# Resistor help

Discussion in 'General Discussion' started by No3ntry, 5 Feb 2006.

1. No3ntry

# Location: Derbyshire

I have an LED which is 3 volts

what resistor will i need to allow it to run on a 12v system

Thanks James

2. Aod

# Location: London

what current does the LED run at?

Last edited: 5 Feb 2006
3. No3ntry

# Location: Derbyshire

30ma

would it matter if i ran a little more current through it ?
say for example 6 or 9 volts what effect is this likely to have long term ?

i have tested it with both 6 and 9 volts and it workd fine but 12v blew one of these leds

Thanks James

4. lordedmond

# Location: Tilchestune

ok 3 volt led
supply 12 vdc

current though network = 30ma or 0.030 amp

you need to drop 9 vdc

thefore ohms law r = v/I

r = 9 / 0.03

= 300 ohm

wattage of resistor p - v * I
9 * 0.03 = 0.27 w

therfor a half watt 300ohm would do the job

increased current will blow the led quicker and give higher output

5. No3ntry

# Location: Derbyshire

I have tried the LED in place at 3V and it wont be bright enough but at 6 it is *just* bright enough and at 9V is spot on (using small 3v watch batteries)

but wiring it directly to the cars 12v burnt it out almost straight away
does the amount of increased current decrease the life of the LED on a linear scale ?

eg 3v - life = 50,000 hours
6v - 25,000 hours

if current gives higher output what does the increased voltage do ?

6. lordedmond

# Location: Tilchestune

OK

power ( watts ) is v * I
eg 1 watt = 1 volt * 1 amp

therfore if the resistance remains the same increase the volts will increase the current as well

if you put the volts up to 6 the network ( led + resistor ) will draw 60 ma

BTW you can get 12 vdc leds high intensity ones as well and save you the trouble

RS sell then as do the other electronic component shop they have a store opp. the old Players office in Notts ( now Nat West office )

7. Aod