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Current Clamp
Adaptor For
Multimeters
By JOHN CLARKE
Looking for a current clamp meter that won’t
break the bank? Here’s a simple clamp meter
adaptor that you can build for about $35. It
plugs into a standard DMM and can measure
both AC and DC currents.
C
LAMP METERS are very convenient when it comes to measuring current, since they do not
require breaking the current path. Instead, they simply clip over the wire
or lead that’s carrying the current and
the reading is then displayed on the
meter.
This is not only much easier than
“in-circuit” current measurements
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but is often a lot safer as well; eg,
where high voltages and currents are
involved. However, clamp meters are
not particularly useful for making
low-current measurements (ie, below
1A) due to their inaccuracy and lack
of resolution.
Unlike this unit, many commercial
current clamp meters can only measure AC. That’s because they are basic-
ally current transformers, comprising
turns of wire around a magnetic core.
This magnetic core is clipped around
the wire to be measured, which effectively behaves as a half-turn primary winding. The winding on the
core itself acts as the secondary and
connects to the multimeter’s current
terminals.
The measured current is a divided
down value of the true current flowing
in the wire. Usually, the division ratio
is 1000:1 so that 1mA shown on the
meter equates to 1A through the wire
that’s being measured.
Clamp meters capable of measuring
DC as well as AC do not use a current
transformer but a Hall effect sensor
instead. This sensor is placed inside
September 2003 53
Fig.1: the circuit uses Hall effect sensor HS1 which produces a voltage at its pin 3 output that depends on the
magnetic field induced into an iron-powdered toroid core. This voltage is fed to op amp IC1a which then drives
the negative terminal of the multimeter. IC1b drives the meter’s positive terminal and provides null adjustment.
a gap in an iron-powdered toroid
core. It measures the magnetic flux
produced as a result of the current
flowing through the wire and produces
a proportional output voltage.
How it works
To make it as versatile as possible,
the SILICON CHIP Clamp Meter Adaptor also uses a Hall effect sensor so
that it can measure both DC and AC
currents. The output of this sensor
is then processed using a couple of
low-cost op amps which then provide
a signal for a standard DMM or analog
multimeter.
When measuring DC current, the
multimeter is set to its DC mV range
and 1A through the wire in the core
equates to a reading of 1mV on the
meter. A potentiometer allows the
output to be nulled (ie, adjusted to
0mV) when there is no current flow.
Similarly, for AC current measurements using the clamp meter, the
multimeter is simply set to its AC
mV range. In this case, the DC offset
potentiometer is not needed, since the
multimeter automatically ignores any
DC levels.
54 Silicon Chip
The high-frequency response of the
adaptor for AC measurements is 3dB
down at 20kHz (ie, 0.7071 of the real
value). However, the actual measurement displayed will also depend on
the high-frequency response of the
Specifications
Output: 1A = 1mV for AC and DC
ranges
Resolution: multimeter dependent
(100mA with 0.1mV resolution on
multimeter)
Maximum DC current: 150A
recommended (up to 900A if core
is demagnetised afterwards)
Maximum AC current: 630A
recommended
Linearity: typically better than 4%
over range at 25°C
AC frequency response: -3dB at
20kHz (meter reading depends on
multimeter AC response)
Current consumption: 15mA
multimeter itself. Some multimeters
give useful readings up to 20kHz,
while others begin to roll off the signal
above 1kHz (ie, frequencies above this
will not be accurately measured).
If necessary, the output from the
Clamp Meter Adaptor can be monitored using an oscilloscope if AC
measurements have to be made at
high frequencies. However, AC current
measurements at 50Hz (ie, the mains
frequency) will be accurate using virtually any multimeter.
Note that most multimeters are calibrated to display the RMS values of
AC current measurements, although
they are only accurate for sinusoidal
waveforms. This unit will not affect
meter calibration, since it does not
change the shape of the waveform for
signals below 20kHz and only converts
the current waveform to a voltage
waveform. However, for non-sinusoidal waveforms, the multimeter will
display an erroneous result unless it
is a true RMS type.
