Need help wiring some LED's

[kinger]

I'm fairly good at wiring things but I'm a little puzzled on these LED's I want to use for activation switches for my nitrous system.

I'm using these LEDs:
http://theledlight.com/panelmount.html

I have a switch that activates the Bottle heater and one for the fuel pump and solenoids. Both have a relay and I'm on the low amperage side.

I wired up a RED led to the bottle heater switch and it works perfect when I flip the switch. The switch is run directly to the battery and to the relay.

I wired up a GREEN led to the fuel pump/activation circuit and it it worked great for 15 seconds then flickered and burnt out. This switch is run to the battery, then to the fuel pump which draws around 2-3 amps, then to the relay to activate the solenoids.

My question is should I be more worried about the amperage or the voltage? The LED place wants to sell me a small regulator to keep 12V and not let it get to 13-14v like I know they system can.

Should I do that?

I measured 496 ohms on the circuit at 12V = 24 mA which I think is close enough to the spec of 20mA

Thoughts?

 

[Rockmeister]

Hey kinger!

Anything over the 20mA will burn out the LED.
LEDs are very unforgiving on too much current.
Usually are real bright for a few milliseconds! lol

Most LEDs internally are 1.4v.
So if you have 14v - 1.4v = 12.6v across your limiting resistor.
12.6v divided by 15mA = 840 ohms
12.6v divided by 20mA = 630 ohms
12.6v divided by your 496 ohms = 25mA
25% over current.

The problem gets worse as in some charge modes, most chargers can get to 15-16v.

16v - 1.4v = 14.4v
14.4v divided by 496 ohms = 29mA
Way over current, almost 50% over.

This is the resistance you need:
14.4v divided by 20ma = 720 ohms total resistance.
720 ohms - 496 ohms = 224 ohms
Nearest standard resistor is 220 or 240 ohms.
This is the resistor you have to add in series to your LEDs, do this and they will last longer than your sled.

The wattage is incredibly low so about any wattage resistor will work.

Usually if the LED is "rated" for 12v most manufacturers know you may use it in/on a vehicle and will make them ok for 16v with a larger current limiting resistor built in.
I know the Radio Shack ones are ok for vehicle use.

Surprised these aren't.

Hope this helps!

 

[kinger]

Thanks rock! I will pick up some resistors and add them to the LEDS, I'll probably do the Red one as well even though it seems to be working OK. I'll try to measure the current before i button everything up and make sure I'm under 20 mA. Thanks!!!

 

[Len Todd]

Good explanation rockmeister!

 

[Rockmeister]

Good explanation rockmeister!

Thanks Len,
I appreciate that!

You are welcome kinger!

How have the LEDs worked out now?

 

[kinger]

I added a 220 ohm resistor in series which at 13V will give me 16 mA So I'm covered. hooked it all up and the green ran twice as long as before and looks like it will last forever! I'm just mad that they dont put a bigger resistor in, why build for EXACTLY 12V?

Anyway thanks again!!

 

[Rockmeister]

Glad it worked out kinger!
Looking great.


http://www.ty4stroke.com/viewtopic.php?t=64698

Check out my sled build, is going well!
Will let you know how the reverse works out.
It's raining now, so I may have some time to design and fabricate it.

 

[euhero987321]

Really good thread... Learn something this day.. Thanks