This is a repost from another thread that I thought would be of wider interest:
Today, however, I modified my my external regulator setup which uses the Van's 35A alternator with their external regulator. This setup is prone to oscillation due to voltage drops and resistance in the field circuit wiring.
(see last paragraphs of this Aeroelectric ariticle
, excerpted below:
"Another common victim of resistance pollution is the
alternator’s voltage regulator. Consider the wiring and
all the components in the wiring between the ship’s main
bus and the voltage regulator on many certified airplanes.
In some of the older Cessnas, it’s not uncommon to
travel through a circuit breaker, field side of a DC Master
switch, overvoltage relay and then out to the regulator.
The production harness includes several AMP Mate-n-
Lock connectors with pins that are NOT gold plated.
Over time, every crimped joint, mated pin, clamped up
ring terminal, switch contact and relay contact increases
it’s resistance with age. No one device goes up a lot. I
think I counted something like 16 assembled connections
in the alternator’s field power path on one particular
An alternator’s field draws about three amps maximum.
Let’s suppose that the field circuit resistance is
something on the order of 100 milliohms. A 3-amp
current draw will induce a 300 millivolt drop in the
pathway from bus to regulator. This can have two
profound effects. First, the regulator thinks the bus is
running 300 millivolts lower than true and jacks up the
alternator’s output to compensate for it. This means that
as the alternator’s load is increased, field current goes up
and the regulator’s perceptions of true bus voltage are
increasingly in error.
Normally, when you load a device or system with heavier
current, you expect resistance associated with the system to
cause voltage to go down. However, when resistance is
included inside the control loop of the voltage regulator, an
increasing load on the system causes the alternator’s output
to rise instead of fall. This is called a “negative resistance”
condition. If it progresses far enough with some regulators,
the system will break into oscillation.
The problem manifests itself as a wiggling ammeter or
perhaps agitated panel lights. More than one airplane
exhibiting these symptoms has suffered replacement of
alternators and/or regulators with no or temporary relief.
Sometimes the problem goes away when the mechanic
replaces the DC Power Master switch (usually the infamous
split-rocker). What the mechanic generally doesn’t know
is that regulator instability is caused by small build-ups in
most of the field supply line’s components. Replacing one
component may bring total resistance down to a stable
condition but the problem returns sooner than necessary
because all of the components continue to age and drive
circuit resistance up."
My fix was to wire a relay into the field circuit that connects the regulator directly to the alternator B lead when the ALT switch is turned on.
See the bottom of this web page
for more information.
My flight test showed rock-solid voltage and no fluctuations. The cockpit lights were steady (before they were not). The voltage on my voltmeter reads 14.1 volts, about 0.1 volts below the regulator set point. This makes sense because the voltmeter is several connections and lengths of wire downstream of the main bus, and is measuring the voltage drops due to loads on the main bus.
Previously, I would see 14.3 to 14.5 volts on the voltmeter, depending on load and phase of the moon (resistance in the circuits and voltage drops across connections and the regulator was several feet of AWG20 downstream from the main bus as well). This would imply 14.6 or more volts on the battery... a tad high.
This problem was so severe at one time due to a Master Switch failure (resistive contacts), that I had overvoltage alarms in flight. Now, a resistive switch will have no effect (within reason).