led's and resistors?

[DEATH_INC.]

If I wanna use a bunch of led's on my bike (say 5,or 10)how big of a resistor do I need to keep 'em alive? anyone know?(out of a broken hi-stop)

[scooterboynz]

When using a Light Emitting Diode (LED) as an indicator, use the following formula to determine series resistance for various voltages: R = (E - 1.7) x 1000/I, where R is the resistance in ohms, E is the supply voltage (DC) and I is the LED current in milliamps.
Eg. to operate the LED at 20mA, for 6V, use 220 ohms

9V use 390 ohms.
12V use 560 ohms
24V use 1.2k ohms

[SlashWylde]

Looking at the sketch in the attachment, you need to subtract the voltage drop across the LEDs (about 1.2 to 1.5v for most LEDs) from the +14v supply. Tip, measure your battery supplyto the light with a multimeter whilst the bike is running to get an accurate figure. This gives about 12.8v across your current limiting resistor. Next, find out how much current each of your LEDs draws at full brightness (usually 10mA to 30mA) and multiply by the number of diodes. i.e 5 diodes at 20mA = 100mA.

Therefore the value of the resistor will need to be R = V/I or R = 12.8v/100mA = 128ohms. You may not find the exact value of resistacne on the shelf given by your calc, so choose the closest available value and err on the high side for the sake of caution. If the light shines too dimly you can always reduce the resistor value a bit, later on.

[DEATH_INC.]

Cheers guys.
One last question,how do I determine the current/voltage drop of the led's?

[FlyingDutchMan]

Just a note: You shouldn't put LEDs in parallel (as in SlashWylde post), unless they're exactly identical (never going to happen) as some will have lwer impedances tha thers making them shine at different brightnesses. You'll have to put them in series.


+14.5V--RES--LED--LED--LED--GND

So thats 14.5v - 1.2v*3 = 10.9v to go across resister. @ 20mA that means it will have to be 545 ohms, so round it up to 560 ohms.

[scooterboynz]

i quite often use led's at work , i use one resistor per led, they only cost a few cents so it doesnt break the bank , also this means if a resistor or a solder joint fails only one led fails , so just buy the 560 ohm 1/4 watt resistor and you will be fine.

[Virago]

Cheers guys.
One last question,how do I determine the current/voltage drop of the led's?

This info is specified on the LED when you buy it.

An example from Dick Smith:

http://www.dse.co.nz/cgi-bin/dse.sto...uct/View/Z4013

[FlyingDutchMan]

Cheers guys.
One last question,how do I determine the current/voltage drop of the led's?

The voltage drop depends on the colour - doesn't matter if 2mA or 20mA is going through, the voltage will be very close t 1.3v for a red LED. Because of this, it is pointless to try and control LEDs by controlling the voltage - it much simpler to control the current - hence why you'll hardly ever see a LED without a current limiting resistor in series.

[Virago]

Depending on what you want to do with them, you can get some LED's that don't require resistors. Another example from Dick Smith (a flashing LED):

http://www.dse.co.nz/cgi-bin/dse.sto...uct/View/Z4044

I use these ones frequently.

[notme]

What are ya trying to do mate? With more info I could do the calcs for you...although there's enough info already posted by others

If the LEDs are out of a hi-stop light, can you just use that as is? or do you want them all seperated around the bike or something? Reason being, the hi stop light has everything you need already, you just feed it 12V !

[notme]

http://www.kpsec.freeuk.com/componen....htm#calculate

[DEATH_INC.]

What are ya trying to do mate? With more info I could do the calcs for you...although there's enough info already posted by others

If the LEDs are out of a hi-stop light, can you just use that as is? or do you want them all seperated around the bike or something? Reason being, the hi stop light has everything you need already, you just feed it 12V !

I only wanna use some of 'em,and it's a busted one so some of it's missing....