From MP3Car.com Wiki
One of the greatest obstacles to operating a PC in the automotive environment is solving the issue of providing power to the system. Inadequate power supply to the CPU and peripherals of the system result in unstable operation including freezes, crashes, and loss of data.
Although most vehicle electrical systems operate on 12 volts DC (direct current), and many CPUs require 12 volts DC, simply connecting the PC to a 12 volt power source will not work. First, computer systems require stable voltages for reliable operation and automobiles do not output stable voltages. Typical ranges for automotive voltages range from 9 to 14.5 volts DC. Second, most computers are intended to be plugged into a wall outlet in a home or office and their power supplies are designed to use 110 volt AC (Alternating Current) U.S. or 220 volt AC in Europe AC power. Use of a laptop system rather than a desktop system can simplify the issue of powering a PC in the car but even laptops will require inverters or cigarette lighter power adaptors to charge their internal batteries.
Fortunately, there are several options for providing power to a PC in a vehicle, each with distinct advantages and disadvantages.
[edit] DC - AC Inverters
One method is the use of an inverter, which uses automotive 12V DC to create a 120V/220V AC and emulate an in-home plug. Inverters are consider to be inefficient. Thus, a variety of DC-to-DC voltage converters have been created so you do not have to use an inverter.
One significant plus to inverters is that they are inexpensive and readily available. Inverters can be purchased at nearly any computer or electronics store, as well as hardware stores, auto supply stores and even large discount stores like Wal-Mart.
The major downside to using an inverter is inefficiency. Consider that the vehicle supplies DC power to the inverter. The inverter converts that to AC power. The computer power supply converts that back to DC to power the computer. The amount of energy wasted doing a double conversion like this varies from unit to unit.
Another drawback to inverter usage is heat. The process of converting DC power to AC power generates heat. This presents a potential safety hazard. Too much heat could result in a fire. Most inverters have a heatsink as part of the casing, but still get quite warm.
The design of some inverters can cause electrial noise due to the way that the 12V DC is stepped up to 120/220V AC. This can give you what is termed "noisy" power. You then feed this "noisy" power to your PSU which inturn supplies power to your computer and it's audio card. It is very difficult to remove this noise from your audio signal. This problem is more noticable in cheaper inverters. Usually the more expensive the inverter, the better they are designed to combat this noise.
Inverters are also more prone to EMI, or electromagnetic interference. EMI can induce 'noise' into an audio signal, resulting in a static humming, which nobody wants to hear for hours on end during a cross-country road trip.
[edit] DC - DC Regulators
DC regulators take your vehicle's variable voltage of 11V - 16V and provide a constant output volt, typically 12V or 5V. It ensures that voltage spikes and dropouts are handled, smoothing out the power to provide a steady voltage. Many power supplies include built-in regulators, but devices that do not regulate voltages will require a regulator.
Wait, I thought cars used 12V. Why does a car PC require power regulation? Automobiles use alternators to generate the power for the electrical items in the car. An alternator's output varies depending on the engine speed (often as high as 14 volts or more. In addition, battery voltages vary as well, usually from 13.5 volts at the high end to as low as 9 volts when cranking the engine. Computer electronics require stable voltages and are sensitive to spikes and dips in the voltage supplied to them. Typical symptoms of low voltage to a computer is instantaneous and unexpected shutdowns, freezes, and generally unreliable operations. Voltages that are too high can cause electronic components to fail, sometimes with spectacular and pyrotechnic results.
[edit] CarNetix
Micro power supplies such as the PW200 do not include power regulation as part of their design. The CarNetix corporation, realizing the shortcomings of the widely distributed micro power supplies, developed a regulator product to work in conjunction with them. The CarNetix regulators feed clean, regulated power to these micro power supplies.
CarNetix has gone to great lengths to write an excellent document about how to choose the correct power supply for your vehicle PC project. [1]
P1900
Designed for mini-PCs like some small form factor HP units, Mac Mini, Cappuccinos, Laptops / Notebooks, and any other computers that utilize a 'transformer brick' that supply 18V - 20V. This regulator includes a start-up/shutdown controller, acceptance of a remote turn-on.
