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How to Pick a Power Supply

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  • How to Pick a Power Supply

    Picking a Power Supply for your CarPC

    Before you start, you need to answer a few questions first.

    • What's the total power required for your system?
      This includes motherboard, fans, CPU, RAM, HDD, floppy, CD-ROM, DVD-ROM, DVD-RW, CD-RW, ALL serial and USB devices such as GPS receiver, IR receiver (IRman), USB hubs, serial hubs, thumb drives, memory card readers, LCD monitor, VGA monitor, character LCD screen, ANYTHING and EVERYTHING that is connected to the motherboard and the PSU.
    • How often do you listen to your system while the engine is not running?
    • What programs will you be using?
    • How much can you spend?
    • What electronic skills do you have (can you solder, read diagrams)?
    • How much room do you have for the PSU?
    • Where do you live, How's the weather?


    Once you've answered all the questions then you can shop.

    Find ALL the power supplies that you could possibly order either online or mail order. This is where you have to do A LOT of search on the forum to see what are all the possible PSU candidates. You should draw a chart for comparing specs. You should try to find the following specs from ALL the PSU candidates:

    • Maximum output wattage
    • Maximum output amperage for 12V, 5V, 3.3V, -5V, -12V, -5vSB
    • Minimum/maximum input voltage
    • Efficiency
    • Temperature range
    • Dimensions

    Maximum Output Wattage

    Things to remember about TOTAL wattage:

    It is important to remember that while all power supplies list Total Wattage, that figure is a combination of all the wattages available on the various voltage rails (rails=wires) of the power supply. You CAN NOT exceed the individual rails wattage maximum. Here are specifications for various types of processors and their power draw.

    So, while your system may not exceed 150 watts, you can still draw too much power from an individual rail. In many cases, this situation happens because of a power hungry video card. Use this this power supply calculator to figure the wattage drawn on EACH RAIL. If your system draws too much power on one of the rails, it will not work. Sometimes it will not turn on, other times it will start up and then shut down quickly. Occasionally, it will start and then shut down during boot or shortly thereafter. This can occur even if your TOTAL system wattage is less than that of the power supply's rating.

    Output Amperage

    The power supply output amperage depends on what components you have. You will want a power supply with a high amperage rating on the 12V line and 5V line if you will be powering a LCD screen, a hard drive or two, a CD drive, and all of the USB components that are so common.

    Input and Output Voltages

    The input voltage should be as low as possible on the minimum side. This allows your PSU to supply a regulated power to your computer during engine start ("cranking") even if your battery is low and it barely cranks over. If the PSU has a high minimum input voltage like 9 or 10 volts, your carpc will reboot every time you start your car. Maximum voltage input should be as high as possible. When the engine is running, the alternator puts out 14.5v-15v at highway speed. When you're charging the battery using a plug in the wall charger, the charger can put out 16-17 volts when the battery is fully charged. If the maximum input voltage is 14v or 15v on the PSU you run the risk of frying the PSU. Today's car electronics are made to operate at as low as 7 volts to as high as 20 volts. That's why no matter what happens to the alternator or the battery, your car's computers and electronics will operate like they should.


    Too often people overlook power supply efficiency. Efficiency can be defined as how well the unit can convert input power into output power. Power can not be created or destroyed. This is a basic law of physics. It can only be changed from one form to another.

    We will use car audio amplifiers for this example.

    Class AB amps are 50-60% efficient. That means a 1000watt amp actually takes 2000 watts of power from the battery and converts 1000 watts into music and 1000 watts into heat. Class D on the other hand is more efficient at 85-95%. A 1000 watt class D amp takes 1100 watts power from the battery and converts 1000 watts into music and 100 watts into heat. A power supple acts the same. A power supple that is not very efficient will take more power to do the job than a power supply that is more efficient. A more efficient power supple uses less power and saves battery power. This means you can use your car pc longer with the car off. This is an advantage of a DC-DC power supple even if it's a less efficient power supply.
    Using an inverter and AC power supply
    A typical 120V ATX power supply is about 70% efficient and an inverter is about 90-95% efficient. Let's do our math backwards. A 250watt AC ATX power supple takes 357 watts of input power if it is rated at 70% efficiency. That means the inverter has to produce 357 watts to power the 250watt ATX power supply. The inverter, at 90% efficient would require nearly 400 watts to convert to 357watts of AC power to power a 250 watt AC ATX power supply. Most of the inverters have a minimum input voltage of 10V. That means every time you start your car, the PC will reboot.
    Tank Circuits
    In order for a tank circuit to work, you would need a 500 watt diode or a 40 amp diode. Diodes have a .7 volt drop when you run power from anode to cathode. If you go with an AC power supply and an inverter, your whole power supply from the battery to the motherboard will yield a 60-65% efficiency.
    Temperature Ranges

    If you live in the desert, you'd probably know that during summer time a car's interior could get as hot as 160 F or higher. What does this mean for a power supply?

    Electronic components in a power supply such as caps, transformers and, coils might not be rated to tolerate the temperatures inside a car with windows up during summer time.


    The dimension of the power supply is important to consider if you don't have a whole lot of room to work with. If you have the room, by all means buy the biggest power supply you can find.

    Shutdown Controller

    A shut down controller is only as good as how your programs react when you hit the soft-off power button. Some programs such as OBD-II data logger and Street Atlas 2003 Plus will not shut down after you have pushed the power button. It always ask if you want to save the current map or if you want to exit and save the data that's being logged. Some programs will freeze up (Winamp2.83 with Ao plugin) when you hit the power button. In this case, a shut down controller might not be the answer for you. You still have to stop the program manually or the computer will wait for the 60 seconds or 2 minutes for the shutdown controller to cut power to the power supply and cause the computer to scan disk every time you boot.

    What about hibernate and suspend to RAM?
    It all depends on how your hardware reacts when it's being waken up. Some hardware drivers will not work after being put into hibernate or suspend mode.

    Cost is an issue for most people. Some people would just spend the money and buy the Opus with warranty and call it a day since it's made for car use. Some people would just build one from a diagram. It's not that hard if you can read the diagram and know how to solder.

    With all that said, you should go back and look at all the specs of all the power supplies and then decide on which power supply is the best for you.

    Common Misconceptions About Electricity


    Will a high wattage power supply damage a low wattage electrical device? Your electrical devices, whatever they are, will only draw as much wattage/amperage as they need. That means a 10 watt device is easily powered by a 100 watt power supply. If the wattage rating of the power supply is too low, the device won't work. However, if the wattage rating of the power supply is higher, the device will only draw what it needs.

    THIS IS DIFFERENT THAN WATTAGE. Do not power a device that requires 5 volts with 12 volts. IT WILL DEFINITELY FRY IT. This is the reason that voltage regulators are recommended on all senstive electronic devices such as motherboards and LCD screens. The regulator supplies a stable voltage to these devices and prevents higher voltages and surges from frying your expensive goodies. The rule of thumb with voltage is: For the most part, voltages must be equal. That means a 12 volt device should get 12 volts. A 5 volt device should get 5 volts, and so forth.
    Undersupplying a device (e.g. supplying 5 volts to a 12 volt device) will generally NOT harm the device, although it is unlikely to work properly, if at all.

    Article written by cproaudio, WiredWRX, BugByte