Hack1.Understand Car Electrical Systems


Hack 1. Understand Car Electrical Systems

Different electronic systems, such as those for houses, cars, and computers, use very different connectors, voltages, and power levels.

If you are familiar with only home or computer electronics, you will need a little bit of background before you start playing with the wires in your car. This hack will introduce you to automobile power. Because you will probably want to "bench test" your in-car computers in the house before installing them in your car, it will also show you how you can hack a computer power supply to power your in-car accessories indoors.

The first term you need to learn about is voltage, which is simply the amount of potential (work, force, energy, change) in an electrical flow. Different devices are designed around a certain level of electrical potential, which can be thought of as the amount of pressure propelling the electricity. The analogy most often used to explain voltage is water flow, where the pressure of the water coming out of a hose is like the voltage, which can be turned up and down by a valve or spigot. The same hose can release high-pressure water to clean a driveway, or a low-pressure flow to fill a water balloon.

Electricity, however, doesn't flow readily through the air like water does. Electricity travels well only through conductors, such as metal wires, and in order to flow it requires a return path (usually called the ground) going back to the source of the voltage. In our water metaphor, the ground would be represented by a drainpipe. (Of course, water conveniently doesn't require a returning drainpipe to flow.)

Another, slightly more accurate analogy for electricity is that of water pouring over a cliff. The cliff represents the positive (or +) end of a battery, and the electricity flows from the high cliff down to the ground (negative, or). The higher the cliff, the higher the voltage. However, I'll continue to use the hose-pressure metaphor because of the corresponding analogy between wires and hoses.

1.2.1. Computer Voltages

Internally, computers run on several different voltages. The power supply, like the one seen in Figure 1-1, delivers all these voltages through the various colored wires that come out of it and connect to the motherboard. The black wires generally represent the ground, which allows the electricity to go back to the source and complete its round trip, or circuit.

The voltage going into the power supply comes from a wall outlet. This provides between 110V and 240V, depending on the country you are in. The computer power supply converts this high-voltage input into the various output voltages needed by the motherboard and computer peripheral devices. (To use a standard PC power supply in a car, you need to install an inverter to increase your car's 12V to 120V [Hack #11]).

The computer uses many different voltages for different chips and devices. Three voltages usually supplied by the power supply are 12V, 5V, and 3.3V. Many chips run on 5V; others run on 3.3V. Modern CPUs use even lower voltages, such as 2.2V or 1.8V, but the motherboard usually converts the higher voltages from the power supply to provide these lower voltages. The primary two voltages that come out of the power supply to power devices are 12V and 5V. 12V is good for powering motors such as those in hard drives, fans, and CD-ROM drives. 5V is good for powering the electronic circuit boards that these devices use. Figure 1-2 shows the four wires that peripheral connectors on standard computer power supplies have: 5V, 12V, and two ground wires.

Figure 1-1. An ATX power supply


1.2.2. Car Voltages

The electricity in your car comes from its battery. Though this large battery is referred to as a 12V battery, it's actually in the 13V range when fully charged and can dip down to 11V or lower if you leave a light on and discharge it. During engine cranking (starting the car) [Hack #45] voltage can dip down to 8V or lower, and if your PC power supply isn't designed to handle this it will hang or reboot your computer.

Computers are not very tolerant of voltage changes like this. For this reason, computer power supplies ensure that, no matter how the input voltage bounces around, the output remains stable. This is called voltage regulation. Automobiles, however, provide much sloppier, unregulated power. Automotive "12V" is more of an average than a precise voltage. Because of this, most devices built for cars have their own power supplies that also regulate the voltage.

Figure 1-2. Typical PC power supply wires


Traditionally, most third-party car electronic devices plug into the cigarette lighter socket (so named because its original purpose was to provide a method to light cigarettes). Even though most cars don't come with cigarette lighters anymore, the standard power outlet shown in Figure 1-3 remains and continues to be utilized.

In cars, the entire metal body of the vehicle is usually connected in some way to the ground. Older cars run a single 12V wire to the cigarette lighter outlet center, and the outer shell of the outlet is then simply connected to the metal dashboard. Newer vehicles are made of plastic and vinyl, with some paint-coated metal thrown in, so they run both the ground and the 12V wire to the outlet.


