The Oscilloscope Inductance Meter is a simple inductance (L)
So why make this device if great LCR meters exist? Well, this
You will need a 50MHz or better oscilloscope to use it with very
Circuit diagram, including power supply
The power supply section uses the simple, inexpensive MC/TS/AZ34063
switching converter in a buck configuration, converting the nominal 9V
value from the battery to a fixed 2V ±1% supply for the meter circuit.
Inexpensive PET metalized film capacitors are used for C1 and C2; all resistors are ±1% 1/2W metal film resistors, and the inductor L1 is an inexpensive, 1W, resistor-sized axial part. The largest component, C3, is a 16V 2200µF electrolytic capacitor; the large value is chosen to minimize voltage ripple on the output.
The power supply is dimensioned for up to 150mA output at 2V; the actual draw is typically <30mA. The green LED, LED1 indicates that the test device is on; it does not have a resistor, because its forward voltage at 20mA is more than the 2V of our power rail; therefore, the low voltage will limit its current.
L/F converter (“the meter”)
The actual conversion circuit is a simple, two-NPN transistor astable multivibrator circuit. It is a variant of a similar, simple, two-capacitor/two-transistor oscillator circuit, but rearranged to instead use a single inductor.
The device-under-test inductor charges slowly up, until there is a voltage difference of about 40mV across it. At that point, one of the two transistors (can be either) turns off, since its VBE falls below ~620mV – below the threshold for turning on the transistor. This causes the inductor to rapidly discharge its magnetic field back into the turned-off transistor, causing the voltage to fall on the other side; this process repeats at a fixed frequency.
The speed with which the inductor charges depends both on R11/R12 and the input voltage; therefore, the input supply voltage is fixed to 2V in the device. Reducing the size of the two resistors decreases the frequency and the measurement range; the values were chosen to fit regular 50MHz oscilloscopes for most applications.
The secondary, optional output stage (R13 and on), simply
If the circuit fails to start oscillating, add a 1K resistor in parallel with R11 or 12 to help the inductor-under-test to develop the desired potential difference; once oscillating, this can be removed. With the values chosen, I have not experienced a failed start with values from .5µH to 1H. Finally, it is important to keep leads and connectors as short
The prototype was made on a simple 5x10cm piece of TriPad protoboard with cheap components and two S9018 NPN transistors. The secondary (optional) output stage was omitted for clarity.
With the chosen values, you can calculate the inductance of your