Re: Update: Extra Taillight Mod

["Ian Overholt" <ioverholt@hotmail.com>]

From: thescirocco@juno.com

Since LED's have a voltage drop of .7V, couldn't you string 17+ of them
in series?

What do you think Dan, would that work?

I have a bunch of LED's laying around somewhere, I might try it...
(on the bench that is, not on my Scirocco) <g>

Peter
'79 Scirocco 16V project car
'83 GTI "Caged Rabbit"
'80, '84, '88 Scirocco rollers


Yes, the LED voltage drops do add up.  So you need to put the right number 
of LED's in series to get 12v.  All LED's do not have the same drop, though. 
  Typically, LED's are considered to be a 1.3v drop, but they can be as much 
as 2.1v.  You'll have to check the spec on the specific LED you are using.  
LED's also do not want more than 200mA of current, so it might be a good 
idea to put a 1/4 watt current-limiting resistor of 1-5 ohms in series with 
the LED's.

Here are some examples:

If you've got 12v at the lights and the LED voltage drop is 1.3v, then you 
need 9 LED's.  The voltage drop across the resistor will be 0.3v and the 
resistor will want to be 3 ohms, dissipating 30mW of power.
12v / 1.3v = 9.23 LED's, call it 9.
1.3v x 9 LED's = 11.7v, meaning 0.3v across resistor
0.3v / 0.1A = 3 ohms
3 ohms x 0.1A2 = 30mW

If you have 13v at the lights and the LED voltage drop is 2.1v, then you 
need 6 LED's.  The voltage drop across the resistor will be 0.4v and the 
resistor will be 4 ohms, dissipating 40mW of power.
13v / 2.1v = 6.19 LED's
2.1v x 6 LED's = 12.6v
0.4v / 0.1A = 4 ohms
4 ohms x 0.1A2 = 40mW

HTH,



Ian Overholt
'88 16v
'78 Champagne Edition

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