1.2.3. The Key to Car Power: Off, Accessory, On/Ignition, and Crank

You probably don't think about it when you drive a car, but the position of the key switch determines where the electricity of the vehicle goes.

There are actually two kinds of power outlets in modern vehicles: switched and unswitched. Switched outlets are on only when your car key is in the accessory or on positions. Naturally, when the car is on, any devices you have are on. When the key is in the accessory position, all of your accessories (such as the radio and any devices that are plugged in) are running. When the car is off, any switched power outlets are turned off, and usually the radio goes off too.

Figure 1-3. Two cigarette lighter power jacks


Unswitched outlets and hardwired accessories are always connected to the car's battery, whether the car is off or on. Car alarms are all wired this way, which is why they are designed to draw very little power. If you've ever left your lights on and come back to find that you can't start your car, you know why low power draw is important.

When you are cranking the engine to start the car, most accessories (including switched outlets) are turned off temporarily so that all the battery power can be devoted to starting the car. Fans, power windows and doors, radios, amplifiers, and most lights observe a "moment of silence" as the car starts. Because the battery must turn a heavy motor that cranks the entire engine, the voltage to all hardwired and unswitched devices usually sags during engine cranking, dropping from 12V to 9V or even lower. Thus, devices that must always remain on have to use very little power and endure wild variations in car voltage from time to time.

1.2.4. Discovering if Your Car Power Outlet Is Switched

Learning whether your power outlet (i.e., the cigarette lighter adapter) stays on when the car is off will help you determine whether you need to run additional power outlets for the devices you intend to use or install in your car.

To test your outlet, you need a device with a light and no battery, such as a cell phone charger with a light on it, or a small map light that plugs into a car power outlet. Plug in the device and observe it when you turn your car on and off. If the light or indicator is on only when the car is on, you have a switched outlet; if the device stays on no matter what, you have an unswitched outlet. It is generally a bad idea to leave devices plugged into an unswitched outlet.

1.2.5. The Hack: Making a 12V Power Supply for Indoors

If you're going to start hacking around with cars, you're going to want to bench test your in-car experiments and devices indoors. You may be working on your installation projects for several days, and you probably won't want to have your dashboard (or someone else's dashboard) torn open for that whole time. If you like to spend money, you can go buy a cool 12V power supply (such as Radio Shack part #22-504) with a nice car outlet on the front so you can do all of your testing out of the vehicle.

If you're a computer hacker, you probably have a few computer power supplies hanging around. Since these output 12V, it's simply a matter of making these power supplies operate with a switch to power a cigarette lighter jack.

If you have an old AT power supply (usually found in PC computers that run slower than about 200 MHz), you're setjust pull out that old Pentium 90 power supply, front-panel rocker switch and all. Figure 1-4 shows how you can splice, twist, or cleanly solder the car power socket to the black (ground) and yellow (12V) wires of the AT power supply that would normally go to a hard drive or CD-ROM drive. Make sure that the outer shell of the outlet connects to ground and that the bottom center of the outlet connects to 12V. Ignore the power supply's red (5V) wire; the car power socket doesn't use it. You can purchase a car power socket for a few bucks at Radio Shack (part #22-540).

Figure 1-4. A PC power supply with an attached cigarette lighter powerjack


If you have a modern ATX power supply, it is designed to be controlled by software ("soft power") that switches it off and on or puts it to sleep and wakes it up. To override this behavior, you simply need to connect the green wire on the ATX connector to any black wire, as shown in Figure 1-5. Then, the power supply automatically outputs 12V when it is plugged into the wall. (This trick works best with a power supply that has its own switch.) If the ATX power supply does not have a switch at all, you can simply connect some sort of switch between the green wire and ground, and then you can turn the 12V off and on without having to unplug the power supply.

ATX power supplies supply a maximum of around 20 amps for the 12V line. Keep this in mind when connecting devices; you'll damage or at least disable your power supply if you try to draw too much power.


Figure 1-5. The splice you need to disable "soft power" on an ATX power supply




    Car PC Hacks
    Car PC Hacks
    ISBN: 0596008716
    EAN: 2147483647
    Year: 2005
    Pages: 131

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