Power Supply from a computer power supply

[jmytyk]

I looked everywhere for this, and now someone was nice enough to make a nice clean version...

power supply from a computer power supply. should be ok for lots of random uses...



http://wiki.ehow.com/Convert-a-Compu...b-Power-Supply

[madmickiii]

hi jmytyk,

Nice one, I found a few while reseaching cnc stuff, but none as cleanly described as this.
Think it will save me building a power supply from scratch now

[dertsap]

that link is sweet thanks i ve been wanting a 24v power supply i am currently using a pc power supply but running only 12v i didnt realize double up the power by connecting the two 12v wires

[Genguy]

Bear in mind that the -12v rail is only good for .8 amp. So you will only have an 800 mA 24v power supply.
Make sure you use an atx style, the older at and xt require a motherboard to be connected to them.

[madmickiii]

Hi dertsap, Hi Genguy,

I was going to say something about the 24 volts too, but you Genguy have said it lol. I think thou that what you say is not strictly correct concerning AT and XT, the ATX power supply is the (not totally sure , but hey, ) one that requires a motherboard to be connected, something to do with the POST (Power On Self Test) one of the ATX connectors connects to the motherboard to detect if a motherboard is indeed connected, if not it wont switch the various power lines on (to protect it self).

Either way thou its deff a cheap alternative to building a power supply from indivual parts and the prices for higher power outputs are not that much more expensive as the bog standard 300 watt units. I have a few lying around rated between 300 and 500 watts. I'm not sure what the power outputs are for the other rails but believe the higher currents are mostly for the 5 volt rails and possibly the 3.3 volt ones (i Believe are used to power the CPU).

MIke

[dertsap]

are you sure about the amperage, if the =12v is say 20 amp and the -12 is .8 amp
then wouldn t it be 24v 20.8 amp , i can see maybe one cancelling out the others amperage but not really , if it is truely possible to get 24v buy connecting the white and the yellow together i cant see lossing all that amperage , its like adding up batteries in a flash light the more you add the more juice you get

[dertsap]

as far as the mother board goes i m running motors on an old pc power supply got lucky i guess , if your not sure about the power outputs take a look at the link jmytyk offered at the top of the page

[Mariss Freimanis]

Some of the other voltages will cause problems. The 7V output (12V - 5V) will not work correctly. Current will flow out of the 12V terminal but cannot flow into the 5V output to give 7V. The 5V output can only source current, not sink it.

Mariss

[dertsap]

something i have been thinking about is connecting two power supplies in series to get the high output i know guys do this ( not with pc supplies ) to built arc welders , don t know if it will work though for this

[joecnc2006]

Originally Posted by dertsap something i have been thinking about is connecting two power supplies in series to get the high output i know guys do this ( not with pc supplies ) to built arc welders , don t know if it will work though for this

I run 3 in series on my machine titled (joes 2nd CNC Machine) and get 32v, running three HP LaserJet motors 100oz and cut at 40-60 ipm with no problems.

Joe

[gar]

051218-1417 EST USA

dertsap:

You can connect multiple supplies in series if not more than one has its common tied to chassis and thus to electrical safety ground. Keep in mind that PC supplies have several output voltages all referenced to the same common, and different current ratings for different outputs,

If you have one supply with common grounded, vs none, then that supply common should go to your electronics common. Call this the base supply.

From the base supply you can extract all the normal voltages.

Each additional supply common should go to the previous supply voltage you are adding to.

Primarily you should use this technique to add +5 V supplies together, or maybe 12 V. Your maximum current will be the lowest of the cascaded supplies. So if you added 12 V to a base 5 V your maximum current at 17 V would be limited by the 12 V supply provided you did not exceed the 5 V rating due to other loads. Note: +5 V supplies generally have a higher current capability than the other outputs.

Now suppose you cascade two 5 V supplies, each with a 20 A capability, and you have an 8 A load on the base supply, then the maximum load on the 10 V combination would be 12 A.

.

[Genguy]

Re: the AT/XT supply;
I have played with, modified/repaired the ATX style so I know it the best. It could be myth or rumor but I think the XT used external voltage regulation on the motherboard. To be honest, I have not played with the old ones enough to know for sure.

Re: the ampacity
Gar has covered this, but just to clarify a bit if I may;
Take the flashlight battery analogy as an example. If instead of touching the batteries + and - end to end, you make that connection with a tiny stand of wire. The voltage available at the terminals on the ends will be the same, but the current that can flow will be limited by the tiny strand of wire connecting the 2 batteries. It is the same with the 24v on the power supply. You will be limited to the "weakest link" which is the -12v portion.