Using a Resistor to Limit Current

One of the most common uses of a resistor is to limit current flowing into a device. We will use LEDs as an example.

LEDs are rated in how many amps of current you can allow through them before they burn out. By themselves LEDs have virtually no resistance so they will allow huge amounts of current through. However, they will burn up almost immediately if connected to a battery without a current limiting resistor. It is up to you, the system designer, to put the correct resistor inline with the LED to limit the amount of current that the LED receives, because the LED manufacturer has no idea what voltage of battery you are going to use in your project. Lets look at an example using a resistor and an LED.

Schematic 1	Schematic 2	Actual Image

Above we have shown two different schematics and one real life circuit. Notice that all 3 are electrically identical. This is an example of using a resistor to limit current. In this configuration we call the resistor a current limiting resistor.

Using Ohms Law to Calculate Which Resistor to Use

The real question here is what value of resistor do we need to install to properly limit the amount of current getting to the LED. When we bought the LED, the data that came with it told us that this LED can handle 0.050 amps of current (or 50 milliamps). Using Ohms Law, we know that V = 9 volts and I = 0.050 amps. We can use one of the forms of Ohms Law to calculate R.

Ohms Law, R = V / I

Plugging in the values that we know, we get.

R = 9 / 0.050 = 180 Ohms

So R = 9 / 0.050 or 180 Ohms. If we could find a 180 Ohm resistor and put in inline with our LED, then 0.050 amps would flow through the LED and we would not burn up the LED. A 180 ohm resistor is:

• Use the resistor color code to look up resistors
• Use an ohmmeter to measure their value
• Use an appropriate resistor to limit the current flow through devices.