LED Calculator & Configuring Multi-LED Circuits

[JonnyMac]

LEDs are popular, but learning Ohm's Law isn't! -- so here's a nice little LED calculator web page that can make your life easier.

http://ledz.com/?p=zz.led.resistor.calculator

When wanting to control multiple LEDs from a single circuit, it's best to put them in series (if you can) versus parallel. Why? Because the combined voltage drop across the LEDs will reduce the required voltage drop and current through the series resistor, causing it to dissipate less heat and be smaller.

Let's say we to run three red (1.4v) LEDs at 20mA using a 12v source.

Series:
  -- current through each LED: 20mA
  -- current through resistor: 20mA
  -- voltage across resistor: 7.8v

If we multiply 7.8 by 20mA (0.02) we get 0.156 watts,  and this is dissipated in the form of heat. The next bigger size resistor is 1/4w -- which we should use for safety -- and is a standard size. Cheap, too. I get them at All Electronics for a nickel each.

Now look what happens when we reconfigure the circuit an make the resistors parallel.

Parallel:
  -- current through each LED: 20mA
  -- current through resistor: 60mA
  -- voltage across resistor: 10.6

Now the wattage dissipated by the resistor is 0.636! This means you'd need to use a 1W resistor for safety, and these are big beast that are not always easy to find.