Clansman radio reverse polarity protection

The Clansman PRC320 HF man pack radio was designed to be used with detachable batteries, either clipped onto the transceiver body or connected remotely via a battery extender cable. In either case the battery can't be connected incorrectly, so there was no need to include reverse polarity protection in the radio.

Clansman battery & extension cable

For amateur radio expeditions I like to power my PRC320 from a pair of car batteries, connected in series to provide the required 24 Volts. I modified a battery extension cable by cutting off one connector and fitting a pair of crocodile clips in its place. Unfortunately it would be very easy to accidentally reverse the polarity and cause a lot of expensive damage to the radio.

I could have put a diode in the lead, which would have caused a small voltage drop, but instead I decided to use a polarity sensing relay. This has a diode in series with the coil; if the supply is connected incorrectly the relay doesn't operate and power isn't applied to the radio. Of course the relay takes additional current from the battery, but with the very large (around 100A/h) batteries that I use this doesn't actually matter!

Here's the relay box that I built.I wasn't going to fit a switch, but then I found an RS 10A resettable circuit breaker in my junk box. This avoids the need for spare fuses, and also works as a switch.

Internal view

Resettable circuit breaker and power LED

The diecast box cost me around £5 at Maplin, and all the other bits came from recycled equipment. 

The cost of building one like this from all new components (but not including the Olive Drab paint) would probably be around £20, which would still be much less than the cost of repairing the radio after an accident!