The bipolar junction transistor transfers current from the lower resistance emitter to the higher resistance collector. You can use this property to measure the inductance by connecting a series inductor/resistor circuit in the transmitter and applying enough time to the transistor to bring the current to a maximum of at least 5 LR time constants. When the turn-off time of the transistor is equal to its turn-on time, but is still biased by the silicon diode, the LR current is exponentially attenuated to 0A. Using the current source characteristics of the transistor, you can measure this current without hindering the attenuation process in the LR circuit.
The transient analysis of the LR circuit shows that if the current of the LR circuit is reduced to a sufficiently low value, such as 5% or less, during the off time, the average current is directly proportional to the on time plus the off time. The value of the inductor. You can use a timed switch circuit to control the current through the transistor and LR network.
In an inductive measurement circuit, the NE555 is connected as an unsteady multivibrator oscillator, producing a square wave with a frequency of approximately 46 Hz, 230 Hz, 2.3 kHz, and 23 kHz with a duty cycle of approximately 50%, depending on the specifics The position of the range selection switch. These values correspond to full-scale inductance measurement ranges up to 2.5H, 500 mH, 50 mH and 5 mH. The square wave alternately switches four four-packaged CD4066 switches through a pair of CD4011 NAND inverters such that during the on-time, when S1 and S4 are open, S2 and S3 are closed and are open During the period S 2 when S 1 and S 4 are turned on, S 3 is turned on.
At the beginning, S 2 and S 3 are closed, Q 1 is turned on from the 5.5V power rail bias, and the diode and meter are disconnected through S 1 and S 4 . The current LX in the inductor to be tested begins to close for a half cycle after exponentially reaching a maximum value after the resistance is determined. S 2 and S 3 are turned off to eliminate the 5.5V bias, and S 1 and S 4 are close to inserting the meter into the collector current path and placing a small diode drop bias voltage on the base of Q 1 .
Normally, the bias voltage of the diode is a bit too low to keep Q1 on. As the LX maintains the initial current, however, its emitter's emitter negative temporarily maintains Q1 during the current decay process. Most of the exponentially decaying LR current flows through the collector to the meter, and a small portion flows through the base and the bias resistor RB, depending on the current gain at Q1. Due to the mechanical damping of the meter hand, the meter responds to the current average over the entire switching cycle. In this simple circuit, the meter deflection is proportional to the inductance. With this value plot, the meter indicates approximately 100μA of full scale when measuring a 5 mH inductor selected for the 5 mH range.
At the end of the turn-off time, the current through the inductor is almost 0A. This appendix shows current waveforms and other details such as calculating high inductive resistance and meter scale factor.
Editorial explanation:
The NE555 high output appears to be slightly lower than the specified active logic 1 voltage, CD4011, but still well above the switching threshold and drives zero load current.
The meter's resistance was not specified and the author used a moving pointer bench VOM. Digital meters may not work properly unless it has the ability to average pulse signals.
The deviation of Q 1 is just below the threshold. Although the data sheet shows that the forward voltage drop of the 1N4148 diode is typically lower than the 2N3904 V BE, during t, some combinations of diodes and transistors may turn the transistor off and cause additional meter deflection. It may be necessary to manually pick these components.
Remember to ground any unused input pins on the rest of the CD4011 quad NAND package.
