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Beta Geek
11-05-2003, 06:36 PM
Is it a good idea to use "custom" voltage levels from the standard supply voltages of a PSU? For instance are there any problems with using the +12V and –12V levels to effectively power a 24V cooling fan?
david eaton
11-05-2003, 06:48 PM
Can't see there being a problem with this. Providing, of course, that the power ratings of the individual power rails are not exceeded.

I use an old AT power unit as a bench supply, and just use whatever outputs give the voltage I need at the time. Seems to be OK for now.

(It'll probably blow up on me now! :D )
mjc
11-05-2003, 09:16 PM
+12V + -12V = ?

Not 24 V......

You would need to reverse the polarity of the -12V .....:D
Beta Geek
11-05-2003, 10:58 PM
Actually, since voltage is a measure of potential difference, when wired in a series voltage divider setup (similar to a PC power supply’s function), +12V and –12V simply adds rather than algebraically adds, and the polarity is determined by the reference point. If you were looking at a simple series circuit, a +12V and a –12V source, they would algebraically add, and cancel each other out.

Anyway, I knew it works in theory, I wanted to know if anyone had tried it before.

See my crudely drawn schematic (I hope it’s drawn right because I don’t have time to double check right now.): :eek:
mjc
11-06-2003, 01:49 AM
Yeah, I realised after I posted the easy way to do it.......

a brain fart on my part.
Reid
11-06-2003, 10:43 AM
That diagram is incorrect. You cannot get 24V from a 12V battery with just resistors. Also, the ground connections short out the lower resistor.

You can get 24V from a power supply, but need to connect across the +12V and -12V connections, without using ground. (I do not have time to draw that)
Beta Geek
11-06-2003, 10:20 PM
Yeah, this morning I looked at it again and saw my mistake. Thanks for pointing that out. Looks like mjc wasn't the only one with gas of the brain.

Anyway, brain farts aside; the 24V from a PSU will work. However, my A+ instructor said today that he wouldn't recommend it. It's one of those things that will work; just make sure you know what you're doing.