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Measuring inductance via a transistor and ammeter

21 Jun 2012  | Raju Baddi, Tata Institute of Fundamental Research

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Bipolar junction transistors transfer a current from a lower-resistance emitter to a higher-resistance collector. Use this to measure inductance by connecting a series inductance/resistance circuit in the emitter and biasing on the transistor for the current to reach at least five LR time constants. When the transistor's off time is equal to its on time but is still biased by a silicon diode, the LR current decays exponentially toward 0A. Using the transistor's current-source property, measure the current without hindering the decay process in the LR circuit.

LR circuit transient analysis shows that if the circuit current is reduced to 5% during off time then, the average current is directly proportional to the inductance value. Currents can be controlled through the transistor and LR network using timed switching circuitry.

In an inductance-measuring circuit (Figure 1), the NE555 connects as an astable multivibrator oscillator to produce a square wave of approximately 50% duty cycle. This is achieved at frequencies of approximately 46Hz, 230Hz, 2.3kHz, and 23kHz, depending on the position of the range-selector switch. These values correspond to a full-scale inductance-measurement range as high as 2.5H, 500mH, 50mH, and 5mH. This square wave toggles four quad-packaged CD4066 switches alternately through two CD4011 NAND inverters wherein, during on time, S2 and S3 are closed when S1 and S4 are off. During the off time, S2 and S3 are open when S1 and S4 are on.

At the start of the on time, S2 and S3are closed, biasing Q1 on from the 5.5V power rail, and the diode and meter disconnect through S1 and S4. After the current in the inductor under test, LX, has exponentially reached maximum as the resistance determines, the off-time half of the cycle begins. S2 and S3 open to remove the 5.5V bias, and S1 and S4 close to insert the meter in the collector-current path. A small diode-drop bias voltage is also placed on the Q1 base.

Normally, the diode's bias voltage is a bit too low to keep Q1 on. As LX maintains the initial current, however, it drives the emitter to negative to temporarily keep Q1 on during the current decay. The exponentially decaying LR current flows through the collector to the meter, while a small portion flows through the base and bias resistor RB, depending on the Q1 current gain. The meter responds to the current average over the entire on- and off-time cycle due to the mechanical damping of the meter pointer. In this simple circuit, the meter deflection is directly proportional to inductance. With the values in the figure, the meter indicates approximately fullscale 100µA when measuring a 5mH inductor on the 5mH range selection.

At the end of the off-time, the current through the inductance is almost 0A.

Editor's notes:

The NE555 high output appears slightly lower than the specified valid logic 1 voltage for the CD4011, but is still well above the switching threshold and is driving zero load current.

The meter resistance has not been specified, the author used a moving pointer bench VOM. A digital meter may not work properly unless it has the ability to average a pulsed signal.

Q1 is biased just below threshold. While the datasheets indicate a 1N4148 diode that has typically lower forward voltage drop than the 2N3904 VBE, there is a possibility that certain combinations of diode and transistor may bias the transistor on during tOFF and cause additional meter deflection. It might be necessary to hand-pick these components.

Remember to ground any unused input pins on the remainder of the CD4011 quad NAND package.

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