Car PC Hacks

Hack 50. Power Your Portable Devices in the Car

Almost any small consumer device can be run on car power, and if you can't find the right adapter you can usually make one.

There are many portable devices that can bring music to your car, but unless your device is very popular there may not be a specific car adapter for it. Nonetheless, by figuring out a few vital statistics about your device, you can adapt it to work in the car.

The main data you need to get a device working is:

Voltage

You need to know the voltage of the device, and whether it runs on AC or DC. The ideal candidates for running on car voltage are DC and between 1.5V and 12V.

Amperage

You need to know how much current your device wants. More amps, in general, require thicker wires and a bigger adapter. The easiest devices to power are under 1 or 2 amps.

Plug size

The common barrel plug is a cylindrical plug with an inner and an outer metal contact separated by insulating plastic. There are many sizes of these, and you need your power adapter to fit correctly in this plug.

Polarity

Polarity is simply which of the contacts in your barrel plug is positive (+) and which is negative (-). These are usually indicated by a small diagram.

Most consumer electronics devices, especially those that run on batteries, are designed to run on DC voltage in the 1.5V to 9V range. This is fortunate, because it is very easy to "step down" a car's 12V to one of these voltages. Figure 4-17 shows one such adapter from Radio Shack with a switch to select 9V, 6V, 4.5V, or 3V, depending on your device.

The second thing to determine is whether your device wants too much current. Any power adapter you purchase should specify a maximum amperage at which it will run, such as 1000 mA (1A). Any device you wish to power should have a corresponding amperage on its label, such as 800 mA. As long as the device's amperage is less than the maximum for the adapter, you can safely power the device.

Getting plug sizes right is straightforward enough. The 12V, 1A cigarette lighter adapter shown in Figure 4-17 comes with four common plugs on the end. If none of these is the one you need, however, all is not lost. You may need to buy a different plug end, or snip the plug end off the wall adapter that came with the unit and splice it onto the 12V adapter to make it work in your car (you can always splice the original wall adapter back together [Hack #4] if you keep it). If you don't want to cut your wall adapter, you should be able to find the right adapter; Radio Shack has a full collection of common adapters and wires to hook them up.

As long as you have a proper barrel plug that fits your device, the only thing you have left to worry about is respecting the polarity of the wires (positive to positive and negative to negative).

There are a few rules of thumb that can help you figure out the polarity of your power supply wires, to make sure that plus goes to plus and minus goes to minus. On AC-to-DC wall adapters, the negative wire is usually black, and the positive wire is often black with a white stripe. Sometimes there is a little diagram on the device and/or the power supply, with a letter C around a dot. The outer C circle indicates the polarity (+ or -) of the outside of the barrel plug, and the inside dot signifies the polarity of the inside of the plug. The key is to make sure that the + and going into the device are what the device expects.

The most effective way to make sure that you get the polarity right is to purchase and use a multimeter. Set the multimeter to the 20V range, and put one lead on the middle plug and one on the outside of the plug to the wall adapter that came with your device. It should indicate the voltage coming out of the adapter, with a plus or minus sign. Then, when you get your new power adapter wired up, do the exact same test on your new in-car power adapter. The voltage should be around the same (e.g., 5V), and the + or - polarity should be the same as it was for the wall adapter. If you do this, you'll know you've wired it up right.

4.11.1. Powering Hard Drives and Optical Drives

USB is notoriously bad for powering devices such as portable hard drives and DVD drives. The 500-m A limit on many USB ports and hubs is less than the 6001000 m A needed to power a device with a spinning disk. FireWire is much more reliable with regard to bus power, and since most of the car PC motherboards of choice come with FireWire ports built in, it is a good choice for powering both CD drives [Hack #38] and portable hard drives, assuming you can use a short enough cable (less than 15'). FireWire may fail to power your devices if you use a longer (1525') cable.

If you have a dual FireWire/USB device and you don't want to rewire a power supply for it, there is a simpler approach: connect the device to the computer via USB, then power it with 12V on the FireWire port. You can use a device such as the Griffin Powerpod (http://www.griffintechnology.com/products/powerpod) to supply the 12V the FireWire drive needs, even if it isn't an iPod, and then hook the drive up to the USB port connected to the computer (Figure 4-18). Only the two power pins of the six-pin FireWire cable will be used; the data will go over the USB. Note that you should not connect the device to the computer through both the FireWire and USB ports, as the computer will be confused about which method it should use to connect to the drive.

4.11.2. See Also

• "Power Your Car PC" [Hack #42]

• "Install USB Ports in Your Car" [Hack #51]

• "Transfer Data to and from Your Car PC" [Hack #64]