This may be an open switch or a broken wire, among other reasons.īut from any of these sources, the voltage drop will be the full source voltage - it does not give us any indication of how much electricity is flowing. In a control circuit, voltage always tends to be lost either at the load device or at the first point at which the circuit is ‘open’. When used properly, the resistance value can actually tell more than voltage or current alone can show. At the other extreme, a very high resistance value which the meter shows as OL (or ‘over limit’) is an ‘open’ circuit and the current will be nearly nothing. According to the part datasheet, the value should be 98 Ohms, this one works well.Ī resistance value of zero, or nearly zero is considered a ‘short’ circuit and can cause a fuse or breaker to trip with excessive current. High resistance leads to low current, low resistance leads to high current.Ī resistance test of a contactor shows a coil resistance of around 97 Ohms. A resistance, measured in Ohms, is the relationship between voltage and current. Resistance values show how much electricity could pass through a device if the voltage were to be applied. However, if the power must be removed from the circuit then neither of these two measurements can properly show any information and an alternative must be used. Current does require the circuit to have electricity actively flowing, but clamp-type meters allow this to be a simple measurement. Voltage requires only that power is applied, the load does not need to be active. In most situations, a measurement of voltage or current is most useful to track down failure in a control circuit.
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