for delivering the 50 or so Amps at a few Volts? Is there anything preventing me from using a step-down transformer in conjunction with my existing PSU? Or do I need to get a module that specifically takes around 30V DC and delivers a constant current at a lower voltage? Thanks in advance!

author: bd139
author_rank: Super Contributor
timestamp: February 25, 2020, 09:24:14 am
content: Look out for old PC power supplies on the high watts end of things. They can shift 3.3V / 5V @ 100A +

author: anvoice
author_rank: Frequent Contributor
timestamp: February 25, 2020, 09:32:20 am
content: Quote from: bd139 on February 25, 2020, 09:24:14 amThank you! But would they be regulated? If I connect a PC power supply to a coil of wire wouldn't it just short it?I'm not seeing any transformers with a high enough current rating right now, but I did find something here:https://www.ebay.com/itm/50A-DC-step-down-power-supply-module-constant-voltage-constant-current-LCD-/273810462533that might work. I could connect it to something like 30V 5A on my PSU and get out 3V at 50A, if I'm reading the specifications right.

author: HighVoltage
author_rank: Super Contributor
timestamp: February 25, 2020, 09:51:28 am
content: 1. You could use an old welder from the junkjard, since they come with a huge transformer and rectifier2. Build your own, using a high voltage microwave transformer, remove the secondary windings and instead add a new heavy cable with a few turns and then send it through a rectifier.

author: anvoice
author_rank: Frequent Contributor
timestamp: February 25, 2020, 10:00:30 am
content: Quote from: HighVoltage on February 25, 2020, 09:51:28 amI don't have a place to scavenge nearby, but I do have a microwave transformer and some heavy gauge wire. Now that you mention it, it was meant for that very same purpose, to build a high-current step-down transformer. Are there any special considerations for the rectifier, besides the current and voltage ratings?

author: SmallCog
author_rank: Frequent Contributor
timestamp: February 25, 2020, 10:05:21 am
content: What’s the application?I’d use a forklift battery if I needed huge amps and low voltsDepending upon your scrounging contacts these can be quite obtainable at scrap lead prices

author: gbaddeley
author_rank: Regular Contributor
timestamp: February 25, 2020, 10:05:32 am
content: Re considerations for rectifier, it will need to dissipate a lot of power, and not exceed its max temperature rating. At say 50 A (average) @ 1 V drop per diode, 4 diode bridge rectifier, that’s 200 W approx.

author: JackJones
author_rank: Regular Contributor
timestamp: February 25, 2020, 10:09:15 am
content: DiodeGoneWild has a few different designs for high current supplies on his website:http://danyk.cz/index_en.htmlFor example this one:http://danyk.cz/proud_en.html

author: nfmax
author_rank: Super Contributor
timestamp: February 25, 2020, 10:10:33 am
content: Quote from: anvoice on February 25, 2020, 09:10:05 amOr use more turns in your coil with a lower current?

author: anvoice
author_rank: Frequent Contributor
timestamp: February 25, 2020, 10:16:31 am
content: Quote from: SmallCog on February 25, 2020, 10:05:21 amApplication is to create a magnetic field (as large as reasonably possible) in a loop or several loops of copper conductor. I'm only seeing super-expensive forklift batteries, not sure where I'd get a deal on one...Quote from: gbaddeley on February 25, 2020, 10:05:32 amI don't see a Watt rating on most of the rectifiers I'm looking at. I'm guessing one rated at 100A and 1600V (surely it's not actually a 160000W rectifier) will be ok? Also, single-phase?Quote from: JackJones on February 25, 2020, 10:09:15 amMuch appreciated! Those look a tad bit more complicated than rewiring a microwave transformer. Any significant advantage?Quote from: nfmax on February 25, 2020, 10:10:33 amRight, but it needs to be relatively flat. So there's only so many turns I can do until running out of room.Thanks so much!

author: Alti
author_rank: Frequent Contributor
timestamp: February 25, 2020, 10:34:59 am
content: Buy a MMA inverter DC welder.The entry models typically go from 1V up to 80V, the current is from 10A up to 150A(35%) (100A continuous). I suggest getting version with remote control (TIG capable).

author: anvoice
author_rank: Frequent Contributor
timestamp: February 25, 2020, 10:42:34 am
content: Quote from: Alti on February 25, 2020, 10:34:59 amThat seems like a good option. As far as I understand, you can select the current and let it continuously supply the 100A through a length of copper? Is the stability relatively good?

author: Kleinstein
author_rank: Super Contributor
timestamp: February 25, 2020, 10:44:37 am
content: The big transformer (e.g. modified MOT or from a welder) would give AC current with mains frequency.A set of parallel batteries can give a high current for a relatively short time.A PC supply can give a more or less well well regulated 3.3 V DC at some 50 A, maybe more with some modelsIn theory with a PC motherboard there is switched mode regulator for some 100-200 A at some 1-1.5 V. However it may not be easy to to make it work without the CPU.So it kind of depends what is really needed: AC/DC, time, accuracy or stability of the current.

