Powering a car PC is one of the most confusing parts for newbies and those with limited electrical experience. Here's the scoop on how to do it.

Short-attention span version

1. You can use either an inverter or a special DC-DC power supply. Most people here recommend a DC-DC Intelligent power supply because they run quieter, produce less heat and provide automatic startup and shutdown signals to the PC so you don't have to do it manually. They're more complicated and expensive than inverters but are considered 'better' solutions than using an inverter.
2. Inverters convert your car power to an AC outlet that you can plug your PC into. They can often cause noise in the audio signal to your stereo, run hot, have fans that make noise and cheap ones may not output power that will work well with electronics. Don't buy a really cheap one.
3. If your PC is over 250 watts, you'll probably need an inverter. Look at the back of the PC or on the power brick to see how many watts it is rated for.
4. If you aren't careful and try to draw too much power from a cigarette lighter, you will blow the fuse or could cause a fire. If you need to supply more than 120 watts to a PC, you NEED to connect it to the battery and use a fused and appropriately sized cable or else you could also cause a fire.
4. If you want to buy a DC-DC power supply, you will probably need a DC-DC automotive ATX power supply.
5. A REGULATOR is *different* from an ATX power supply. It usually puts out a single (or maybe 2) voltage and is used for stuff that already has an internal power supply like a Mac Mini or a laptop computer or a case that has a DC power supply built in already such as a Morex ITX case.

Scary stuff you should know about before wiring your car.
You can cause a fire if you don't know what you are doing. Things that will cause a fire are: 1) not using the proper gauge of wire (remember, the lower the gauge number, e.g. 8 AWG, the thicker the wire and the more power it can carry); 2) overloading a cigarette power supply port; 3) connecting your PC to your car's radio harness or the accessory wire that switches on and off with the key; 4) running dedicated power cables to the battery but failing to fuse them within a foot or so of the battery and ALSO at the other end just before it supplies your devices (PC/USB hub, etc.). Two fuses. One to prevent a fire in the engine compartment when your cable insulation frays and shorts to the engine block and the other, lower rated fuses to protect each of your precious electronics.

Here is an article that will tell you how to avoid these mistakes and connect devices that need power safely.


The Automotive electrical system.

The universal standard for car electrical systems is 12 volts, DC (Direct Current). Almost all cars follow this standard except for very, very old ones (e.g. Model A's have a 6 volt system) or unusual cars (if you have an electric car, for example). Because 12 volts is the standard for cars the world over, this article assumes a 12 volts system in your car.

There are two components to the car's electrical system. First, is the battery. The purpose of the battery is to start the car. While it is true that if you shut the car off but leave the key in the accessory position the battery will supply power to the car, the PRIMARY purpose of the battery is only to start the car. Turning a starter and supplying power to the spark plugs requires a battery that can provide a large amount of current for a very short time. Your car battery is designed to do this. It is not designed to discharge slowly over time at low power. It is designed to supply lots of current for a very short time, which discharges the battery only slightly.

Once the car is running, the battery no longer supplies power to the car. Instead, an engine driven device called an alternator generates power to the car's electrical system. The alternator is also used to charge the battery back up so it is ready to start the car the next time. Although cars are considered 12 volt systems, alternators actually provide as much as 14.5 volts because you need slightly greater voltage to charge the car battery fully. Thus, although you will see comments about 12 volt systems, the voltage in the system will actually range higher and lower than 12.

An environment that is hostile to electronics

In fact, the electrical environment in your car is quite hostile to electronics. When you try to start your car, the draw on the battery is so great that the voltage of the system will be pulled down to perhaps 11.5 or even lower - especially if it is very cold. After the engine catches and the alternator kicks in, you may see voltage suddenly spike up to higher levels and then settle down. Turning anything electrical on or off may cause a spike or sag in voltage - headlights, high beams, seat heaters, fans, playing the audio system loud - there are a variety of things that can cause sudden changes in voltage. And electronics like computers don't like sudden spikes and sags in voltage. They can cause the system to lockup or even fry a component.

A fundamentally different kind of electrical power

Most pc's are designed to plug into alternating current, or AC. AC is the type of power that comes out of the outlet in your house and it is usually relatively clean with fewer sags and spikes in voltage. In addition, the voltage is much, much higher than the 12 volts in the car. It is 110 volts in the US, 220 in Europe and other parts of the world. For technical reasons that will only confuse you if you didn't already know this, you cannot simply plug a DC device into an AC outlet nor an AC device into DC power. They are fundamentally different types of power.

Desktop PC's use 110 volt AC power. Laptops use DC power but usually come with a power brick that converts AC power to DC power so you can plug it into a wall. The actual battery inside the laptop is DC, but might be quite different from the 12 volts in your car. A common DC voltage for a laptop is around 19 volts.

What to do about power?
Believe it or not, there are TWO solutions to powering a PC in your car. The first is simpler and less elegant, the second is better but a bit more difficult.

