How do I power my car PC when it's not in the car?

So you've got your computer put together (or it arrived UPS) and you're ready to load the operating system, front end, navigation application, support software, and maybe even a pretty skin for the front end. But it's raining outside and you really don't feel like sitting in your car for a Windows installation. You also want to test the shutdown capabilities of the PSU to make certain it's all setup the way you want it to behave.

You can use a spare ATX power supply to provide 12v to the power supply. This FAQ will show you how to accomplish that so you can tweak and customize your vehicle computer inside.

If your system uses an inverter

If you are using an inverter, there's generally no problem. You simply unplug the power supply you are using in the car, bring it indoors with you and plug it into the wall socket.

If you are using a DC-DC power supply


You have a couple of options:

Option 1:

Unhook the DC-DC power supply from the carpc and use a normal ATX power supply (110V - US & 220V - Europe) hooked to the wall with the ATX plug connected to the carpc motherboard. This method is explained in detail below.

Option 2:

Leave the DC-DC power supply connected and provide 12V DC power to it by using an AC/DC converter (pictured below).

Make sure the AC-DC converter provids enough amperage to power your system. A 10 amp system can supply 120 watts (12V X 10A=120W), a 15 amp system can supply 180 watts. In truth, these power supplies often are rated at 13.8 volts, giving 138 watts and 207 watts, respectively.

These AC-DC converters come in various shapes and sizes and run about $100 at Radio Shack.

Option 3:

Required components

  • (1) ATX power supply that provides sufficient power on the 12v rail. Typically, 10A-15A should be sufficient. You can get the ratings from a sticker on the side of the power supply.
  • (1) 3-pin HDD pass-thru adapter like this one.
  • (1) low-voltage switch that will maintain on/off state.
  • (1) paper clip or something to start the computer, if you do not have a momentary switch handy.
  • (3)Terminal connectors that are suitable for your DC-DC power supple or shut down controller.
  • 14AWG wire (thicker wire may be used, but it may be unwieldy to work with)
  • Method of connecting wires (solder/heatshrink or crimp connectors are preferred)

Background Information

An ATX power supply provides 12v DC power to the motherboard and to components like hard drives, optical drives and the like. This is the same input voltage that is required for your DC-DC power supply or shutdown controller. To utilize the 12v DC from the ATX power supply, you need to do two things:

1) Wire the DC-DC power supply's inputs to a 12v source from the ATX power supply. The easiest place to get the 12v is the HDD connector.
2) Fool the ATX power supply to believe it's connected to a motherboard and powered on, so it will supply 12v
In addition, you'll want a switch in order to simulate turning ignition on and off to insure that the shutdown capabilities of your DC-DC power supply (or standalone shutdown controller) work the way you want. A paper clip can be used here as well. You just need something to momentarially short the two pins.

The hard drive molex connector provides power in two voltages, +5v and +12v. The yellow wire is 12v and the red wire is 5v. The two black wires are both grounds. The yellow wire will provide power to the DC-DC PSU, as well as to the ACC line for startup/shutdown control.

This procedure assumes that you have your computer assembled properly and the DC-DC PSU or ShutDown Controller setup is wired correctly to the motherboard, including the two-wire connector that goes to the power switch header on your motherboard!!!


Connector to fool the ATX power supply

1) Cut the three-pin fan header off about halfway along the wire.
2) Strip the wire that goes to the outer pin and connect it to the unused outer pin on the 3-pin fan header.
Connector for the DC-DC PSU

1) Cut the female end off the pass-thru connector as close to the female end as possible. Discard the female connector.If you would like to keep the female connector for any reason, do not cut so close that you will not be able to splice onto the remaining wire.
2) Extend the yellow (+12v) and black (ground) wires to your desired length. Six inches or so should be more than sufficient.
3) Attach terminal connectors to the extended +12v and ground lines.
4) Connect the thinner wire that went to the HDD connector to the input side of the switch.
5) Connect another wire to the output side of the switch and attach a terminal connector to the other end of that wire.
6) Connect the +12v, ground, and switched lines to their appropriate terminals on the DC-DC power supply or shutdown controller, it matches the following diagram:


You now have two connectors. One is connected to your PSU at one end and a HDD connector to the other end. Make certain this is wired up first. Also, make certain the switch is in its off position.

The other connector (the 3-pin fan header) goes into the ATX connector that would normally go into the motherboard. The two outer pins of the fan header should fit nicely into pins 13 & 14 of that connector. You can locate the correct pins in this diagram:

The wire colors may or may not be the same!

Plug the ATX power supply into the wall and turn it on, if required.

At this point, cross your fingers, pray to your deity and turn on the switch in your homemade adapter. When the switch is in the on position, you should get +12v to your IGN line of your DC-DC power supple or shut down controller, which should send the pulse to the motherboard and initate booting of the PC.

Artile written by KByrd, evandude, BugByte, DarquePervert