# How to measure small AC currents from mains?

I want to measure AC current from the mains connected to an isolated source - such as a single 10 watt bulb. The standard multimeter measures the current only in DC, so I needed a device that provides AC current measurement (though, I am trying to avoid buying an expensive multimeter.)

I looked at many current transformers such as this:

However, these have a range of 30A measurement capacity, whereas the currents I am looking at will be very small (in a few mA.)

Are there any alternative solutions that I can build?

• What is the cold Resistance? is that rated for 230V nom.? Compare with my Sim. tinyurl.com/y9l4vutk – Tony Stewart Sunnyskyguy EE75 Jul 14 at 18:56
• "I want to measure AC current from the mains connected to an isolated source - such as a single 10 watt bulb." Can you explain this sentence more clearly? – Transistor Jul 14 at 19:17

10 W at 230 V will draw a current of 10/230 = 43 mA.

A 10 Ω resistor in series with the bulb will give a 430 mV voltage drop across it which may be adequate if your multimeter has a 2 V AC range with true RMS measurement.

simulate this circuit – Schematic created using CircuitLab

Figure 1. Current measurement circuit. Divide the voltmeter reading by 10 to convert to mA.

Wiring the current shunt in the neutral line makes this a little bit safer if you were to accidentally make contact with the shunt resistor. You should not rely on this "protection" for your or anyone else's personal safety but terminate everything correctly and insulate properly.

I will leave the resistor power rating to you as an exercise. Choose one of double the rating you calculate so that it will run much cooler.

• Thank you for this answer. – Dipan Mehta Jul 31 at 17:31

You can loop a thin wire many times through the hole in the core to increase the sensitivity.

For example, you could use this current transmitter from the same supplier with a 10x loop and get 100mA full scale, which would just accommodate a 10W bulb on 230/115VAC mains (about 40 or 80mA).

An alternative would be to purchase a handheld DMM that has an isolated serial interface, but that would require batteries.

• That is usually the easiest thing to do, just loop through the CT a bunch. – relayman357 Jul 14 at 20:01