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Current Clamp Meter Adaptor for DMMs

Looking for a current clamp meter that won't break the bank?

By John Clarke

Clamp meters are very convenient when it comes to measuring current, since they do not require breaking the current path. Instead, they simply clip over the wire or lead that's carrying the current and the reading is then displayed on the meter.

This is not only much easier than "in-circuit" current measurements but is often a lot safer as well; eg, where high voltages and currents are involved. However, clamp meters are not particularly useful for making low-current measurements (ie, below 1A) due to their inaccuracy and lack of resolution.

Unlike this unit, many commercial current clamp meters can only measure AC. That's because they are basically current transformers, comprising turns of wire around a magnetic core. This magnetic core is clipped around the wire to be measured, which effectively behaves as a half-turn primary winding. The winding on the core itself acts as the secondary and connects to the multimeter's current terminals.

The measured current is a divided down value of the true current flowing in the wire. Usually, the division ratio is 1000:1 so that 1mA shown on the meter equates to 1A through the wire that's being measured.

Clamp meters capable of measuring DC as well as AC do not use a current transformer but a Hall effect sensor instead. This sensor is placed inside a gap in an iron-powdered toroid core. It measures the magnetic flux produced as a result of the current flowing through the wire and produces a proportional output voltage.

Output: 1A = 1mV for AC and DC ranges
Resolution: multimeter dependent (100mA with 0.1mV resolution on multimeter)
Maximum DC current: 150A recommended (up to 900A if core is demagnetised afterwards)
Maximum AC current: 630A recommended
Linearity: typically better than 4% over range at 25°C
AC frequency response: -3dB at 20kHz (meter reading depends on multimeter AC response)
Current consumption: 15mA

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