X-Virus-Scanned: clean according to Sophos on Logan.com X-SpamCatcher-Score: 1 [X] Return-Path: Received: from [68.202.132.19] (account marv@lancaironline.net) by logan.com (CommuniGate Pro WEBUSER 5.1.6) with HTTP id 1846422 for flyrotary@lancaironline.net; Mon, 19 Feb 2007 02:27:38 -0500 From: "Marvin Kaye" Subject: Re: [FlyRotary] LED built into an ATC fuse To: "Rotary motors in aircraft" X-Mailer: CommuniGate Pro WebUser v5.1.6 Date: Mon, 19 Feb 2007 02:27:38 -0500 Message-ID: In-Reply-To: References: MIME-Version: 1.0 Content-Type: text/plain;charset="iso-8859-1";format="flowed" Content-Transfer-Encoding: 8bit Hi guys, I've looked all over the place and can't find that article anywhere. Anyway, the little LED circuit I mentioned earlier is extremely simple, costs next to nothing if you buy the LEDs and resistors from Mouser or Digi-Key, and hooks up in a snap. Since I couldn't find the article it means that I also couldn't locate the schematic... that's ok, we don't need one for something this simple. The typical connection to a fuse or circuit breaker (you can use this circuit with them as well, if you have a hard time seeing which breaker has tripped) is from the positive side of the power bus to one end of the fuse (or cb). The other end of the fuse, of course, goes on to the load. The LED and resistor connect directly to the 2 fuse terminals. The anode side of the LED (typically the long lead) attaches to the bus side of the fuse. The LED cathode (short lead) connects to one end of a 1/4 watt 1K ohm resistor, the other end of the resistor connects to the load side of the fuse. 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. I hope some of you get some use out of this... it's probably way cheaper than other options, is quick and easy, and gets the job done. Enjoy! PS... I apologize for the basic electricity lesson to all of you who already know that stuff. I thought someone might like the full explanation. "Chris Sargent" wrote: """ Marvin mentions a great capability below in having an LED light up if an ATC fuse trips. """