MOSFET enhances low-current measurements using moving-coil meter

Maxim MAX4172 MAX495

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, IE and IC, respectively, are not identical because base current, IB, adds to the emitter current.

This updated version of an earlier Design Idea uses a MOSFET to drive an analog meter display, offering great flexibility in power-supply-current measurement.
Figure 1. This updated version of an earlier Design Idea uses a MOSFET to drive an analog meter display,
offering great flexibility in power-supply-current measurement.

You can express these current components as IE = IC + IB and then as IC = IE – IB. Whether base current adversely affects the measurement accuracy depends on the magnitude of IB and the magnitude of the common-emitter current gain, β, because base current IB = IC/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 IB 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, ID, equals its source current, IS. 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 2 V. The remainder of the circuit design proceeds as in the original Design Idea; that is, for a maximum voltage drop of 1 V across R1, you calculate RSENSE2 as follows:

where RSENSE2 is in ohms, 1 V represents the voltage drop across R1, and IMETER is the full-scale meter reading in amps.

Note that a 1-kΩ resistor at R1 develops 10 V/1 A output across sense resistor RSENSE1. In this application, 100 mA produces 0.1 V across RSENSE1, and the voltage across R1 thus corresponds to 1 V for full-scale deflection of the meter.

Reference

  1. Bilke, Kevin, “Moving-coil meter measures low-level currents

Materials on the topic

  1. Datasheet Maxim MAX4172
  2. Datasheet Maxim MAX495
  3. Datasheet onsemi BC182
  4. Datasheet NXP BSN254

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