How to measure capacitance and inductance in a large capacity range

Most simple circuits used to measure reactive components can cover a limited range of component values. Although the circuit described in this article is only made up of some cheap components, the capacitance and inductance values it can measure can span seven orders of magnitude. Whether it is a capacitor with a capacitance range of about 1pF~10μF, or an inductor with an inductance value range of about 200nH~4H, you can use this circuit to measure its component value.

However, to cover such a large value range will be a little troublesome, because to determine the value of the device under test, you need to adjust the variable resistor first, and then check the corresponding capacitance or inductance value on the calibration curve instead of Direct reading.

Regarding the operation of this circuit, please first look at the basic schematic diagram shown in Figure 1. In Figure 1a, a square wave voltage source drives the bottom terminal of the capacitor under test. The top terminal voltage is a series of positive and negative pulses that decay exponentially above and below the +5V power rail. The decay time constant is naturally the product of R and CTEST. Similarly, in Figure 1b, a square wave voltage source is fed into the inductor under test, which causes a similar transient above and below the +5V power rail. At this time, the decay time constant is equal to LTEST/R. During the exponential decay of the voltage, the respective proportions of the two half cycles of the occupied square wave are determined by the relationship between the time constant and the oscillation period.

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Figure 1: The basic principle diagram of measuring capacitors and inductors with frequency conversion square waves.

Please see the complete schematic diagram shown in Figure 2 below. IC1 is arranged as a simple Schmitt trigger resistor-capacitor oscillator and output buffer, and a square wave is generated in this circuit. The frequency is set by the variable resistor R9, and the frequency range spans from A to F of the six decimal capacitors. R9 should have linear resistance distribution characteristics, so that the oscillator period increases with the clockwise axis rotation.

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Figure 2: The complete schematic diagram of the capacitance/inductance meter.

With a double-pole double-throw switch, you can choose between capacitor and inductor measurement modes. According to the basic principle diagram shown in Figure 1, the voltage directly output by IC1 or the current generated by Q1 are respectively fed into the capacitor or inductor under test. The resistor R2 with a resistance value of 510Ω is used as the attenuation resistance R in the inductance measurement mode in Figure 1, while the series connection of R5 and R2 forms the attenuation resistance in the capacitance measurement mode (the role of R5 is to maintain the voltage offset of the base of Q2 At a low enough level to avoid saturation. Bias resistor R7 and diodes D3 and D4 maintain the base of Q3 at a level of about 2VBE below the +5V power rail. At this bias point, Q2, R3, and Q3 are formed A rectifying transconductance block, the transconductance block carries a small amount of reactive current, and this current can only sensitively sense the positive transient from the device under test and above the +5V power rail. The collector current pulse from Q3 is It becomes weaker when passing through R4, and the resulting voltage is measured by an external voltmeter after being balanced by C2 and C3.

The exponentially decayed transient in a certain period of the square wave cycle will produce the corresponding output DC voltage, but the non-linear relationship between the duty cycle and the output voltage is not important. Since Q2 and Q3 are in high-speed common collector and common base configurations respectively, the response speed of the circuit is very fast, and the duty cycle measurement is basically independent of frequency.

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After adjusting the oscillation period with R9, the output voltage can stay at a fixed reference level (for example, 1.00V), so that the exponentially decayed time constant forms a fixed correlation with the oscillation period.

Since the decay time constant changes linearly with the reactive value of the component under test, the measured capacitance or inductance value will have a linear relationship with the oscillation period, and therefore a linear relationship with the R9 axis angle. By applying appropriate scale marks on R9 and calibrating the circuit with reference to several known capacitance and inductance values, a calibration chart can be drawn to determine any component value. Figure 3 shows the R9 scale marking diagram, which is included in the available sample calibration package (see the end of the article).

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Figure 3: R9 scale mark example diagram.

The oscillator range switch will cover six decimals, but the period will be limited by the IC1 transmission delay. Therefore, it can cover six orders of magnitude of capacitance or inductance from A to F from low to high. Deploy the component under test in the circuit, find the setting of the range switch and the variable resistor when outputting a voltage of 1.00V, and view the measured value in the graph corresponding to each frequency band. The measurable value of the A segment is about 10pF or 2μH, and the measurable value of the F segment is about 10μF or 4H.

If you want to measure low component values around 1pF and 200nH, you can use another method. The small bias components C1 and L1 are always the time constants formed in the capacitance or inductance measurement mode. Therefore, when the device under test is added to these small bias components, by comparing the changes in the voltage readings on the external voltmeter, the Very low component values draw another calibration chart.

The method of measuring the above-mentioned component value range is: firstly, by opening the capacitor test clip or short-circuiting the inductor test clip to exclude the component under test from the circuit; then, set the frequency of the oscillator to the A section and pass R9 Adjust the oscillation period until the circuit reaches the target voltage value of 1V by only relying on the bias component;, connect the component under test to the circuit and observe the change in the voltmeter reading. By looking at the offset voltage on the calibration chart, the small component value can be determined.