Ohm's Law

Ohm's Law

Ohm's Law is a formula that relates the basic electrical properties. These properties can be measured with a multimeter. The common formula for Ohm's Law is:

V=IR

Voltage = Current x Resistance

Ohm's Law Diagram

The following diagram shows the relationship between the various electrical properties.

Voltage (V)

Voltage is measured in volts. (V) There are two basic type of voltage:

• AC Alternating Current
  • Example:
    • The voltage of a typical wall power outlet in the United States is 120 volts AC (VAC).
• DC Direct Current
  • Example:
    • The voltage of a typical AA battery is 1.25 volts DC (VDC).
    • Many Integrated Circuits require 5 volts DC (VDC) to power them.

Current (I)

Current is measured in amps (A). Current is often measured in many electronic circuits is measured in milliamps (ma). A milliamp is one thousandth of an amp (0.01A).

• Example:
  • A typical LED draws 20 milliamps (0.02A or 20mA).

Resistance (R)

Resistance is measured in ohms (Ω). Resistors are the most common form of resistance in circuits.

• Example:
  • A common current limiting resistor for driving optoisolators is 680 ohms (Ω).

Power (P)

Power is measured in watts (W). The common formula for Power is:

P=VI

Power=Voltage x Current

• Example:
  • A common table lamp is 100 watts (100w).

LED Calculator

Use these calculators to calculate the resistor needed to include in a circuit with a LED.

Single LED

LED Calculator for 1 LED.

Array of LEDs

LED Calculator for an array of LEDs.

Pixel Voltage Drop Calculator

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