Bob,
Was reviewing your article:
<http://www.aeroelectric.com/articles/Failure_Detection_and_Annunciation.pdf>http://www.aeroelectric.com/articles/Failure_Detection_and_Annunciation.pdf
Alan, sorry for the delay on this. I'm fielding input from
the List on three different computers which scatters
my to-answer list over three in-boxes. I have to scan
the forum archives from time to time to make sure I
don't have any dangling conversations!
I want to annunciate the failure of a 12v scavenge pump to draw
current. You mention such a circuit being used on a heated pitot but
I was wondering if it mattered if the load was a pump motor? Full
load would be approximately 5 amps and it needs to be running
whenever the engine is running. I figured the LED would come on
whenever I put power to the ECU and Ignition and would quickly go out
if the scavenge pump was pulling power as it should. Thus, even if I
"forgot" to flip the switch for the scavenge pump, the LED would
illuminate until I did AND the pump was pulling power.
Sounds like a plan
Any problem with a motor as opposed to a resistance load? Any other
things I should consider?
There are many ways to deduce whether or not an accessory
is drawing the expected current (and therefore assumed
functional). Since your pump has a 100% duty cycle,
perhaps it would be better to rig the annunciation such
that you have a warning light which speaks to the pump
NOT running. It's a human factors thing that an undesirable
condition be annunciated by turning a light ON as opposed
to watching to make sure a light doesn't turn OFF.
The reed-switch current relay I illustrated in that
article has been incorporated by the thousands in all
manner of monitored systems for many years. A friend of
mine has been manufacturing those sensors in a wide
variety of current detection levels for decades.
I would not discourage you from crafting a similar
device . . . but they DO require a bit of tinkering
on the bench to make sure that the reed is going to
be closed when it's supposed to . . .
Further, keep in mind that the warning light side
of the sensor system needs to be powered from some
source OTHER than power that runs the motor. You don't
want a failure warning light to be dark just because
the supply fuse/breaker is open.
Further, you'll want to have a pre-flight test item
that checks the light for operation by turning the
pump switch OFF.
Emacs!
Power to the light doesn't need "protection"
with fuse or breaker if you mount the resistor
close to the bus-tap . . . the resistor's
current limiting qualities protects the wiring.
A 1K resistor can be installed in the source
wire like . . .
http://www.aeroelectric.com/articles/Homeless/Homeless_Components.htm
Build your relay so that it pulls in with
anything over 2A or so.
http://jameco.com sells reed capsules.
Bob . . .
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