Demagnetising the core
One problem with clamp meters is
that the core can remain magnetised
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after making high DC current measurements; ie, even when the current
flow has been reduced to zero. In fact,
this effect becomes apparent when
measuring DC currents above about
150A. It is easily detected because
the output from the sensor remains
at several millivolts after the current
ceases flowing.
Fortunately, there’s an easy solution
to this. If the core does become magnetised, it can be demagnetised again
by momentarily reversing the current
flow in the core.
This is done by un
clipping the
core from the wire, replacing it over
the wire upside down and applying
the current again for a brief period
of time.
Modified battery clamp
To keep costs down, the SILICON
CHIP Clamp Meter Adaptor uses a
modified car battery clip as the current
clamp. This is fitted with an iron-powdered toroid core which is cut in half
so that the clip can be opened and
slipped over the current-carrying
wire. The Hall effect sensor sits in a
gap in the toroid, near the front of the
clip –see Fig.2.
The output from this sensor is fed
to a processing circuit which is built
on a small PC board and housed in a
plastic case, along with the battery.
This circuit in turn connects to the
meter via two leads.
By the way, commercial clamp meters using Hall effect sensors usually
place the sensor at the hinge end of the
core. This can be done when the clamp
material is non-magnetic. However,
when the clamp is magnetic, as in this
design, the magnetic flux is conducted
through it instead and bypasses the air
gap where the sensor sits – see Fig.2
(top drawing).
This problem is solved by simply
placing the sensor in an air gap at the
front of the clamp, so that it cannot
be bypassed.
Circuit details
Refer now to Fig.1 for the circuit
details. It’s relatively simple and comprises a dual op amp (IC1a & IC1b), a
3-terminal regulator (REG1), the Hall
effect sensor (HS1) and a few resistors
and capacitors.
Power for the circuit is derived
from a 9V battery and is fed to REG1
which provides a regulated +5V rail.
This then powers the Hall effect sensor
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Fig.2: if a steel (ie, magnetic) clamp is used, the Hall sensor
must be placed in an air gap in the toroidal core as shown in
the bottom diagram. This is necessary to ensure that it is not
bypassed by magnetic flux flowing through the clamp instead.
and op amps IC1a & IC1b. Note that a
regulated supply is necessary, since
the Hall sensor output will vary with
supply rail variations.
In operation, the Hall effect sensor
produces a voltage at its pin 3 output
that depends on the magnetic field
in the core. If the marked face of the
sensor faces a south magnetic field, its
output voltage will rise. Conversely, if
it faces a north field, the output voltage
will fall.
The sensor’s output with no magnetic field applied to it will sit between
2.25V and 2.75V, depending on the
sensor. This voltage remains stable,
providing the supply voltage remains
stable.
The output of the Hall effect sensor
is fed to op amp IC1a. This stage is
wired as an inverting amplifier and
it attenuates the signal by an amount
that depends on the setting of trimpot
VR1 (calibrate). Note that the gain of
IC1a is set by the resistance between
pins 1 & 2 divided by the 18kΩ input
resistor.
This means that if VR1 is set to
half-way, IC1a has a gain of (2.5kΩ +
1kΩ)/18kΩ = 0.19.
In practice, VR1 is adjusted so that
it produces an output of 1mV per amp
flowing through the current-carrying
wire.
Op amp IC1b and its associated
circuitry compensate for the initial
DC voltage at the output of the Hall
effect sensor (ie, with no magnetic
field applied). As shown, IC1b is connected as a unity gain buffer with its
output connected to its pin 6 invert
ing input. The non-inverting input at
pin 5 connects to a resistive divider
network consisting of VR2, VR3 and
a 22kΩ resistor.
The output from IC1b (pin 7) goes
to the positive meter terminal and
is also used to bias pin 3 of IC1a via
a 10kΩ resistor. This bias voltage is
nominally about 2.5V (ie, 0.5Vcc) and
allows the output of IC1a to swing up
or down about this voltage, depending on the sensor input. It also effectively allows the quiescent voltage
from the Hall sensor to be nulled so
that we get a 0V reading on the meter
September 2003 55
Fig.3: install the parts on the PC board as shown here.
The Zero Adjust pot (VR3) is installed by soldering its
terminals to three PC stakes.
