# How do I find out how many volts a computer power supply has?

#### joulian

Joined Aug 27, 2009
53
I have an old 2001-02 hp computer

I have been keeping it for parts.

What is the average voltage and wattage of one of these things?

I have a 48v 300cfm .49amps fan I was trying to figure out how to power.

#### joulian

Joined Aug 27, 2009
53
I see the amps are 1.1amps.
This transformer i have only goes to .49amps

What do you guys think?

I am no expert at matching these things.

#### Duane P Wetick

Joined Apr 23, 2009
435
If the power supply is rating is 48 vdc @ 1.1 amps, it will run your 48 vdc. 0.49 amp fan easily, assuming the power supply does not have to power something else. If you load your power supply too much, the voltage will sag and it will overheat.

Cheers, DPW [ Spent years making heaters out of op-amps.]

#### BMorse

Joined Sep 26, 2009
2,675
I have an old 2001-02 hp computer

I have been keeping it for parts.

What is the average voltage and wattage of one of these things?

I have a 48v 300cfm .49amps fan I was trying to figure out how to power.

If you look on the power supply of the PC you will see how many watts and how much current each rail can put out....

If you are looking for something to power the 48 volt fan from anything in that PC, you are out of luck! The highest voltage the PC power supply puts out is 12 volts, you can probably power it off of it, but the fan will run 4 times slower than normal (if it even runs at all!) and would probably draw too many amps and toast the power supply from the PC anyway.....

My. 02

#### joulian

Joined Aug 27, 2009
53
DOes the amps have to match exactly?

In any case.

Lets say if it where 60amps would that hurt the .49amp fan?

#### joulian

Joined Aug 27, 2009
53
So i can use a power supply to test all my little IC projects?
I took DC and Digital this past semester and was looking for a power supply other than a battery or something to use.

Any advice on how to go about doing this. Maybe set up a project box with a switch and then some terminal post that are labeled so i can plug up a breadboard or something?

#### Mike33

Joined Feb 4, 2005
349
Hi Joulian,

To keep it simple, look at a power supply this way. Devices (your fan) DRAW current. The power supply rating says it WILL GIVE .49A at 48V, so if your device needs about 1/2 ampere, the voltage will be 48V or so. BUT, if you only draw .2A, your voltage might well be higher! Like, 55V or something. The voltage is essentially "Pressure".

The device pulling the rated current will make the supply sag down to its rated voltage. That current rating is just saying where the voltage will lie when some device draws that much current.
And, if it could give more current, it wouldn't hurt the power supply to attach it to the fan...BUT, the FAN might burn up, because the voltage would be higher!

One caveat - you can use a voltage regulator to bring a higher voltage down to a useable level for a device (Look up LM317, etc), provided you are within the appropriate levels for the regulator. With that supply, you're close to the maximum differential voltages for an LM317, tho - wouldn't go any lower than about 9V output. You will need the regulator, caps to filter (see the LM317 data sheet...), a heatsink for it, small PCB, a nice enclosure, and some means of connecting the power to a breadboard, as you mentioned. Not a bad use for the power supply, really. You could work it out to get plus and minus voltages...

#### joulian

Joined Aug 27, 2009
53
Mike33

So if it says positive voltage regulator from the wikipedia write-up on the LM317 does that mean its converting and regulating DC current?

"LM317 is a positive voltage regulator supporting input voltage of 3V to 40V and output voltage between 1.25V and 37V." source http://en.wikipedia.org/wiki/LM317

#### joulian

Joined Aug 27, 2009
53
Ahhh...

Mike33 I know what that is now, voltage reg. My tankless hotwater heater had one on it when i opened it up a week ago to reset it after it tripped the breaker for some reason. Its 220, so i guess thats wrong about assuming they are used for DC devices. hmmm

Hmm now i am a little confused. I just replaced a large capacitor on my drill press a month ago. The motor wouldn't even turn. I was thinking the capacitor was only used for kicking it over from a dead start. The voltage reg. sort of does the opposite. Instead of a large spike by a capacitor it regulates it to make sure to much doesn't go in at once.

But am i confused about the job of that type of capacitor on my Drill press?

#### davebee

Joined Oct 22, 2008
540
48 volt supplies aren't as easy to find as 5 or 12 volt ones. They are available, but can be expensive.

Radio Shack sells a 25 volt 450 mA transformer for about $6.50. If you bought two of them, wired their outputs in series with the right phase then you'd have about 50 VAC. Rectify that and you could run your fan. #### Paragon Joined Dec 8, 2009 45 REgulated supplies like a ATV supply or others from computers will stay at their rated voltages. That is what they are designed to do. If your computer didnt dray a lot of current at a certain time and the voltage jumped to 13v or higher, it would likely fry something. Other wallwart supplies will not be as regulated (I always keep them around for testing stuff) I have one that is rated for 10.6V, 1.3A and just having the multimeter on it reads 13V. That would be very distructive if that type of supply was in a computer. Anyhoo, I ahve an old 200W supply from a dell (I think a PII Proc) and it has the 12V at 6A (which is always going to be the hightest rated rail) and the -12V at .8A What you can do is connect the +12 and -12 to get 24V. #### Paragon Joined Dec 8, 2009 45 I forgot to mention, you have to connect 2 pins together on the largest connecter (the one that would go to the motherboard) with a wire in order to turn the supply on when it is not connected to a motherboard. Search the internet as I forget which ones. Thread Starter #### joulian Joined Aug 27, 2009 53 48 volt supplies aren't as easy to find as 5 or 12 volt ones. They are available, but can be expensive. Radio Shack sells a 25 volt 450 mA transformer for about$6.50. If you bought two of them, wired their outputs in series with the right phase then you'd have about 50 VAC. Rectify that and you could run your fan.
I was thinking of the same

thanks

#### shortbus

Joined Sep 30, 2009
9,377
Why not just take the fan out of your computer? It is made to work with the power supply from that computer.

#### Mike33

Joined Feb 4, 2005
349
The above answers are correct...the PS from the computer is likely regulated. But on the topic of regulation - the drill capacitor is not for regulation, it is involved in 'kicking it over' as you guessed. That's a discussion about how motors work!

If you were to drop the voltage with a divider, or make it variable, by using an LM317 with your PS, you will generate heat (wasted power, actually). The suggestion to buy a smaller Radio Shack transformer and build a 'real' benchtop experiment supply is a great one. You only need the voltage to be a few volts higher than what you want for an output when using a regulator. More than that is wasted as heat.

For this purpose, I'd get a 12VAC transformer, rectify the output, and use the LM317 in 'adjustable mode' from the application notes on the data sheet - and then you'd have an adjustable benchtop supply giving 5, 9, 12 volts or in between. There are other schematics for wiring them up to give + AND - voltages if your chips need that, search around (could get the 25VAC CT transformer)...but we're getting off the original topic about the fan!!