Determining LED circuit resistor

08/12/2007 Determining LED circuit resistor
I have a small spreadsheet on my PDA into which I can enter a source voltage, the forward voltage of the LEDs I am using and the number of LEDs. It then indicates the resistance required to allow a current either 15mA, 20mA or 25 mA. I can enter the resistance of the actual resistor used and it will show me the current predicted. Seems simple enough and it seemed to match reality nicely.

I noticed however that one in a circuit that has a string of cheap white LEDs connected in series, one LED would flicker occasionally. I measured the current and found that only about 5mA was flowing - insufficient for one of them to light up. By measuring the voltage drop across each LED I found that they did not create a 3V drop as I had entered into my calculations - but they varied from 2.9V to 4.9V.

Further testing has also shown that the current in the circuit wouldsteadily increase over time and eventually level off. If the plateaux is above the maximum current that the LEDs can handle, they will burn out!

So here is my process for handling such beasts.

Assume a forward voltage of 4V for white/blue LEDs (surface mount LEDs may be different)
Calculate the resistor needed based on the supply voltage.
For example: to produce 20mA (0.02A) with a 17V supply and 4 LEDs

(17V - (4*4V)) / 0.02 = 50 Ohms

Then, starting with a resistor >= the answer (50 in the example above), measure the current in the circuit. Let it sit a good 3 or 4 minutes. If the mA reading starts climbing above 20mA remove the power and increase the size of the resistor, and try again. (To measure the current your ammeter must be in series with the circuit.) If the stable current reading is too low ( < 15mA) then decrease the size of the resistor and try again.

A variable voltage power supply comes in very handy when reducing resistor size, as one can start with a low voltage and bring it up to voltage while watching the mA reading, thus preventing LED damage by using a resistor that is too small.

If the characteristics of the LEDs are well known and accurate, one can simply determine the resistor through calculation. By bench testing a set of LEDs together at the correct voltage, one can compensate for gross variations and optimize the resistor value.

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