Fig.4: the full-size etching pattern for the PC board.
when no current is being measured.
VR2 is initially adjusted with VR3
set to mid-range, so that the multi
meter reads 0V with no magnetic field
applied to the Hall sensor. VR3 is then
adjusted during subsequent use of the
clamp meter – it can vary IC1b’s output
by about 25mV to null out any small
voltage readings.
In effect, trimpot VR2 acts as a
coarse offset adjustment, while VR3
allows fine adjustment to precisely
zero the reading.
Looked at another way, VR2 & VR3
are simply adjusted so that the voltage
on pin 7 of IC1b is the same as the
voltage on pin 1 of IC1a when there is
no magnetic field applied to the Hall
effect sensor – ie, the voltage between
pins 1 & 7 is 0V.
The outputs from both op amps
are fed to the multimeter via 100Ω
resistors. These provide short-circuit
protection for the op amp outputs and
also decouple the outputs from the
cable capacitance.
Construction
Building the circuit is easy since all
the parts are mounted on a small PC
board coded 04109031 and measuring
75 x 30mm. Begin construction by
Check your completed PC board assembly carefully to ensure that all polarised
components have been correctly installed. These parts include IC1, REG1 and
the two electrolytic capacitors.
56 Silicon Chip
checking the PC board for any shorts
between tracks and for any breaks in
the copper pattern. Also check that the
hole sizes are all correct for the various
components, particularly those for the
PC-mount stereo socket and the on/off
switch (S1).
Note that two of the corners on
the PC board need to removed, so
that the board later clears the corner
pillars inside the case. If your board
is supplied with these corners intact,
they can be cut away using a small
hacksaw and carefully finished off
using a rat-tail file.
Fig.3 shows the assembly details.
Install the resistors and wire link
first, using Table 1 to guide you on the
resistor colour codes. It’s also a good
idea to check the resistor values with
a DMM, just to make sure.
IC1 can go in next, taking care to
ensure that it is ori
ented correctly.
That done, install the trimpots and
the capacitors, noting that the electrolytics must be oriented with the
polarity shown. The trimpots are usually labelled with a code value, with
502 equivalent to 5kΩ (VR1) and 503
equivalent to 50kΩ (VR2).
Next, install PC stakes at the two
power supply inputs, the +5V terminal, the three VR3 terminal positions
and the two multimeter outputs. These
can be followed with the switch and
the PC-mount stereo socket.
Finally, complete the board assembly by installing potentiometer VR3
– it is mounted with its terminals
soldered to the top of its PC stakes.
Position it so that the top of its mounting thread is at the same height as the
top of the switch thread.
Drilling the case
The front panel artwork can now
be used as a template to mark out and
drill the lid of the small plastic utility
case that’s used to house the board.
You will need to drill two holes – one
for the switch and the other for the
potentiometer.
In addition, you will have to drill a
4mm hole in one end of the case for
the multimeter leads, plus a 7mm hole
in one side to accept the stereo socket.
The latter should be positioned 14mm
down from the top of the case and
21mm in from the outside edge.
Note that it’s always best to drill
small pilot holes first and then carefully enlarge them to size using a tapered
reamer.
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Fig.6 (below): a 60mm-length of 3-way
rainbow cable is used to make the
connections to the Hall sensor. This
cable is then joined to a 300mm length
of 2-core shielded cable which is then
terminated in 3.5mm stereo plug.
Fig.5 (above): this exploded diagram shows how the toroid
core and Hall sensor are fitted to the clamp. Each core half
is secured in position using builders’ adhesive, as are the
Hall sensor and the adjacent plastic rectangle. Note the
earth connection to the metalwork of the clamp.
Next, the integral side clips inside
the box need to be removed using a
chisel. Be sure to protect your eyes
when doing this, as the plastic tends
to splinter and fly out. You can then
attach the front panel label and cut the
holes out with a sharp knife.
The next step is to solder the battery
clip leads to the supply terminals (red
to positive, black to negative). That
done, connect the multimeter leads to
the output terminals, then feed these
wires through the hole in the box and
attach banana plugs to each free end.
