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Multiple-Turn Primaries

When measuring DC currents with a transducer, it isn’t uncommon to encounter operating currents that are too small to provide an input that is within the transducer’s optimal measurement range. This can compromise accuracy. In such cases, the first instinct is to buy another smaller transducer. However, a larger transducer can still provide an accurate measurement by adding additional turns to the primary winding.

The correlation between the number of windings and the input and output values is expressed in the equation

T.R. = n = Np/Ns = Ip/Is

where T.R. and n are the transformer rating, Np is number of turns on the primary winding, Ns is the number of turns on the secondary, Ip is the input value from the primary, and Is is the output value at the secondary.

From this we can derive the formula for the output value at the secondary as

Is = Ip(Np/Ns)

and we see that when Ns is an arbitrary number, each additional turn at Np increases Is by a factor of 1 plus the value of Is at a single turn.

Given this, for a transducer rated at 300A-to-5mVac, for example, which is not designed to measure values as low as 100A, adding three turns to the primary winding effectively allows the transducer to measure the 100A current as if it was rated 100A-to-5mVac.