How to Choose the Right Power Supply for your CarC
When designing your CarComputer system (CarPC and CarMac systems) one of the most important elements to consider, as well as one of the least understood, is the power supply. We will discuss the various types of power sources and how to select the best one for your CarComputer system.
The Car Power System In order to properly design your CarComputer system you need to know a little about the cars 12 volt electrical system.
The typical passenger car electrical system provides 12 volts to power the various electrical elements of the car. However, the real definition of 12 volts is important to understand when designing a CarComputer system. The actual voltage available to power accessories within a car can have the following range:
During Engine Cranking As low as 6-5 volts if the battery is old, partially discharged, damaged, and/or very cold Typically 7 C 9 volts if the battery is in relatively good condition When the Engine Is Off (see battery chart) 12.7 volts if the battery is fully charged (100%) 11.9 volts if the battery is at 40% of its full charge 11.58 C 11.75 volts if the battery is nearly discharged (30-40% charged) 10.5 C 11.3 volts if the battery is fully discharged.
When the Engine Is Running 14+ voltage spikes (noisy alternator or other anomalies like electric door locks, electrical window operation, etc.) 13.8 volts (the typical output from the alternator)
As you can see the 12 volt electrical system is anything but that. It ranges from about 7 volts during engine cranking to well over 14 volts when electrical impulses are present. Your CarComputer power supply must be tolerant of these input variations and must continue to provide a well regulated output, even at the extremes. Otherwise your CarComputer will crash and perhaps sustain damage on one or more of its components.
How are Computers Powered In a Normal Enviroment?
It is also important to understand how a typical computer is powered under normal (indoor) conditions so that you can better judge how to power your computer in an automotive environment.
The primary input power is provided by the mains (110VAC or 220VAC). This alternating current (AC) is converted by the PSU (Power Supply Unit) to the voltages and currents required by the various components of the computer. For example, most motherboards require several different DC voltages to run them. Typical voltages include:
3.3 volts DC 5 volts DC 5 volts standby DC 12 volts DC
Notice that all of the voltages produced by the PSU are DC (direct current). Since the PSU converts the incoming AC (mains) voltage to the appropriate DC voltages, the typical PSU is an AC to DC converter.
The computer manufacturing community has established limits on both the voltage tolerances (ie regulation), and maximum currents for each rail (the 3.3V, 5V, 5VSB, and 12V PSU outputs are typically referred to as rails). The specification that governs these tolerances (as well as many other aspects of a PSU) is called the ATX12V Power Supply Design Guide.
The typical voltage tolerances defined by the ATX12V are as follows:
Notice that the +12V1DC and +12V2DC tolerances are +/-5%. This means that motherboard manufacturers design their motherboards to accept a maximum input on the +12V rail of 12 x 1.05 = 12.6V, and a minimum input on the +12V rail of 12 x .95 = 11.4V. Thus, a PSU that directly provides a +12V rail to the motherboard must fall with the voltage limits of 11.4 volts to 12.6 volts. This is one very important criterion for choosing a PSU for automotive use. We will explain this in more detail in the following sections.
