LED Resistors what value?

[modelman]

Hi all
I have a large quantity of Led's with no spec, any idea how I can find out what value resistor to use to run these on 12v DC.
Any suggestions gratefully received.

 

[Styx]

You can either get a DVM and set it to th DIODE function and find out what its diode-drop is.

Or the rule-of-thumb tha tI usually use is:

diode drop of 1.2V, forward current of 10mA

so from 12V

you want: 12V -1.2V (to give you the voltage drop across the series resistor) = 10.8V

for 10mA: 10.8V/10mA = 1.08kOhms - make it 1k1 Ohms

 

[stevez]

If they are visible light LEDs (they could be IR) then I'd use 10 or 20 ma as suggested then see how they look. If you have lots of them and can afford to waste a few then take the current up somewhat and let it run there for a while. If it lasts then back down from that, maybe 25% and use that value.

I was looking at some curves at one time that plotted current and possibly light output. As I recall there was a knee or other significant hump in the curve that might be a characteristic of all LEDs. Finding that knee or hump might give you some clues. If you have enough of the LEDs it might be worth looking into this method.

 

[hotwaterwizard]

Try this website out.

**broken link removed**

 

[gary_choo]

My suggestion, using a variable resistor (range from about 100ohm to 1Kohm or higher). Connect your 12V battery, variable resistor and a LED in series (before this, please make sure that the variable resistor is set to it's MAX resistance). After the connection, slowly turn the variable resistor to decrease the resistance until u get the desired brightness of the LED.
Without changing anything further more, measure the Voltage across the LED, the value u get is the typical forward voltage of the LED.
Also, measure the current passing through this connection, the value u get is the typical forward current.
u can use this method for ur further LED cct design to determine the suitable series resistor value for ur desired LED brightness.