nuts & bolts
building basics
Choosing a Switch
Poles, Throws, and Power Capacity
RICHARD KOEHLER, EAA 161427
Over the last few months we have been looking at electrical wiring in your plane. The basic layout is for power to come from either the battery or alternator
to a main distribution “buss” and from there through an
appropriately sized fuse or circuit breaker to, in most cases,
a switch that turns on or activates the circuit.
Some circuits do not have a switch, such as cockpit
gauges, but most do. In some cases, such as avionics, the
unit may have its own switch, but for many systems, such
as lights, pitot heat, and boost pump, each item has a
switch. It is important that the switch be of the right type
and capacity rating. Let’s look
into the various options for
switches.
Poles and Throws
Switches are listed by the
number of poles and throws
they have. Think of a pole
as a circuit. If the switch is a
single pole, then it controls one
circuit. If it is a double pole, it
controls two circuits. Throws, on the other hand, list the
number of directions the power can be sent, usually beyond
“off.” For instance, a single-throw switch will allow current
to go to one place only, whereas a double-throw switch will
allow current to be sent to two possible locations.
The simplest switch is a single pole single throw,
abbreviated SPST, which is used to turn one circuit on or off,
such as the pitot heat or landing light. In fact, most of the
switches in your plane will be SPST switches. A single pole
double throw (SPDT) switch could be used to run two fuel
quantity inputs onto one gauge, perhaps a left tank with
the switch to the left and right tank with the switch to the
right.
A popular use for a double pole single throw (DPST) is your
master switch. With one flick of the switch you energize the
battery solenoid and connect the voltage regulator circuit
so that the alternator will generate power when the engine
rpm is high enough. This DPST switch controls two separate
circuits at the same time.
If you’re getting this, you can probably tell me where a
double pole double throw (DPDT) switch is located in your
plane, especially if you’re a Piper owner. DPDT switches
are sometimes used in reversible motor circuits, such as
electric gear or flap actuation, but they are also commonly
found in light switches on older Pipers. These Pipers use a
DPDT switch to turn on the nav lights and cockpit lights at
the same time. In the middle position both are off. When
switched up, both the nav and cockpit lights are on bright,
Think of a pole as a circuit. If the
switch is a single pole, then it controls
one circuit. If it is a double pole, it
controls two circuits.
but when switched down, the nav lights come on bright
but the cockpit lights have a resistor added in the circuit to
make the lights dim. This way, Piper offered cockpit light
dimming, without the expense of a heavy variable resistor.
Again, you will commonly use a DPST for the master
and a bunch of SPSTs for everything else, but there may be
exceptions. I once removed a four-pole six-throw switch
that had been wired into the cockpit intercom system to
allow either pilot to independently transmit or receive on
any radio in the plane!
Power Capacity
Power capacity is also important; the switch must be able to
handle the current being switched through it. The switch
has a rating in amps and volts. For instance, a common
ultra-miniature switch is rated at 6 amps at 125 volts AC and
4 amps at 28 volts DC. The rating is not power scalable and
