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10-24-2016

Current sensor measures 0 to 500A

Microsemi » 2N2060, 2N3810

Jose Carrasco

EDN

The simple circuit in Figure 1 can sense both low and high current levels without low sensitivities or loss of accuracy either at the low or the high end of the scale. The circuit is useful for discerning either low or high currents in noisy environments.

Current sensor measures 0 to 500A
Figure 1. A simple current sensor can sense both low and high current levels.

The circuit comprises a current mirror formed by complementary pair Q1 and Q2 and the feedback provided by Q3. When a current I flows through RS, the voltage at the emitter of Q2 increases. The voltage at the base of Q3 then increases, which increases the current, IEQ3, through Q3's emitter. This process continues until the circuit restores its equilibrium by positioning Q2 at the same operating point as Q1, which is working as a diode. Consequently, the following relationship holds:

I × RS = IEQ3 × R1.

Therefore, Q3's emitter delivers a current proportional to the current through RS. Further, because Q4 also works as a diode, Q5 has to work at the same operating point as Q4 so that the current that the collector of Q5 delivers is also proportional to I. The collection of series diodes provides the current-to-voltage converter in logarithmic scale. Figure 2 shows the dc transfer function, which is the output voltage, VOUT, versus the current through RS.

Current sensor measures 0 to 500A
Figure 2. A sensor measures currents of 0 to 500 A with a logarithmic output.

It is important to use complementary pairs for Q1- Q2 and Q4 - Q5 because the operating point of each transistor in the pair should be the same, even if temperature varies within its unions. The circuit uses low-power passive components except for the 5-mW resistor RS, which is manufactured with calibrated wire of well-known ohms/meter characteristics.

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