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This easy-to-build test
instrument can measure
inductances over the
range from 10µH to
19.99mH with an
accuracy of about 5%.
It uses readily
available parts and
has a 4-digit LCD
readout.
By RICK WALTERS
Build this:
10uH to 19.99mH
Inductance Meter
26 Silicon Chip
A
N INDUCTANCE METER can
be a handy test instrument in
many situations. It can be used
for servicing (eg, in TV sets), selecting
coils for RF circuits, checking coils
for switchmode power supplies and
for measuring coils in many other
applications.
The instrument to be described here
measures from 10µH to 19.99mH over
two ranges and has the twin virtues
of being easy to build and easy to
use. As shown in the photos, there
are just three front panel controls: a
range switch (µH or mH), a pushbutton
switch and a potentiometer. An AC
plugpack is used to supply power,
so there is no on/off switch to worry
about.
To make a measurement, you first
connect the inductor to the test terminals and switch to the µH range.
You then press the “Null” button and
rotate the knob until the LCD panel
meter reads zero, or as close to zero
as you can get (ie, a null). This done,
you release the button and read the
inductance directly off the display.
If the meter over-ranges (ie, it only
displays a 1 at the lefthand digit), you
simply switch to the mH range before
reading the inductance value from
the meter. The value indicated on the
scale by the potentiometer is the DC
resistance of the inductor (although,
in practice, this reading may not be
all that accurate).
Block diagram
Fig.1 shows the block diagram of
the Digital Inductance Meter. It uses
a 3.2768MHz crystal oscillator (IC1a)
to generate a precise clock frequency
and this is divided by 20 and filtered
by IC5 to give a 163.84kHz sinewave
signal. In addition, the signal from the
divide-by-20 stage is divided by 100
and filtered by IC6 to give a second
frequency of 1638.4Hz.
Main Features
•
Two ranges: 10-1999µH &
1-19.99mH
•
Indicates inductor DC
resistance
•
Operates from a 9V AC plugpack supply
•
Accuracy typically 5% from
10µH to 19.99mH
Range switch S2a selects between
these two frequencies and feeds the
selected signal to a nulling circuit.
This circuit is used to null out the
DC resistance of the inductor being
measured. The output from the nulling circuit is then fed to positive and
negative peak detectors and these in
turn drive a digital panel meter (DPM).
Circuit details
Let’s now take a look at the circuit
diagram of the Inductance Meter – see
Fig.2.
NAND gate IC1a and its associated
components function as a square wave
oscillator. It oscillates at a frequency
of 3.2768MHz, as set by crystal X1.
The 33pF, 270pF and 100pF capacitors
provide the correct loading for the
crystal and ensure that it starts reliably
when power is applied.
Pushbutton switch S1 is used to
disable the oscillator. Normally, the
output of IC1a (pin 3) clocks the pin
15 (CA-bar) input of IC2b. However,
when S1 is pressed, pin 1 is pulled
low and IC1a’s pin 3 output remains
high. We’ll explain why this is done
later on.
IC2b, part of a 74HC390 dual 4-bit
decade counter, divides the clock
signal from IC1a by 10. The divided
327.68kHz output appears at pin 9 and
in turn clocks pin 1 of IC3a.
IC3a is one half of a 74HC112 dual
J-K flipflop. In operation, it toggles its
Q and Q-bar outputs on each falling
edge of the clock pulse and thus divides the frequency on its pin 1 input
by 2. The resulting 163.84kHz square
wave signal on the Q output (pin 5)
is then applied to op amp IC5 which
is configured as a Multiple Feedback
Bandpass Filter (MFBF).
Because a square wave is made up
of a fundamental sinewave frequency plus multiple harmonics, we can
configure IC5 to recover virtually any
harmonic. In this case, we are using
IC5 to recover the 163.84kHz fundamental frequency, as determined by
the three resistors and two capacitors
between the output of IC3a and its
inverting input.
The recovered 163.84kHz sinewave
output appears on pin 6 of IC5 and
due to the bandwidth limitations of
the IC, it is a little “notchy”. For this
reason, it is further filtered using a
1.5kΩ resistor and a 470pF capacitor
to remove these high frequency artefacts. This filter circuit also reduces
the amplitude of the sinewave to
around 5V peak-to-peak. The filtered
sinewave is then fed to VR1 which
is the calibration control for the µH
(microhenry) range.