Don’t fit the board to the case lid at
this stage. That step comes later, after
calibration has been completed.
Clamp assembly
The clamp assembly comprises a car
battery clip, the toroidal core and the
Hall effect sensor. Figs.5 & 6 show the
assembly details for this unit.
The first step is to cut the core in
half using a fine-toothed hacksaw
blade. That done, the Hall sensor
This view of the
completed current
clamp clearly shows
the general
arrangement. If the
toroid core becomes
magnetised
during use, it can
be demagnetised by
momentarily
reversing the
current flow in the
core.
should be wired using a 60mm length
of 3-way rainbow cable which should
be sheathed in heatshrink tubing (see
Fig.5). The other end of this cable is
then connected to a 300mm length of
2-core shielded cable which in turn is
terminated with a 3.5mm stereo plug.
As shown in Fig.6, the cable shields
are joined together and connected to
the earth lead of the rainbow cable.
They are also connected to the metal
work of the clip using a short length
of hookup wire. Small pieces of insulating tape should be used to prevent
shorts between the wires where the
Table 2: Capacitor Codes
Value μF Code EIA Code IEC Code
100nF 0.1μF
104
100n
1nF 0.001μF 102
1n0
Table 1: Resistor Colour Codes
o
o
o
o
o
o
No.
1
1
1
1
2
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Value
22kΩ
18kΩ
10kΩ
1kΩ
100Ω
4-Band Code (1%)
red red orange brown
brown grey orange brown
brown black orange brown
brown black red brown
brown black brown brown
5-Band Code (1%)
red red black red brown
brown grey black red brown
brown black black red brown
brown black black brown brown
brown black black black brown
September 2003 57
ground and shield.
As it stands, the clamp can be
slipped over leads up to 7mm in dia
meter. A larger clamp with jaws that
open wider than the specified unit
will be necessary if you intend measuring currents flowing in leads that
are thicker than 7mm.
Note that the clamp adapter is not
suitable for use with 240VAC mains
when the wiring is uninsulated.
Testing
Fig.7: this simple setup can be used to calibrate the Clamp Meter
Adapter. Null the reading first using potentiometer VR3, then switch
on the 12V supply and adjust trimpot VR1 for a reading of 66.7mV.
cables join, after which the join should
be covered using heatshrink tubing.
The next step is to glue the Hall
sensor to one of the core pieces using
some builders’ adhesive (it can go in
either way up). That done, glue a small
piece of plastic to the remaining part of
the core gap to protect the Hall sensor
from damage when the clamp closes.
Naturally, this piece of plastic needs to
be slightly thicker than the Hall sensor
to provide this protection.
The two core pieces can now be
glued in position on the jaws of the
battery clip, again using builders’ adhesive. Make sure that the two halves are
correctly aligned before the glue sets.
Once the core pieces are secure, the
wiring for the Hall sensor can be glued
in position and secured at the end of
the clip with a cable tie. In addition,
the metal tabs on the clip should be
bent over to hold the wire in place.
This must also be done on the other
handle, so that the jaws of the clamp
can be opened as wide as possible.
The 3.5mm stereo plug is wired as
shown, with the tip and ring terminals
connecting to the red and black wires
respective
ly. If your twin shielded
wire has different colours, take care
to ensure that pin 1 on the Hall sensor
goes to the tip connection. Pin 3 must
go to the ring terminal and pin 2 is the
There’s plenty of room inside the case for the PC board and a 9V battery. The
board is held in position by slipping the case lid over the switch and pot shafts
and doing up the nuts.
58 Silicon Chip
The unit is now ready for testing.
First, connect the battery and check
that there is +5V at the test point on
the PC board (ie, 5V between this test
point and ground). There should also
be +5V on pin 8 of IC1.
If these measurements check OK,
plug the clamp assembly into the
socket on the PC board and check the
voltages again. If they are no longer
correct, check component placement
and the wiring to the Hall sensor.
Next, connect the output leads from
the unit to the voltage inputs on your
multimeter and set the range to mV DC.
That done, set VR3 to its mid-position
and adjust VR2 for a reading of 0mV.
