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 metermovement 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 voltagecontroller 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, IR, adds to the emitter current.

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, ©¬, because base current IB=IC/©¬. When ©¬ is greater than 100, the base current's contribution to emitter current is generally negligible. However, ©¬ is sometimes smaller. For example, the general-purpose BC182, an NPN silicon transistor, has a lowcurrent ©¬ 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 ©¬ would amount to 0.375mA. Subtracting base current from collector current introduces a 2.5% error.

But if you use a moving-coil meter that requires 150¥ìA for fullscale deflection, the measurement error increases considerably because ©¬ decreases as collector current decreases. For the BC182, reducing collector current from a few milliamps to 200¥ìA, current gain decreases ©¬ 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 gatesource 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 RSENSE2 as follows: RSENSE2= (1V/IMETER), where RSENSE is in ohms, 1V represents the voltage drop across R1, and IMETER is the full-scale meter reading in amps. Note that a 1k¥Ø resistor at R1 develops 10V/1A output across sense resistor RSENSE1. In this application, 100mA produces 0.1V across RSENSE1, and the voltage across R1 thus corresponds to 1V for full-scale def lection of the meter.

Reference

1. Bilke, Kevin, ¡°Moving-coil meter measures low-level currents,¡± EDN, March 3, 2005, pg 72.