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This Low Ohms Tester plugs directly
into a digital multimeter and can
accurately measure resistances down
to 0.01Ω. It’s easy to build and runs
off a 9V battery.
By JOHN CLARKE
Build a low ohms
tester for your DMM
The ability to measure low resistance values is necessary when items
such as meter shunts, loudspeaker
crossover networks, inductors and
contact resistances are to be checked.
Unfortunately, a standard digital
multimeter can only accurately measure resistances down to about 5Ω.
Resistors with lower values will give
misleading results due to a lack of meter resolution. A couple of examples
will serve to illustrate this point.
First, let’s assume that a resistance
of 0.1Ω is to be checked on a standard
3-1/2 digit multimeter. In this case,
you would have to switch down to
the 200Ω range (the lowest you can
select) and the reading would be 0.1Ω
±1 digit (ie, ±0.1Ω). In other words,
40 Silicon Chip
Fig.1: block diagram of the
Low Ohms Tester. It works by
applying a constant current
through the test resistor (Rx).
The voltage across Rx is then
measured using a DMM.
the resolution of the DMM limits the
accuracy of the reading to ±100%
which is ridiculous.
This situation quickly improves
with increasing resistance values. For
example, a value of 1Ω will result in
a reading of 1.0Ω ±1 digit, assuming
that the 200Ω range is used. This
represents an accuracy of 10%. For
values above 10Ω, the accuracy of
the instrument will be 1% or better
since the resolution of the reading is
considerably improved.
This Low Ohms Tester overcomes
the limitations of conventional digital multimeters for low values of
resistance. It does this by applying
a constant current through the test
resistor Rx. The resulting voltage de-
Fig.2: the full circuit
for the Low Ohms
Tester. REF1, IC1 and
Q1 form a constant
current source for
the test resistor Rx.
The resulting voltage
across Rx is then either
measured directly or
amplified by IC2 before
being applied to the
DMM.
veloped across Rx is then amplified
and applied to the DMM which is set
to read in millivolts. Fig.1 shows the
basic scheme.
As shown in the photos, all the circuitry is housed in a compact plastic
case. This carries a power switch,
a 4-position range switch and two
binding post terminals for the test
resistor. The output leads emerge from
the top of the instrument and are fitted
with banana plugs. These simply plug
into the COM and VΩ terminals of the
DMM.
The output from the Low Ohms
Tester is a voltage (in mV) which is
directly proportional to the resistance
being measured. In practice, you simply multiply the reading on the DMM
by the range setting on the tester to
get the correct value. For example, a
DMM reading of 5.6mV when the 0.1Ω
range is selected is equivalent to 5.6
x 0.1 = 0.56Ω.
From this, it follows that if the 1Ω
range is selected, the reading on the
DMM is directly equivalent to the
value in ohms.
Values from 100Ω down to 0.01Ω
can be measured via the tester. Below
this, errors start to be significant due
to contact and lead resistance.
Values above 100Ω can also be
measured via the tester but this is
rather pointless. That’s because the
DMM alone can be used to accurately
measure values above this figure.
Circuit details
Refer now to Fig.2 for the complete
circuit of the Low Ohms Tester. It
consists of a constant current source
(which supplies the current through
test resistor Rx) plus an amplifier stage
to drive the DMM.
IC1, REF1 and Q1 are the basis of
the constant current source. REF1
is a precision voltage source which
provides a nominal 2.490V between
its “+” and “-” terminals. This device
is connected between the positive
supply rail and ground via a 5.6kΩ
current limiting resistor. VR1 allows
•
•
•
•
Main Features
Measures from 0.01Ω to 100Ω
Four ranges
Outputs to a digital multimeter
Battery operated
the reference voltage to be adjusted
slightly and is used for calibration.
Op amp IC1 and transistor Q1 function as a buffer stage for REF1. Because
this stage is simply a voltage follower,
the voltage on Q1’s emitter will be the
same as the voltage on pin 3 of IC1.
This means, in turn, that the voltage
across the resistance selected by S2b
is equal to the REF1 voltage.
As a result, a constant current flows
through the selected resistance and
this current also flows through Q1,
test resistor Rx and diodes D1 & D2
to ground.
In greater detail, when S2b selects
positions 1, 2 or 3, the 2.4kΩ resistor
is in circuit and so has the REF1 voltage across it. If REF1 is adjusted to
2.4V, then 1mA will flow through the
resistor and thus through Q1 and Rx.