Calibration
The Current Clamp Adaptor is calibrated using a 12V power supply, a
5m length of 0.5mm enamelled copper
wire and an 18Ω 5W resistor.
First, wind 100 turns of the ECW
around the core and connect it to the
12V supply via the 18Ω resistor as
shown in Fig.7. The current through
the wire will be 12/18 = 0.667A and,
as far as the clamp meter is concerned,
this is effectively multiplied by 100
due to the number of turns on the core.
All you have to do now is adjust VR1
for a reading of 66.7mV. And that’s it
– the calibration is complete!
Note that if the power supply is
not exactly 12V, you can compensate
for this by calibrating to a different
reading. Just measure the supply voltage, divide the value by 18 (to get the
current) and multiply by 100 to obtain
the calibration number.
For example, if you are using a 13.8V
supply, you will have to set VR1 for
a reading of 76.7mV on the meter (ie,
13.8/18 x 100) = 76.7).
Once the calibration has been completed, the PC board can be attached to
the case lid. It’s held in place simply
by slipping the lid over the switch
and pot shafts and doing up the nuts.
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Parts List
Fig.8: this full-size artwork for the
front panel.
Using the clamp meter
Note that before making a measurement, the DC Zero potentiometer must
first be adjusted so the multimeter
reads 0mV when there is no current
flow. Note also that the core may need
to be demagnetised after measuring
high DC currents, as described previously. This will be necessary when
the DC Zero control no longer has
sufficient range to null the reading.
When measuring relatively low currents (eg, between 100mA and 10A),
increasing the number of turns of the
current-carrying wire through the core
will improve the resolution. However,
this will only be possible if the wire
diameter allows the extra turns to be
fed through the core.
Note that the readout on the multimeter must be divided by the number
of turns through the core to obtain the
correct current reading. Note also that
the accuracy of the unit will vary according to the temperature of the Hall
sensor, particularly when making high
current measurements.
It's a good idea to mark the top of
the clamp with an arrow to indicate
the direction of positive current flow
once you have the unit working correctly. This can easily be determined
by trial and error.
Finally, remember to switch the unit
off when it is not in use. There’s no
power indicator LED to warn you that
the unit is on, so take care here! SC
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1 PC board, code 04109031, 75
x 30mm
1 front panel label, 80 x 52mm
1 plastic box, 82 x 54 x 30mm
1 iron powdered toroidal core, 28
x 14 x 11mm (Jaycar LO-1244
or equivalent)
1 50A car battery clip (DSE
P-6424 or equivalent)
1 3.5mm stereo PC board mount
socket (Jaycar PS-0133 or
equivalent)
1 3.5mm stereo jack plug
1 SPDT toggle switch (S1)
1 5kΩ (code 502) horizontal
trimpot (VR1)
1 50kΩ (code 503) horizontal
trimpot (VR2)
1 1kΩ 16mm linear potentiometer
(VR3)
1 red banana line plug
1 black banana line plug
1 9V battery clip
1 9V battery
1 potentiometer knob
1 4 x 4 x 2mm piece of soft
plastic
1 300mm length of twin core
shielded cable
1 60mm length of 3-way rainbow
cable
1 200mm length of red heavy
duty hookup wire
1 200mm length of black heavy
duty hookup wire
1 50mm length of green heavy
duty hookup wire
1 50mm length of 4.8mm
diameter heatshrink tubing
1 100mm cable tie
8 PC stakes
Semiconductors
1 LM358 dual op amp (IC1)
1 UGN3503 Hall effect sensor
1 78L05 5V regulator (REG1)
Capacitors
1 100μF 16V PC electrolytic
1 10μF 16V PC electrolytic
1 100nF MKT polyester
1 1nF MKT polyester
Resistors (1% 0.25W)
1 22kΩ
1 1kΩ
1 18kΩ
2 100Ω
1 10kΩ
Calibration parts
1 5m length of 0.5mm enamelled
copper wire
1 18Ω 5W resistor
There’s no power LED on the front panel to warn you when the power is on,
so be sure to switch the unit off when it is not in use to save battery life. Also,
be sure to null the reading on the multimeter (ie, when there is no current flow
through the core) before taking a measurement.
September 2003 59
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