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re:Window Switch Lamp Repair for Archives - Long

KSMITH1@mailgw.sanders.lockheed.com writes:

... much usefull stuff deleted ...

<< In the switch I just repaired, the 
LED was in series with a 270 ohm resistor.  This would allow 50 mA to 
flow with 13.5 volts across the switch.  I don't know if this was a 
fabrication error or Audi uses particularly robust LEDs (evidently not 
given their propensity to fail), but for the new Radio Shack lamp, a 
larger resistance is called for.  Replace the resistor with a 1 kohm 
0.25W resistor, or thereabouts (up to 1500 ohms would be fine). >>

I don't mean to be quarrelsome (there's enough people on this list
for that!), but a 270 ohm resistor won't give you 50ma.  An LED has a
voltage drop of around 1.6 volts.  The actual current is then:

i = (13.5V - 1.6V)/270ohm = 44ma.

I do agree that this is quite a bit.  You can find bright LED's that
can handle this much, but for the common 10-20ma LED you would want
a larger resistor:

R = (13.5V - 1.6V)/10ma = 1200 ohms  - 1/4 watt.  So anywhere from 1K
to 1.5K would work, like you said.

Thanks for your procedure.  This is something I plan to tackle on my
'88 90Q soon...


Steve Valin	http://reality.sgi.com/steve_engr	steve@sgi.com
    		"Time wounds all heels"  -Crackerbash