15 volts from a pc p/s???

[Intimidator#3]

How could you get 15 volts from a pc p/s<p>I know doing this with batteries you just add them in series to the volts you want, but how would you do it with a p/s with a 12v an 3.3v out leads??connecting the 3.3v an 12v together??<p>Thanks
Kevin

[rshayes]

It is possible that you could connect the two outputs in series to get 15.3 volts. However, the circuit board may have the negative leads of both outputs connected together on the circuit board with printed circuit traces. These may be difficult to separate. It depends on the board layout.

[Externet]

Hi.
Joining the 3.3 and the 12 V together won't work.
Better try to calibrate any potentiometer on the power supply circuit board or trick the reference level.
Miguel

[Intimidator#3]

ok i found out in testing that if i use the +3.3v an the -12v i get 15 volts, using the +12v an the -12v gives me 24 volts i can get anywhere from 3.3-24 volts out of this by using the -12v an -5v as the ground an the + side for possitive

[Mike]

What brand and model PS is that? Did you have to make any modifications?<p>If that works it would be great for big electronic things. No more overpriced transformers!

[Engineer1138]

Mike: I've done that without having to make any mods to the PS. As long as your circuit doesn't try to share the same ground that the PS uses, you're OK. The real problem is that you're limited to the current capacity of the lowest supply you use, so if you are using +12 & -12V to try to get a 24V power supply, and +12 can supply 3A and -12 can supply 1A, the most you can draw is 1A.<p>I went through this a few months ago trying to
configure PC power supplies to get a 12V@6A source to test a battery charger I was building. I finally used a 12V SLA battery. Kinda funny: using one battery to charge another.

[Intimidator#3]

only mod i did, was instead of using 2 10ohm resistor blocks on the 5v wires for load(couldnt get 12v using them) I am using 10 1157 automotive bulbs in series for my load, they draw 13 amps!!!!! but i get 12v on the nose sometimes 11.99v

[Intimidator#3]

the p/s is a Power man 300 watt atx

[Mike]

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by Engineer1138:
Mike: I've done that without having to make any mods to the PS. As long as your circuit doesn't try to share the same ground that the PS uses, you're OK. The real problem is that you're limited to the current capacity of the lowest supply you use, so if you are using +12 & -12V to try to get a 24V power supply, and +12 can supply 3A and -12 can supply 1A, the most you can draw is 1A.<hr></blockquote><p>Oh, ok I see. I was hoping to be able to use these to power amps I build. Using two power supplies to get +/-24V at 350W or so would be cheaper than one large torrid.<p>Power supplies use a torrid transformer to get the +12V rail. I wonder if I can take a few windings off of it and make it output +24 or possibly even more volts.

[Mike]

Another idea. i'm looking at an old 200W AT power supply. It has +12V 7.5A and +5V 20A. Would I be able to hook those in series to get +17V 7.5A?<p>edit: how dangerous are PC power supplies that have not been plugged in for a long time and do not work anymore? This one I have does not work at all and I want to pull some parts off of it.<p>[ March 29, 2005: Message edited by: Mike ]</p>

[Dean Huster]

Mike, if you hooked the +12 and +5 together, you'd get a 7 volt supply. To find out what you'd get, find the algebraic difference of the supplies in question, including the signs.<p>+12 -(-12) = +12 + 12 = 24
+12 -(+5) = +12 -5 = 7
+5 -(-12) = +5 + 12 = 17<p>Dean

[dacflyer]

Uhhhh... how bout this... most supplies that i have come across usually have the LM7812 type regulator for the 12v line.. if your not going to be using this PS in a computer..why not just change the LM7812 to a LM7815... its a direct swap out of you can see it in the supply..
but check the input 1st to see if you will have enough voltage to use. ( more than 15v input to the LM7812 before swapping out)

[Mike]

It usually uses the 7912 regulator for the negative, right, not the 7812 for the positive, because a 7812 can only supply 1A or 1.5A of current, while the +12V supply is near 20A on most PSUs.

[Dean Huster]

I'm not saying you guys are wrong about the linear regulator in the switcher. This is just an observation on SMPSs:<p>It just seems very odd to me that they'd have any kind of linear regulator at all. The whole idea of a high-efficiency, switch-mode power supply is that you regulate the primary through the output of one of the secondaries and then have a set number of turns for each supply needed and those turns set the voltage. It's usually done that way to reduce the parts count, complexity and cost. Throwing in a bunch of linear regulators flies the face of that. Other than screwing up and making secondary windings too small of a gauge and making it "soft", the system otherwise works well.<p>Dean

[dacflyer]

Hmmmm. the PS i played with were really old.. like the AT types.. so i am not up to date on the ATX etc..