Is that whine in your earphones driving you nuts? It might well be alternator-induced radio noise. Here's how to identify alternator and regulator noise, what causes it, and how to get rid of it.
February 5, 1996
|About the Author ...
John Schwaner is AVweb's powerplant expert. John is a world-class
authority on piston aircraft engines, and a specialist in the
engineering analysis of engine failures. John runs Sacramento
Sky Ranch, Inc., a leading distributor of aircraft and engine
parts, and probably the foremost aircraft hose shop and magneto
overhaul facility in the U.S. John and his wife live in Sacramento,
John has also written two superb technical books:
Sky Ranch Engineering Manual
The Magneto Ignition System.
Both can be previewed in and ordered from the
AVweb Online Bookstore.
Identifying the problem
Alternator induced radio noise is a high pitched whine whose pitch
and intensity increases and decreases with changes in engine speed.
Turning the alternator master switch off also turns off the radio
Solid state regulators that use a pulse-width-modulated field
control system can also create a whine in the radios. Regulator-caused
whine can be distinguished from the alternator-caused whine in
that the intensity and pitch of regulator-induced noise changes
with changing current load at a constant engine speed. Thus, turning
on the landing lights won't increase alternator whine but will
increase regulator whine.
How the alternator works
Current generated in the alternator stator windings is three-phase
alternating current, but diodes convert it from AC to DC before
it leaves the alternator. Six diodes are required to rectify the
three stator phases. Each of the three stator windings is connected
to a pair of diodes. Three diodes are connected to the positive
output terminal of the alternator, and the other three are connected
to the negative (ground) terminal.
As the voltage of each stator winding increases, the corresponding
pair of diodes becomes forward biased and allows alternator current
to pass. Which stator winding and diode pair is conducting at
any moment depends upon rotor position. After the diodes rectify
the three AC phases and sum them all together, the combined result
is a DC voltage with only a slight amount of AC ripple voltage
The best way to detect ripple voltage on the electrical bus is
with an oscilloscope. Another method is to use an ordinary volt-ohmmeter
(VOM) set to measure AC volts. You may have to connect a capacitor
in series with the positive meter lead to block out the DC voltage
so that only the ripple voltage gets to your meter. (Some meters
do this automatically when you select AC volts.) The capacitor
is an open circuit to DC but passes AC, so the voltmeter reading
you see is the amount of AC ripple voltage on the bus. You will
need to do comparison readings with other aircraft to determine
what AC voltage level is normal.
What causes alternator whine?
Normally, there is not enough ripple voltage to cause radio noise.
But, there are two conditions that can cause an increase in ripple
voltage sufficient to create radio noise. These are diode failure
and increased circuit impedance.
If an alternator diode fails, the amount of ripple voltage increases
markedly. Alternator whine can be a symptom of a bad alternator
diode. Two test methods can be used to test the alternator without
There is a hand held unit with a probe that clamps over the alternator
output wire. A bad diode will show up on the meter. These meters
were originally sold as the Ward Aero Alternator Tester model
647. They are currently sold by Support Systems Inc. as model
The second test method is to use an oscilloscope to check the
alternator output for excessive voltage ripple or rectifier spikes
caused by a bad diode.
Checking the diodes
With the alternator apart, the diodes can be checked with a VOM
set to measure ohms. This test makes sure that each diode conducts
in only one direction. You need to unsolder the stator leads from
the each diode. Calibrate the VOM on the R x 1 multiplier range
scale so that there is zero reading with the VOM leads shorted
Connect one test probe to the alternator's positive output terminal
and touch the other test probe to each of the three solder terminals
of the diodes mounted to the positive rectifier plate. Note the
three ohmmeter readings: they should be identical. Now reverse
the test probes and repeat the test. Note the three ohmmeter readings:
again they should be identical to each other, but not the same
as in the previous step. Three of the ohmmeter readings should
show a low resistance reading of approximately 6 to 20 ohms and
three should show an infinite reading (no meter movement).
Repeat the same test procedure for the three diodes on the negative
rectifier plate, connecting one test probe to the negative output
terminal and checking all three diodes with the other probe. Then
reverse the leads and check again. The diodes should show low
resistance in one direction, and infinite resistance in the opposite
Alternator whine can also be caused by poor electrical connections,
especially at the battery. Normally, the low impedance of the
battery keeps the aircraft's electrical circuits at a DC potential.
(Impedence is simply resistance to an AC current.) Any AC ripple
voltage in the aircraft bus is absorbed by the battery. Thus,
the aircraft battery acts as a big ripple absorber.
If the battery provided zero impedence (i.e., a short-circuit
for AC current), alternator noise could not occur. In the real
world, there will always be some impedance. But the lower it is,
the less ripple voltage there will be.
