MPBS1 - ATX DC-DC 12 Volt Car Power Supply by Jeff Mucha

Overview:

 

This page is dedicated to a 70-Watt Power supply that can be used in the car to power a ATX based computer from the cars 12 volt electrical system. This power supply is the perfect solution for really small places. It measures 1.5" x 3.8" x 2.5" making it the smallest solution avaliable!

 

Addressing the need and Motovation:

 

There are several reasons why I designed this power supply I will discuss a few of them.

The origional Mpegbox used a standard ATX AC-DC power supply with an inverter to generate an AC voltage of 120 volts only to convert it back to 3.3 to12 volts. While this system worked very well, it was big an bulky and not very efficient.

 

The second Mpegbox used laptop hard drives which were designed to operate on batteries with a DC range of 11 to 14 volts. This method worked pretty well, but the supplies would cut out as soon as the voltage at the supply was less than 11. This often happened when the car was started. Most inverters I have seen have a cutout range of 10.5 to 11 volts. Because the power requirements of a standard motherboard and hard drive are different than that of a laptop, I had to use two laptop power supplies. This system worked very well, but still had inefficiencies and drawbacks.

 

Here you can see what the voltage at the terminals of my car's battery does when the car is started. Every division of the screen is 2 volts. The battery starts out at 12 volts and as soon as the solenoid engages and the starter motor draws current, the voltage at the battery drops down to 8 volts for a few milliseconds. While the engine cranks, the voltage stays around 10 volts and as soon as the car starts, the alternator kicks in and brings the voltage at the battery to the charging voltage of 13.8.

 

Not every car will drop as low as 8 volts, it depends on the condition of the battery and how hard it is for the starter motor to turn over the engine. If the voltage drops this low, many power supplies commercially available will switch off causing the comuter to reboot. The inverter method and the laptop power supplies proved to be inadequate for this application.

 

The final reason that I decided to design my own power supply was that my second Mpegbox was stolen. I was determined to make a box that was smaller and better. There are several companies that make commercial grade DC-DC power supplies, Keypower and Arise come to mind. But they are big and clunky just like a standard AC-DC ATX power supply measuring 5.9" x 5.9" x 3.3". Mpegbox 3 measures 8.5" x 9" x 3" and that includes the power supply, hard drive, and motherboard.

 

Solution:

After addressing all the needs, I spent countless hours designing, laying out, and redesigning my DC-DC power solution. The supply I came up with I call the Mpegbox Supply One or MPBS1 for short. It is very small, not much bigger than a deck of cards. It has a big aluminum heat sink which allows it to "run cool". It measures a mere 1.5" x 3.8" x 2.5" and the input voltage can go as low as 8 volts allowing the supply to not cause the computer to crash or reboot when the car is started or restarted (engine cranking). The high range of the input can go as high as 25 volts without damaging the supply but above 15 volts the negative supplies start to "brown out." I have used this supply in my car for months and the computer never reboots when I pull into a gas station, go pay and come back to restart my car. This supply doesn't seem to care if my battery is in really bad shape!!!

 

Specifications:
Input Power	80 Watts Max
Input Voltage	8-15 Volts Non-Regulated
Input Current	10 Amps Max
Output Power	70 Watts Max
Efficiency	Approx 80%
Individual Supply Outputs	Max Output Current
5.0 Volts +/- 5%	5 Amps Nominal, 6 A peak
3.3 Volts +/- 5%	5 Amps Nominal, 6 A peak
12.0 Volts +/- 5%	2 Amps Nominal, 3 A peak
5.0 Volts Standby +/- 5%	1.5 Amps
-12.0 Volts +/- 10%	100 mA
-5.0 Volts +/- 5%	100 mA

Mounting:

To attach this supply to your system, it comes with four mounting screws coming out of the top of it. It can be attached to sheet metal or plexiglass that is 1/8 inch thick or less. It can also be attached with double stick tape to the inside of an existing casing. Care has to be taken not to let the bottom side of the circuit board touch any metal as it will short out. It is held together with standard 1.5 inch 4-40 screws which can be replaced to suit the application.

For more information on the MPBS1 Click Here

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