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Date: Fri, 23 Feb 2007 08:19:14 -0500
From: Ernest Christley <echristley@nc.rr.com>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: LED built into an ATC fuse
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Marvin Kaye wrote:

>
> Here's how it works... let's assume this circuit is for a single 100 
> watt landing light and the circuit is protected by a 10 amp fuse.  
> When you turn on the light virtually all of the voltage is dropped 
> across the light bulb because of its very low resistance (maybe 1.5 
> ohms... 12v / 1.5 ohms = 8 amps).  At this point there's just about a 
> direct short across the LED/resistor part of the circuit in the form 
> of the fuse (something like .1 ohm) so the current takes the path of 
> least resistance...  just about all of it flows through the fuse,  and 
> next to none through the LED and resistor.  So the landing light is 
> dropping 11.99 volts, and there's maybe a 8-9 millivolt drop across 
> the LED/resistor, so there's nowhere near enough current there to 
> light the LED.  Somewhere along the line we were working on our fuse 
> panel and accidentally put a 5 amp fuse in the landing light circuit.  
> We go to turn on the landing light and bango!, we blow the fuse.  (100 
> watts / 12 volts = 8 amps... the fuse gives up the ghost at 5, maybe a 
> little more.)  The current path is now through the LED and resistor 
> instead of the fuse, so virtually all of the voltage is dropped by 
> them and there's maybe 15 millivolts left over for the landing light, 
> which is nowhere near enough for it to light up.  The LED, however, is 
> merrily glowing away to let you know that the fuse has failed.  Pretty 
> cool, eh?  There is a catch.  If the fuse is blown or removed _and_ 
> the circuit is powered down, (ie, the switch to the landing light is 
> off) the LED will not glow.  The path through the load must be 
> complete for current to flow through the LED and resistor.  As soon as 
> you flip that switch, though, the LED will light up telling you to 
> chech that fuse and life is good once again.

Here's another option...lighted switches.  There's a little LED in the 
bat handle of the toggle switch that lights when the switch is turned 
on.  As long as the light comes on, I know that there is power coming 
into the switch (ie, the fuse is still good).  It also works as a dandy 
reminder that something needs to be shut down on postflight.

-- 
         ,|"|"|,              Ernest Christley       |
----===<{{(oQo)}}>===----    Dyke Delta Builder      |
        o|  d  |o        http://ernest.isa-geek.org  |