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-5v, -12v, needed?

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carangil

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What are the -5v and -12v power supply terminals used for on most PC clone motherboards? Can it live without them?
 
The -5v is probably unnecessary unless you're going to be using really early hardware, or as lucasdaytona mentioned, a Roland LAPC-i.

The -12v, OTOH, is used for serial ports and all sorts of stuff that involves analog signals, i.e., sound cards, video capture, even some VGA cards. That's not to say that all such devices need it, but you're likely to have at least some hardware that does. And it's not just ISA hardware either... there are even a fair number of PCI cards that use it.
 
Unless you're running 4016 DRAM, it's not really critical. You're unlikely to break anything if you don't supply -5 or -12, so try it out, and if it works with your hardware, go with it. As was pointed out, you may have issues with serial ports, but if you don't care about that...
patscc
 
Serial ports require -12 to be within spec. -5 might work depending on the device.


Mike
 
Consumption of -5 is usually in the miniscule range (a couple of miliamps, tops), so you can take a LM7905 and drop the -12 to -5. That way, you have all bases covered.
 
Some minus 5 volt use:

* 16/64K 5150 motherboard: DRAM chips plus chip U1. Lack of minus 5 volts prevents motherboard working due to DRAM not working.
* 64K-256K 5150 motherboard: Only chip U1 uses minus 5 volts. Lack of minus 5 volts to U1 results in a 131 error (cassette wrap test).
* Early version of the IBM 5.25" Diskette Drive Adapter uses minus 5 volts
* 5160 motherboard: No use.

Crypticalcode0 said:
If the PSU has white wires on it's connector you should have -5 volt.
Click to expand...
I'm glad you used the word, "should" rather than "will".
In the thread [here], the white wire turned out to be the POWER GOOD line.
 
modem7 said:
Some minus 5 volt use:

* 16/64K 5150 motherboard: DRAM chips plus chip U1. Lack of minus 5 volts prevents motherboard working due to DRAM not working.
* 64K-256K 5150 motherboard: Only chip U1 uses minus 5 volts. Lack of minus 5 volts to U1 results in a 131 error (cassette wrap test).
* Early version of the IBM 5.25" Diskette Drive Adapter uses minus 5 volts
* 5160 motherboard: No use.
Click to expand...

Some of the IBM-branded memory expansion cards also require -5; the 32K and 64K boards definitely do. Something in the back of my mind seems to say that one of the really special boards also does (Data acquisition adapter, maybe?). As far as third-party boards go, it's a case of who knows? Best to have -5 available and not take chances.
 
Sound Blaster 2.0 (CT1350) uses -5V (apparently for op amps). Other earlier Sound Blaster cards might use it as well...

And if I remember correctly -12V is also used by some VGA cards (Paradise VGA1A, Trident TVGA9000).
 
Last edited:
sergey said:
Sound Blaster 2.0 (CT1350) uses -5V (apparently for op amps). Other earlier Sound Blaster cards might use it as well...

And if I remember correctly -12V is also used by some VGA cards (Paradise VGA1A, Trident TVGA9000).
Click to expand...

SB2.0 is the only sound card I can think of that uses -5v, but most other sound cards use -12v instead.
 
To quote one of my posts on the VOGONS forums:

"Despite generating -5v onboard for the analog section of the card, most* PAS16/Studio variants draw -5v directly from the bus to provide negative voltage for a single, itsy-bitsy op-amp in the digital section... that just happens to be part of the external timing circuit for the 16-bit CODEC.

(* - Does not apply to the 650-0097-xx, "Pro Audio Studio 16XL" cards.)"
 
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