Power group
Figure 4 expands the view of the power group. Collectively, the power group exists to supply and manage power to the devices in your system, which turns out to have three elements:
FIGURE 4: Power group
Provide 12 VDC power — The cleanest approach to powering on-board electronics is to drive them from the 12 VDC vehicle source directly. That requires different power supplies than usually found in consumer electronics, but eliminates the inefficiency and cost of a double conversion from 12 VDC to 120 VAC and then back down to the necessary DC voltages inside the electronic assemblies.
Provide 120 VAC power — If you can’t adapt to 12 VDC power, you’ll need to source 120 VAC (or 240 VAC, depending on where you are and the characteristics of the units). Power conversion units, called inverters, are commonly available from 100 to 400 watts.
Supervise PC operation — DOS didn’t care much, because it didn’t often cache important data, but you shouldn’t just cut power randomly using operating systems such as Linux and Windows. You’ll need to wire the server directly to the battery — a relatively permanent source of power — and use the on/off status from the switched power to control PC operation.
The 12 VDC supply in a car isn’t fixed at 12 volts. Instead, you’ll see a range from 10 (battery essentially discharged, and possibly during engine crank) to 15 volts (past full charge; typically the maximum input for radios), and any 12 VDC power supply you connect should be capable of accepting that full range. You may find the supply you have can’t hold up its outputs during engine crank; if so, you can use a diode and auxiliary battery (the diode prevents current from flowing back into the car from the auxiliary battery) between the car power source and your power supply.
The current you’ll draw from the 12-volt source depends on the efficiency of your power supply or inverter. Assuming it’s 85 percent efficient, a 400-watt total load will draw 400/12/0.85 = 39.2 (or about 40) amps. You must use wire and automotive spade connectors rated for the current draw, with some surge reserve, and must fuse the circuits appropriately.
The spade connectors are important because, although they’re easy to connect and disconnect, they’re vibration resistant and won’t drop off at an unfortunate time. It’s common to use red for unswitched +12 volts and black for ground. You could use other colors for switched +12 volt lines.When you’re running grounds that collect multiple circuits, be sure to add the total draw for each circuit to calculate the current for the ground wire.
A power inverter drives any electronics that can’t operate directly from 12 VDC. There are directions on how to make your own on the Internet.
Wiring into the electrical system isn’t a matter of picking a random wire and splicing. You should run the primary power feed directly back to the fuse box, tapping into a spare fuse of adequate capacity. If you don’t have any, consider adding a new auxiliary fuse panel. The circuit from the fuse will always be powered so long as the battery is connected; use a power supervisor such as the one shown in Figure 5 to manage when equipment is on or off.
The power supervisor draws little to no power from the ignition switch connection; it’s just using it to determine whether the engine is on or not. Be sure to examine the fuse block carefully — the electrical shop manual for your car would be ideal — because some cars have connection lugs for both battery and ignition power ready for your use.
The power supervisor will sequence the PC off whenever the ignition power signal goes off by simulating pushing the power button on a PC case through the ACPI power switch connector. You need to set up the Windows ACPI power management configuration (or its Linux equivalent) to shut down the PC when the power button is pushed, but that’s all that’s necessary.
FIGURE 5: Power supervisor block diagram
If you’re using a 12-volt supply in the PC case, and powering everything outside the PC via USB, all the power supervision you need is likely built into the power supply itself. For example, the Opus case’s DC-DC Power supply includes a supervisor to monitor automobile battery voltage to protect against deep discharge and control the ACPI signals. The supervisor can’t itself know how long the PC takes to shut down after the switch signal, however, so it simply uses a timer and, after the timer expires, shuts down power to the PC leaving only the required ACPI standby power lines active. You can program the power supply to shut down the PC after a delay of up to one hour. The input power is filtered for transients and other problems so the PC does not reboot during engine start or cranking.
If you’re using an inverter to produce 120 VAC power for use with a conventional PC power supply, you’ll need a separate power controller. The Micro Shutdown Controller (Figure 6) sold by MP3Car works for this purpose, and includes not only a header connector for the computer connections, but also a fuse and spade connectors for the automotive wiring.
FIGURE 6: Micro Shutdown Controller
Something else to consider when wiring your system, mundane as it may sound, is grounding. Not only do you need to use big enough wire, good connectors, and tie to a robust connection with the frame (or run the ground wire directly back to the battery cables), you also need to consider the relationship between grounds for all the interconnected equipment in your system. The problem you’re trying to avoid is that, depending on the relative resistance and current flow between each component and the common ground reference (say, at the battery ground terminal), the voltage drop to the ground point from the components can vary. The difference in ground reference voltage can cause noise when you connect two components — especially audio — at different ground potential together and form what’s called a ground loop. The easiest solution is to make sure all ground connections tie together at a single point, rather than to random points on the chassis, but the best solution is to isolate equipment with fiber optics. That’s often easy to do for audio, because the S/PDIF (Sony/Philips Digital Interface) specification has been widely adopted and may be present on both your PC and your car stereo head unit.
Laptops have their own batteries, but most don’t have the endurance for more than a relatively short trip — especially if you’re keeping the display on all the time. If you’re using a laptop, you’ll want to get a power converter. You can get them from iGo and many other sources. You can then configure your laptop so the display stays on at all times when working from external power and you’ll never be interrupted by a screen saver, standby, or hibernate.