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 Voltage

  • AC Alternating Current
    • Common values:
      • The voltage of a typical wall power outlet in the United States is 120 volts AC (VAC).

DC Voltage

  • DC Direct Current
    • Common values:
      • 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.

Voltage Formulas

  • V=IR
  • V=√PR
  • V=P/I

Voltage Examples

  1. What is the voltage drop across a 4 Ω resistor that has a current of 0.3A?
    V=IR, V=(0.3)(4)=1.2V
  2. What is the voltage across a 5W lamp with a resistance of 2 Ω?
    V=√PR=√(5)(2)=√10=3.16V
  3. What is the voltage across a 1 W LED that has a current of 0.35A running thru it?
    V=P/I, V=1/0.35= 2.86V


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).

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

Current Formulas

  • I=V/R
  • I=P/V
  • I=√P/R

Current Examples

  1. How much current flows thru a 680 Ω resistor that has 5V applied to it?
    I=V/R, I=5/680=0.007A (7 ma)
  2. How much current is used by a 75W floodlight using 120V?
    I=P/V, I=75/120=0.625A
  3. What is the current thru a 5W lamp with a resistance of 2 Ω?
    V=√P/R=√(5)/2=2.24/2=1.12A

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.

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

Resistance Formulas

  • R=V/I
  • R=V²/P
  • R=P/I²

Resistance Examples

  1. What value resistor do you need to drop 3V and allow 20ma thru the circuit?
    R=V/I=3/0.020=150Ω
  2. What is the resistance of a 120V light string that draws 40W?
    R=V²/P=(120)²/40=14400/40=360Ω
  3. What is the resistance of a 1W LED that draws 350ma?
    R=P/I²=1/(0.35)²=1/0.123)=8.16Ω

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

Power Formulas

  • P=VI
  • P=I²R
  • P=V²/R

Power Examples

  1. How much power does a 100 count string of 120V incandescent lights that use 0.33A use?
    P=VI=(120)(0.33)=39.6 W
  2. How much power does a 4 Ω resistor dissipate as heat that has 0.3A running thru it?
    P=I²R=(0.35)²(4)= (0.123)(4)=0.49 W
  3. How much power does a 10 Ω resistor dissapate as heat that has 12V across it?
    P=V²/R=(12)²/10=144/10=14.4W


LED Calculator

By using Ohm's Law you can calculate the current limiting resistor needed to include in a circuit with a LED. LEDs need a resistor in a circuit to limit the amount of current flowing though it to prevent it from being destroyed. The typical current used by LEDs depends on the type of LED being used. Common values are:

  • Optoisolator LED - 6ma
  • Display LED - 20ma
  • "1/3W" LEDs - 100ma
  • 1W LEDs - 350ma

All LEDs have a characteristic voltage drop across them called the VF that depends on the color of the LED. Commoon values are:

  • Red
  • Green
  • Blue
  • White


Single LED

LED Calculator for 1 LED.

Array of LEDs

LED Calculator for an array of LEDs.

Pixel Voltage Drop Calculator

Due to the high current used by pixels and the small guage wires used by many people, there can be issues due to voltage drops caused by the wire resistance.
Thread
Voltage Drop Calculator