Many newbies don't realize it but powering the car PC setup is possibly the most critical step in building a stable and working system. If the power system does not work properly, supplying both a stable voltage and clean supply, free of voltage spikes, the result will be a system that freezes randomly, shuts down unexpectedly, or worse, damages your PC components or display.
These days, most car PC hobbyists purchase the components from a professional source rather than build their own. While building your own power supply can be done, it takes considerable time and effort and the savings is negligible over the cost of a new power supply.
First, it is important to explain that there are several components that come to mind when discussing "power systems". However, not all of these components are power related but are often confused as part of the power system. Here is a set of requirements for a PC power system:
Battery --> Regulator --> startup/shutdown controller --> Power supply --> Computer
NOTE: Many "Power supplies" incorporate regulation, startup/shutdown, and power supply all into a single box. See the table below for more information, of if you are unsure, post a question asking about the product you intend to buy. It will help improve this FAQ and may clear up misunderstandings by others.
Note that the regulator is not, per se, a power supply component. It's job is to REGULATE the power that comes from the battery and alternator to ensure that voltage spikes that could damage the computer don't get through. It also may supply additional power if the voltage momentarily drops below 12 volts, as when starting the car (known as "cranking").
Rule of thumb: If you need over 200 watts of power, you are looking at using an inverter. If you need less, you may be able to use a DC-DC converter.
Words of wisdom: You need BOTH power regulation AND a power supply if you want your system to work properly and reliably.
DC-DC Converters
A DC-DC converter does exactly what it says. It takes in DC power of a certain voltage (usually 12 volts) and outputs not just 12 volts but other voltages, commonly 5 volts and 3 volts. Your computer needs several voltages to operate, not just 12 volts. The DC-DC converter supplies each of the various voltages needed to run different parts of the computer including disk drives, USB ports, etc.
A DC-DC converter is
more efficient than an inverter, discussed below. However, there are several types of converters and an explanation of the differences is beyond the scope of this discussion. Go to the wikipedia for more information on regulators.
What makes choosing a DC-DC converter so confusing is that they often combine different functions in the chain shown above, into a single product. This makes comparing features difficult. The table attached to this post lists popular DC-DC converters and their functions:
Many commercial units have other features such as low battery monitoring to shut your PC down if the car is off and the PC is turned on or in standby mode. This prevents the PC from draining the battery.
Other devices such as digital startup/shutdown controllers (DSSC’s) or power controllers (CarPC EZ) are not PSU’s but control the sequencing and control of power, such as preventing “thump” when the amplifiers in the car are on but the PC hasn’t yet powered up. These are not strictly DC-DC converters but they work with the components in your system to control the power flow.
User feedback (qualitative) indicate that the Opus is the “Cadillac” of converters, followed by the M1-ATX. The Carnetix is used in conjunction with an existing power supply, such as one that comes in a Casetronix case or a PW-70 type supply. The ITPS is inexpensive but widely regarded as worthless.
Here is a link to how to wire an Opus to your car. The other devices are wired in a similar fashion.
All regulators cause the voltage input to drop as it feeds through the regulator. The ITPS is particularly egregious in this regard as it drops voltage by approximately 1.5 volts. When the car is operating, the alternator generally puts out 13.5 volts. This drop then supplies 12 volts to the system. However, when the car is off, the voltage is only 12 volts from the battery and the ITPS only supplies 10.5 volts to the system. This is not sufficient and can cause random freezing and cause failure of data writes to the hard drive.
Calculating how much power your system draws
Keep in mind that the maximum wattage DC-DC power supplies can muster at this writing is around 150 watts with one or two (Maestro, DSATX) emerging in the 200 watt range. This is generally insufficient for Pentium 4 systems. Here are several:
1.
Carpc's calculator
2.
JS custom PC's power calculator
3.
Oh my god my arm fell off's power calculator
4.
Erol's power calculator
Here are electrical specifications for a large variety of processors.
**"But wait!" you say. Here's a PW200 that claims 200 Watts of power!. WRONG. They are making claims that are at best misstatements, and at worst, outright lies. The PW200 gets to 200 watts by simply routing 161 watts of UNREGULATED power straight to the 12 volt rail!
here is a an explanation of what is going on. Short answer: this regulator only puts out 60 watts of regulated power. So beware.***
Inverters
Inverters are a cheap and easy way to power your computer. An inverter takes DC power from your car and converts it to 120 volt AC power, just like in your house. To use the inverter, simply plug in your home power supply to the inverter. The power supply converts your AC power BACK into DC power that your computer can use. Naturally, converting DC power to AC just to convert it back to DC is inefficient and the inefficiency manifests itself as heat. Many also report that inverters introduce noise in your audio lines, distorting the sound. In addition, the manner in which inverters convert DC power to AC power can be unacceptable to certain computers and they will not work with an inverter. In addition, automatic startup and shutdown of the PC by an inverter requires additional hardware such as a DSSC (see above).
However, inverters are easy to install, inexpensive and many will survive crank, making them popular among many members. It doesn’t really matter how one powers the computer, though. What works for one may not work for another.
**** Here's the answer to one of the most FREQUENTLY asked questions about inverters: Do they draw power when the car is off? The answer is yes. Maybe. Read about it
here. *********
Links to some articles explaining power supplies:
HardwareDecrets.com
ExtremeOverclocking.com