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An Inductance Measurement Adapter For Digital Voltmeters
This adapter, as presented can measure 0.1 microhenry to 2 millihenry on a single 2 volt scale
Figure 1. The DVM indicates 462 millivolts, which corresponds to 462 microhenries on an inductor that is marked 470 microhenry.
Related: A detailed description of the inductance-to or capacitance-to voltage converter operation
Introduction
This is an inductance-to- or
capacitance-to-voltage adapter for digital voltmeters. Its measurement method
is the most popular method in use in low cost LC meters designs for the home
lab. With these meters the part to be measured is made to resonate with a known
reference part, for example to measure an inductor, a capacitor of a known
value is placed in parallel with it in an oscillator. Measuring the oscillation
frequency and knowing the value of the capacitor is enough to calculate the value
of the inductor using the widely recognized relashionship shown in Formula 1 .
Formula 1. Find value of inductance and capacitance in an LC oscillator.
Where L is inductance,
C is capacitance, and
f is the oscillation frequency.
The meter circuits that I have seen use a microcontroller to perform the
calculation and display the result on a numerical or alpha-numberic display. It can be difficult or impossible for some people,
especially students and hobbyists in developing countries to obtain and
program microcontrollers. At the same time good and inexpensive digital voltmeters are
readily available. For some of us, assembling the microcontroller with a
display is just too much trouble to be bothered with. This simple digital/analog
switched-voltage circuit performs the calculation of inductance or capacitance
as a function of oscillator frequency as shown in Formula 1, generating a voltage that is linearly
proportional to the inductance of capacitance so that the value may be read
directly on the display of a digital voltmeter.
No precision components are need to build the
adapter but a component with a known value at or below the full scale value is
needed to perform calibration. The full scale input of the demonstration circuit
is 2.000 millihenries. Multiple ranges can be obtained with a single pole
multiple position switch and three passive components for each range.
The Circuit
This
circuit can be easily adapted to measure either inductance or
capacitance or both with a switch. To simplify things a little I will
describe ths as an inductance meter. It should be understood that using a
stable inductor may be used to resonant with unknown capacitors.
Figure 2. The entire circuit is made of six integrated circuits, not counting the voltage regulator.
The heart of the circuit and the section that has to most to determine
how linear the circuit works is the LC oscillator that includes the
LM319. The high speed and high gain of the LM319 makes it a good choice
for LC oscillators that, like this one, at times is called upon to
oscillate at over 1 MHz. a small u or 2 uH inductance is placed in
series with the inductor under test to provide some offset so that the
circuit that follows the oscillator will not clip at ground. The power
supply to the LM319 is well bypassed and decoupled with a 1 mH inductor
to reduce interaction between the LM319 and the LM555 reference
oscillator.
The purpose of U1 LM555 is to provide timing reference which U2 uses to
measure the period of the LM319's oscillation. From inspection of
Formula 1 that the L is linearly proportional to the period of the LC
oscillator (which is 1/f). As such, the period of U1 may be varied to
adjust the full scale output voltage.
U5A lights the red LED when the LM319 LC oscillator's period exceeds the
dynamic range of the frequency-to-period converter U2, thus an
erroneous reading if flagged.
The Q and not-Q outputs of U2B are complimentary duty cycle modulated
pulses. The duty cycle is proportional to the period of the LC
oscillator. These pulses are used to switch the digital inputs of the
pulse height-width multipliers made of U3 and and the 0.1 uf capacitors.
U2A is a unity gain buffer that is used to lower the impedance of the
output of the RC filter made of the 20k resistors and the 0.1 uf
capacitor so as to be low enough to drive the analog input of the second
pulse height-width multiplier. This combination of the
frequency-to-period converter and the pulse height-width multipliers
results in a voltage proportional to The Period of the LC oscillator
squared, which is the solution to Formula 1 and is restated a little
more simply in Formula 2.
Formula 2. Simplified relationship between oscillator frequency and inductance. K is a scale adjustment and is done by adjusting the frequency of the NE555 reference oscillator.
Figure 3. The output stage.
The output stage is a simple 2x voltage amplifier with an offset
adjustment and a 900 nanoamp current from the 10 meg resistor from the
unswitched 9 volt battery. This allows the battery voltage to me checked
by merely measuring the output terminals anytime power to the unit is
off. The offset pot is used to set the output voltage just over zero
with the inductor terminals shorted.
Figure 4. The output stage.
The power to the adapter is off and the DVM displays 34.6 millivolts. To
find the battery voltage, take the meter reading and multiply it by
251,000, meaning the battery voltage is 8.69 volts.
U4 should be a low offset opamp with an output that can swing all the way to ground.
The better the offset and drift specifications, the less you will have
to worry about in terms of how often you have to recalibrate it.
C1, the resonating capacitor is very important to linearity. This should
be a capacitor that maintains its total reactance over a wide range of
frequencies. My current thinking is that an X7R ceramic capacitor would
be the best in this application. If you have experience that suggests otherwise, pleaes let me know.
If you want to learn more about the converter circuit and the
calibration procedure, and information useful in modifying the circuit click on this link:
A detailed description of the inductance-to or capacitance-to voltage converter operation
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First posted in March, 2016
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