How do I get 24 volts from ATX PS? EE's please?

[Zebo]

This guy states you can achive various voltages with the PS

There is a +12v, -12v, +5v, -5v, +3.3v as well as ground, zero v. You can use any combination of these to get voltages of: 1.7, 3.3, 5, 7, 8.7, 8.8, 12, 15.3, 17, or 24 volts.

One clear example is the 7 volts obtained by using the potential difference between the 12V and 5V on the molex connector. Is it possible to get 24V though?

Reason I ask is there is a great water pump made by Iwaki which needs 24V to run on in my water cooling
setup. http://www.oberread.com/IW_RD_Series.htm

 

[rimshaker]

Combine two 3.3V lines with one 5V and one 12V line. Total would be 23.6V, but that's considered 24V for all practical purposes.

 

[CTho9305]

Originally posted by: rimshaker
Combine two 3.3V lines with one 5V and one 12V line. Total would be 23.6V, but that's considered 24V for all practical purposes.

Wha? How are you going to add up the voltages? Unless you used multiple power supplies with offset grounds, I don't see that working. If you hook up the pump with the red/+ on the +12V line, and the black/- on the -12V line, you'll get the 24V you need. HOWEVER, most power supplies are meant for only the tiniest load on -12V (mine will supply 0.5A, meaning your pump can't use more than 6 watts, or slightly more than an 80mm case fan), so this is probably not a good solution.

 

[Zebo]

Originally posted by: CTho9305

Originally posted by: rimshaker
Combine two 3.3V lines with one 5V and one 12V line. Total would be 23.6V, but that's considered 24V for all practical purposes.

Wha? How are you going to add up the voltages? Unless you used multiple power supplies with offset grounds, I don't see that working. If you hook up the pump with the red/+ on the +12V line, and the black/- on the -12V line, you'll get the 24V you need. HOWEVER, most power supplies are meant for only the tiniest load on -12V (mine will supply 0.5A, meaning your pump can't use more than 6 watts, or slightly more than an 80mm case fan), so this is probably not a good solution.

Ahh so there is a -12V line (which is it BTW?). So If I hook the red/+ on the +12V line, and the black/- on the -12V line of the pump how is the load divided between the PS lines? Say the pump draws 2.5A or 28 watts @24V.

 

[Mark R]

Ahh so there is a -12V line (which is it BTW?). So If I hook the red/+ on the +12V line, and the black/- on the -12V line of the pump how is the load divided between the PS lines? Say the pump draws 2.5A or 28 watts @24V.

There is a -12V line, but it is not designed for supplying significant loads - the only thing they are there for is for COM (serial) ports, and even they are 5V powered these days. Most PSUs are rated for 0.5A max for the -12V line (some even less). Attempting to draw more than that, will cause the voltage to drop dramatically and potentially shut-down or damage the PSU.

 

[pm]

The color of the -12V wire on a standard ATX power supply is supposed to be blue. It's the only blue wire on the long 20-pin primary power connector. It is pin #12.

 

[Zebo]

Originally posted by: Mark R

Ahh so there is a -12V line (which is it BTW?). So If I hook the red/+ on the +12V line, and the black/- on the -12V line of the pump how is the load divided between the PS lines? Say the pump draws 2.5A or 28 watts @24V.

There is a -12V line, but it is not designed for supplying significant loads - the only thing they are there for is for COM (serial) ports, and even they are 5V powered these days. Most PSUs are rated for 0.5A max for the -12V line (some even less). Attempting to draw more than that, will cause the voltage to drop dramatically and potentially shut-down or damage the PSU.

My PS is 1.2A on the -12V line (thanks for pointing out the color pm)...That's why i asked how it "divides" the load between the power supply. If it's an equatorial relationship, ie 2.5A / the two lines = 1.25A each I should'nt have a problem...should'nt However, If both lines "see" the full 2.5A it's not going to work.

Well ...then there's also the start up of the pump which I belive will use more power than it's "continuous duty" power rating (2.5A) so it may be all for not regaudless.

 

[Mark R]

The current is the say at all points in the circuit - so if the pump needs 2.5A, it will need 2.5A from both rails.

