Tuesday, June 22, 2010

I have a receiver that I like to power with NiCd batteries ...

Q Craig Cochran, N5KYF, asks, “I have a receiver that I like to power with NiCd batteries. But since the radio can operate over a very wide range of supply voltages, the batteries are exhausted before the radio starts to sound weak. (It takes six ‘D’ cells but seems to work okay down to 4 or 5 V.) “So, I need to build a circuit that will turn the radio off (disconnect the batteries)when battery voltage drops below about 6 V. I also need a circuit that will not draw much current (the whole idea is to save the batteries!). Could I use a 6-V Zener diode and a switching transistor?”

A If you were going to make a circuit that switched off your radio, the energy to measure the voltage and perform the switching function would come from where? The battery! It is an ironic fact that any kind of battery indicator ultimately runs the battery down faster than if it weren’t there at all. Users of H-Ts who like the security of a built-in battery checker beware! I’m not saying the amount of current drain is significant, but it’s there nonetheless.

I think a better approach is to use an indicator that draws very little current and leaves the act of shutting off the radio to you. That way you can at least have a little control over the process. The lowest current-drawing indicator that I can think of is a single segment of an LCD display. Although it would draw current itself in the act of monitoring the battery voltage, the amount should be miniscule. Just think of how long the LCD watch face on your wristwatch runs off that tiny cell inside. In the October QST there is a also nifty little circuit for monitoring the condition of your battery. It draws very little current. See “A Battery-Voltage Indicator” by Donald G. Varner, WB3CEH, on page 50.

How about an extreme “low tech” approach? It could be something as simple as putting a subminiature momentary contact switch in series with the battery so that it only works for the split second when you press the button. A tiny meter movement could take the reading for you. You could accurately measure the battery voltage and it wouldn’t be drawing anything except during the moment you pressed the button. Even then the current drain would be insignificant.

From January 1999