by Chris Cox, N0UK, G4JEC
I can't believe I never thought of this before! This simple modification is applicable to pretty much any 12 volt operated, positive earth equipment.
It was described to me yesterday by Bruce, W9FZ, as a method that many European radio amateurs use to protect against connecting the power cable up in reverse for lowish-power equipment, whilst not incurring any forward voltage drop as exhibited by using the ubiquitous inline diode in the positive voltage line.
In the circuit diagram below, the Radio Shack reed relay specified is rated at carrying up to 1A at 125 Vac. It can probably handle a couple of amps at 12Vdc.
Theory of operation
Operation is very straightforward. When the input power is connected up correctly, the diode inline between the +12Vdc input connector and the relay is forward biased, supplying about 11.4V to the relay's solenoid. This then pulls in the reed, allowing current to flow on to the protected equipment, The second diode is normally reverse biased and is used to clamp the back EMF generated by the relay's solenoid when power is removed.
If power is applied in reverse, the inline diode is reverse biased, preventing current from flowing through the relay's solenoid, therefore, preventing any current from flowing into your protected equipment.
You should note, using this circuit will incur a small additional current drain from your power supply. The relay specified, is rated at drawing 11mA at 12Vdc.
Construction
Construction is absolutely non-critical. You could, if you so desired, build it by just soldering the components leads together, on perfboard or Veroboard(tm), or, if you're really enthusiastic, on a PCB!
Disclaimer
It is a sorry state of affairs to have to write a disclaimer, but due to the over-zealous legal profession in the USA, it's probably wise.
Any modification that you perform (including this one) are made at your own risk. If you damage your equipment you cannot blame anyone other than yourself.
Good luck!