The primary output is jumper selectable for 18V, 19V, or 20V operation and can provide up to 6.32 amps (125 Watts @ 20V ). The secondary output is jumper selectable to provide either +12V to power your LCD screen or +5V to power USB devices with up to 3 amps (15 Watts). The P1900 can accept battery input as low as 7 volts under full load (140 watts) during cranking while providing a well-regulated output.
P1290
The CNX-P1290 provides 6.3A of 12v current, for a maximum output of 75.6w. It also provides 3A of 5v current (15w) for those devices that require it.
P1260
The CNX-P1260 provides 5A of 12v current, for a maximum output of 60w.
[edit] DC - DC ATX-Compatible Power Supplies
The ATX-compatible DC - DC power supplies convert your vehicle's power (11 - 16V, typically) to the necessary volts required to run your motherboard. There are many options for DC-DC Power, listed below. Note that there are some models of PSU that require regulated voltage to operate properly.
[edit] Mpegbox DSATX
The DSATX manufactured by Mpegbox has the most features of any of the DC-DC power supplies on the market, however it costs a lot due to manufacturing in the US and low volume. It is also the only supply on the market with a serial interface allowing users to tweak settings like temperature cutout, low voltage, and delay times.
Mpegbox's line of vehicle PC power products are designed by zootjeff, a valuable member of mp3car.com.
DSATX
12v rail = 12A (144w)
3.3v rail = 10A (33w)
5v rail = 12A (60w)
[edit] Mini-box micro power supplies
PicoPSU-120 & PW-200M
These units are designed to power a miniITX motherboard, such as the VIA EPIA-series motherboards. They are small (especially the PicoPSU), which gives them a decided advantage for vehicle installations, where space is usually at a premium.
However, these units do not regulate the 12v line and will require a regulator for safe operation.
Since this PSU is unregulated the vehicle's voltages are feed directly to any 12V devices that are connected to the power supply.
There are some hobbyists that use these models without regulation. This can only be accomplished with systems based on Via's C3 CPU, as it draws power from the 5V rail, while other CPUs draw power from the 12V rail. Any components that require 12V connected to a system powered in this way are likely to have problems up to and including failure of the component.
PicoPSU-60-WI
This newer version of the PicoPSU does not require regulated 12v input, making it a better choice for a vehicle installation. However, it is designed to power a motherboard with no other components such as hard drive or optical drive and only a minimal number of USB devices. It has a very low power output, making it a suitable choice for the EPIA line of motherboards with the C3 CPU.
12v = 0.4A (4.8w)
3.3v = 6A (19.8w)
5v = 6A (30w)
[edit] Mini-box M-series
The M1-ATX and M2-ATX manufactured by Mini-box are the workhorses of car computing - they are small, having the same form factor as the Morex supplies common in Mini-ITX cases. They allow the PC to stay on during engine cranking and they also watch battery voltage and shut off it goes too low. They are a bit underpowered - the largest wattage is 160W and they are susceptible to damage if incorrectly hooked up. Both version include a start-up/shutdown controller.
M1-ATX
12v = 2A (24w)
3.3v = 10A (33w)
5v = 10A (50w)
M2-ATX
12v rail = 8A (96w)
3.3v rail = 8A (26.4w)
5v rail = 8A (40w)
[edit] Opus Solutions
The Opus Solutions power supplies are the oldest and defacto "standard" supplies for car computing. The latest units go up to 250W. They are a bit large and awkwardly sized unless they are used in the Opus industrial cases.
Opus Solutions 90w
12v rail = 2.5A (30w)
3.3v rail = 6A (19.8w)
5v rail = 6A (30w)
(Note that the Opus 90w PSU is no longer in production. It has been replaced by the 120w unit. The information on the 90w is included here because there are plenty of them around and can be purchased / used.)
