A word of caution for those of you planning to make modifications of the
electrical circuit recommended for the Rotax 912 series engine and its
associated voltage regulator. Please remember that, although Rotax state
that the low voltage warning light is optional (and therefore could be
omitted without affecting the operation of the regulator), the circuit was
designed to operate with a 12 volt/3 watt lamp. If you substitute a device
such as an LED that draws a much lower current, then it may not reliably
indicate when there is a low voltage condition -- for example, it may give a
false warning and light when the alternator output voltage is normal.
If you are determined to use an LED, then it could be wired in parallel with
a resistor that would draw additional current to bring the total up to the
250 milliamps of the 3 watt lamp, say 50 ohms -- but it would of course have
to be one of 3 watts rating (say, how about using a 3 watt lamp?).
This brings us to the question of why you would wish to make any change --
are you trying to improve reliability, or to save current when the
alternator has failed and you are relying on the battery to get your home?
You might experiment to see how little current you can get away with, but I
suspect individual regulators would vary in their requirement. I am sure
that there are many electronics experts out there who would be only too
happy to offer you a circuit that would take very little current to drive an
LED when there was a low voltage condition.
Mike
-----Original Message-----
From: Fred Fillinger
Subject: Re: Alternator warning lamp
Hi, Javier --
Light-emitting diodes have a voltage drop of about
2 volts, and the typical variety wants no more
than 20 milliamps. So, 10/.02 = 500, or a 470 or
510 ohm resistor (1/4 watt OK) is what you want.
Regards
Fred Fillinger, A063
> Barahona Alonso, Francisco Javier wrote:
>
> Dear friends,
>
> Months ago somebody advised me about the
> possibility of substituting the alternator
> warning lamp (3W, 12V) for a diode plus a 480
> Ohms resistor. Making calculations I obtain
> that I need a resistor of 48 Ohms instead:
>
> R = (V
2)/P = (12
2)/3 = 48
>
> I do not know which is the resistance of the
> diode. Could somebody confirm me which of both
> resistors, 48 or 480 Ohms should I use?
>
> Best regards.
>
> Javier Barahona (Classic #187)
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