Similarly, for the mH range, IC3a’s
Q-bar output is fed to pin 4 of IC2a
which in conjunction with IC1c and
IC1d is wired as a divide-by-5 counter.
Its output appears at pin 3 and clocks
decade counter IC4. IC4 divides the
frequency on its pin 15 input by 10
and in turn clocks JK flipflop IC3b
which divides by two. The signal is
then fed to MFBF filter stage IC6, in
this case centred on 1638.4Hz.
The output from pin 6 of IC6 is a
1638.4Hz sinewave (also at 5V p-p)
and this is fed to calibration control VR2. Range switch S2a selects
between the two output frequencies
Fig.1: the block diagram for the Digital Inductance Meter. Two precise sinewave frequencies are
derived and these are fed to a null circuit which contains the inductor under test. The following
circuitry then measures the impedance of the inductor and displays its inductance in µH or mH.
JULY 1999 27
Parts List
1 PC board, code 04107991,
124mm x 101mm
1 plastic case, Jaycar HB6094
1 front panel label
1 Digital Panel Meter, Jaycar
QP5550 (or equivalent)
1 9V AC plugpack
1 chassis mount power socket, to
suit plugpack
1 DPDT toggle switch (S1)
1 pushbutton switch, (PB1),
Jaycar SP0710 (or equivalent)
1 speaker connector panel, Jaycar
PT3000 (or equivalent)
1 knob to suit front panel
1 ferrite core set, Altronics L5300
(or equivalent)
1 bobbin, Altronics L5305 (or
equivalent)
20m 0.25mm enamelled copper
wire
2 5kΩ multi-turn trimpots (VR1-2)
1 10Ω wirewound potentiometer
(VR3) (see text for alternative)
3 20kΩ vertical mounting trimpots
(VR4-VR6)
1 3mm x 20mm bolt
1 3mm nut
1 3mm flat washer
1 3mm fibre washer
13 PC stakes
Semiconductors
1 74HC00 quad 2 input NAND
gate (IC1)
1 74HC390 decade counter (IC2)
1 74HC112 dual JK flipflop (IC3)
1 4029 binary decade counter
(IC4)
and applies the selected signal to the
bases of transistors Q1 and Q2 via a
10µF capacitor.
Nulling circuit
OK, we now have two precise
frequencies, either of which can be
selected and fed to the bases of PNP
transistors Q1 and Q2. These are wired
in a nulling circuit. Let’s take a closer
look at their operation.
The thing to remember here is
that the emitter of a PNP transistor
is always 0.6V more positive than
its base (0.6V more negative for an
NPN transistor). Thus, if the base of
Q1 is at 5.7V, its emitter sits at 6.3V.
Because the supply voltage is 9V, this
means that 2.7V must appear across
28 Silicon Chip
4 LM318 op amps (IC5, IC7-IC9)
1 TL071 op amp (IC6)
1 TL072 dual op amp (IC10)
1 7809 TO-220 9V regulator (REG1)
1 78L05 TO-92 5V regulator (REG2)
1 79L05 TO-92 -5V regulator (REG3)
2 BC559 PNP transistors (Q1,Q2)
4 1N914 silicon diodes (D1-D4)
2 1N4004 1A power diodes (D5,D6)
1 3.2768MHz crystal (X1), Jaycar
RQ5271 (or equivalent)
Capacitors
4 470µF 16VW PC electrolytic
7 100µF 16VW PC electrolytic
1 10µF 16VW PC electrolytic
7 0.1µF monolithic ceramic
5 0.1µF MKT polyester
3 .01µF MKT polyester
1 .0047µF MKT polyester
1 470pF ceramic or MKT polyester
2 270pF NPO 5% ceramic
1 220pF NPO 5% ceramic
3 100pF NPO 5% ceramic
1 33pF NPO 5% ceramic
2 22pF NPO 5% ceramic
Resistors (0.25W, 1%)
1 8.2MΩ (select on test)
1 1MΩ
2 5.6kΩ
2 820kΩ
3 4.7kΩ
2 200kΩ
1 1.5kΩ
5 100kΩ
2 1kΩ
1 68kΩ
2 470Ω
1 47kΩ
2 270Ω
1 33kΩ
1 180Ω (calibration)
2 20kΩ
4 100Ω
14 10kΩ
1 3.3Ω (calibration)
1 7.5kΩ
the associated 270Ω emitter resistor
and this translates into a current of
10mA through the resistor.
