Resistors

Last week we indicated here that you had to protect your LED with a resistor of between 200Ω and 1kΩ.

This week, we will tell you why! Let's get started with a simple assembly...

Assembly

The following assembly uses a light-emitting diode or LED, a switch, and a resistor. When the switch is closed, the current passes through the resistor and the diode, so the diode lights up. This same assembly can be drawn as an electronic diagram as follows (the switch is drawn in the closed position here):

If we look at the documentation for standard LEDs - like those ones supplied with a lot of Arduino kits - we see that they usually require a voltage between 1.5V and 3.5V to work (this depends on the color of the LED, as well as its manufacturer).

However, we don't want the current to exceed a few tens of milliAmperes (written mA: thousandth of an ampere) otherwise the LED may be damaged. That is why a resistor is placed in this circuit, to limit the current through the LED.

To calculate a good value for a resistor, so that the LED will be at a maximum brightness without being damaged, we are going to use

Ohm's law: R = V⁄I .

For the following discussion we want the voltage in the LED to be at 2.2V (a reasonable value) at its terminals.

The voltage of this circuit provided by the two AA batteries is 3V. The voltage drop that we want to have across our resistor is therefore 3V - 2.2V = 0.8V. Often, we limit the current through an LED at 20mA (= 0.02 A). This gives us R = 0.8V / 0.02A. The result is that it is possible to reduce the resistance by up to 40Ω without damaging the LED.

In the case of an LED attached to a pin of an Arduino board, we have 5V coming from the pin. The voltage drop we want across the resistor is 5V – 2.2V = 2.8V. That gives us R = 2.8V / 0.02A = 140Ω (minimum).

Resistor resistances

Resistors are manufactured in a wide range of standard values. For example, there is no commercially available 40Ω resistor; the closest value is, in fact, 39Ω. In addition, the value of a resistance is very rarely exactly the value indicated: there is always a tolerance (normally 5%). A 5% tolerance on 39Ω means it can have a resistance in the range 37.05Ω to 40.95Ω!

You can find out the resistance of a resistor by looking at the striped bands of colour around it. For a nice clear guide to reading resistor bands, click here (external link).

For more info...

You can read the articles on our wiki on:

Further References

Adafruit - using LEDs