Question on a computer PS for 3.3 Volts.

[Hemlock_Mike]

I asked my computer friend for a PS which has 3.3 volts at 20 to 25 Amps for a diode bar.
Today, he delivered a small Dell PS which has 3.3V @ 9 amps max, 5V @ 11 Amps max and 12V @ 11 amps max. Thanks to a previous long ago post here, I got it to run with that green wire.
The PS label says that the load sharing of the 3.3 and 5 volt outputs can not exceed 80 watts. With no load on the 5 v line, can I exceed 9 amps on the 3.3 volt output?

HMike

 

[Bionic-Badger]

I would not. The maximum amperage rating is probably based on the wiring gauge, not so much what the PSU can supply. Here is a nice chart of maximum amps for certain gauges. In the chassis wiring column, the 9A load is at 21 gauge, which is about right for the PSU's 3.3V wiring since breadboard wiring is 22 gauge. They're conservative numbers, but a good estimate.

 

[Hemlock_Mike]

BB ---
Thanks for the info. The wire size isn't a problem because there are at least 5 orange (3.3v) wires exition the PS. In total, they can handle over 9 amps. I am a bit concerned about the circuit board traces and components.
I was hoping for maybe 15 amps with only the 3.3 volt portion under load.

Thanks again --- HMike

 

[Meatball]

That should work... the main way that you can tell if it doesn't, is by trying it. Most of the PSUs are sophisticated enough to shut themselves down for protection, even after a fuse is blown. You can find even cheaper and more powerful supplies from just about any computer store with a recycling program.

Don't forget to put about 10ohms across one of the rails, or the PSU won't work properly. It wants to see some resistance!

If all else fails, I've had good luck with connecting several of these PSUs' rails up in parallel for some very nice amounts of current capacity.

I hope this helps,

-Tyler

 

[pseudolobster]

Can you not step down a 12v rail to 3.3 with some sort of regulator? Even with a horribly inefficient regulator I'd imagine any PSU these days would shrug off that kind of amperage on the 12v rail... I've seen <$100 power supplies with 80+ amps on the 12v rail, so even if you're throwing away a lot as heat, you can certainly achieve 25A at 3.3v... Heck, AFAIK you could use a LM317T for this so long as you bolt a heatsink to it.

 

[Meatball]

I think that the LM317t for example would fry itself (heatsink or not) at about 2 amps. It could be done, but you'd need to find a regulator that could handle the current. It regulator would most likely be more of an entire circuit, rather than a IC or two..

 

[pseudolobster]

I think that the LM317t for example would fry itself (heatsink or not) at about 2 amps. It could be done, but you'd need to find a regulator that could handle the current. It regulator would most likely be more of an entire circuit, rather than a IC or two..

Oh yeah, you're right... 1.5A at 35V doesn't work out to 25A at 3.3V in any way shape or form... I need to stop posting late on a Saturday night after getting home from a night out with friends. So yeah, something other than a LM317T, maybe some beefy diode or something, but I think you'd have better luck on the 12V line with the way computer power supplies are designed these days.

 

[Bluefan]

Old 200w AT power supplies have a 20A 5V line, those are dirt cheap, I have them for less than 5 euros here. They also switch easier on and off.

 

[goninanbl00d]

Actually, the reason why is because on those crappy switchmode supplies, the 3.3V and 5V rails are regulated by one component, with the 3.3 often being derived from the 5V rail.

It's cheaper to do it that way, and that's why it's done like that.

Bear in mind, though, because it's group regulated (not sure if that's the correct term), you won't be able to 'pool' the power (in the sense that you can 'borrow' wattage from one rail to use in another).

If you do that, you'll likely damage the secondary (or even primary) switchers.

 

[Flaminpyro]

Go ahead Mike let the smoke out of it, it's been trying to get out for years

 

[anselm]

Don't people ever check the date before they post?