A common failure point of our charging system is the Over Voltage switch, a little known and hard to find device that has the power to shut down the alternator at will. Let's take a look at this pesky little critter:
Nessled in the wire bundle of most of our aircraft is a small black
tube with three wires and a simple goal in life: to protect us from
a runaway alternator. Should the alternator start to produce higher
than desired voltages on the buss, this box will shut down the
alternator until it is reset. It does this by interrupting the field voltage, and as such it is in series with the alternator field. It senses the voltage of this line as it's source for monitoring for over voltage as well. It is reset by turning the master switch off and back on again.
It is located in the bundle of wires behind the circuit breakers, over the feet of the copilot. Often it is surrounded by the wires in that bundle and is not visible without removing a large number of wire wraps. The two pictures here are typical installations in two different aircraft.
The failure mode of this device is simple: the alternator stops
producing current and the voltage drops to battery levels. If
the overvoltage switch is defective this will often be a time or
temperature driven event, occurring a certain
number of minutes after startup or once the cabin reaches a
certain temperature. A quick check of the overvoltage sensor can be performed in a simple way: on the ground, with the engine off, stand beside the left side of the airplane and turn on the master switch. Listen for the click of the regulator. You may wish to turn on the battery portion of the master switch first, then cycle the alternator side while listening for the click.
If you do not hear a click, there are only a few possible causes: either you have a very fancy solid state regulator, the master switch is not working, the circuit breaker is not passing current, there is a wiring problem or the overvoltage relay is open. By far the most common reason is the last.
By the way, the master switch is another frequent source of problems, but they are most often intermittent and can be observed in the same way, listening for the click as you cycle the switch.
What is the solution? A new overvoltage relay is one possibility, but the most common solution is removal of the switch altogether (connecting the field wires together to remove the switch from the circuit) and replacement of the old 'Ford-style' regulator with a more modem version that has built-in overvoltage protection.
One example is the Zeftronics regulator shown here. It
performs the functions of the overvoltage switch internally,
and includes indications via it's LED when it does so. Do we really need this protection? Perhaps we should simply remove the overvoltage protection and let the alternator run? Not a good idea: remember that certain cars use an unregulated alternator to defrost their windshields, and these systems operate at over 90 volts.
Even worse than loosing a charging system would be loosing all your radios and the battery at the same time...
Keith Peterson
Over Taiwan, April '98