LEDs and Circuit Breakers
In the March issue, Shop Talk, Skip
Pardee shows an illustration depicting
an LED light across a circuit breaker
(page 93). The LED will “come on”
when a fuse or breaker pops. While
having a red light come on when a
fuse or breaker pops is a great visual
attention-getter, installing the LED
across the breaker may cause issues
with some loads that may be “down
line” from the breaker.
Fuses and breakers are normally
designed to protect the wiring of
the circuit, not the load (there may
be one, two, or more loads on one
circuit). Wiring the LED across
the breaker allows current to flow
through the LED, then through
the load. (The LED is “grounded”
through the load.) Although this
current would be small, it may
have a detrimental effect on some
aviation equipment that has suffered
a failure. If the equipment’s failure
has created a very high resistance
or “open” through the load, the
LED may not be bright enough to
attract attention or even illuminate.
In the case where popping a breaker
is required to reset the equipment,
such as an alternator field, the LED
may allow enough current through
the equipment to cancel out the
attempt to reset the circuit.
There are two solutions. One is to
connect a green LED on the load side
of the fuse/breaker to ground. The
green LED would indicate the circuit
is functional. While not as good at
attracting attention to a problem as
a red light indicating failure, this
installation does not have the LED
current passing through the load to
ground. The second solution would
be to install a field effect transistor
(FET) in conjunction with the LED.
The FET has a source, drain, and gate
connections. The source would be
connected to the bus, the gate to the
load side of the fuse/breaker, and the
drain to the positive lead of the LED.
The negative side of the LED would
be connected to ground.
With a good fuse or breaker, the
gate has 12 volts applied to it, which
does not allow current flow from the
source to the drain. When the fuse or
breaker pops, the gate has no voltage
applied to it, and current flow is
allowed from the source to the drain,
which causes the LED to illuminate.
The downside to this arrangement
is the extra cost for the FET and the
extra connections.
I do not know if there are
combination FET/LEDs available yet.
If there are, this would simplify the
connections.
Terrence Werth, EAA 711799
Bradshaw, Nebraska
I just wanted to comment on the
Shop Talk article in the March 2009
issue. The battery with a switch to
reverse polarity is a good idea, but a
battery like that should never be used
without overcurrent protection. The
battery could likely supply in excess
of 100 amps were it short-circuited,
which could do serious damage to
the wiring and/or the battery. An
inline fuse holder could easily be
added, and the fuse chosen to match
the expected current draw.
For example, if you are connected
to a load that should draw no more
than 5 amps, you could fuse it at 5
amps. If the fuse blew, you’d know
there was a problem without doing
the kind of damage you would
otherwise.
For reference, I am a registered
professional electrical engineer,
a former electronics technician,
and a private pilot; am building a
RANS S- 19; and teach electrical and
computer engineering at University
of Wisconsin-Madison.
Mike Morrow, EAA 784139
Cross Plains, Wisconsin
Beware Ethanol
On page 61 of the March 2009 issue,
Gordon White asks “…what is so
different about aircraft engines from
all the other engines, auto and the
many others that we run on autogas,
