Current sensor measures 0 to 500A

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.

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 R_S, the voltage at the emitter of Q₂ increases. The voltage at the base of Q₃ then increases, which increases the current, I_EQ3, 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 × R_S = I_EQ3 × R₁.

Therefore, Q₃'s emitter delivers a current proportional to the current through R_S. 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, V_OUT, versus the current through R_S.

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 R_S, which is manufactured with calibrated wire of well-known ohms/meter characteristics.

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