FAQ: Power Supplies Explained (Part 1)
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.
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 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:
FAQ: Power Supplies Explained (Part 2)
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 didnít 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 thatís 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
Powering your Car PC when it's not in the car
An often asked power-related question is "How do I power my car PC when it's not in the car?" For example, if you are building the system prior to installation, or have removed it from the car for maintenance.
If your system uses an inverter
If you are using an inverter, there's generally no problem. You simply unplug the power supply you are using in the car, bring it indoors with you and plug it into the wall socket.
If you are using a DC-DC power supply
If your system uses a DC-DC power supply, you have to deal with the fact that your computer's power supply is expecting 12 volts as input. You should know better than to plug it into the 110 (US) or 220 (Europe) wall socket in your house, but just in case DON'T PLUG YOUR DC-DC POWER SUPPLY INTO THE WALL SOCKET.
You have a couple of options (thanks to members KByrd and evandude for this info):
Option 1. Unhook the DC-DC power supply from the carpc and use a normal ATX power supply (110V (US) 220V (Europe) in hooked to the wall)
with ATX connection to the carpc motherboard. Manually momentarily short the power-switch pins on the motherboard with a paper clip or something to start the computer, if necessary.
Option 2. Leave the DC-DC power supply connected and provide 12V DC power to it by an AC/DC converter. http://www.erusa.com/images/products/PS104.jpg Make sure the AC-DC converter provided enough amperage to power your system. A 10 amp system can supply 120 watts (12V X 10A=120W), a 15 amp system can supply 180 watts. In truth, these power supplies often are rated at 13.8 volts, giving 138 watts and 207 watts, respectively.
These AC-DC converters come in various shapes and sizes and run about $100 at Radio Shack.
In terms of affordability, option 1 is probably a bit more affordable than option 2.