Saturday, June 22, 2013

Reading Voltage Across Switches

Voltmeters read the difference in potential from one point to another. When a voltmeter indicates a reading of 120 volts, this means that one lead is 120 volts higher than the other lead. We often refer to this as voltage drop. I find that people often misinterpret voltage readings across switches. A switch is designed to either allow current flow, or stop it by opening the circuit. A voltage reading across a switch indicates that the switch is open (off). Let’s take a circuit with a light and a standard toggle switch controlling the light. With the switch off, you will read 0 volts at the light. If you read voltage across the light switch, you will read 120 volts with the switch off. The voltage is dropping across the switch. Now if you turn the switch on, the voltage reading across the switch will change to 0 volts. Checking the light, you now read 120 volts at the light. When current travels through a switch, there should be NO drop in voltage. When current travels through a load, there SHOULD be a voltage drop. A closed switch should have a resistance of close to 0 ohms, while a load should have a measurable resistance. Using Ohm’s law to calculate the expected voltage drop across a switch you would get 0 volts because 0 ohms times any amount of current would still be 0 volts. I have often seen students looking for the break in a circuit read voltage across a switch and declare that switch is OK because they got a reading. In fact, that switch is open because they got a reading. One point to keep in mind is that the voltage across all devices in the circuit should add up to the source voltage. If you are reading 120 volts across a switch in a circuit wit a source voltage of 120 volts, there is no voltage left for the load.


  1. if call for heat and the switch to the motor is closed reads 0 volts and the motor next to that switch reads 120 volts it means its defective?

  2. This helped me out today, Thank you for this blog.