Rainbow Inductance Adapter
taken from the ARRL Handbook (2002 Edition)
Many of us have a DVM (digital volt-meter) or VOM (volt-ohm meter) in the shack, but few of us own an inductance or capacitance meter. If you have ever looked into your junk box and wanted to know the value of the unmarked parts, these simple circuits will give you the answer. They may be built in one evening and will adapt your DVM or VOM to measure inductance or capacitance. The units are calibrated against a known part. Therefore, the overal accuracy depends only on the calibration values and not on the components used to build the circuits. If it is carefully calibrated, an overall accuracy of 10% may be expected if used with a DVM and slightly less with a VOM.
Inductance Adapter for a DVM or VOM
Construction...
The circuits may be constructed on a small perf board (Radio Shack dual mini-board (#276-168), or if you prefer, on a PCB (Printed Circuit Board). Layout is non-critical - almost any construction technique will suffice. Wire-wrapping or point-to-point soldering may be used.
Desciption...
The schematic shown below converts an unknown inductance into a voltage that can be displayed on a DVM or VOM. Values between 3uH and 500uH are measured on the L (low) range and from 100uH to 7mH on the H (high) range. NAND gate ICA is a two frequency RC square-wave oscillator. The output frequency (pin 3) is approximately 60 KHz in the L (low) range and 6 KHz in the H (high) range. The square-wave output is buffered by ICB and applied to a differentiator formed by R3 and the unknown inductor; LX. The stream of spikes produced at pin 9 decay at a rate proportional to the time constant of R3-LX. Because R3 is a constant, the decay time is directly proportional to the value of LX. ICC squares up the positive going spikes, producing a stream of negative going pulses, at pin 8 whose width is proportional to the value of LX.
They are inverted by ICD (pin 11) and integrated by R4-C2 to produce a steady dc voltage at the + output terminal. The resulting dc voltage is proportional to LX and the repetition rate of the oscillator. R6 and R7 are used to calibrate the unit by setting a repetition rate that produces a dc voltage corresponding to the unknown inductance. D1 provides a 0.7 volt constant voltage source that is scaled by R1 to produce a small offset reference voltage for zeroing the meter on the L (low) inductance range.
When SW1 is L (low), mV corresponds to uH, and when H (high), mV corresponds to mH. A sensitive VOM may be substituted for the DVM with a sacrifice in resolution.
Test and Calibration...
Short the LX terminals with a piece of wire and connect a DVM set to the 200-mV range to the output. Adjust R1 for a zero reading. Remove the short and substitute a known inductor of approximately 400uH. Set SW1 to the L (low) position and adjust R7 for a reading equal to the known inductance. Switch SW1 to the H (high) position and connect a known inductor of about 5mH. Adjust R6 for the corresponding value. For instance, if the actual value of the calibration inductor is 4.76mH, adjust R6 so the DVM reads 476mV.
Inductance Meter Schematic
All components are 10% tolerance. 1N4146 or equivalent may be substituted for D1. An LM7805 may be substituted for the 78L05. All fixed resistors are 1/4 watt carbon composition. Capacitors are in uF. R3 value may need to be increased or decreased if calibration cannot be achieved as described in text
