The other day, I needed to measure the AC current of my water heater without having to disconnect the wiring. Typically, you would need a clamp meter to do this. Since I don’t have a clamp meter, I thought I would just make a simple current transformer myself.

In fact, making a current transformer is a very easy task. All you need is a magnetic core and a single winding. A ferrite bead is a convenient choice for the magnetic core as the opening allows the core to wrap around the wire and thus enable easy measurements.

I used some thin magnetic wire (35 AWG) and made a 50-turn winding on half of a ferrite bead (see picture below):

Current Transformer Winding
Current Transformer Winding

And the two halves of the ferrite bead can be snapped onto wires to measure the AC current:

Current Transformer
Current Transformer

The reading was fairly linear for the range I was testing with. when the primary winding current is 1A the current reading on the secondary winding is around 3mA and when the primary winding current is 10A the current reading is around 30mA.

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