In-Car USB Power Final Assembly

This post is part 3 of the In-car USB Power series. You may want to read part 1 and part 2 first.

When all the parts for the power converter arrived, I breadboarded the circuit, and it worked as expected. It could charge my cellphone in about the same amount of time as the wall charger. After a few sketches, I came up with a reasonable way to connect all of the components on a small piece of perf board. The leads for the buck converter IC are pretty close together, so I decided to connect some on the top and some on the bottom to give myself more room and minimize the chance of shorting two connections together. A little work with the Dremel made a reasonable slot for the leads to pass to the underside.

Drilled perf board

A bolt holds both the IC and the heatsink in place on the board, while also holding the IC firm against the heatsink. Before placing the IC, I put some leftover thermal paste I had lying around after my last computer build on both the IC pad and the heatsink surface to create a good thermal connection.

LM2576 mounted on perf board

I made the rest of the connections by just soldering the leads together. The result isn’t exactly a work of art, but it’s compact and works great! I held everything in place with hot glue for mechanical stability (these things are going to be vibrating around a lot in the car) and for electrical insulation so the connections don’t short together or to other metal behind the panel if it shifts around. It’s important to not cover the diode or the IC/heatsink, as they need to be able to dissipate heat.

Top view of buck regulator circuit
Bottom view of buck regulator circuit

The easiest place to mount the USB and audio ports in the car is in the lower dashboard near the cigarette lighter. This gives me easy access to a power line, and is a nice place for the cables to connect without getting in the way of the rest of the dash. There also happens to be a sizeable cavity behind this panel, so there’s room for everything to fit.

After measuring everything carefully, I put down some masking tape and marked off the holes I wanted to cut. I drilled out the corners of the square hole, then used a spiral bit to mill out the middle. After milling out most of the material, I cleaned up any jagged edges and squared out the round corners with a utility knife.

Cutouts drawn on car panel on masking tape
Holes for USB and audio jack drilled in car panel

I didn’t know of a better way to hold the jacks in the panel than to just epoxy them in place. I roughed up both surfaces with sandpaper, then glued the jacks in. This is pretty irreversible, and if I were doing it again, I’d probably do something a little different. It would be better to glue in standoffs to either side, then mount the jacks to another piece of perf board, which could then be screwed into the standoffs. This would probably be a little less mechanically rugged, but would be removable if I needed to change anything about the jack connections.

To connect to the power line in the car, I used a molex connector from an old PC power supply. This allows me to unplug and remove the circuit, instead of having it forever attached to the car. After disconnecting the battery, I clipped both the power and ground wires leading to the cigarette lighter. After stripping both ends of the clipped power wire and the wire from the molex connector that I wanted to splice in, I twisted all three together and then connected them with a screw-on wire cap. I did the same with the ground wire. The mating molex plug is soldered to the input side of the converter circuit, so it can be easily connected and removed from the car power.

Molex connector spliced into power line

With everything installed, I reconnected the battery and gave it one last test before putting the panel back together. The charging works great! I used zip ties to collect the extra wire length and secure the perf board in place.

Converter installed behind dash

I am really pleased with the whole build. The jacks fit into the look of the panel really well; almost like they might have been designed there.

Final installation in the car

After putting all of this together, I realized that I hadn’t accounted for Apple devices, which require a certain resistor configuration to charge. I just put up with this for a while, but eventually pulled the panel back out to add the resistors to make my iPod work too. (This is where it would have been really nice to be able to remove the USB jack from the panel. :))

It also turns out that if you try to use the audio input jack to play audio from a device that is charging from the USB port, there is a LOT of noise on the line. It increases in frequency with engine RPM, so I believe that it’s noise on the power line from the ignition system. It’s possible that if you used another power circuit in the car, it might be quieter; I’m not sure. I also think that adding an inline choke on the power output or maybe a few smaller decoupling capacitors on the output of the voltage regulator might help filter out some of the noise. With all of the hot glue on there, there’s really no way I can get back in to test this out. It isn’t a big deal; I just don’t charge a device while I’m listening to it. If anyone knows what might fix this, please post in the comments to let me know!

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