Question about amperage/voltage

Fly-n-hi · Post by **Fly-n-hi** » Fri Oct 24, 2008 12:49 am

This is for you guys that know more than I do...which is probably most of you.

Let's say that you have a circuit with 55 LEDs. Each LED runs on 20ma. That would be 1.1 amps, correct? Let's assume that the supply voltage is 12v, the LEDs are wired in parallel, and the correct resistors are used.

In this set up, if your power supply was 12v and only 1 amp, what effect will this have on the LEDs?

I ask because this is happening in my model. I have 154 LEDs (3.08 amps, if I'm not mistaken) being powered by a 3.0 amp power supply. It has worked for several weeks but now the lights will not come on. I can tell power is running through the circuit because I get a spark when I connect the power supply to the leads in the model. My guess is that there is a short somewhere in the model but I took good care to keep everything insulated. In fact, I cut open the model in a few places to chek for any wires that may be touching and I can't find any shorts.

I've tested the power supply with other LEDs and it works fine. I'm very confused. Would a power supply with more amperage fix this?

Thanks.

Fly-n-hi · Post by **Fly-n-hi** » Fri Oct 24, 2008 12:58 am

Also, this may be a bit of a stretch, but I was using wet sand paper to do some filling and sanding. Now if water got in there it could short it out. If that were the case is the circuit already ruined or will it work fine when dry?

I wanted to mention that when I apply the power source to the leads on the model the lights flicker on very briefly.

Madman Lighting · Post by **Madman Lighting** » Fri Oct 24, 2008 10:18 pm

I'll go out on a limb here and guess that you may have used too small a resistor to limit LED current and one or more LEDs has over loaded and is now a short circuit.

Did you consider tolerances when you sized everything? Resistors typically have a 5% tolerance which means that if you used a 1k resistor, its true value can be anywhere between 950 ohms and 1050 ohms. If you sized the resistors to allow 20mA into an LED, and its absolute max is 20mA, there's a 50% chance you've overloaded an LED somewhere.

A rule of thumb for "robust" designs is to run 15% below abs max.

Another way to guess is the LED brightness. I know LEDs are very bright these days but if the resistor was undersized, the LED would be "extra" bright and burn out eventually.

Hope that helps and your're not doing this:

Good luck!

-John C

jwrjr · Post by **jwrjr** » Fri Oct 24, 2008 10:42 pm

If you wire up the leds in series strings of 2 each you will use 1/2 as much current and generate less heat from the resistors. Leds are fairly reliable, so failures should not be a problem. A 12 volt power supply will easily power 2 of any common led in series.

Fly-n-hi · Post by **Fly-n-hi** » Sat Oct 25, 2008 2:52 am

Madman,

You could be right. The resistors are right at the limit. I did not take into account the 5%. I did notice one LEDs flicker very quickly just before it happened.

Maybe I'll start with that LED, replace it and the resistor, and lower the voltage. I'll let you know if that works.