This (constant) current will also
flow in the collector circuit of Q1,
regardless of the load resistance (provided this resistance is not too large).
If the base of Q1 is now modulated by
a sinewave, its collector current will
vary sinusoidally, the average still
being 10mA.
Q2 has the same value of emitter
resistor as Q1 so its collector current
will be the same as Q1’s; ie, 10mA.
This collector current flows through
potentiometer VR3 to ground.
Note that high beta (gain) transistors
are used for Q1 and Q2 to reduce the
base current, which is a small fraction
of the emitter current.
Because the current through Q2 is
10mA, VR3 (10Ω) will have the same
voltage across it as an inductor with
a 10Ω resistance connected between
Q1’s collector and ground. This position is labelled on the circuit as
“DUT”, which means “Device Under
Test”. The scale for VR3, on the front
panel, is calibrated from 0-10. We will
come back to it shortly.
Q1’s collector is connected to the
positive (red) input terminal of the
inductance meter, while the other
input terminal is connected to ground.
When an inductor is connected across
these terminals, a voltage appears
across it. This voltage consists of two
components: (1) a voltage due to the
DC resistance of the inductor (as just
described); and (2) a voltage due to
the inductive reactance.
In operation, Q1 drives pin 3 of
differential amplifier stage IC7 via a
resistive divider (10kΩ & 20kΩ), while
Q2 drives the pin 2 input via VR3. IC7
and the following parts, including
the LCD readout, function as a digital
voltmeter.
Before taking a measurement, the
resistive voltage component must be
cancelled out. This is done by pressing
switch S1 which shuts down oscillator stage IC1a and effectively “kills”
the sinewave signals selected by S2a.
Potentiometer VR3 is then adjusted so
that the signal on pin 2 of differential
amplifier stage IC7 is the same as the
signal on pin 3, as indicated by a 0.00
reading on the LCD readout.
Note that when the meter reads
zero, the control knob on VR3 indicates the inductor’s DC resistance on
the calibrated scale.
Making the measurement
If S1 is now released, the selected sinewave modulates the 10mA
collector current of Q1. This in turn
generates a sinusoidal voltage across
the inductor (DUT), the amplitude of
which is proportional to the inductance. The resulting sinewave signal
from IC7 is subsequently rectified by
peak detectors IC8 & IC9, summed
Fig.2: the complete circuit diagram
of the Digital Inductance Meter. IC1
is the oscillator, while ICs2-5 divide
the oscillator signal to produce the
two precise sinewave frequencies.
Constant current sources Q1 & Q2
form the null circuit.
JULY 1999 29
Fig.3: install the parts on the PC board as shown here, taking care to ensure
that all polarised parts are correctly oriented. Note that two 8.2MΩ resistors are
shown connected to pin 2 of IC7 but only one is used in practice and is selected
on test (see text). Note also that the metal case of the pot is connected to earth
via one of its terminals.
30 Silicon Chip
in IC10b and applied to the digital
panel meter.
IC8 is used to detect and rectify the
positive sinewave peaks. It works like
this: when the output of IC7 swings
positive, pin 6 of IC8 swings negative
and charges a 100µF capacitor via D4
and a series 100Ω resistor to the peak
level of the waveform. As a result, the
voltage across the 100µF capacitor is
equal to but opposite in polarity to the
peak positive input voltage.
D4 prevents the 100µF capacitor
from discharging as the input level
falls and the voltage on pin 6 starts to
rise. In addition, D3 is reverse biased
during this time and so has no effect.
Conversely, when IC7’s output
swings negative, IC8’s output swings
positive and is clamped by D3 so that
it is 0.6V above the virtual earth input
at pin 2. As a result, the voltage across
the 100µF capacitor is “topped up”
only during positive signal excursions
at the output of IC7.
