Ohm's Law

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Ohm's Law

Ohm's Law is a formula that relates the basic electrical properties of Voltage, Current and Resistance. 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). Voltage is a measure of the electric potential difference between two points.
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 NiMh AA battery is 1.25 volts DC (VDC).
    • Many Integrated Circuits require 5 volts DC (VDC) to power them.

Formulas

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Current (I)

Current is measured in amps (A). Current is a measure of the flow of electric charge. Current is often measured in milliamps (ma) in many electronic circuits. A milliamp is one thousandth of an amp (0.001A).

• Example:
  • A typical LED draws 20 milliamps (0.02A or 20mA).
  • A typical string of 100 count incandescent lights draws 0.33A.

Resistance (R)

Resistance is measured in ohms (Ω). Resistance is a measure of the opposition to the passage of an electric current through a circuit component. 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). Power is a measure of the rate of doing work.
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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