Conversely, when S2b selects position
4, the constant current source delivers
10mA to Rx (assuming that VR2 is
correctly set).
IC2 functions as the amplifier stage.
This operates with a gain of either x10
or x100, as set by switch S2a. Switch
S2c selects between the collector of
Q1 and the amplifier output at pin 6.
Thus, when position 1 is selected,
June 1996 41
Fig.4: this is the full-size etching pattern for the PC board.
4 are selected, IC2 amplifies the
voltage across Rx and drives the
DMM via its pin 6 output. IC2
operates with a gain of 10 when
position 2 is selected and a gain
of 100 when positions 3 or 4 are
selected. These gain values are
set by the 1MΩ, 10kΩ, 1kΩ &
91kΩ resistors in the feedback
network.
In position 2, all four resistors
are connected in parallel to give
a feedback resistance of 900Ω.
IC2 thus operates with a gain of
1 + 900/100 = 10. In the other
Fig.3: install the parts on the PC board
three positions, only the 1MΩ
and complete the wiring as shown here.
and 10kΩ resistors are connected and these give a feedback
the amplifier is bypassed and the DMM resistance of 9.9kΩ. The gain is now
directly monitors the voltage across 1 + 9900/100 = 100.
Rx. Because the constant current
Note that the 0.1µF capacitor is
source supplies 1mA through Rx in
always connected across the feedback
this position, the reading in millivolts path, to reduce any high frequency
is directly equivalent to the value of
noise.
Rx in ohms.
The 91Ω resistor at pin 3 matches
Conversely, when positions 2, 3 or
the impedance seen by this input to
that seen by the pin 2 input. This ensures that equal currents flow in the
two op amp inputs and this in turn
minimises the output offset voltage.
VR3 nulls out any remaining offset
voltage and is adjusted so that the
DMM reads 0mV when Rx is 0Ω (ie,
when the test terminals are shorted
together).
One interesting point is that the
lower end of Rx is two diode drops
above ground, due to series diodes
D1 and D2. This ensures that IC2
operates correctly when the output
is only 1mV above the lower Rx connection point.
Power for the circuit is derived from
a 9V battery via power switch S1. Two
47µF capacitors across the supply
provide decoupling and lower the
impedance of the 9V rail, while LED1
provides power on/off indication.
Construction
Most of the parts are mounted onto
a small PC board coded 04305961 and
RESISTOR COLOUR CODES
❏
❏
❏
❏
❏
❏
❏
❏
❏
❏
❏
No.
1
1
1
1
1
1
1
1
1
1
42 Silicon Chip
Value
1MΩ
91kΩ
10kΩ
5.6kΩ
2.4kΩ
2.2kΩ
1kΩ
200Ω
100Ω
91Ω
4-Band Code (1%)
brown black green brown
white brown orange brown
brown black orange brown
green blue red brown
red yellow red brown
red red red brown
brown black red brown
red black brown brown
brown black brown brown
white brown black brown
5-Band Code (1%)
brown black black yellow brown
white brown black red brown
brown black black red brown
green blue black brown brown
red yellow black brown brown
red red black brown brown
brown black black brown brown
red black black black brown
brown black black black brown
white brown black gold brown
PARTS LIST
1 PC board, code 04305961, 60
x 100mm
1 front panel label, 62 x 125mm
1 plastic case, 130 x 66 x 43mm
1 9V battery holder
1 9V battery
1 SPDT toggle switch (S1)
1 3-pole 4-way PC mount rotary
switch (S2)
2 10kΩ horizontal trimpots
(VR1,VR3)
1 100Ω horizontal trimpot (VR2)
1 12mm knob
2 banana plugs
2 banana panel sockets
6 PC stakes
1 6mm ID rubber grommet
1 20mm length of 0.8mm tinned
copper wire
1 300mm length of hook-up wire
3 2.5mm screws and nuts
Semiconductors
2 CA3140E Mosfet input op
amps (IC1,IC2)
1 BC328 PNP transistor (Q1)
1 LM336Z-2.5 reference (REF1)
2 1N914, 1N4148 signal diodes
(D1,D2)
1 5mm red LED (LED1)
Capacitors
2 47µF 16VW PC electrolytic
1 0.1µF MKT polyester or
monolithic ceramic
The PC board carries nearly all the parts and is mounted by clipping it into
the guide notches of a standard plastic case. Note that the locking collar of the
rotary switch (under the mounting nut) must be set to position 4, as described in
the text.
measuring 60 x 100mm. The board
clips into the integral side pillars of
a plastic case measuring 130 x 66 x
43mm.
