Ohm's Law

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There is a good description of Ohm's Law on Wikipedia.

Ohm's Law is a statement of the relationship between Voltage, Current, and Resistance in an electrical circuit. It can be represented by this diagram:

Ohm's Law Diagram

Note: You may also see Voltage represented by the letter E. This stands for "Electromotive Force," an older name for what is now called Voltage. So, you may see the equations as:

  • E=I×R
  • I=E÷R
  • R=V÷I

Examples

OhmsLaw.svg

One common question is: "How much resistance is needed for an LED?

To determine this, you begin with the data sheet for the LED in question. Specifically, the values for the continuous forward current, IFWD, and forward voltage VFWD.

For example, the data sheet lists IFWD as 30mA maximum, VFWD at 20mA is from 3 to 3.6V.

The power source is 16VDC. The maximum voltage across the LED, VFWD, is 3.6V. For safety, the VFWD will be 3.3V. The series resistor (Rseries) will be determined by the 3.3V at 20mA rating. Rseries will have the difference between 16 and 3.3V across it.

VRseries = 16 − 3.3 = 12.7V

Using R = V÷I, calculate the value: 12.7 ÷ IFWD = 12.7 ÷ 20mA

= 635 Ohms

Using I= E ÷ R: 12.7 ÷ 635 = 0.02A Using E = I × R, 0.02 × 635 = 12.7V

There is no 635 ohm resistor available on the market, so the closest value would be 680 ohms.

Using 680 ohms, the current is 12.7 ÷ 680 = 19mA.

Power Dissipation

The series resistor must be able to handle the power that will flow through it. Using the above calculations, the result is found as follows:

Watts = I2 × R = 0.22 × 635 = .0004 × 635 = 0.25 W

In this case, a ¼W resistor or better will suffice.

See Also

Electronics Primer