Measure. - Here is Circuit and Project Current Transformer AC Load Indicator LED. A current transformer (CT) is a simple, practical and inexpensive means of detecting the presence of AC current. Detecting load current rather than applied voltage is a superior method of determining normal operation because the load (e.g. motor or heater) could be either disconnected or otherwise defective. Described here are two circuits – the 1st may be tailored for a known and fixed load current, while the 2nd is suitable for a wide range of currents.
What is a current transformer (CT)?
A CT is generally a toroidal core transformer with an opening for a single turn (or multiple turns if required) rather than a primary winding. The secondary winding often has 1000 or 2000turns. Some that are intended for switch panel instrumentation using panel meters have a 5A secondary. Current transformers require a resistive load (ballast) in order to output the desired signal voltage. Avoid operating them without a load resistor as they can generate self-destructive voltages in the order of several kV. Like all magnetic transformers, they operate only on AC.