External EMS Warning Light (pin 29)

maddog

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I'm installing an external EMS Warning light (LED) as depicted in the Installation Instructions.
The install manual (page 7-75) states the following:
Aircraft Spruce P/N 17-410 is an example of a light that will work for this application. An LED and resistor in series will also suffice. If you use an LED as the indicator, you must choose a resistor that delivers the appropriate current to the LED, and can accommodate the power required for its current and voltage drop. Also note that the power and ground connections on LEDs are not reversible.
Can anyone tell me the purpose of installing a resistor in series with the LED? Why is it necessary? I've experimented with several resistors: 10k, 5k, and 3.3k and they all seem to allow the LED to operate but at a much reduced intensity.
Any advice will be appreciated.
 

Rhino

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A series resistor is primarily to prevent burning out your LED, or reducing it's life, by allowing too much current through it. It can also pop breakers, and conceivably damage other equipment it's connected to, because an LED effectively becomes a short when it's forward voltage is reached.

 
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Rhino

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By the way, the 17-410 is not an LED. It's incandescent. So if that's what you're using, it explains why the brightness changes so much when you change resistor values. You should use something like the 13-15812, which is an LED, already has a built in resistor, and comes in different colors.
 
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maddog

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Thanks Rhino. Makes sense. I'm using a Lone Star LED indicator. No mention of an internal resistor with the literature. I'll have to check other sources or give them a call.
 

Rhino

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Unfortunately Lone Star doesn't appear to provide resistor information. I'd ask them directly.

 

Rhino

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You mean the LED you got from SuperBrightLEDs was too bright? Whod'a thunk? ;)

The 13-15812 from Aircraft Spruce I mentioned above is dimmable. The one from SuperBrightLEDs might be too (I didn't check).

But maddog may just need to decide what resistance value gives him his desired brightness level.
 
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Rhino

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Nevermind. Apparently it's the Firefox browser on my tablet. Works in Chrome. Weird. Never noticed that before.
 

Rhino

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The LED from SuperBrightLED is just a bulb used to replace existing incandescent bulbs. So it may well be dimmable since it isn’t using PWM.
 
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