HOW TO DO IT

Low level signal inputs are subject to damage caused by inadvertent high level signals or transients. Neil, Radio8Z, offers this simple, low cost circuit that could prevent damage from those occurences.

By Neil Radio8Z

There are devices such as metal oxide varistors (MOVs), spark gaps, and gas discharge tubes which can be used to provide protection from voltage surges on signal lines. These work by clamping the voltage on the signal lines to approximately 80 volts and above referenced to a protective ground but such voltages can still damage electronic equipment. The risk of electronic damage can be reduced if the voltages on the signal lines is limited to smaller values such as 3 volts above circuit ground. Such limiting needs to be high enough in voltage so as not to interfere with the signal but low enough to protect the electronics.

One very simple technique is to use diodes and resistors to limit the currents and voltages on a signal line. For a typical single ended line level connection the circuit in Figure 1 can be used. The diodes, typically 1N4004, limit the voltage and the series resistor, 100 ohms or thereabouts, limits the current. If the signal voltage is higher than 3 volts peak then more diodes can be added in series with each adding 0.7 volts before clamping. The intent of this circuit is to limit the voltage difference at the input to the electronic device and it is not intended for personnel or property protection so if used with wires from outside the building it should only be used in conjunction with properly installed building entry protection.