Inductance meter adapter
All resistances are in ohms
7805 positive regulator 5v o/p
2SC9018 is a high frequency transistor with FT of 1.1 Ghz . you
need high frequency transistors in the circuit where it is used
in case you want to substitute for it.
I guess 2n2857, 2n5179 ,BF180 can be substituted
The following circuit enables me to measure inductance of the
inductor labeled LX which is the inductance to be measured. The
o/p of the circuit is a TTL square wave whose frequency relates
to the inductance being measured.
The inductance meter adapter output is connected to a frequency
meter and the inductance is calculated from the frequency.Hence
you need a frequency meter and some calculation to get your
HOW IT WORKS
The heart of the circuit is the buffer colpitts oscillator(the
first stage) which resonates with the unknown inductance to give
a Sine wave of a particular frequency .
The frequency of the sine wave is a function of the unknown
inductance and the four 1000pF capacitors.
The output sine wave is amplified by the second transistor and
is then rectified by the capacitor and diode combination that
The rectified sine wave now having only positive excursions is
buffered by the third transistor and is then fed to the 74ls393
Counter ic which is configured as a divide by 256 counter.
The output of the ic pin 6 and ground is connected to the
The 7805 regulator powers the IC and the last 2 transistors
The circuit operates from a 9V battery which also feeds the
The Ic consists of two counters in one package of 14 pins hence
you need just one.
PIN 14 is VCC and PIN 7 is ground
The ic is TTl IC
PERFORMANCE AND GETTING YOUR READING
My own digital meter can read frequency so I use it to take the
I programmed my calculator to get me the inductance value from
the reading. A simple computer programme can do this as well.
Actual frequency= Frequency reading*256 (the counters have
scaled down the frequency by 256 times)
Actual frequency =
So the last equation allows you to solve for the value of the
inductance Lx which will be in microhenries.
This inductance measuring scheme works well for inductances of
even low values less than 1uH. However the lower
Values could have error due to the circuit including test lead
inductance. The circuit is that sensitive. I play with some
tricks to get around this which I will discuss later if people
are interested in the circuit. I wish it was a stand alone unit
but here in my country NIGERIA(IN WEST AFRICA) and my city
Ibadan there are no fancy PICS or Digital displays to make it a
stand alone unit so I just have to make do with Simple ICíS and
Sorry the equations donít look too neat I hope you get them
Any comments about this circuit can be mailed to me and I hope
somebody finds it interesting enough to construct And possibly
make a stand alone unit with microprocessors or PICS and
displays that I havenít got. Thanks to Bernardo Dimacali for
helping me put the equation in a good and clear form.
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