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measuring current across diodes

Posted 6th March 2010 at 03:09 PM by klewis

Do you know what is the Vf of your LEDs? R1 should be roughly equal to (3 x Vf) / desired_current. So for testing, if your LEDs have Vf = 1.8V or even Vf = 2.1V, and we're aiming for something around 20mA, then you can use R1 = 300R.

For 1.8V LED
I = 3 * 1.8 / 300 = 18 mA

For 2.1V LED
I = 3 * 2.1 / 300 = 21 mA

The 300R resistor can be 1/2W or whatever you have around.

You will know it works well if you measure the voltage across R1 and divide it by 300 and if the result is close to about 20. Of course you should see the LEDs on.

Try to implement just the CCS portion with a 300R for R1.
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  1. Old Comment
    jan.didden's Avatar
    Does the value of R1 (which I assume is the R in series with the LED) not depend on the voltage you have available to drive the LED?
    If I have 12V and an LED with a Vf of 2V @ 20mA, I need to drop 10V @ 20mA across R1. That makes R1=500 ohms.
    So, in other words, you can't say what resistor value you need unless you know your supply voltage.

    Possibly what you meant is that to stabilize the current through an LED, you should make sure that the voltage across the R is a few times greater than Vf. If you do that, temp and Vf spread has very little effect on Iled from one LED to another. Is that what you meant?

    jd
    permalink
    Posted 7th March 2010 at 08:31 PM by jan.didden jan.didden is offline
 
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