Opus Solutions 120w
12v rail = 3.5A (42w)
3.3v rail = 8A (26.4w)
5v rail = 8A (40w)
Opus Solutions 150w
12v rail = 5A (60w)
3.3v rail = 10A (33w)
5v rail = 10A (50w)
Opus Solutions 250w
12v rail = 10A (120w)
3.3v rail = 15A (49.5w)
5v rail = 15A (75w)
[edit] KeyPower
KeyPower makes ATX-compatible DC-DC PSUs that are primarily for industrial use. They lack many of the features of the other products listed (start-up/shutdown controller, battery protection, etc.), and are not designed to be as efficient as comparable products. Until recently, they were the only manufacturer to offer a 250w DC-DC PSU. Their current product line includes DC-DC PSUs rated as high as 350w, but are not available at their online store as of this writing. The KeyPower units are the same size as a standard ATX power supply (150mm x 86mm x 142mm / 5-7/8" x 3-3/8" x 5-1/2").
Keypower KP-DX250H
12v rail = 10A (120w)
3.3v rail = 14A (46.2w)
5v rail = 25A (125w)
[edit] Choosing a DC-DC Power Supply
The main factor in determining power needs is your CPU. Some CPUs draw lots of power, namely P4 and Athlon64 units. The CPU is the biggest variable in systems, as far as power consumption.
Look up your CPU on Chris Hare's PROCESSOR ELECTRICAL SPECIFICATIONS page. He lists the electrical specs of just about every CPU produced in the past ten years or so. Look at the Max Power Dissipation column. That's the peak power usage of the CPU under heavy load. Using this figure in your calculations will allow power overhead for the CPU to perform any tasks you throw at it without choking off your power supply.
Now that you know the max power draw of your CPU, you need to know how much power the PSUs provide on the 12v rail. The 12v rail is the one that will provide power to the CPU. It's stepped down to 1-2v (typically) with more amperage so the CPU doesn't burn up. There's circuitry on the motherboard for this.
Typically, the PSU will tell you how many amps it will output on the individual rails. To calculate wattage output, you multiply amps by volts (A x v = w). If the wattage output of the PSU on the 12v rail is less than the max dissipation for your CPU, it won't work.
The other thing you need to consider is other 12v devices. There aren't that many. 3.5" HDDs, full-sized optical drives, case fans, external audio like a Creative SB Audigy NX, possibly an LCD screen, etc. There are other power calculators that will give you power usage estimates for these devices. These need to be added to the total wattage load on the 12v rail to determine if the PSU will suit your needs.
EXAMPLE:
Let's say you want to use an Opus 150w PSU with a P4 2.8C CPU. If you look up that CPU on Chris Hare's page, you can see that the max power usage is either 68.4 or 69.7w, depending on flavor. Let's call it 70w for simple math. ALWAYS ROUND UP!!!!
So you look up the specs for the Opus 150w and it says that the output on the 12v rail is 5A. 12v x 5A = 60w.
You'd get 60w out of the Opus PSU and the CPU requires 70w. It's not going to work, so there's no point in calculating how much power the other components are going to draw from the 12v rail, as the CPU itself uses more than the PSU can supply.
The ratings of all the major DC-DC power supplies are listed above, under their respective manufacturer product listings.
[edit] Power Calculators
These make things easy
[edit] Start-up / Shutdown Control
Another function required to make PCs work in a car is start-up and shutdown control. Making a PC start with the car and shut down with the car requires special circuitry, which is often integrated with the power supply. When the key is turned on in the car's "accessory" wire (IGN) provides 12V, which triggers the controller to "press" the power button on the PC. When the car turns off, that button is "pressed" again. Some units provide a delayed off, preventing shutdown during short stops. If the unruly PC fails to shut off, the shutdown controller will eventually cut power to the PC, ensuring that the battery of the car doesn't get drained.
[edit] Tank Circuit
A tank circuit solves problems associated with using a second electrical storage device such as a battery for backup power. While batteries are the most common form of tank circuit, capacitors can also be used. Tank circuits are used to solve two common problems -how to power a PC with the vehicle off without discharging the battery, and to prevent rebooting of the PC during car start.
While tank circuits are sometimes used to provide low voltage drop-out protection for the PC, preventing system reboots when starting the car, this problem has largely been solved by the development of modern car PC power supplies, eliminating the need for a second battery and tank circuit.