Option 1: Use an inverter

An inverter is a device that takes 12 volt DC power and converts it to 110 volts AC so that devices that run on AC power can be run in the car. In theory, you could connect an inverter to your cigarette power plug, turn it on, and plug your PC into it. Inverters can be easily found in automotive supply stores, Wal-Marts, at camping supply places like Bass-Pro and online.

Naturally, it isn't necessarily as simple at buying one and connecting your PC to it. For technical reasons, (Google "true sine wave inverter" if you want the details) the power that an inverter outputs is SORT OF like 110 volts AC and electronics are very sensitive to "sort of" solutions. If the inverter is of low quality (usually indicated by the LOW PRICE), your PC may simply refuse to boot or your laptop will not charge.

In addition, the way that an inverter converts DC power to AC is inefficient and wastes much of the power as heat while generating magnetic interference that can sometimes be heard over the car's audio system. Whether the inverter does this or not is found out through experimentation. Again, low price inverters seem to be more prone to this problem than quality inverters. Also, inverters often use fans to cool themselves and that generate audible noise in the cabin.

In addition, inverters supply power to the PC in accordance with how power is supplied to them. That is, if the power outlet they are plugged into turns on and off with the car, your PC will turn on and off the same way. There is no signal to the PC to startup - you will need to press the power button to start the PC and before you turn off the car, you will need to tell the PC to shutdown or else the inverter will simply quit powering the PC -the equivalent of pulling the plug on a running PC in your house. This will sometimes corrupt the hard disk on the PC and it will fail to boot properly the next time it is powered up.

Inverters may be the only solution if you need more than 250 watts of power to run your PC. However, if you need this kind of power, you should NOT plug an inverter into a cigarette lighter power outlet unless the outlet is specifically labelled as being able to provide that wattage that your PC needs. Even then, a fuse on the inverter should be used and good inverters will have fuses on them. If the power outlet is not labeled, don't try to plug a pc that uses over 120 watts into it. Instead, run dedicated cables to the battery and fuse them appropriately. Using a fused power distribution block (Google it) is the only good solution unless you enjoy extinguishing car electrical fires inside your dashboard.

Option 2: Use a DC-DC power supply

Most electronics run on DC power. Even your desktop PC runs on a range of DC voltages (3 volt, 5 volt, 12 volt among others) that is supplied to it by your Power Supply Unit (PSU), which is what you are plugging into the wall outlet. The PSU converts the AC power from the wall outlet to the variety of DC voltages and routes them through a connector to the motherboard of the computer, the disk drives, etc.

AC power is useful because it is easy to step the voltages up or down to meet the needs of the electronics, converting them to DC as part of the process. It is more difficult and far less common to step DC power up and down. However, to power a pc in the car, it has to be done and the devices that do it for our car pc's are called DC-DC power supplies.

DC-DC power supplies cut out the 'middle-man' of the inverter. The compensate for the varying sags and spikes in the car - generally from around 10 volts up to 15 volts or higher, and provide the necessary power to the PC. They are generally much more efficient than inverters, quieter, and run cooler than inverters. However, they have one main drawback that can make them unsuitable for powering a PC in the car -they don't output as much power as an inverter can.

For systems requiring over 250 watts, DC-DC power supplies generally don't exist. So, powering a leftover desktop tower type unit with a DC-DC power supply may not be possible if, for example, it requires 400 watts of power. The inverter will be the only option.

When you select power supply, you should take into consideration the following things:

* DC-AC inverters (12->120 or 12->230) usually have very low productivity level (about 30%) and produce a lot of noise into car electric network. This means that CarPC will work on the battery for much less time with an inverter than with DC-DC; the electric noise could interfere with in-car LAN and block it.
* There are some devices that require DC-DC 12->5 inverters: CD drive, USB HUB, Camera. The inverter should have enough power for these devices. CD power should have 5V 2A, USB HUB - 5V 0.45A x Number_of_Ports (7-port HUB takes up to 3A)
* If your carputer uses DC-DC power supply it should be connected directly to accumulator (both "+" and "-") with 4-6 mm2 cable. If you follow this rule exactly, you could avoid possible noise in car sound, radio and display.

One of the greatest obstacles to operating a PC in the automotive environment is solving the issue of providing power to the system. Inadequate power supply to the CPU and peripherals of the system result in unstable operation including freezes, crashes, and loss of data.

There are several options for providing power to a PC in a vehicle, each with distinct advantages and disadvantages.

DC - AC Inverters

DC - DC Regulators
DC - DC ATX-Compatible Power Supplies
Start-up / Shutdown Control
Tank Circuit
Point of Load
Wiring
Powering outside of vehicle


There are many providers of power solutions.
CarNetix
KeyPower
Mini-box micro power supplies
Mini-box M-series
Mpegbox DSATX
Opus Solutions

Choosing a DC-DC Power Supply

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