IC9, the negative peak detector,
works in exactly the same way but
with opposite polarity. It charges its
100µF capacitor to the positive peak
of the applied waveform. Thus, the
positive peak voltage is represented
by a negative DC voltage, while the
negative peak voltage is represented
by a positive DC voltage across the
lower 100µF capacitor.
Due to the bandwidth limitations of
the ICs, this rectification is not perfect
at the higher frequency. This limits the
accuracy below 10µH and readings
below this value should only be used
for comparison measurements.
The output signals from the positive and negative peak detectors are
summed in amplifier stage IC10b. This
stage operates with a gain of .056, as
set by the 5.6kΩ and 100kΩ feedback
resistors, to match the signal to the
sensitivity of the DPM (200mV FSD).
IC10b drives op amp IC10a which
operates with a gain of two and this
then drives the IN+ input of the panel
meter.
Note that the IN- input of the panel meter takes its reference from the
9V supply rail and normally sits at
about 6.3V. As a result, IC10a must
also operate as a level shifter. This is
achieved by biasing pin 3 of IC10 to
half the IN- reference voltage (using
two 10kΩ resistors). Thus, under no
signal conditions, pin 1 also sits at
6.3V and the meter reading is zero.
Trimpot VR6 is used to compensate
Table 1: Capacitor Codes
Value
IEC Code EIA Code
0.1µF
100n
104
.01µF 10n
103
.0047µF 4n7
472
470pF
470p
471
270pF
270p
271
220pF
220p
221
100pF
100p
101
33pF 33p 33
22pF 22p 22
for any offset voltage at the output of
IC10a and allows us to set a zero reading on the DPM when the output of IC7
is at ground. Similarly, VR4 and VR5
compensate for any offset voltages at
the outputs of the peak detectors.
Range switch S2b switches the decimal point on the panel meter, so that
it displays the correct value when we
switch from µH to mH. In effect, this
switch divides by 10 while S2a divides by 100, so that we get an overall
range division of 1000 when switching
from the µH to the mH range.
Power supply
Power for the Digital Inductance
Meter is derived from a 12VAC AC
plugpack supply. Its output is halfwave rectified by diodes D5 and D6
to derive +12V and -12V rails and
these are filtered and fed to 3-terminal
regulators REG1 & REG3 respectively.
Quite a few changes were made to the PC board of the Digital Inductance Meter
after this photograph was taken.
Table 2: Resistor Colour Codes
No.
1
1
2
2
5
1
1
1
2
14
1
2
3
1
2
2
2
1
4
1
Value
8.2MΩ
1MΩ
820kΩ
200kΩ
100kΩ
68kΩ
47kΩ
33kΩ
20kΩ
10kΩ
7.5kΩ
5.6kΩ
4.7kΩ
1.5kΩ
1kΩ
470Ω
270Ω
180Ω
100Ω
3.3Ω
4-Band Code (1%)
grey red green brown
brown black green brown
grey red yellow brown
red black yellow brown
brown black yellow brown
blue grey orange brown
yellow violet orange brown
orange orange orange brown
red black orange brown
brown black orange brown
violet green red brown
green blue red brown
yellow violet red brown
brown green red brown
brown black red brown
yellow violet brown brown
red violet brown brown
brown grey brown brown
brown black brown brown
orange orange gold brown
5-Band Code (1%)
grey red black yellow brown
brown black black yellow brown
grey red black orange brown
red black black orange brown
brown black black orange brown
blue grey black red brown
yellow violet black red brown
orange orange black red brown
red black black red brown
brown black black red brown
violet green black brown brown
green blue black brown brown
yellow violet black brown brown
brown green black brown brown
brown black black brown brown
yellow violet black black brown
red violet black black brown
brown grey black black brown
brown black black black brown
orange orange black silver brown
JULY 1999 31
This photograph shows the
completed Digital Inductance
Meter with the calibration
inductor connected to its test
terminals – see text.
REG1 provides a +9V rail, while REG3
provides a -5V rail. In addition, REG1
feeds REG2 which provides a regulated +5V rail.