author: digsys
author_rank: Unknown
timestamp: February 25, 2020, 10:45:45 am
content: Quote from: anvoiceWe've made a few electro-magnets for Industry over the years - some in the many KW range, huge buggers, but we've used 3+3phases (STAR PLUS DELTA) and a sheetload of Semikron big-ass diode blocks. They make them in incredible high currents. Heatsinks were huge !! with several fans. It was for our now defunct car Industry.We've also made "smaller" ones using Meanwell Power supplies - LRS-350-3.3 (= 3V3 at 60A) output or LRS-350-4.2 (4.2V at 60A), off-the-shelf units. I think these (or can be customized) to be SYNC'd, providing near unlimited current. They have other Voltages / models as well. Just another idea.Oh, and for testing, we had a 3 Phase ~5KW variac :-)

author: Alti
author_rank: Frequent Contributor
timestamp: February 25, 2020, 12:46:10 pm
content: Quote from: anvoice on February 25, 2020, 10:42:34 amYes, you select the current (in amps) by turning a pot and whatever load you connect, it will keep the current constant. Of course only up to rated voltage, usually 80V. The TIG capable welders additionally have this remote input (where you can connect sth like arduino) and control current remotely. The inverter welders can work 100% of the time but usually the 35% current is placed on a sticker (check the datasheet). My welder does 140A at 35% and 100A at 100% and it will shut down to cool itself if you exceed the limit.This is essentially a regulated DC current source powered from mains. Of course it is isolated from mains galvanically but be careful, 80V is not pleasant to touch and 100A DC arc can ruin not only your day.As for the current stability - good question, but I am pretty sure you can find some current ripple screenshots on the internet. I do not have adequate shunt but I do have such welder and a scope.. Hmm, maybe 0.1R resistor in a glass of water?

author: Gyro
author_rank: Super Contributor
timestamp: February 25, 2020, 12:50:57 pm
content: Quote from: anvoice on February 25, 2020, 10:16:31 amI'm don't think you've given quite enough detail about the application. We know you want DC, but how smooth? A forklift battery or whatever will give you a smooth DC current, whereas a transformer + rectifier, welder etc. will give you a pulsating DC current (at 2 x mains frequency if you use a bridge/full wave rectifier (reservoir capacitors don't sound practical at these sort of currents). An SMPS (eg heavily loaded PC supply) [Edit or Inverter welder] would give you smooth DC with some high frequency ripple.Does this matter for your application?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 25, 2020, 01:02:57 pm
content: Why not use thinner wire with more turns (for the same total cross section) and a more common power supply rating, or an offline supply (basically just a rectifier, since the coil is its own filter)?If it's about cooling (water), cooling tubes can be added regardless. Mind to use a reasonably conductive filling or potting. It is a bit more convenient to use tubing, true. As long as the total tubing length isn't so much that you need 10atm to force the water through it... (in which case you need bigger (and thicker walled, may be special order?) tubing, and even more current).Or if this is really hot stuff, you'll have to use high pressure water and lots of it, and those perforated plates they use for the big boy magnets.Tim

author: anvoice
author_rank: Frequent Contributor
timestamp: February 25, 2020, 08:08:11 pm
content: Thanks for the input so far everyone! The application is basically to try to create a low-field, low-cost MRI machine. I know an MRI expert who believes it will be possible to image with such a machine given a field of at least a few mT. Using superconductors for this is really impossible because I'd have to cool them with nitrogen and helium and that takes low-cost out of the equation.Quote from: Kleinstein on February 25, 2020, 10:44:37 amFor the main coil, I'll need DC. It will ideally need to be on for half an hour to an hour at a time, with good current stability. I'll have to ask that MRI expert I mentioned what the exact stability needs to be, but the more stable, the better. I will also need AC at some point for an orthogonal magnetic field, but that's probably a question for another post or this one will get cluttered.Quote from: digsys on February 25, 2020, 10:45:45 amThat sounds like it was fun! I need something as cheap and simple as possible as I don't want to invest a ton.Quote from: barycentric on February 25, 2020, 10:52:19 amYes, but cost would become prohibitive... I have heavy wire that I was planning to use, which should be able to handle 50A. I can calculate how hot it will get, but it doesn't sound impossible right now.Quote from: Alti on February 25, 2020, 12:46:10 pmI think the current stability is the main issue. I should have mentioned that requirement in my opening post, but yes, it needs to be reasonably stable. I'll have to dig around for spec sheets on those.Quote from: Gyro on February 25, 2020, 12:50:57 pmMy apologies, I was trying to be concise but seems like it backfired. Since this is for an small MRI machine (think imaging berries, or at most your head), I think the current needs to be pretty stable. I'll get back with the exact smoothness requirement once I've talked to my expert. I was thinking of running the transformer, if I go that route, off a DC variable PSU capable of 30V and 10A, which should be more than enough power. Would anything be introducing additional instability at this point?Quote from: T3sl4co1l on February 25, 2020, 01:02:57 pmThat would certainly be easier. I don't have a full grasp on all the MRI mechanics yet, there are some requirements like field gradient and I'm not certain if a longer solenoid will meet all of those, but I'll definitely look into it.