Let's assume that the battery positive terminal is corroded. Although
DC resistance as measured with an ohmmeter may still be low, the
high-frequency resistance (i.e., impedence) may be very high.
The higher this impedence, the greater the ripple voltage on the
bus and the more whine you hear in your radios.
Circuit impedance can be lowered by making sure the battery posts
are clean and making good contact. DC resistance should be less
than 0.01 ohm...virtually zero. Also check the alternator ground
connections, including the engine grounding strap. DC resistance
between the alternator and the negative post of the battery terminal
should be as low as possible.
The ideal low-noise circuit would have the alternator power output
going directly to the battery's positive terminal. This dumps
ripple voltage into the battery, where it is absorbed. The radio
power lead would also go directly to a pure DC source, the battery.
If the alternator power lead and the radio power lead connects
to a bus, then voltage ripple can go from the alternator to the
radio power lead. The amount of voltage ripple at the bus depends
upon the impedance between the bus and the battery. This impedance
is higher than at the battery.
The return path is from the alternator to the engine, engine mount,
firewall, and through the fuselage to the battery. These connections
should have low resistance. Flat braided ground straps are ideal
for grounding the airframe to the engine mount. Flat braided straps
are used because impedance is less with a braided, flat conductor
than a round wire conductor.
There are two methods of filtering ripple voltage: bypassing the
ripple voltage back to the source, or blocking the voltage ripple
so that it cannot pass. Capacitors are used to bypass ripple voltage,
whereas inductors are used to block noise currents. The most effective
approach depends primarily on the circuit impedance.
Capacitors bypass noise currents back to the alternator return
path (commonly referred to as ground). To be effective, a capacitor
must have a low impedance path back to the alternator. Consequently,
a filter capacitor must be mounted as close to possible to the
alternator. The capacitor is installed with one lead connected
to the power output and the other lead to ground, so that it is
in parallel with the circuit.
For DC voltages the capacitor forms an open circuit (high impedance)
and doesn't allow any current to pass. At noise frequencies the
capacitor forms a short circuit (low impedance) and bypasses noise
currents back to the alternator. In this manner we have formed
a low-pass filter. The effectiveness of using a capacitor as a
noise filter depends upon matching the capacitance rating of the
capacitor to the frequency of the noise currents.
The frequency at which the capacitor's capacitance and inductance
are equal is where it has the lowest impedance and the best filtering.
This is the resonant frequency. The correct size capacitor is
one where the frequency we wish to bypass is the same or less
than the resonant frequency.
Smaller size capacitors (picofarad range) are effective at high
frequencies, while larger size capacitors (microfarad range) are
effective at lower frequencies. If you're filtering conducted
interference (as you are in an alternator), then this is low-frequency
and the capacitor should be in the microfarad range. If you're
filtering radiated interference (where the conductor is acting
as an antenna), this is high-frequency and the capacitor should
be in the picofarad range.
Typically, an alternator filter uses a .5 to 50 microfarad capacitor.
Cessna has a 5.72 microfarad capacitor filter available as part
The best types of capacitors for filtering are ceramic and tantalum
capacitors, ceramic for the picofarad range and tantalum for the
microfarad range. Electrolytic capacitors are relatively poor
noise filters, and also have a short life.
Capacitor resonance can be approximated with the following formula:
resonant frequency (in MHz) equals 1/2 pi times the square root
of lead length times capacitance. Notice that lead length has
a significant effect on the capacitor's resonant frequency. For
example, a 500 pf capacitor with 1/4 inch leads resonates at 100
MHz. But with 1 inch leads, it resonates at 50 MHz. So capacitor
lead lengths used in filter circuits should be kept as short as
The other way to filter radio noise is to block the ripple with
a series inductor. The most common style of inductor for noise
filtering is a ferrite core. These come in many different styles
but typically the wire with the noise currents is wrapped around
the core, creating an inductor in series with the circuit. DC
current passes through the core but high frequency currents induce
a magnetic field in the ferromagnetic material of the core. This
magnetic field raises the impedance and effectively blocks noise
currents. Ferrites are effective on radio power input leads and
strobe power input leads. In the first case they prevent noise
currents from entering the radio, and in the second case they
prevent noise currents from exiting the strobe.
To be effective, ferrite impedance must be larger than circuit
impedance. To filter the output of an alternator would required
an impractically huge ferrite core. So alternator voltage ripple
is usually bypassed to ground by use of a capacitor. However,
ferrites are simple to use and have an amazing filtering ability.
Ferrites are best used in low impedance circuits whereas capacitors
are best used in high impedance circuits. It is best to install
ferrites on the radio power input leads, and to use a filter capacitor
at the alternator output terminal.