Ghost Voltages

I know everyone who has used a digital meter has taken some measurements that don't always make sense. I believe we just learn to ignore them, recognizing the "real readings" from the junk. But when students see the odd readings, they become confused. Here is a great article by Fluke explaining what causes strange voltage readings when there really is no voltage.

GHOST READINGS

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.

Are You Seeing Ghosts?

Have you ever taken a voltage reading with a digital meter and gotten something that really did not quite make sense? With high impedance digital meters, it is common to get readings where you don’t expect them. This can be very confusing for students. Volt-meters are basically electrical loads. Normally, for them to read anything, they have to be in a complete circuit. The impedance rating of a meter determines the amount of current required to operate the meter. The higher the impedance, the less current required to operate the meter and get a reading. High impedance meters are preferred for electronic work because they don’t add any load to the circuit they are testing. However, high impedance meters can behave differently from older analog meters. A phenomenon called capacitive coupling can cause high impedance meters to read voltages that low impedance meters don’t read. These voltages are sometimes referred to as ghost voltages. Unenergized wires and devices in close physical proximity to energized wires and devices are charged by the energized wires in their proximity. This is a static voltage which cannot move any appreciable current. Since high impedance digital meters really don’t require a current flow to operate, they can read the ghost voltage. To differentiate between a ghost voltage caused by capacitive coupling and a hard voltage that will move current, place a load across the voltage. When loaded, a ghost voltage disappears. Some digital meters actually have settings for both high and low impedance to allow technicians to distinguish between a ghost voltage and a hard voltage. Fluke makes an adapter, SV225,  that can be used with high impedance meters to effectively turn them into low impedance meters by placing a small load across the leads. Low impedance analog meters do require a small current to operate, so they typically will not read the ghost voltage built up by capacitive coupling.