Sunday, June 2, 2013

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. 

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