Begin construction by checking
the PC board for shorted tracks or
small breaks. Check also that it clips
neatly into the case. Some filing of the
PC board sides may be necessary to
allow a good fit without bowing the
case sides.
Begin the board assembly by
installing the PC stakes. These are
located at the three external wiring
points and at the con
nections for
switch S1. This done, insert the
single wire link (it sits immediately
beneath VR3).
Next, install the resistors (see table
for colour codes), then install the
diodes and ICs, taking care to ensure
that they are oriented correctly. The
capacitors can go in next – note the
polarity of the two 47µF electrolytic
types.
REF1 and Q1 can now both be installed. Note that these two devices
look the same so make sure that you
don’t get them mixed up. LED1 is
mounted on the end of its leads so
that it will later protrude through a
matching hole in the front panel. For
the same reason, switch S1 is soldered
to the top of the previously installed
PC stakes.
Rotary switch S2 is mounted directly on the PC board. Ensure that it
has been pushed fully home and sits
Resistors (0.25W, 1%)
1 1MΩ
1 2.2kΩ
1 91kΩ
1 1kΩ
1 10kΩ
1 200Ω
1 5.6kΩ
1 100Ω
1 2.4kΩ
1 91Ω
1 1Ω 1% (for calibration)
Miscellaneous
Hook-up wire, tinned copper
wire.
flat on the PC board before soldering
its pins. This done, loosen the switch
mounting nut, lift up the star washer
and rotate the locking collar to position 4. This turns what was a 12-position rotary switch into a 4-position
rotary switch. Check that the switch
operates correctly, then do the nut up
tight again so that the locking collar
is secured.
The board assembly can now be
June 1996 43
LOW OHMS TESTER
POWER
+
+
VALUE per mV
+
0.1Ω 0.01Ω
1Ω
1mΩ
Rx
+
+
Fig.5: this full-size artwork can be used as a drilling template for the front panel.
completed by mounting the trimpots
and fitting the battery holder. Note
that VR2 is a 100Ω trimpot, while
VR1 and VR3 are both 10kΩ types
so be careful with the values here.
The battery holder is secured to the
PC board using the 2.5mm mounting
screws supplied with it.
Final assembly
It’s now just a matter of installing
the board and the ancillary bits and
pieces in the case. First, attach the
front panel label, then drill holes for
the LED, switches S1 & S2, and the two
test terminals. A hole will also have
to be drilled in the top of the case to
accept a small grommet.
The PC board can now be clipped
into the case, the test terminals
mounted in position and the wiring
completed as shown in Fig.3. This
done, check that the switches and
the LED line up with the front panel
holes. Adjust the height of the LED
and switch S1 if necessary, so that
they fit correctly.
The leads to the meter run through
the grommetted hole in the top of the
case. Keep these leads reasonably short
and terminate them with banana plugs.
It will be necessary to trim the shaft of
switch S2, so that the knob sits close
to the front panel.
Test & calibration
Now for the smoke test. Apply
power and check that the LED lights
(if it doesn’t, check that the LED has
been oriented correctly). Now check
the supply voltages on IC1 and
IC2 using a multimeter. In each
case, there should be about 9V
between pins 7 and 4.
If everything is OK so far, check
the voltage between pin 3 of IC1
and the positive supply rail (ie,
the voltage across REF1). Assuming VR1 is centred, you should
get a reading of 2.4-2.5V. Pin 2 of
IC1 should be at the same voltage
as pin 3.
To calibrate the unit, follow
this step-by-step procedure:
(1) Monitor the voltage across
REF1 and adjust VR1 for a reading of 2.4V (this sets the constant
current.
(2) Plug the Low Ohms Tester
into the DMM and short the Rx test
terminals using a short length of 1mm
tinned copper wire.
(3) Select the 0.01Ω range and adjust
VR3 for a reading of 0mV on the DMM.
Check for a similar reading when the
1mΩ range is selected.
(4) Connect a 1Ω 1% resistor between the test terminals, select the
0.01Ω range and adjust VR1 again for
a reading of 100mV.
(5) Select the 1mΩ range and adjust
VR2 for a reading of 1V.
(6) Short the test terminals again
and verify that the DMM reads close
to 0mV for all ranges.
That completes the calibration procedure. The lid can now be attached
to the case, the knob fitted to S2 and
SC
the unit pressed into service.
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