usb charging in an old car

Posted by 54clipper On 2011/8/17 17:54:04

I have been wondering how I could power a gps or charge a cell phone when in my Packard. As you know, as do all cars prior to 1955, it has the wrong polarity (positive ground) and wrong voltage (6v).

I did some searching on the web, and found no practical solution available. Years ago (not long after cars switched from 6v to 12v in 1955) some 6v to 12v converters were made, but they were very inefficient, large clunky things. I found some of the old ones still available on ebay. Building a 6v to 12v converter today is easier, but upconverting voltage is inherently inefficient and takes a bit of circuitry. What was needed here is a very steady 5v (actually 4.75v to 5.25v) supply for a usb device charging - virtually all modern devices charge from usb - iphone, blackberry, gps systems, etc... The voltage supplied at the cigarette lighter (or cigar lighter as it was called in 1954) can vary quite a bit. Simply using a voltage divider circuit with a few resistors is tempting, but since the cars voltage isn't constant, neither would be the voltage that the circuit produced. I would hate to ruin my iphone or gps by supplying it with incorrect charging voltage.

So I did a little digging, built a circuit based on a low dropout voltage regulator chip and gave it a try. Success! Works like a charm. Constant 5v dc output from the cigarette lighter in my old car.

Here are some pics. Maybe I'll build a neater version now.

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