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LED lighting basics

 

What is a LED?

LEDs are Light Emitting Diodes. Yeah, you knew that. But do you really realize that it is a diode, and what that implies? Being a diode, it is basically an open circuit in one polarity and a short circuit in another. Yes, there are "voltage" ratings, but this is a NOMINAL voltage. If you were to try to regulate a voltage of 3.4 volts (typical nominal forward voltage on a white LED), and tried to hook an LED to it, you would probably burn it out.

This is because of how a diode works, to be simplistic, you cannot regulate the current through a short circuit by varying the voltage.

LEDs work on current. You add a resistor in series to guarantee a maximum current, through the circuit, and thus the current through the LED.

(This is completely opposite from incandescent lamps, which DO work on voltage, and DO need a fixed voltage source)

 

Check: If you get anything from this page:

LEDs need the current through them regulated. This is most easily done with a resistor. You CANNOT do it with a regulated voltage. Their "voltage" is a nominal value, not an operating constraint.

 

"Quick and dirty" resistor calculation

Ignore the voltage drop of the LED, and just calculate the current for your circuit. So if you have 12 volts and you want 20 milliamps through your LED,  then just use Ohms law to calculate.

Ohms law is V = I * R (Voltage equals Current in amps (I) times Resistance in ohms)

You can express Ohms law solving for Resistance:  R = V / I

So R = 12 volts divided by  0.02  (20 milliamps is 20 thousandths of an amp)

R =  600 ohms.

This is a fail-safe calculation, and you do not have to know the voltage drop of the LED. You can't use this where the "input" voltage is very low, or you are using a number of LEDs in series. 

 

Don't ignore wattage!

I'm telling you this FIRST because many people do not even consider the wattage needed to handle the heat dissipated by the resistor, and burn stuff up.

Wattage is  Volts times Amps, or Amps squared times Resistance.

Easy way to use the current (because that is your real target) and the resistance (because that is your result)

So, in the example above, a 600 ohm resistor and 0.02 amps:

W = 0.02 * 0.02 * 600 or 0.24 watts. That's about 1/4 watt.

Now wait! do not go out and buy 1/4 watt resistors!!! Running a resistor at maximum wattage makes enough heat to burn you and melt plastic!

So always try to DOUBLE the resistor wattage. In this case, get a 1/2 watt resistor! Trust me!

 

More accurate way to calculate your resistor:

There is actually some voltage "used" by your LED, so to be more exact, you subtract that voltage (or voltages if LEDs in series) from the "input" voltage. So suppose your LED has a forward voltage drop of 1.7 volts for red, on up to about 2 volts for high brightness, to about 3 for some white ones, to even higher for certain white or blue LEDs.

So, let's say that your LEDs manufacturer specifies 2 volts as the "average forward voltage drop". So your resistor does not need to "control" all 12 volts, because the LED uses 2 of them.. so calculate using Ohms law again, but subtract the voltage drop of the LED from the input voltage.

R = (12 - 2) / 0.02 =  500 ohms...

Suppose you have 2 LEDs in series... then you just add all the voltage drops in:

R = ( 12 - 2 - 2) / 0.02 =  400 ohms...

Led Calculators

Calculate for a single LED: http://led.linear1.org/1led.wiz

Calculate for several LEDs in series: http://led.linear1.org/led.wiz

An even better one: http://ledcalc.com/

Cool device to avoid calculating resistors:

The Supertex CL2 is a small current regulator in a package the size of a transistor. It always limits the current to 20 milliamps, at input voltages up to 90 volts! This is ideal for many applications, and obviously will work on varying input voltages, which a resistor will not do. 

Click here for the CL2 datasheet You can get the Supertex CL2 from Mouser Electronics: http://www.mouser.com/Search/Refine.aspx?Keyword=supertex+cl2

 

Chip resistor package sizes:

  • 0.1 watt: 0603
  • 0.125 watt: 0805
  • 0.25 watt: 1206
  • 1 watt: 2512

LEDs I have used:

 

Osram lwt6g v2-5k-0-20 , this is the Osram TOPLED series, surface mount, PLCC-2 package, 120 degree dispersion, 5k is the color