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iPaq™ auto adapter

August, 2000

I am sure you are aware that the Compaq auto adapter is $70 or more. Already spending more than I should have on my iPaq, I didn't want to fork over that much for an auto power supply. So I built my own for around $8 worth of parts. Although you might be able to find a 5V 500-1000ma supply it is very rare to find the 2 amps that the iPaq specification states. I'm also sure that you will find numerous how-to pages on how to build one using a standard voltage regulator. This is ok if you don't mind having a huge heat skin and having to worry about it getting too hot.

So what is the better solution? Simple, build a switching voltage regulator. This makes almost no heat, compared to the voltage regulator, but does require more parts and work. Also, standard voltage regulators are not rated over 1 amp and pulling more over that will cause problems. This simple switching regulators can be used up to 3 amps. More than enough my iPaq.

Step 1

Get the parts from Radio shack or your local electronics store.

• Switching Regulator PN LM2576
• Heat sink
• 100 uH inducer
• 100 uF (16v) and 1000 uF Capacitors (6v+)
• Schottky Rectifier (fast diode) PN 1N5822
• Small PCB predrilled
• Small Case (your choice of shape and style)
• 2 Amp fuse. (I used standard glass fuse)
• Cigarette Plug (I used lighted pass through but standard will work)
• 4.0 mm plug (Type B)
• Connection wire
• Solder and iron

It is important to get the 16V 100uf capacitor because the input voltage for your auto adapter. While the voltage is only 12V DC when the engine is off, when running it jumps to 13.8V! Therefore make sure you get a 16V or higher rated capacitor. The higher the voltage, the larger the cap. So 16V is good voltage to shoot try and find. All components on the output side can be rated 6V or less. While the input needs to be 16V, or better. The nice thing about this project is it takes 7V - 40V and makes it a regulated 5V. This is important because the iPaq needs a regulated supply of power. It also handles the heat because it doesn't use a linear voltage regulator.

Step 2

Assemble the parts following this schematic.

 

You should get something like the right image. Then moved the parts to fit the box, but you can use any configuration. Then started by drilling the center hole for the regulator. This is what you will use to bolt the regulator to the heat sink and to the PCB. After you have the hole drilled, then very carefully use needle nosed pliers to bend the pins to fit the predrilled holes in the PCB. I staggered them to prevent them from touching and to minimize the amount of bending I had to do. You will see a small circle on one side of the regulator. This marks the side that pin 1 starts. Keep that in mind and start adding components to the board in position. I would suggest placing the diode next because of it's small size. After you have added all the components, make sure you complete the connections with wires or traces. I added a 2 amp fuse to protect my iPaq, even though this circuit can handle 3 amps. Before you solider the cigarette plug you need two do two things. One drill the hole for the wires in the box and PCB. Then check to see which wire is positive. My part I had a ridged side which marked positive lead. A small metal clip was added on the inside of the box to keep the wires from pulling out. On the exit lead tie a lose knot to help keep it from pulling out as well.

Step 3

Test and re-test! I can't stress this enough. Although I had no problems and it worked the first time, you can damage your iPaq if something isn't right. Never hurts to check it a few times before you try it. You should get 5V on your voltmeter. The iPaq is center positive, which means the center of your plug must be positive and the outside is negative. Be sure you have your wires running to the right side and solider them in place on the PCB., If everything is in place, you should get a steady 5V+ from your center lead.

Step 4

Put the cover on and you're done. It only cost $15 total worth of parts for a really nice setup. Even used a cigarette plug that was lighted and had a pass through plug so other devices could be plugged into it without losing my socket. This is just a personal preference, you can use the standard plugs as well. Let me know what you think.

Disclaimer

I am not responsible if you burn down your house, car, or catch your iPaq on fire. This information is provided as is, and you must use it at your own risk!