nuckolls.bob(at)aeroelect
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 Posted: Sat Nov 14, 2009 7:00 am Post subject: Failure Detection and Annunciation |
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Bob,
Was reviewing your article: 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.
[img]cid:.0[/img]
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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( It's MATRONICS FUND RAISER MONTH! )
( Do your part to keep this marvelous )
( tool sharp and available to all our )
( brothers in the OBAM aviation )
( community. )
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