Power Supply:

The power system actually was one of the most difficult decisions of the entire project. I initially wanted to use a DC-DC power supply, but I really wanted to use an ATX motherboard and finding DC-DC power supplies that could handle the +3.3V required were hard to find and expensive. My initial plan was to build the MP3 system with an AC-DC power supply and see how much noise there was with it. If there was noise in the system I would try to solve the problem. If I failed to remove enough noise, I could then switch to a DC-DC supply in hopes of solving the problem.

Here are a few reasons to get a DC-DC power supply:

• They are more efficient.
• They should induce less noise into the system.
• They may be smaller.
• They can be wired directly into car power supply without the need for an inverter.

However, there are also some disadvantages with DC-DC power supplies:

• Much harder to find than AC-DC supplies.
• The +3.3V ATX output is very rare.
• They cost much more.
• I've heard of problems of brown-outs when idling, or when turn-signals on, etc.
• Many of them will require custom wiring for motherboard and hard drive power connectors.
• Have to have a spare AC-DC supply and re-wire the computer when working on it outside the car.

Using a regular AC-DC power supply has advantages too:

• They are much easier to find.
• They are much less expensive.
• The comptuer can be used indoors with regular AC outlets.
• Much easier to find ATX power outputs than DC-DC supplies.
• No brown-outs, even when starting the car.

Of course AC-DC power supplies have disadvantages too:

• DC-AC inverters add cost.
• DC-AC inverters add heat.
• Less efficient than DC-DC, because converting DC to AC and back to DC.
• But the main reason is that there generally is noise induced into the system as a 60Hz hum.

After weighing all of the options, I decided to go with an regular AC-DC ATX power supply. My only remaining concern was noise in the system. Since I decided to use a MicroATX motherboard, I decided to get an InWin Development 145W MicroATX power supply and an "AC Traveler" 140W DC-AC inverter. The power supply was $15 and the inverter was $20, so I didn't spend too much to power my system.

I cut off the cigarette lighter plug on the inverter and wired it directly into the main power of the car, without going through the ignition switch. I sent a wire up to the driver for a master power kill switch that turns off the inverter. The inverter takes about 3 seconds to warm up before I can turn on the computer.

I've installed a 4-pole spring loaded connector to the back of the computer, just like the connectors used on some speaker enclosures. I've taken the +5V and ground from a 4-pin Molex hard drive connector and wired that to the connectors in the back of the computer. In addition to 5V power, I've connected the ATX power switch so I can have an external momentary pushbutton switch near the driver to turn on and off the computer itself. I use the 5V leads to power the LCD display/backlight module.

I've recently added an in-line fuse to the 5V output on my computer. I don't want to fry anything and did have a close call with some sparks when I crossed some wires connecting things up. I also put a toggle switch in the case in-line with the top cooling fan. I won't need the fan in the winter and it is quieter without it. The fan is about two inches below the thin carpet on my rear deck, and I could hear it when my engine wasn't running.

Noise:

After first wiring up the system, I found there was some noise, but not as bad as I have read from others. Still, it was unacceptable to me. My first task was to check the grounds. I directly wired the inverter's ground to the chassis, so that was OK, but the computer's ground wasn't connected to any ground other than through the inverter. I took a ground wire and just touched the metal power supply cage and the noise was drastically reduced. I then touched the external ground post that I had wired directly from a 4-pin hard drive connector to the back of the computer. This seemed to reduce the noise by at least the same amount as when I grounded the power supply cage. I would subjectively say that 90% - 95% of the noise is removed by properly grounding the system. I find it acceptable, and don't think that I will take any further measures to reduce noise. About the only thing left would be to put an in-line noise filter on the RCA cables going to the CD auxiliary input. I can still hear a thump when the inverter warms up and is ready to power the system, but only at very high sound system volumes. Likewise, I can also hear the buzz of the computer's power supply fan and hard drive spinning, but again, only at very high volumes.

Actually, my car had more alternator whine coming through the regular sound system than what the MP3 player added. I purchased an alternator noise suppressor, but haven't gotten the chance to install it yet. I hope that that will reduce the noise to a point where it isn't noticable at all.

I'm not sure that using a DC-DC power supply would have any less noise than mine. The hard drive spin-up and power supply fan noise might still be present. A friend is building his system with a DC-DC power supply, so I guess when he is done we can compare noise levels. I'll probably try it with my next project.

A couple of notes about my power system...

I attempted to perform a test to see how long I could leave my MP3 system running before the battery went dead. I was suprised to find out that it lasted so long. The MP3 player, as well as the LCD display/backlight, were on, but not the sound system. It was connected to my network, and I was actively uploading new MP3s to it for most of that time. After six hours I shut it down and went to bed. The next morning the car started up just fine. I have a relatively new 60-month battery, but suspect that I could get maybe 10 hours before draining the battery.

One weekend I accidentally left the power inverter on, but not the computer itself. After arriving home I could start the car, but did hear some hesitation from the starter. The inverter was probably running off the battery for about 36 hours or more. I guess this comes from the poor efficiency of using an inverter.

I've read that others have had difficulties with their DC-DC power supplies, many of which were home-built, clipping and shutting down the computer when the voltage gets too low, like when idling while using the brake lights or turn signal. I'm fairly certain that these same systems would shut down if using the starter also. On my system, I can start the car with the computer running, so in that regard the inverter system is quite nice. I've never had any brown-out situations at all. If the engine is cold in the morning and it takes a lot to start it (did I mention how old my car was?), it may turn off the inverter on me by dipping the voltage way low, but any other time it is just fine. I suspect that the professional DC-DC supplies that are 8V-16V would work OK in this regard.

In my research, I did find some good choices for DC-DC power supplies that may be helpful if you decide to make your system that way...

Here are some ATX DC-DC power supply options for car MP3 players:

• Arise Industrial Computers offers many DC-DC power supplies, but the only ATX DC-DC supply is the ACE-828T 250W ATX PS/2-style (regular type that fits into desktop computers) power supply. It may not work because it requires 18V - 32V for input voltage which is too high for most car's 12V - 15V systems. It also costs $150 which is more than you should need to spend.
• Amtrade Products' CP-150D1 is a 150W ATX industrial power supply that fits in a 1U rack mount chassis. It costs $85 and would probably work great for a custom built case. They also sell a 250W ATX unit sized for a 2U rack for $155, although I don't know why you would need that much power for an MP3 system.
• The SDD and SDX Series supplies from Sunpower are AT and ATX DC-DC power supplies for $160.

Here are some AT DC-DC power supply options for car MP3 players:

• The Arise Industrial Computers ACE-916V 160W AT PS/2-style power supply would fit the bill for standard cases.
• The ACE-865V 65W AT power supply and the ACE-845V 45W AT power supply are both industrial mini-size power supplies that offer AT voltages. These are two nice candidates that would work for many custom-built computer cases. The 45W supply costs $70 and the 65W supply costs $80. I think I will use the 65W supply for my next project - a MP3 player for my truck.

This page was last updated on Monday, November 13, 2000

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