Date:         Thu, 7 Jun 2012 15:12:33 -0700
Reply-To:     mark drillock <mdrillock@COX.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         mark drillock <mdrillock@COX.NET>
Subject:      Re: more mysterious circuitry
Comments: To: pickle vanagon <greenvanagon@GMAIL.COM>
In-Reply-To:  <CA+n284Npku-9O_43M7Jz5j9XY8j_kQm+bSjmbpsfNa2FRzCxmA@mail.gmail.com>
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No mystery. The parallel resistors at 60 ohms are what provides the
excitation current for the alternator to begin charging. This current
path goes AROUND the led. The 470 resistor in line with the led is to
limit the amount of current that flows THROUGH the led, to give the led
a longer life. So the 2 things are related but have different important
functions.

Replacing the 60 ohm set with a 50 ohm set will increase the current to
the alternator field during the time the key is on but the alternator is
not yet charging.

Resistors heat up when current flows through them. They used 2 resistors
so the heat would be shared by 2 and neither would get as hot as if it
was just 1. This really only matters during the time the alternator
light is on. Earlier clusters had only one resistor but it must have
sometimes been a problem so they went to 2.

Mark

pickle vanagon wrote:
> Mysterious to me at least... I'm sure to the Beierl's of the word it makes
> perfect sense!
>
> The alternator led circuit has me confused.  (This is shown, for example,
> on 97.74).
>
> It goes from 12 volt power supply, splits into two tracks.  The first track
> goes through a 60 ohm resistor, the second track goes through a 470 ohm
> resistor and then through the alternator led, and then they meet up again
> and go through a diode on their way to the alternator.
>
> (The 60ohm resistor is actually two 1watt 120ohm resistors in parallel).
>
> Why does this need to be like this?
>
> My curiosity stems from the fact that I have two 1 watt 100ohm resistors
> available, and I'm wondering if I can replace those 25yr old 120ohm
> resistors with them, or if I need to leave them be.  I'm also wondering
> what parameters I need to worry about from the standpoint of upgrading that
> LEDto something brighter.
>