author: james_s
author_rank: Super Contributor
timestamp: February 25, 2020, 09:08:50 pm
content: I have gotten very high current at low voltage by using a toroidal transformer and winding my own 2-3 turns of heavy wire around the core. In my case I did it to heat up a heavy copper bus bar so I could solder to it.

author: anvoice
author_rank: Frequent Contributor
timestamp: February 25, 2020, 09:15:05 pm
content: Quote from: james_s on February 25, 2020, 09:08:50 pmThank you! Since I have a microwave transformer and heavy wire, I might try going that route. I would hook it up to a DC variable power supply. I'm mostly concerned about stability since if the field wildly fluctuates it'll be trouble.

author: Gyro
author_rank: Super Contributor
timestamp: February 25, 2020, 09:21:45 pm
content: Thanks for the clarification anvoice.Small MRI sounds a very interesting application. I suspect that you are going to need a very smooth current (superconducting MRI magnets of course give averysmooth current by their very nature).I suppose it is possible that a pulsating field might be possible to use, or even give some sort of gradient enhancement effect if properly synchronised with the gradient coil signal generator. However I don't think that things tend to fall out so conveniently in reality.

author: anvoice
author_rank: Frequent Contributor
timestamp: February 25, 2020, 09:32:58 pm
content: Quote from: Gyro on February 25, 2020, 09:21:45 pmCouldn't agree more. I'm guessing smooth is the way to go, since the spectrum will be assuming constant field at any given point from the DC coil. At the very least I would have to compensate for pulsation by constantly measuring it, which would add another layer of complexity.

author: Gyro
author_rank: Super Contributor
timestamp: February 25, 2020, 09:47:23 pm
content: Just one other thought. With the ready availability of strong NdFeB magnets these days, have you considered the possibility of a permanent magnet implementation? These are used on medical open bore MRI machines, so could be feasible for a very small one, maybe even cheaper than a PSU and heavy coil.https://www.mriquestions.com/types-of-magnets.html

author: anvoice
author_rank: Frequent Contributor
timestamp: February 26, 2020, 06:38:42 am
content: I did think about that. In fact, this idea might be very usable. There are some disadvantages though: if I stack up, say, a ring of magnets, the field will be some weird linear combination of the fields of the individual magnets, rather than a nicely modeled field of a solenoid. Unless I use something like a huge c-shape magnet (would have to be very big and expensive to fit anything larger than a berry, and probably custom-made).

author: SmallCog
author_rank: Frequent Contributor
timestamp: February 26, 2020, 06:59:50 am
content: Quote from: anvoice on February 25, 2020, 10:16:31 amTo get them cheap you’d need to find someone that replaces the batteries in electric forklifts (and similar big battery arrays like some UPS systems and off grid solar systems).Often when the set of cells is replaced there’ll be a mix of good and stuffed cells.You probably won’t get them for free as the lead is worth money for scrap, but if you find the right bloke you should get them for a lot less than a new one.Not saying it’d be any good for a home brew MRI machine but it’s a feasible way to get big smooth amps at about 2V for a limited amount of time.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 26, 2020, 07:23:35 am
content: Quote from: anvoice on February 26, 2020, 06:38:42 amA solenoid isn't a very good field. Have you modeled it?If you're looking at mT then much cooling won't really be required.Stability will, for which a current regulator circuit is probably adequate.You'll most likely want a Helmholtz coil, or any higher order equivalent. In the same way that one coil can be split into two to give a first-derivative zero in the middle, two can be split into four (of appropriate spacing and diameter) to give first and second derivative zeroes, and so on.At some point, mechanical tolerances will overwhelm the magnetic design and you'll have to shim it to desired flatness in the active region.Tim

author: anvoice
author_rank: Frequent Contributor
timestamp: February 27, 2020, 04:24:15 am
content: Quote from: SmallCog on February 26, 2020, 06:59:50 amThanks for the info! Doubt I'll be using a battery for this project, but it's a nifty idea.Quote from: T3sl4co1l on February 26, 2020, 07:23:35 amIt obeys simple mathematical formulas, which makes it a bit easier to work with. Do you mean the shape is not good for MRI?The idea is to try to reach 5-10mT and see what's attainable with that. Any regulator scheme you would recommend?I did think of a Helmholtz configuration (didn't know the name though), and I know I'll have to do shimming.Thanks for the input!