 

[Mday]

setup up dc-dc converter should run a few bucks to a ~$100 depending on how much current you wanna draw. also, you're better off using an AC-DC converter. it's the cheapest way you'll get it, especially after looking at the power that the pumps require. check out surplus stores like allelectronics.com or the like. i am sure someone will point you towards that direction. the only bad thing is that the pump may still be off if the computer is on.

 

[Mday]

Originally posted by: Mark R
The current is the say at all points in the circuit - so if the pump needs 2.5A, it will need 2.5A from both rails.

yup, remember kirkov's (sp) current law. current in = current out for this case.

 

[KalTorak]

Egads!

If your pump wants to draw 2.5A at 24V, and you connect it between the +12V and -12V rails, you're going to draw 2.5A from the +12V rail, and you're going to push 2.5A INTO the -12V rail. And I don't know whether that's gonna trip the power supply or not (tho if i thought long enough about how a full-bridge rectifier behaves, I could probably figure it out.)

When ya hook up a fan between +12V and +5V, for instance, you're drawing, say, 500mA from the 12V rail and pushing it into the +5V rail, but since you're probably pulling more than that from the 5V rail with other stuff, you're still drawing a net current FROM the +5V rail in that case, and that's okay (that's what a PSU is designed for.)

In the case you're describing, that's almost certainly not true; you'll be pushing net current into the -12V rail, and I'm not sure how a PSU behaves under those conditions.

 

[FrankSchwab]

Originally posted by: KalTorak
Egads!

and you're going to push 2.5A INTO the -12V rail. And I don't know whether that's gonna trip the power supply or not (tho if i thought long enough about how a full-bridge rectifier behaves, I could probably figure it out.)

Do you have the foggiest idea of what you're talking about? Pushing 2.5A into the - 12v rail? Full-wave bridge rectifier? WTF?

All ATX power supplies reference their output voltages to the same ground. So, measuring between the +12V and -12V pins, you'll have 24V. As long as your pump doesn't pull more current than the power supply is capable of, you'll be fine. Should the pump pull more current than the lowest-rated rail is rated at, one of two things will happen:
1. the rail with the lower current rating will not hold it's rated voltage. For example, the -12V rail may drop to -10 or -8 volts. In this scenario, there are one or more components in the power supply that are running above their maximum ratings, and will likely fail soon. This may or may not be fatal to your computer.
2. The power supply will shut down. This is the scenario for a well-designed power supply that monitors overcurrent on ALL of it's outputs, not just one or two.

I wouldn't try this on the power supply plugged into my computer.

You might try a few other options. For example, MPJ is a good source for power supplies.

 

[Zebo]

Nice transitors on that baby frank

If it has a fan I'm SOL since I don't use them.

 

[KalTorak]

Originally posted by: FrankSchwab

Originally posted by: KalTorak
Egads!

and you're going to push 2.5A INTO the -12V rail. And I don't know whether that's gonna trip the power supply or not (tho if i thought long enough about how a full-bridge rectifier behaves, I could probably figure it out.)

Do you have the foggiest idea of what you're talking about? Pushing 2.5A into the - 12v rail? Full-wave bridge rectifier? WTF?

Uhm, yeah, actually, I do. For instance, here's a lovely picture of what I spent my summer designing... Anyway:

Full-wave bridge rectifier? That's what's inside the power supply; it's the most common topology for an AC to DC converter. I've never designed a silver box myself, so a full-wave bridge rectifier is the best approximation I can use for how it behaves. A basic electronics text will cover the topology and how it works.

Pushing 2.5A into the -12V rail? Well, yeah - you could call it returning those amps into the -12V rail, but it all amounts to the same thing.

Let's start with a more basic problem - suppose you've got a fan intended to draw 2.5A and run off 12 volts, and you connect it from +12V to GND. Electrically, you could choose to model it either as a resistor (something like 4.8 ohms) or as an ideal 2.5A current source, but either way you represent it, your fan takes 2.5A out of the silver box on the +12V rail, and sends 2.5A back into the silver box on the GND rail. This is no big deal; GND's meant to be used as a rail for returning current.

