LED Problem

Here is a stupid question: Is there a current limiting resitor in series with these LED's?

If not, they are just forward biased diodes and will pull huge amounts of current. For a very short time until they die.

Some panel mount LED's have built in resistors but most do not.

The forward voltage of most LED's is around 2V (this varies with LED type and color but this is a good approximation). With a 12V battery that leaves 10V to be dropped by the series resistor. The next thing you need to know is what current you want going through the LED. Check the specifications to find the maximum allowed current and do not exceed it.

Ohms Law: E =IR

E = voltage (volts) I = current (amps) R = resistance (ohms)

R = E/I = (12V-2V)/.01A = 1000 ohms

Power disipation by the resistor might be a concern, so lets compute that as well.

P = E*E/R = 10V*10V/1000 = .1W

So a 1/4 Watt resistor will be fine.

If the continuity test LED is designed so that the LED is in series with the ignitor, I recommend using two 500 ohm resistors in series. This way the current through the ignitor will still be limited even if one resistor fails. Very unlikely but having motors ignite while you are a couple feet from them is no fun.

A schematic would have been extremely helpfull.

Stephen wrote:

Well, I have the main control box, and one pad box done for my relay controller. I had to test it. I hooked up the 4 conductor phone wire between the 2 boxes, and plugged 12v into each box (pad and main). The 12v was provided by a marine starting battery. I hook the igniter leads together, push the continuity check button on the pad box and I hear the beeping. I unhook the leads and push the button. No sound. Great so far. I then open up the main control box and push the continuity test button. The LED that indicates continuity (green) flashes yellow then goes red, and then nothing. All while holding down the button. When I tested the ARM feature, the same thing happened to the red LED. It flickered and then dead. Did I just blow them? The battery I am using is rated at around 400amps. Would putting in a fuse help? I would just constantly blow the fuse then. What should I do, what is the problem?

snipped-for-privacy@aol.com (GCGassaway) wrote in news: snipped-for-privacy@mb-m12.aol.com:

Ohm's Law; E=IxR. Take a red LED with a 2.3V forward drop,so subtract 2.3 from 12=9.7V Then for 10ma LED current; solve for R; R=E/I 9.7/.01=970 ohms.Us ethe closest value=1K ohm.

This is a great suggestion David. !!

Well, since the continuity check is not inline with the igniter (relay controller) I am just going to use a single 1k ohm resistor.

Thanks a lot for the help