The ±5V rails supply most of the
op amp stages, while the +9V rail
supplies the digital panel meter and
the constant current sources in the
null circuit. The +12V rail is used
for the positive supply to IC10, as its
output needs to swing up to near the
9V supply of the DPM.
Putting it together
Building the circuit is a lot easier
than understanding how it works.
32 Silicon Chip
Most of the parts are mounted on a
single PC board and this is coded
04107991. This, together with the digital panel meter, fits inside a standard
plastic case with a sloping front panel.
As usual, check the PC board for
etching defects by comparing it with
the published pattern (Fig.4). Any
defects should be repaired before
proceeding. In addition, part of the
PC board will have to be filed away
along the bottom lefthand and bottom
righthand edges, so that the board
will fit between the mounting pillars
of the case.
Check also that the body of switch
S1 fits through its matching clearance
hole in the board. Enlarge this hole
with a tapered reamer if necessary,
so that it clears the switch. The same
goes for the threaded bush of pot VR3.
Fig.3 shows the assembly details.
Begin by fitting 13 PC stakes for the
external wiring points, then fit the
11 wire links on the top of the board
(including the one under VR3). This
done, fit the resistors, diodes and
transistors. Table 2 shows the resistor
colour codes but check them with a
DMM as well, just to make sure.
Take care to ensure that all the
transistors and diodes are installed the
correct way around and make sure the
correct part is used at each location.
Once these parts are in, install the
capacitors (watch the polarity of the
electros), the regulators and the ICs.
We used IC sockets in the prototype
but suggest that you solder your ICs
directly to the PC board. Again, be
sure to use the correct device in each
location and note that the ICs don’t
all face in the same direction.
The trimpots can now all be installed, followed by poten
tiometer
(VR3). As shown in the photo, VR3 is
installed from the component side of
the PC board and is secured using a
nut on the copper side. Its terminals
are connected to their pads on the PC
board using short lengths of tinned
copper wire.
Once the pot is in, you have to run
two insulated wire links between its
terminals and points CT & CW on the
PC board – see Fig.4. These points are
located near Q2, towards the bottom
righthand corner. Note also that the
metal case of the pot is connected to
earth via one of its terminals.
That completes the board assembly.
Before placing it to one side though,
go over your work carefully and check
for errors. In particular, check for
missed solder joints and incorrectly
placed parts.
Final assembly
Next, attach the artwork to the front
panel and use it as a drilling template
for the switches, the potentiometer,
the test terminals and the panel meter. The square cutout for the meter is
made by first drilling a series of small
holes around the inside of the marked
area, then knocking out the centre
piece and filing the edges to shape.
This done, use a sharp chisel to remove the short mounting pillar inside
the case, to prevent it from fouling the
panel meter. You will also have to drill
a hole in the top rear panel for the
3.5mm power socket – see photo. Be
sure to position this hole so that the
socket clears the panel meter when it
is mounted.
The various components can now
all be installed in the case, starting
with the switches and the input connector block which carries the test
terminals. Bend the lugs on the input
connector block so that they are parallel to the front panel, to prevent them
shorting to the PC board. The board
can then be fitted inside the case and
secured using two self-tapping screws
into the short mounting pillars.
Before fitting the digital panel meter, it should have a link fitted from
N to OFF (to disable the polarity
indication). In addition, you have to
fit three 100kΩ resistors from P1, P2
and P3 to OFF. These modifications
are all shown on Fig.3 (do not forget
the link).
The panel meter we used has an
external dress bezel with two captive mounting screws. This bezel is
mounted from the front and the panel
meter then fitted over the screws and
secured using nuts and fibre washers.
The assembly can now be completed by running the point-to-point
wiring. Note the connections between
S2 and the panel meter. In particular,
the middle lefthand terminal of S2
goes to the ON pad on the meter board
(not to resistor P3). By contrast, the
top and bottom lefthand terminals
are connected to the resistors on P2
and P1 respectively.
Fig.4: two insulated flying leads must be run on the copper side of the
PC board, between the pot terminals and points CT & CW, as shown
in this diagram.
Test & calibration
Before you begin testing, you need
to wind an inductor which is used
later during the calibration procedure.