Now, the 24V fan is the same idea, except for the 2.5A is being sent back into the silver box on the -12V rail.
This is a little different, since it'll probably mean that there's net current going INTO the supply on that rail (it's doubtful that anything else is drawing nearly 2.5A out of the -12V rail). That's probably not a big deal - it seems kinda reasonable that the negative-voltage rails would be designed to do that. Frankly, they show up so rarely in motherboard design that I've never had occasion to check. (It'd be more clear if the power supply designers would flip the sign of the rated current on the negative voltage rails to indicate this, but whatcha gonna do?)

In any case, if that fan really does want to pull 2.5A from the +12V rail and return them on a -12V rail that's only rated to 1.5A, then it doesn't matter whether that rating's source or sink - you're likely to trip the overcurrent protection.

 

[Geniere]

KalTorak- I must say when I read your first post I dismissed it, sharing Frank Schwab?s opinion. However, if I had replied, I would have been more politic. Your second post more clearly indicated your knowledge.

 

[damonpip]

The pump would overload the -12v line, because it is only 1.5A. Putting a 2.5A load on a 1.5A line will not work.
KalTorak - Computer power supplies are switchmode power supplies, which convert the AC line voltage to very high frequency/(voltage?) AC, and then step it back down to the appropriate voltages. The high frequency allows much smaller step down transformers to be used.

 

[KalTorak]

Thanks, Geniere.

Damonpip - while you're right about silver boxes being switch-mode supplies, and I assume you're right about why that's so (transformers and magnetics really aren't in my area of expertise), I still think the full-wave bridge rectifier is the right way to model the power supply as you're looking back into it from the motherboard. Even with what you're saying about how the supply goes from 120VAC down to 12VAC and such, the AC-to-DC conversion step's gotta get done somewhere, and I'm pretty sure that's done in practice with a full-wave bridge rectifier. I also believe that's the last step before presenting the different voltage rails out of the box, since doing DC to DC conversions (after the rectifier) is kind of a pain - changing voltages while you're still working with AC is much simpler.

 

[kevinthenerd]

You all are overcomplicating this...

Hook it up on the +12 and -12 on a spare AT or ATX power supply, and run it that way. If it fries the spare, oh well. It will have been fun while it lasted. I personally wouldn't run anything like that on my main computer PS.

 

[Mday]

i still stand by having the pump be powered by another source.

 

[kevinthenerd]

Originally posted by: kevinthenerd
Hook it up on the +12 and -12 on a spare AT or ATX power supply, and run it that way. If it fries the spare, oh well. It will have been fun while it lasted. I personally wouldn't run anything like that on my main computer PS.

Originally posted by: Mday
i still stand by having the pump be powered by another source.

Yep...
I figured that a spare power supply would be something everybody on here has in one place or another.

 

[greenman100]

is a quarter second of fun worth it? no ATX PSU will push 2.5 on the -12 for very long.

 

[Dman877]

I happened to be looking at an Enermax 431 watt PS when I saw this thread and thought I'd throw this out. It's rated at 20 amps on the +12 and 1 amp on the -12.

 

[kevinthenerd]

I just had an idea.

Is there some way to throw out the common ground on a power supply and run the two power supplies off of eachother with the ground of one hooked up to the positive of the other? Then, you'd be running one at ground and the other at 12 volts above ground, and they might add up the potential to run your device?

This would definitely require removing the ground connector from your power supply cable, though, and I don't know what else you'd have to do.

Edit: Again, don't use your main PS.

 

[kevinthenerd]

Originally posted by: greenman100
is a quarter second of fun worth it? no ATX PSU will push 2.5 on the -12 for very long.

Ratings don't mean jack when you're willing to accept a lot of voltage drop and a lot of heat.

 

[CTho9305]

Originally posted by: kevinthenerd
I just had an idea.

Is there some way to throw out the common ground on a power supply and run the two power supplies off of eachother with the ground of one hooked up to the positive of the other? Then, you'd be running one at ground and the other at 12 volts above ground, and they might add up the potential to run your device?

This would definitely require removing the ground connector from your power supply cable, though, and I don't know what else you'd have to do.

Edit: Again, don't use your main PS.

Yes, I read an article detailing how to do this a while back. Warning: this may kill you and the usual other bad stuff.