Powering ATX Motherboard on 12V Battery

[Drycola]

Hello everyone,

I need to run an old ATX motherboard on 12V DC batteries instead of the powersupply. I need that for a robotics project.
I know that power supply provides +12V, -12V, +5V, & -5V DC to the motherboard and other components, but I'm little confused about the +12 and -12 issue, I mean can these be directly connected to the +ve and -ve output of the 12V battery??

My question in simple words: How can I run motherboard on 12V battery?

 

[crutschow]

No. You need a separate +12V and -12V supply, as well as a separate +5V and -5V supply.

The easiest way might be to use a 12VDC to 120VAC inverter from the 12V battery to power the ATX power supply.

 

[Drycola]

Thanks for your reply..
You are absolutely right, I've already done that using a UPS unit (that is run on 12V DC) but that would be a lot of weight as well as much power losses during multiple conversions (12VDC -> 220VAC -> 12VDC & 5VDC).
So, is the difference between +12V and -12V equals 24Volts ??? or it is just the + and - connectors of the 12V ??

 

[crutschow]

You can look at it as 24V with the center tap grounded. It is not the + and - connection to the 12V.

Whenever a plus and minus voltage is listed in a circuit, it always means a separate plus voltage and a separate minus voltage, both referenced to a common (gnd) point.

Edit: To save weight you could use separate switching regulators (four total) operating from the 12V battery to generate all the voltages.

 

[Drycola]

So, If I want to get the -12V, can I use 2 batteries as in the figure below???
**broken link removed**

 

[Reloadron]

Unless the system is very, very old as in about 10 to 12 years old there is likely no need for a negative 12 volt power. This is also true of the negative 5 volts. The negative 5 volt requirement was dropped from the PSU design guide around 2002. The last system I saw that used negative 12 volts was a ten year old HP system and it only used the -12 volts for the audio out card before systems used self powered speakers. Unless of course your PC has a custom card in a slot with a need for -12 volts. Even systems that used -12 volts only drew a few hundred mA on that PSU rail.

I would just be real sure the system needs the -5 or -12 volt rails before you worry too much about it.

Yes on your drawing if you need it.

Ron

 

[MrAl]

Hi there, and hi there Ron,

I was going to say something similar, and if the -12v line was needed perhaps a low current dc to dc converter would do it, or yes, another 12v battery. The -12v doesnt require that much current draw.

The +5v supply could come from a buck converter for the required current, and if a 3.3v is needed the same there.

 

[cr0sh]

Sometimes the -12 volt side was also used for the serial ports; but yeah, likely you don't need either negative voltage. I once hooked up a 486 motherboard (over 10 years ago) using a power supply that only output +/- 5V and +12 volts, and it worked OK with the -12 volts (never tried it without the -5 volts).

 

[antknee]

Did you know VMWare emulates operating systems using the ATX board? Not so long ago I had my Vista PC running a copy of Windows XP, Windows Server 2008 and another copy of Vista all simultaneously. VMWare chose the ATX board because it was so incredibly reliable and robust. My first computer had one, it ran for 9 years without fault, then it just got obsolete and just too slow so I put it out to pasture.

My point is that it might help simulate what you are doing in software.