Single IC forms inexpensive inductance tester
A simple Pierce oscillator measures the inductance of a device under test.
Luca Bruno, ITIS Hensemberger Monza, Lissone, Italy; Edited by Charles H Small and Fran Granville -- EDN, August 2, 2007
This Design Idea shows how to build a reliable, low-cost, and simple inductance tester. The basis for the tester is a Pierce buffered CMOS oscillator (Figure 1). Instead of using the usual quartz crystal, you connect the inductor under test. This oscillator uses a single CMOS inverter biased through resistor R1 in its linear region to form a high-gain inverting amplifier. Because of its high gain, the inverter dissipates lower power than an unbuffered gate; even a small signal drives the output high and low.
The LCπ network forms a parallel resonator that ideally resonates at the frequency fO=1/2π
which corresponds to a period, TO, of 2π where CS=C1||C2=50 nF. So, you can calculate the inductance, LX, by measuring the resonant frequency, fO, or the period, TO. At the resonant frequency, the LCπ network provides a 180° phase shift from input to output. To oscillate, the phase shift at frequency fO around the oscillator loop must be 360°, and the gain of the oscillator loop must be greater than one. Inverter IC1A provides an additional 180° phase shift from input to output and a high gain to compensate for the attenuation of the network.
Resistor R1 is not critical, and its value can be 1 to 10 MΩ. Resistor R2 isolates the output of gate IC1A from the LCπ network so that you can obtain a nearly clean square wave from the output of the gate itself. In addition, R2 improves frequency stability because it increases the slope of phase shift around the resonant frequency. For best performance, use film capacitors with low self-inductance, such as the MKP1837 polypropylene-film-capacitors series with 1% tolerance from Vishay. You can also use other film capacitors with standard tolerance provided that you select the value with a precision capacitance tester for best accuracy. The low supply current of the circuit allows you to use a battery as a power source.
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For best accuracy supply voltage must be as high as possible, because the propagation delay of the inverter decreases when supply voltage increases.
I have tested several coils with unknown inductance values and compared measurements with the simulation of the circuit with Micro-cap8 software. The overall accuracy is about 2.5% in the mH range up to 250mH, while it doubles in the uH range down to 0.5uH.
Luca Bruno - 2007-3-9 07:18:00 PDT -
Nice circuit. Have you evaluated its upper and lower measuring limits? I did some quick checks of inductors in my junkbox against their labeled values (not precision parts though). I got pretty close comparisons (within a couple percent or so) down to about 2.2uH on the low end and 27mH on the upper. Samples at 1.0uH and 125mH showed high errors. At the bottom end, it's probably test lead inductance. Not sure about the upper end.
Nick Kennedy - 2007-10-8 09:23:00 PDT -
The two capacitors C1 and C2 are obviously in series.
The formula Cs = C1//C2 means only how to calculate the equivalent series capacitance which is
Cs=(C1*C2)/(C1+C2)= 50nF
Luca Bruno - 2007-3-8 07:28:00 PDT -
The author says that Cs = C1//C2 = 50 nF. He should say that Cs is equivalent to the series of C1 and C2, i.e. 50nF. The parallel of C1 and C2 is 200nF.
Mario Pazzini - 2007-3-8 00:16:00 PDT
