20A Ajustable Power Supply

Hello,

I tried to search around but could not find any, so I tried to build one myself, but I’m very electronic noob :slight_smile:

I’ll start with the 12V (28A) from my Computer PSU, then put the line into the circuit attached… do you think it will work?

LM338 will have heatsink and fan.

Thanks! :slight_smile:

No, sorry. When you're looking at >1A, it's time to think about moving away from linear regulators. Paralleling them isn't good for much more than making a portable heater.

Building (or modifying) a 20A PSU is not a trivial exercise. If you're an electronics newbie, be sensible and do two things: 1) Buy a power supply with the current you need (and be sure you really need that much -- 20A is enough to get yourself into trouble); 2) Start building linear PSUs to power small loads, and work your way up gradually to bigger better faster designs.

Don't be in too big of a rush, you'll get there. Experience takes time.

I tried to find something that does 20A directly, but I could not find any. Looking at the TI datasheet of the LM338, I found I could parallels them.

btw, I'm already using that PSU. I just want to have the output variable. Right now, I have 12V, 5V et 3.3V Regulated. I wish to make the 12V output variable.

Do you have a better idea?

Maybe I could just use it regulated like this an do that later... but well, I was thinking it was a cool project :)

The problem with paralleled regulators is that the current sharing isn't equal without some additional means of keeping them balanced. (Usually a low-value resistor on the output of each.) The datasheet should have suggestions on using external transistors for high(er) current applications, which is a far better (and usually simpler/cheaper) way of improving current capacity.

As it stands, one of those ICs is doing more work than the rest. As such, when you load them down, it will fail first. And then the second most aggressive IC will take the brunt of the load, and fail shortly after. Then the next will die very quickly. And the next...

The only reasonable way to generate a variable 20A supply is with a beefy switching regulator design that can vary its PWM duty cycle enough to provide the voltage range you want. Although, you then have to compromise on the output filtering stage, rather than tuning it for a particular voltage or small range of voltages. With higher-current supplies, this balancing act gets more complicated. Which is why you probably won't find many out there.

Seriously, any time you're talking about 20A regulated supplies, it should ring alarm bells. Why do you need 20A? Typically, with that large of a load, it either 1) doesn't really need to be regulated (e.g., relays, motors, heaters, lights, power amplifiers, etc.); or 2) is a large project that can probably be broken down into more, smaller power supplies (e.g., a car computer where you can supply power to drives separately from the motherboard, and probably get by with a low-power CPU like an Atom or VIA EPIA).

Finally, there's good reason not to have that much power on tap. A short circuit on a 3A supply will blow an output fuse or kick it into over-current protection. With a 20A supply, depending on the resistance between the supply and the fault, you could set things on fire while the supply just keeps on truckin' like nothing happened.

I’ve bought reasonably priced power supplies from here before:
http://www.mastechpowersupply.com/

Just look around for something. Also good for cheap scopes and function generators. Rigols are Agilents off-brand.

SirNickity: The problem with paralleled regulators is that the current sharing isn't equal without some additional means of keeping them balanced. (Usually a low-value resistor on the output of each.) The datasheet should have suggestions on using external transistors for high(er) current applications, which is a far better (and usually simpler/cheaper) way of improving current capacity.

Oh, that must be why there was a 0.1 ohm resistor at each output. My initial design had those.

SirNickity: As it stands, one of those ICs is doing more work than the rest. As such, when you load them down, it will fail first. And then the second most aggressive IC will take the brunt of the load, and fail shortly after. Then the next will die very quickly. And the next...

Oh... they have peak at 7A and "should" receive 5A each.

SirNickity: Seriously, any time you're talking about 20A regulated supplies, it should ring alarm bells. Why do you need 20A? Typically, with that large of a load, it either 1) doesn't really need to be regulated (e.g., relays, motors, heaters, lights, power amplifiers, etc.); or 2) is a large project that can probably be broken down into more, smaller power supplies (e.g., a car computer where you can supply power to drives separately from the motherboard, and probably get by with a low-power CPU like an Atom or VIA EPIA).

Yeah... it's just because I wish to use computer power supply I have "stock" and I wish to have it adjustable (no real use in fact, since I'll design all my stuff to the right voltage). Maybe it was more a challenge and "cool factor" to have it.

SirNickity: Finally, there's good reason not to have that much power on tap. A short circuit on a 3A supply will blow an output fuse or kick it into over-current protection. With a 20A supply, depending on the resistance between the supply and the fault, you could set things on fire while the supply just keeps on truckin' like nothing happened.

Yeah okay. So I guess I should only use one LM317 to get 1A ? I'll get variable + I'll be more safe ?

Paralleled regulators can really be bad news - I had some LDO 5V regulators accidentally paralleled and blew up all the 5V chips, I think they coupled together to form an oscillator, putting the full 12V input voltage onto the output, or somesuch failure mode.

The way you boost current output from a standard linear regulator is place a bigger pass-transistor inside the feedback loop, which means you need a 4-terminal regulator (input, output, ground and sense) in order to get inside that loop.

A linear regulator is just a voltage reference plus an opamp with large asymmetric current output stage (and some protection circuitry).

excessnet: Yeah... it's just because I wish to use computer power supply I have "stock" and I wish to have it adjustable (no real use in fact, since I'll design all my stuff to the right voltage). Maybe it was more a challenge and "cool factor" to have it. ... Yeah okay. So I guess I should only use one LM317 to get 1A ? I'll get variable + I'll be more safe ?

Definitely. :) No one's going to stop you if you absolutely have to have a monster PSU, but you should be aware that it's not all fun and games. Respect the energy source Jedi mind trick. :D

Everyone here has blown up a 1A regulator during experimentation. If they haven't, they will.

You're better off with a couple of switching regulators linear regs are not up for this job unless you buy some beefy resistors.

Hi, using the 12V 28A from a computer power supply might sound okay, but if you use it to supply a linear regulator you will be lucky to get to 0 - 9V regulation. This because the regulator needs some overhead to work, the specs for the LM338 should tell you this.

I agree with most of what has been said about building electronic projects and being new to the subject, start with small projects and build up.

Tom... :)

ps, Why do you need 20Amps?