Computing stuff tied to the physical world

CT’s and burden resistors

Let’s open up that Current Transformer and see what’s inside:

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Here is what is in the upper half of that blue plastic case – this unit is rated at 30A:

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There are two resistors in there, of 10Ω and 52Ω respectively. Being in series, that means there is 62Ω of resistance placed across the secondary winding. This is a “voltage output” type CT: the current is internally passed through a “burden” resistor, so that we can measure the voltage on the end of the wire coming out.

The above unit is rated for 30A. The winding ratio is 1:2000, so those 30A are converted to a 15 mA output current, and 15 mA through 62Ω leads to a 930 mV output voltage under full load. Which is only 31 mV for 1A, i.e. with a 230W AC mains load.

Here is another unit, rated at 100A. It too has a 1:2000 winding ratio, so the current output will be 50 mA at 100A. Unlike the previous model, this unit has two SMD components:

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A quick check on a very useful analysis instrument called a Component Tester (a simple form of Curve Tracer) shows that each component is a zener diode of about 8V:


The way to read this diagram is that the applied voltage is on the horizontal scale (from -10V to +10V), while the resulting current is on the vertical scale (up to ±10 milliamps).

With two of these zeners placed back-to-back, you get a circuit which is normally open, but starts conducting when the voltage drops below -8.6V or rises above +8.6V:


In other words, the current is being passed on to the output wire for measurement, but the zeners will kick in as protection if nothing is connected and voltages end up rising too far.

We can easily confirm proper operation (and the 1:2000 turns ratio) of this second CT by hooking it up to a multimeter in AC current measurement mode:

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And indeed, with a 0.6A current through the primary, we get about 0.3 mA coming out. Note that the multimeter is taking over the role of burden resistor in this case, since it has been set to a current measurement range.

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