The following is a tutorial from member cproaudio:
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? (You'll listen to it more often than you think). 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 PSUs 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, max output amperage for 12V, 5V, 3.3V, -5V, -12V, -5vSB, min/max input voltage, efficiency, temperature range, operating humidity, dimension
input voltage should be as low as possible for minimum. This allows your PSU to supply a regulated power to your computer during engine start even if your battery is low and it barely cranks over. If the PSU has a high min 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 puts out 16-17 volts when the battery is fully charged (the heavy duty charger at my shop does). When your voltage regulator takes a dump, the voltage could go as high as 17 or 18 volts. If the maximum input voltage is 14v or 15v on the PSU you're gonna fry that thing more often than you think. 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 it should.
The PSU output amperage is depends on what components you have. You want a PSU with high amperage rating on the 12V line and 5V line if you're gonna power an LCD screen, several hard drives and CD drives and all the USB crap you got hidden underneath the carpet.
Too often people overlook PSU efficiency. efficiency can be defined as how well the unit can convert input power into output power. Power can not be created or destroyed. It can only be changed from one form to another. Take car amps for example. Class AB amps are 50-60% efficient. That means 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% efficient. A 1000 watt class D amp takes 1100 watts power from the batter and converts 1000 watts into music and 100 watts into heat. PSU is the same way. A low efficient PSU will take more power to do the job of a high efficient PSU. A light efficient PSU uses less power and saves battery power. This means you can listen to your MP3 longer. This is an advantage of a DC PSU even if it's a low efficient PSU. If you're going with an inverter and an AC PSU, think of this. A typical 120V ATX PSU is about 70% efficient and an inverter is about 90-95% efficient. Let's do our math backwards. a 250watt AC ATX PSU takes 357 watts of input power at 70% efficient. That means the inverter have to produce 357 watts to power the 250watt ATX PSU. The inverter, at 90% efficient would require nearly 400 watts to convert to 357watts of AC power to power a 250 watt AC ATX PSU. Most of the inverters have a minimum input voltage of 10V. That means every time you start your car, the PC will reboot. What? what? A tank circuit you say?? 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 PSU and an inverter, your whole PSU from the battery to the motherboard will yield a 60-65% efficiency.
Temperature is also important. If you live in the desert, you'd probably know that during summer time a car's interior could get as hot as 160 degrees Fahrenheit or higher. Don't believe that "yeah but it's dry heat" crap, 160 is freaking hot. Hell, 110 is already freaking hot during the summer here where I live. What does this mean for a PSU? Well, The glue that the manufacturers use might not hold well at such extreme heat. If the glue holding the caps or transformers gives, the caps, transformers and other crap in the PSU are held together at the solder joints. Put that in a car going 80MPH on a freeway that's designed for 55MPH (they changed the law but they didn’t change the road) and its gonna vibrate loose sooner or later. You're gonna end up with a phucdup PSU with bad solder joints. Electronic components in a PSU such as caps, transformers, coils might not be rated to tolerate the temperatures inside a car with windows up during summer time as well.
Humidity is probably not as big of a deal unless you live at a place with high humidity over 90% all year round. AC will fix that if the PC is getting that AC action.
Another spec to look for is operating G. This basically tells you how much G force will the PSU take before it blows up or something. In a car install is very important. This probably has something to do solder joints and what G force the PSU will take before the transformer falls off the circuit board.
Dimension is kind of important too if you don't have the room for a brick. There are tiny DC-DC ATX PSUs out there. Jeff has one that’s pretty small. If you have the room, by all means buy the biggest PSU size wise you can find.
A shut down controller is only as good as how your programs react when you hit that soft off power button. Some programs such as OBD-II data logger, Street Atlas 2003 Plus will not shut down after you 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 would just freeze up (Winamp2.83 with Ao plugin) when you hit the power button. In this case, a shut down controller won't do squat for you. You still have to stop the program manually and properly or else the computer is just going to sit there and wait for the 60 seconds or 2 minutes for the shutdown controller to cut power to the PSU and cause the computer to scan disk every time you boot. Edit: 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 put into hibernate or suspend.
Cost is an issue too for most people. Some people (me) 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. Getting the right parts may be a problem. Rat shack is not like it used to be. The cheapest way is order Jeff's shut down controller, buy an inverter and rip a power supply from a retired computer.
With all that said, I think you should go back and re-look at all the specs of all the PSU and then decide on which PSU is the best for you. I have TONS of USB crap for my system and my Opus seems to be working just fine. It's been 4 months since I had it installed. The extra 20 bucks you spend on Opus will save you ALOT of headache later on. (unless you get a defective one. Even at, Kris will still take care of you)
If I said anything wrong please correct me