MOSFET enhances low-current measurements using moving-coil meter

A previous Design Idea describes an interesting and useful method for using a moving-coil analog meter to measure currents in the less-than-1A range (Reference 1). The design offers considerable flexibility in the choice of meter-movement sensitivity and measurement range and simplifies selection of shunt resistors. Although the design uses a bipolar meter-driver transistor, under some circumstances, a MOSFET transistor represents a better choice. The original circuit comprises a voltage-controller current sink that measures the bipolar transistor's emitter current, but the transistor's collector current drives the analog meter. A bipolar transistor's emitter and collector currents, I_E and I_C, respectively, are not identical because base current, I_R, adds to the emitter current.

You can express these current components as I_E=I_C+I_B and then as I_C=I_E–I_B. Whether base current adversely affects the measurement accuracy depends on the magnitude of I_B and the magnitude of the common-emitter current gain, β, because base current I_B=I_C/β. When β is greater than 100, the base current's contribution to emitter current is generally negligible. However, β is sometimes smaller. For example, the general-purpose BC182, an NPN silicon transistor, has a low-current β of only 40 at room temperature. If you were to use a 15-mA-full-scale meter in the transistor's collector, full-scale base current I_B at minimum β would amount to 0.375 mA. Subtracting base current from collector current introduces a 2.5% error.

But if you use a moving-coil meter that requires 150 µA for full-scale deflection, the measurement error increases considerably because β decreases as collector current decreases. For the BC182, reducing collector current from a few milliamps to 200 µA, current gain decreases β by a factor of 0.6 and adversely affects the meter reading's accuracy.

To solve the problem and improve the circuit's accuracy, you can replace the BC182 with an N-channel MOSFET, such as the BSN254 (Figure 1). Because a MOSFET draws no gate current, its drain current, I_D, equals its source current, I_S. When you select a MOSFET for the circuit, note that the device's gate-source threshold voltage should be as low as possible. For example, the BSN254 has a room-temperature gate-source threshold-voltage range of 0.8 to 2V. The remainder of the circuit design proceeds as in the original Design Idea; that is, for a maximum voltage drop of 1V across R₁, you calculate R_SENSE2 as follows: R_SENSE2=(1V/I_METER), where R_SENSE is in ohms, 1V represents the voltage drop across R₁, and I_METER is the full-scale meter reading in amps. Note that a 1-kΩ resistor at R₁ develops 10V/1A output across sense resistor R_SENSE1. In this application, 100 mA produces 0.1V across R_SENSE1, and the voltage across R₁ thus corresponds to 1V for full-scale deflection of the meter.