Electrical

Introduction

This section is intended to highlight some of the electrical challenges I have faced while putting together the car computer. Hopefully if you read this you can save yourself a lot of time by learning from my mistakes :)

Wiring Tips

Tip #1: Hook your power supply up directly to your battery. Don't bother trying to run the car computer off your cigarette lighter or something like that. Inevitably you will end up blowing the fuse on that line. In my car the cigarette lighter is fused at 7.5 amps. This falls way short of the 40 amps my power supply may need!

Tip #2: Get the correct gauge wire for your power supply. Mine will draw up to 40 amps. Don't try to pass this through 22 gauge speaker wire as the resistance will be too high and your wire will likely melt. Not that I have ever had a wire melt on me before, but I hear that is what happens :)

Tip #3: For ultra small gauge solder jobs, consider bending the wire back on its self to prevent the wires from pulling apart. If you don't, you will likely accidentally pull the wires apart when wrapping then in electrical tape or running them into the tight recesses of your car. The below photo of my front panel audio cable illustrates the point. I soldered it 2 times (headphone, + microphone for 14 solder connections) without bending the wire back on it self. Because the wire is 1 million gauge, at least 1 solder joint would always pull apart. Trust me and bend the wire back on it self to take the stress off the solder joint.

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Tip #4: You can extend USB cables by soldering in twisted pair network cable.

Tip #5: You can ONLY extend front panel audio connections (headphone + microphone) using shielded wire. If you try to use some else, oh lets say network cable, it will sound horrible. You can get shielded wire of the appropriate gauge at radio shack.

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Tip #6: Ground your motherboard to the car's frame. The motherboard is designed to be grounded to the metal computer case at the screw holes. You should figure out a way to ground the board to the car's frame. This may help reduce interference, which will be apparent on audio lines.

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Tip #7: If you like listening to the radio, then place your power supply, or run any power lines near your antenna line. In my car my power supply is actually sitting right on top of my antenna line. When I turn the computer on, my radio receptions drops by half. Its very annoying.

Tip #8: If you are having problems with interference on a line, you can wrap the wire around some ferrite. I dont know the physics behind it but resistence is reduced the more loops the wire makes around the ferrite. You can get the ferrite things at Radio Shack.

Choosing the Power Supply

You will need to make sure to get a power supply for your systems that can handle all your power needs while remaining small, efficient, cool, and quiet. The first thing you have to do is determine how much power you need. You may be familiar with the max wattage rating of PC power supplies. This is actually not a very helpful number. Is is far more helpful to measure the amperage supplied on 3.3, 5, and 12 volts lines.

The 12 volt line is usually used for powering the processor, hard drives, and CD-Roms. The 5 volt line is usually used for powering hard drives and most of the motherboard components. The 3.3 is usually used for the rest of the motherboard components and add-on cards (i.e. PCI cards).

Most computer components should give some idea of how much power they draw and at what voltage. Using this information it should be possible to determine your max power draw on each individual line.

Here is a photo of a hard drive and CD-Rom that clearly show how much power they draw on 5 and 3.3 lines. Note that these devices are ancient and not what I am using in my car:

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The power needed for the processor can be a little more difficult to determine. You can visit the manufactures web site to determine how many watts it needs however they do not tell you which line these watts are drawn from. For example, my AMD Sempron 2800 needs 62 watts. I have found some internet sources that claim this all comes from the 12 volt line. Since watts = volts * amps this equates to 62 watts / 12 volts = 5.17 amps. If you are planning on using a Pentium 4 processor, then we know for sure all the power is drawn from the 12 volt line. On P4 motherboards there is another 2 prong power plug called (oddly enough) the P4 plug. This is another 12 volt line for the P4 processor to suck power from since the standard 12 volt line may not pass sufficient amps over that small of a wire.

The motherboard can tricky to determine its power needs. If you are lucky your manufacturer may list how many watts their board needs. Just like the processor they do not list on what line the power comes from. If your manufacturer does not give the wattage required you can use this on-line calculator (Power Calculator). The motherboard is assumed to be around 25 watts I believe should vary based on the on-board components. For example, mine has onboard audio, video, and LAN. My motherboard with 2 sticks of DDR memory should require a minimum of 56 watts. Lets just arbitrarily say 80 watts to be safe. If that is all on the 5 volt line then that is 80 watts / 5 volts = 16 amps. In reality this is probably split on the 3.3 volt line but I don't know for sure *.

If you plan on running anything else off your power supply, such as GPS or a screen, then you also need to figure in its power needs. My screen gets its power elsewhere. My Delorm Earthmate GPS requires 175 mW off the 5 volt line. This is 0.035 amps which is almost negligible.

You should also consider any USB devices you may have. All USB devices use power from the 5 volt line. In my setup I have a Griffen Powermate rotary knob. I have no idea how much power it draws but I imagine it would be pretty close to 0.

Below is a summary of what I think my power needs are for my car computer. Remember that the motherboard power is probably split between the 5 volt and 3.3 volt lines but I don't know for sure.

The above power requirements is maximum usage. I actually powered my entire system with the power supply shown below before getting my DC power supply:

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I should mention that while using that power supply, the car computer would shut down periodically, presumable due to insufficient power.

Once you have mapped out your maximum power needs on all the different lines, you are ready to pick your power supply. Needless to say, power supplies that operate on 12 volts DC incoming, rather than 120 volts AC, are much preferred for car computers. That limits your choices significantly. Most of the 12 volt DC power supplies are not made buy name brand power supply companies. Keep in mind that you probably want a power supply that is ATX compatible. This the motherboard to tell the power supply to turn off. If a power supply is not ATX compatibly, then after shutting down your computer from windows you will have to manually cut power to your power supply. You may also want a power supply that provides the P4 line.

At the time I bought my power supply (January of 05) there were only 3 DC units capable of producing the power I needed. Thinking back on it I probably should have purchased a micro-ITX board rather than a standard micro-ATX system. This would have reduced my power needs significantly. I ended up buying a Keypower 250 watt unit. Although its big, has a built in fan (thus is somewhat noisy), and is not very efficient (60%), I have no doubts I will be able to power my system now and with whatever upgrades I decide to do in the future. Here are the stats for the unit:

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