John,
I used
a relay on mine. I saw a
schematic somewhere, or read that the best to do was to use the alt warning
lead from the alternator to pull a relay. I then used a normally open contact on
the relay to provide 12VDC to an LED on the I/P through a 680 ohm
resistor.
Mine
seems to work fine.
Steve
Brooks
Cozy
MKIV #1071
CH
25-finishing
-----Original
Message-----
From: Rotary
motors in aircraft [mailto:flyrotary@lancaironline.net]On Behalf Of John Slade
Sent: Friday, March 12, 2004 8:10
AM
To: Rotary motors in
aircraft
Subject: [FlyRotary]
charging
I'm
still having trouble with my alternator warning light.
I know
we discussed this before, but would someone refresh my mind?
Currently I have an
LED in the circuit as the warning light. It doesn't go out. Buss voltage seems
fairly stable at 14.2v with engine running. Do I need a regular bulb rather an
LED, and what are the specs of the diode shown in the
schematic?
John
Slade (hit 25 mph last night)
John,
Not certain about your setup, but as Steve
indicated its probably best not to have a LED in series with the lamp or
idiot light wiring into the alternator. It appears that the Mazda
Alternator uses the current through that line to bootstrap the field coils and
start the alternator producing electricity. After the alternator starts
producing then it no longer needs the voltage from that wire.
In the car circuit a relay and diode are
in that line and as Steve said using the points of the relay to control the
voltage to your lamp (type probably doesn't matter LED or
incandescent).
If you have a lamp or LED or lamp in that
line and should it fail - then no startup voltage will be supplied to
your field coils on start up and no voltage from the alternator. It will
appear that your alternator has failed and normally no voltage would be
produced by the alternator.
The probably reason your LED
stays lit is you do not have the diode in the circuit. With
the diode there it would be back-biased by the voltage from the
alternator. If back-biased, then the diode cuts off ALL current flow and
the light will not stay lit. I suspect that without the diode there is
still sufficient current flow after the alternator starts to cause a voltage
drop a cross your LED sufficient to keep it lit.
I would recommend using the circuit that
has a 12V relay and a 16 volt diode in series. You can get both from
most Radio Shack stores (or used to be able to). The base (sharp point
of the diode symbol) should point toward the alternator and away
from the battery voltage. This results in 12Volts of
forward-bias across the diode. That way when the alternator is not
producing voltage (say at start up), the diode is biased so that current flows
through it and therefore the relay causing the relay to close which in turn
lights on your lamp/LED (and also provides the bootstrap voltage to your
alternator's field coil). Once the alternator starts
producing voltage, the voltage on the alternator side of the diode
starts to increase, when that voltage approaches the 12Volts on the other
(battery) side of the diode the voltage difference across the diode approaches
zero (12V on battery side minus 12Volt on alternator side of diode =
zero) the diode stops conducing current . When the diode stops
conducting , the current flow through it and the relay stops. The relay
no longer can stay closed so the points open and your lamp goes out indicating
the alternator is functioning.
Again, if your alternator is similar to
the Mazda alternator set up. A failed bulb in that line can fail and your
alternator may not produce voltage. Using the LED without the diode
probably permits sufficient current to flow even after alternator startup to
keep the LED lit. If you just want to make the light go out then inserting the
diode as I mention would do that - but, I strongly recommend not have a lamp
in the direct line but use a relay to interface to the lamp.
My 0.02 worth.
Ed Anderson