Date:         Wed, 9 May 2012 17:48:47 -0400
Reply-To:     David Beierl <dbeierl@ATTGLOBAL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@ATTGLOBAL.NET>
Subject:      Re: Charge warning light LED question - David B.
Comments: To: Scott Daniel - Turbovans <scottdaniel@TURBOVANS.COM>
In-Reply-To:  <4FAADA94.1080806@turbovans.com>
Content-Type: text/plain; charset="us-ascii"; format=flowed

At 04:59 PM 5/9/2012, Scott Daniel - Turbovans wrote:
>it works fine.
>I know an LED would be brighter.
>LED's have polarity ..
>so wondering if an LED will work there for a warning light.

You could use an LED the way the Vanagon does, but in addition to the
series resistor for the LED you'd have to add another resistor in
parallel to the pair, to get enough current to excite the
alternator.  Take a look at 90.8 fig 4.  R3, 150 ohms one watt or
better (notice the space around it) is wired from panel hot to
D+.  K2, the indicator LED in series with 470 ohm R4 is also wired
from panel hot to D+.  You can see this on 97.91 track 50-51.   As of
'87 they apparently changed to two shunt  resistors instead of one,
presumably 300 ohms each (97.108 track 50-51).  I suspect they did
that because they were getting overheating problems on the flexible
circuit, so they put in two half-watters in parallel to spread out
the heat load a little.

In this setup the 150 ohm resistor is supplying the "tickle" current
to get the alternator going; the contribution from the LED and its
resistor is pretty minor.

Yrs,
d