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?

[madsci1016]

Google 'pico atx power supply' and you should find what you need.

Its called a picoPSU.

[billhowl]

http://www.mini-box.com/DC-DC

[Drycola]

Thanks for the answers...
I know about this, but I can't buy it.
I need to make it my own, how it is done WITHOUT the adapter mentioned above??

[waltr]

+12V, -12V, +5V, & -5V DC

Each of these voltages are referenced to the ground wire. So you need four voltages and ground. Only the +12V can come directly from the battery. The +5V is easy to obtain with a simple linear regulator like a 7805 but would need the high current version and/or the use of a pass transistor to provide the required current. This is not an efficient method due to the way a linear regulator drops the voltage and dissipates the excess power as heat. [power loss = (12V - 5V) * current] To improve the battery life a switching regulator would have better efficiency.

The negative voltages will require a switching type power supply. The ATX board doesn't require much current for the negative voltages so these wouldn't be too difficult to build.
Be sure to find or measure the current for each voltage that the ATX board needs. Then you can start working on the regulators.

[Drycola]

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

[waltr]

Yep, two batteries wired as you show will work and would be the easiest solution.

Then you could use a negative linear regulator for the -5V, like the LM7905.

[Soeren]

Hi,

Just a quick warning before you ditch your PC.
The 12V side can NOT be taken directly from a battery, as the PC have much smaller tolerance than the battery can deliver (An SLA will be between 12.6V and ~14.4 over its discharge life).

The way to do it is with a multi-tap SEPIC or Ćuk converter - and that is not something you are able to design if you have to ask how, so bite the bullet and pony up for a Pico 

[billhowl]

Read this PDF, the circuit diagram in page 14.

Mini-ITX Power Solutions

apart from providing the voltage, other signaling need to take care too.

in short to supply the ATX motherboard with the 12V DC batteries is not that simple of adding a few voltage regulator will to.

+5 VDC / +3.3 VDC Power Sequencing
The +12 VDC and +5 VDC output levels must be equal to or greater than the +3.3 VDC output
at all times during power-up and normal operation. The time between the +12 VDC or +5 VDC
output reaching its minimum in-regulation level and +3.3 VDC reaching its minimum in-regulation
level must be =< 20 ms.

PWR_OK
PWR_OK is a “power good” signal. It should be asserted high by the power supply to indicate
that the +12 VDC, +5VDC, and +3.3VDC outputs are above the under-voltage thresholds a
minimum of 17 ms. and that sufficient mains energy is stored by the converter to guarantee continuous
power operation within specification for at least the duration specified, “Voltage
Hold-up Time.” Conversely, PWR_OK should be de-asserted to a low state when any of the
+12 VDC, +5 VDC, or +3.3 VDC output voltages falls below its under-voltage threshold, or
when mains power has been removed for a time sufficiently long such that power supply operation
cannot be guaranteed beyond the power-down warning time.

PS_ON#
PS_ON# is an active-low, TTL-compatible signal that allows a motherboard to remotely control
the power supply in conjunction with features such as soft on/off, Wake on LAN*, or wake-onmodem.
When PS_ON# is pulled to TTL low, the power supply should turn on the five main DC
output rails: +12VDC, +5VDC, +3.3VDC, -5VDC, and -12VDC. When PS_ON# is pulled to
TTL high or open-circuited, the DC output rails should not deliver current and should be held at
zero potential with respect to ground. PS_ON# has no effect on the +5VSB output, which is
always enabled whenever the AC power is present.
The power supply shall provide an internal pull-up to TTL high. The power supply shall also
provide de-bounce circuitry on PS_ON# to prevent it from oscillating on/off at startup when
activated by a mechanical switch. The DC output enable circuitry must be SELV-compliant.
The power supply shall not latch into a shutdown state when PS_ON# is driven active by pulses
between 10ms to 100ms during the decay of the power rails.

+5 VSB
+5 VSB is a standby supply output that is active whenever the main power is present. It provides
a power source for circuits that must remain operational when the five main DC output rails are in
a disabled state. Example uses include soft power control, Wake on LAN, wake-on-modem,
intrusion detection, or suspend state activities.
The +5 VSB output should be capable of delivering a minimum of 2.0 A at +5 V ± 5% to
external circuits. The power supply must be able to provide the required power during a "wake
up" event. If an external USB device generates the event, there may be peak currents as high as
2.5A lasting no more than 500mS.
Overcurrent protection is required on the +5 VSB output regardless of the output current rating.
This ensures the power supply will not be damaged if external circuits draw more current than the
supply can provide.