LED lighting basics


As usual, I try to interject some humor in with all of these dry facts. Also, as usual, there are all kinds of old wives tales about LEDs and you will find people with "information" which contradicts what I say here.

All I can tell you is that in this area, like many in the hobbies, there is a ton of misinformation based on second hand knowledge, extrapolation of something "similar", and just plain made up stuff.

This page is intended to help people who are trying to understand how to use LEDs and want to use them "right", i.e. your installation won't blow up in a week, or stop working for an unknown reason.

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. If your "expert" buddy says I am wrong here, you should either keep reading and realize he knows nothing, or stop reading and take your chances.

This is because of how a diode works. This should help you understand, you cannot regulate the current through a short circuit by varying the voltage. Yes, you have a friend that did this. It's dangerous, most likely he was running way below the rated current. A diode is, again, basically a short circuit in one direction, so regulating the voltage through a short circuit is impossible.

LEDs work on current. They work the "opposite" of incandescent lamps. You MUST add a resistor in series to control the 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 "dropping" resistor. You CANNOT do it with a regulated voltage. Their "voltage" is a nominal value, not an operating constraint. Ignore people who claim anything other than these statements, say "thank you very much" and walk away.

LEDs on AC:

Simple rule, don't do it unless you not only have a resistor to regulate current, but also a regular diode in series? Why? Because LEDs are not your typical diode/rectifier. All diodes have what is called a "PIV rating", which stands for Peak Inverse Voltage. Your typical diode used in the hobby, the 1N4001 is a 1 amp diode with a PIV of 50 volts. Well, LEDs do have a PIV rating, but normally it is very low, so low that they can short out at 12 volts AC or more. Yes, I know you have a friend that has never had this happen. It can and it does. (don't take any more advice from that friend too)

"Quick and dirty" current limiting resistor calculation

All components in a complete circuit "use" voltage, and the voltage they "use" is called the "voltage drop". A "complete" circuit is just a path that goes around and around, sort of like a race course. Normally you have a power supply, and a number of components hooked in series. The thing about a complete circuit is that the sum of the voltage "drops" (things that use energy) has to equal the voltage source.

So in a 12 volt circuit with one thing connected, that thing will "see" 12 volts.

In another 12 volt circuit, suppose you have 2 identical resistors hooked in series (one after the other), then each resistor "sees" 6 volts, the 2 summed up = 12 volts (6+6)

To do the calculation for a LED you should know the NOMINAL forward voltage drop, which can be about 3.4 volts or so, but you can simplify,

For a quick and dirty way to figure the resistor needed, ignore the voltage drop of the LED (assume it is a dead short, 0 volts), and just calculate the current for your circuit. So if you have 12 volts and you want 20 milliamperes through your LED,  then just use Ohms law to calculate.

Ohms law is V = I * R

Voltage equals Current (in amps) times Resistance in ohms
(I stands for current)

You can also express Ohms law by solving it for Resistance: 

R = V / I
Resistance equals Voltage divided by current

Standard current in most LEDs is 20 milliamps, or 0.02 amps

Let's calculate for 12 volts:

R = 12 volts divided by  0.02 

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. 

Attention: Don't ignore resistor 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:

Watts = 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 DOUBLE the resistor wattage. In this case, get a 1/2 watt resistor! Trust me!

Be careful, at higher voltages you may need a 1 watt resistor, and most people miss this.

Another thing to remember, no matter WHAT the size of the resistor, if your circuit dissipates 1 watt, that 1 watt of heat will be generated in your model, that heat has to go somewhere, into the air, melting plastic it is touching, etc.

More accurate way to calculate your resistor:

There is actually some voltage "used" by your LED, the "voltage drop" of the LED itself, since it is not a "perfect" short circuit.

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. (These are typical voltage drops)

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.

So your 12 volts on the input is "split" between the resistor and the LED, 2 volts in the LED and the rest in the resistor. So when we calculate the resistor, it only needs to "drop" 10 volts in the circuit.

Below you see (12 - 2) volts in the calculation of the resistor:

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 resistor Calculators

Now you know how do do it yourself, I'll give you a few links to online calculators for LEDs and resistors:

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/

Hey, this is a pain in the butt! Is there an easier way? A gadget to limit the current for me?

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. 

You just put it in series with your LED(s) and no matter what the input voltage, you have a guaranteed 20 milliamps through the LEDs. This way you can string a whole bunch of them in series and use just one CL2.

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

 LEDs I have used and misc info:

on ebay, look for bestshop2008hk for individual leds...

I found incredible prices on led strip lights from newyork_lin,  $8.50 for 5 meters!

https://www.superbrightleds.com/ - look under components... then through hole for your "traditional" types, note they have a 360 degree led!

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

Chip resistor package sizes:

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

Prototype lighting:


here's a cheap rotary beacon used for legos, 4.5 mm in diameter


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