However, tank circuits are in common use to power the PC from a separate battery when the vehicle is off, preventing the battery from going dead. This setup is often seen in RV's or marine vehicles and is used to power the entertainment or nav systems while preserving the original battery for engine startup.
A tank circuit provides a solution to an electrical problem that occurs when connecting batteries together. When two electrical storage devices such as batteries are connected together, if they have differing voltages (e.g. 11.5 volts in battery A and 12.2 volts in battery B), the lower voltage device will draw from the higher voltage device. In the case of this example, battery B will charge battery A until both voltages are equal. In addition, when starting the engine the drop in voltage on the battery circuit will cause both batteries to supply power to the starter, possibly causing a reboot of the PC.
The solution to this problem, obviously, is to keep the two batteries separated, or isolated. This presents the problem of how to charge the second battery. A tank circuit solves this problem by allowing current on the circuit to flow only one direction -to the second battery for recharge, and preventing it from flowing out of the second battery and into the starter circuit.
Tank circuits aren't always 100% sucessful in preventing reboots during engine cranking. The best solution to avoid this, is to buy a PSU with the ability to surive cranking.
Here is a link to a thread on tank circuits at mp3car.com
Image:Tank Circuit.jpg
[edit] Point of Load
Point Of Load DC-DC power supply's are designed specifically for powering LCDs, USB hubs, DVD drives, and other peripheral devices that require regulated output. (i.e. 5v, 12v, etc)
[edit] Opus Solutions
Opus Solutions 15w PoL
Output Voltage: +5V, +7.5V, +9V or +12V (voltage set by 2mm shunt jumper)
Output Power: 3A max, 3.5A peak or 15W
[edit] CarNetix
CNX-P5V 15w 5V PoL
Output Voltage: +5V
Output Power: 3A max or 15W
For applications that require more than about 1 amp, a small heatsink will be required (16 degC/Watt).
MatrixPC created a PCB based on Mastero's design for a 5v & 12v PoL Link to Forum Post You need some basic experience with assembling PCB's to build this.
There is good information on how DC-DC converters work and their design principleson the net. Have a look here
There are other designs on the net depending on what voltages you need. Google is your friend.
[edit] Wiring
Electrical_Tech_Tips Electrical Wiring Tips
'Do's and Don'ts'
Do's:
- Use a proper rated wire. If you are not sure, get larger wire.
- Use a proper rated fuse suited to the wire you are using.
- If you are not sure how to wire up our system, ask someone who does. Even if you have to pay for the advice or even someone else to do it for you. It may be cheaper than replacing parts of your CarPC or your whole car.
- Check and double check your wiring before powering your system for the first time.
- Put electrical tape or sleeving (heatshrink) over connections and terminations so your wiring can't short out to the car body.
- Secure wiring and hardware properly.
- If your wiring is going through a hole in the body of you car, protect it with a grommet or some sleeving.
- Use connectors, crimps or solder wires together where possible.
Don'ts:
- DO NOT RUN YOUR SYSTEM WITHOUT A MAIN POWER FUSE. This fuse should be no more than 18" from the battery.
- Don't rush to get your install finished. This is when most mistakes can occur. Especially when it is late at night.
- Don't cut corners. You will most likely have to redo your wiring because you have tried to finish it quicker or cheaper. This ties in with the point above.
- If you are using a soldering iron to solder wires together, be carefull where you place the soldering iron. It will burn through carpet and interior plastics before you know it. They can also give you really bad burns
Determining proper wire gauge / fuse size / battery size alternator size
This is to be used as a guide only. If you are not sure what to make of all the numbers in the following tables, consult an auto electrian, car audio installer or a car audio shop.
[edit] Powering outside of vehicle
Powering outside of your vehicle is easy with one of these options:
- Dedicated workbench power supply. Workbench units are sold at RadioShack or equivilate. Workbench units convert 120V/220V AC to 12V or 13.5V DC @ different amps depending on model choosen.
- Create your own with a spare AT or ATX PSU. - Example here - Power a DC-DC PSU on a workbench
- You can use a spare car or motorcycle battery to power your PSU. make sure that if you choose this option that you keep your battery charged with a battery charger.
