Exceptionally Good LED Power Tutorial

The reason for not using current limiting resistors to drive LEDs.

Assume one 3.3V LED running off 12.5V at 20mA using a current limiting resistor.

Voltage drop across the resistor, 12.5V – 3.3V = 9.2V.
Resistor value, R=V/I, R= 9.2V/.02A, R = 460 Ohms.
Power dissipated in LED, P=VI, P=3.3V*.02A, P=.066Watts.
Power dissipated in resistor, P=VI, P=9.2V*.02A, P=.184Watts.
Total power from power supply, P=VI, P=12.5V*.02A, P=.25 Watts.
Efficiency = Pled/Pin, .066W/.25W, Efficiency = 26.4%! Not good.

If voltage goes up to 14V then:
Voltage drop across the resistor, 14V – 3.3V = 10.7V.
Resistor value from last calculation, R = 460 Ohms.
New current in LED and resistor, I=V/R, I=10.7V/460 Ohms, I=.023A.
Power dissipated in LED, P=VI, P=3.3V*.023A, P=.076Watts.
Power dissipated in resistor, P=VI, P=10.7V*.023A, P=.25Watts.
Total power from power supply, P=VI, P=14V*.023A, P=.322 Watts.
Efficiency = Pled/Pin, .076W/.322W, Efficiency = 23.6%! Not good.

From this it can be seen that the LED current will go from .02A with a 12.5V source to .023A with a 14V source. A current increase of .003A or an increase of 15%. Not too bad.

A current regulated power supply will hold the current to .02A and still have an efficiency of around 90%. Note it has to be a switching supply, a linear supply will be no more efficient.

So what happens if you put several LEDs in series, lets say 3 LEDs.

Total voltage across the 3 LED string, 3 * 3.3V = 9.9V.
Voltage drop across the resistor, 12.5V – 9.9V = 2.6V.
Resistor value, R=V/I, R= 2.6V/.02A, R = 130 Ohms.
Power dissipated in LEDs, P=VI, P=9.9V*.02A, P=.198Watts.
Power dissipated in resistor, P=VI, P=2.6V*.02A, P=.052Watts.
Total power from power supply, P=VI, P=12.5V*.02A, P=.25 Watts.
Efficiency = Pleds/Pin, .198W/.25W, Efficiency = 79.2%! Much better.

If voltage goes up to 14V then:
Voltage drop across the resistor, 14V – 9.9V = 4.1V.
Resistor value from last calculation, R = 130 Ohms.
New current in LEDs and resistor, I=V/R, I=4.1V/130 Ohms, I=.0315A.
Power dissipated in LEDs, P=VI, P=9.9V*.0315A, P=.312Watts.
Power dissipated in resistor, P=VI, P=4.1V*.0315A, P=.129Watts.
Total power from power supply, P=VI, P=14V*.0315A, P=.441 Watts.
Efficiency = Pled/Pin, .312W/.441W, Efficiency = 70.7%! Not as good as with 12.5V.

From this it can be seen that the LED current will go from .02A with a 12.5V source to .0315A with a 14V source. A current increase of .0115A or an increase of 57.5%. Not very well regulated with using a resistor and may well destroy the LEDs

A current regulated power supply will hold the current to .02A and still have an efficiency of around 90%. Note it has to be a switching supply, a linear supply will be no more efficient.


If you look at a voltage from 10.5V, a dead battery to 14.8V when charging the current regulation with a resistor is really bad and most likely will cause a disaster.

Hope I did not make a math error!

Mex, I don't remember exact specs but IKEA have a whole bunch of LED panels that you plug into 120 VAC. Not cheap, but nice looking.

The so-called 10-watt 12 volt chips I purchased manage to get the munchies and consume 15 watts at 12.03 volts. My dream is to find a reasonably priced adjustable voltage regulator 120vac to 30-33vdc, 100 watts.

Talk About Being A Dreamer :)

Most LED assemblies have the needed regulators in them, either a simple resistor or a current regulator (Which may be nothing more than a 3 terminal voltage regulator with a resistor) ...

His info is for those who BUILD the assemblies themselves.. IE: ME.

And it is a good start. You also need to know Mr. Ohm's Law.. you know, Resistance is futile.. Oops. that was Davros' Law. Ohm's is I=E/R so R = E/I you also need to know the forward bias voltage drop for the LED. By my calcs a JUMBO High Brightness LED from Radio shack needs about 450 Ohms for a RV. The standard size LED in my light up grab bar cane with a 150 Ohm. and promptly burned out from too much current.. So did the one I repalced it with, Then I metered the resistor and upgraded it and have not replaced one since. NOTE teh Jumbo High Brightenss led sucks a bit more current than the standard ones.

The neat part about knowledge is having the ability to establish educated compromises.

So he wants to say that if the current per LED is over 0.02A, you should add a resistor to bring it down to 0.02A. No problem.

OTH, if it's already lower than 0.02, you should regulate the voltage to bring it up to 0.02A. Many users won't like the idea of voltage regulation.

In other words, - order a few dozen of Chinese boards, throw away those that are not close to 0.02A at whatever is prevailing voltage in your DC system, and use the rest.

Well written and excellent calculator for multi led projects