A 12V Power Supply For CB Radios And The Like

Now and then, old CB radio sets come my way, and I needed a suitable power supply to bench test them with. Here's a view of the rig that I came up with.

The unit at the left is an ancient Canadian Tire Motomaster battery charger, Cat. No. 11-1568-8. A battery charger of that type has no filtering or voltage regulation; it's just a transformer and a full-wave rectifier. At the 2A setting, average dc voltage out is about 11.9V. At the 12A setting, average dc voltage out is about 13.1V. I don't use the 70A setting; I suspect that it's stressful for the transformer.

I modified the charger for my purpose here by nipping off the original clip leads, then adding an output terminal block, output banana jacks and a chassis-ground/earth terminal.

Here's an inside view of the rear of the charger's front panel.

I added fork terminals to the clip leads, so I can still use the charger for its original purpose, like so.

The Regulator

The regulator box in the centre of the first photo above is something I cobbled together from odds and ends that I had on hand, and a new Hammond No. 1411PU aluminum utility case. Here are some views of it.

Output voltage range is about 1.2VDC to 16VDC. The LM317 regulator IC is good for about 1.5A.

Here's the schematic.

1.5A is plenty of current supply capability for a receiver. It appears to be marginal for transmission, though. I may want to look into boosting the regulator's current capability.

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Current Capability Boosted -- WEDNESDAY, FEBRUARY 20, 2019

I was using the regulator to power a vintage xenon-tube timing light, and having trouble with the timing light overloading the regulator. So, I decided it was time to beef up the regulator's current capability.

The way I did it is considered to be bad practice, but I went ahead and did it anyway. I added two LM317s directly in parallel with the first one. That more-or-less works. I've got regulation up to well over 3A output.

The argument against directly paralleling three-terminal regulators is that there's likely to be imbalances -- the regulators won't share the load equally because of tolerance variations. No doubt that's true, but the method appears to work adequately well enough for my purposes. It may have helped that I used three regulators from the same manufacturer, all with the same date code.

Here are views of the modified regulator.

It's not the most elegant wiring job I've ever done, but it works.

And here's the revised schematic.

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