Resistance is Futile!

LEDs will consume as much current as is fed to them. This is where resistors come in to save your LED from burning out prematurely. Resistors are passive components, without polarity, which have the ability to reduce current. The higher resistance, the less current. Lower resistance, more current.

Not to scare anyone away with a little math but choosing the correct resistor for your LED involves the Ohm’s law equation: V = I x R (Voltage = Current x Resistance).

In order to calculate resistance there’s a few variables we need to know first:

  • Voltage of power source.
  • LED’s forward voltage.
  • LED’s forward current.

Looking at another equation, here’s how we’ll calculate resistance needed:

Resistance = (Power Source Voltage - LED Voltage) / LED Current

As a more concrete example, let’s say we have a 3 volt input from a power source and a red LED. We know from the charts in this article that we’re dealing with 20mA of current and for a red LED, a 2.0 forward volts. A quick note, when calculating milliamperes we need to first convert to Amps (A) by dividing by 1000. So let’s look at our equation one more time:

Resistance = (3v - 2v) / 0.02

The result of the above would be 50, so you’ll need a 50 Ohm resistor. It’s possible that your result based on your application has a resistance value of something like 104 Ohms, which resistors of that value would be very hard to come by if they exist at all. In these cases you’ll want to round up. If you’re feeling dangerous, you could round down but remember – too much current will fry your LED eventually!