To do this, wind around 300 turns of
30 B&S wire on the L5305 bobbin, then
fit the cores and clamp them together
using a 20mm bolt, flat washer, fibre
washer and nut.
Once the coil has been wound,
clean and tin the ends, then connect
a 180Ω 1% resistor in parallel with it.
Now put the coil to one side – you’ll
need it shortly, for Step 7 of the following procedure.
To test the unit, apply power and
check that D5’s cathode is at about
12V. This voltage will depend on the
particular plugpack you use and is
not too critical. Next check the +9V,
Fig.5: check your PC board by comparing it with this full-size etching
pattern before installing any of the parts.
JULY 1999 33
H
SILICON
CHIP
INDUCTANCE METER
4
5
6
7
3
2
8
9
1
PRESS AND ADJUST
FOR METER NULL
+5V and -5V rails – these should all
be within 5%. The panel meter should
show a reading of around 16.00 or
160.0, depending on the range.
Now check the supply rails at the
IC pins. If these are OK, you are ready
to calibrate the instrument using the
following step-by-step procedure:
Step1: connect a multimeter across
the test terminals and set it to a range
suitable for measuring 10mA DC.
Step 2: press S1 and check the current on the multimeter. It should be
close to 10mA.
Step 3: release S1, rotate VR3 fully
anticlockwise (0Ω), remove the multimeter and connect a 3.3Ω resistor
across the test terminals.
Step 4: switch your multimeter
34 Silicon Chip
0
10
Fig.6: this full-size artwork can be used as a drilling template for the front panel.
mH
to a low voltage range and connect
it between pin 6 of IC7 and ground.
Short switch S1’s terminals using an
alligator clip, then adjust VR3 (on the
front panel) for a 0V (or as close as you
can get) reading on the multimeter.
Step 5: connect the multimeter
across the 100µF capacitor at the output of IC8 and (with S1 still shorted)
adjust VR4 for a reading of 0V. Now
adjust VR5 for 0V across the 100µF
capacitor at the output of IC9.
Step 6: adjust VR6 for a zero reading
on the panel meter and remove the
shorting clip from S1.
Step 7: remove the 3.3Ω resistor
from the test terminals and fit the
inductor that you wound earlier (with
its parallel 180Ω 1% resistor).
Step 8: rotate VR3 to the zero ohms
position and measure the voltage on
pin 6 of IC7. It must be adjusted to
zero by fitting a resistor between pin
2 and either the +5V or -5V rail. Two
sets of pads have been placed on the
PC board for the resistor, from pin 2
to each supply. Our unit needed an
8.2MΩ resistor to the negative rail.
Step 9: set S2 to µH and adjust VR1
until the panel meter reads 174.9.
Step 10: switch to the mH range
and adjust VR2 for a reading of 17.49.
That completes the calibration procedure. You can now close the case
and begin using your new inductance
meter.
By the way, if you find that you
cannot zero (or null) the panel meter
when measuring an inductor, even
with VR3 rotated fully clockwise,
it means that the resistance of the
inductor is greater than 10Ω. Despite
this, the inductance reading displayed
when S1 is released should be close
to the correct value.
What if it won’t work?
If you have problems, the first step
is to check your sol
dering. In particular, look for missed solder joints
and shorts between adjacent tracks
and IC pins.
A few voltage checks can also help
pinpoint problems. First, check for
+ 2.5V on pins 5, 6 and 9 of IC3. Pin
6 of IC5 and pin 6 of IC6 should be
around 0V DC and 4-5V AC. Most
meters will give quite a low reading
on the AC output of IC5. As long as
you get an indication, the signal is
probably OK. The bases of Q1 and Q2
should be at 5.7V and their emitters
at 6.3V. The collector of Q2 should
read 100mV.
Note that when the unit is working
properly and there is no inductor
across the terminals, the meter will
read around 16.00 or 160.0, depending on the range. This is due to the
positive peak detector swinging to
full output and is normal.
Variations
VR3 can be changed if you wish
to measure inductors with DC resistances greater than 10Ω. For example,
a 25Ω pot will allow inductors with
resistances up to 25Ω to be measured.
Naturally you will have to recalibrate
the potentiometer scale or you can
simply multiply the front panel readSC
ing by 2.5.
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