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By NICHOLAS VINEN
High-Current Adaptor
For Scopes & DMMs
If you want to measure and monitor mains current of up
to 30A using your DMM or scope, this is the safe and easy
solution. It works just as well with DC and it has significantly
better resolution and bandwidth than most clamp meters.
I
N THE SILICON CHIP laboratory, we
often need to hook our digital storage oscilloscope (DSO) up to mainspowered equipment to examine the
current waveforms. The two most
common ways to do this are with a
shunt resistor and differential probe
or a clamp meter. But both approaches
have drawbacks.
A shunt resistor connected in series
with one of the mains conductors (eg,
Neutral) provides the best bandwidth
and resolution but you need a differential probe (which can be expensive),
even if you are measuring on the Neutral leg since Neutral is usually a few
volts above or below Earth potential.
The resistor also limits how much
current you can measure depending on
its value. For example, a 0.1Ω 10W resistor limits you to measuring around
7A RMS (after de-rating by 50%). This
70 Silicon Chip
option can also be quite unsafe as the
wiring between the shunt and probe
is connected directly to mains.
A clamp meter is safer since it
doesn’t require any exposed mains
wiring. But they tend to have a fairly
low output voltage, eg, 1mV/A. This
gives you lousy resolution and noise
performance with scopes which usually have a maximum sensitivity of
5mV/div. Clamp meters also typically
have quite limited bandwidth (eg,
10kHz) which is no good for loads with
fast-changing current waveforms such
as switchmode supplies.
Also, you need to separate out the
mains conductors to use a clamp meter
since if you just clamp it over the cable,
the Active and Neutral currents are of
identical magnitude and opposite in
direction so the magnetic fields effectively cancel. So you need some kind
of special cable or adaptor to measure
mains current with a clamp meter.
Our solution
With our adaptor, you get much
higher bandwidth and resolution than
a clamp meter (80kHz, 100mV/A) with
better safety than a shunt resistor, no
need for a differential probe and at a
fairly low cost.
We use an Allegro ACS712 IC,
which like a clamp meter operates on
the Hall Effect principle but the whole
shebang is within a single chip. One
side of the IC contains a 1.2mΩ shunt
which can handle a continuous current of at least 30A and pulses up to
100A for 100ms. On the other side is
a fully isolated Hall Effect sensor and
amplification circuitry.
There is no electrical connection
between the two halves; sensing is
siliconchip.com.au
purely based on the magnetic field
generated by current passing through
the shunt. The chip has an isolation
rating of 1500VAC between the two
halves so the output can safely be
hooked up to a scope or other device
even if you are measuring mains current at up to 250VAC.
There are three versions of this IC,
designed for sensing currents up to
±5A, ±20A and ±30A. They are otherwise identical. For our prototype, we
used the 20A version since its output
is 100mV/A and this makes it easy to
set up our scope to read out directly
in amps (by telling it we have a 10:1
current probe). We run it from a 5V
supply, giving readings of up to ±25A
although linearity is a little degraded
at the extremes.
The 30A version has an output of
66mV/A and can read up to ±38A. You
can use this one if you prefer but then
you may need a calculator to interpret
the readings.
Power comes from a 9V battery because this is much more convenient
than a plugpack when setting up a
test. We fitted ours with a mains plug
and socket for measuring the current
drawn by mains devices however it
could also have been fitted with DC
connectors if that’s what we wanted to
measure. The output is a BNC socket,
making it easy to hook up to a scope.
For connection to a DMM, we use a
BNC plug to banana socket adaptor.
So that you can’t accidentally leave
the unit on and drain the battery (easy
to do!), we incorporated an automatic
time-out which switches the unit off
after about 15 minutes. If you want to
use it for a longer period, you just have
to remember to periodically press the
power button to keep it on.
Specifications
Accuracy: approximately 2% error
Bandwidth: typically 80kHz
Range: ±25A* (linear over ±20A)
Output: 100mV/A*
Noise: ~40mV peak-to-peak
(equivalent to ~400mA)
Power supply: 9V battery, approximately 20 hours life
Resistance: ~2mΩ plus cable
resistance
Isolation: 2.1kV RMS (suitable for
use up to 250V AC)
Withstand current: 100A for
100ms
Other features: power indicator,
auto-off to preserve battery life
* With alternative shunt IC, range increases to ±38A (linear over ±33A)
with 66mV/A output
Circuit description
Refer now to the circuit diagram
in Fig.1. The power supply is shown
at left while the actual current sense
portion of the circuit is at lower right.
IC3 is the ACS712 shunt monitor IC.
In addition to a 100nF power supply
bypass capacitor, it has a 1nF filter capacitor from pin 6 to ground. This sets
its bandwidth to 80kHz and provides a
good compromise between bandwidth
and residual noise. The shunt side
of the IC, at left, is connected to two
terminals of a 4-way terminal barrier,
which is then wired to the mains plug
and socket.
If you increase the value of the filter
capacitor at pin 6, the residual noise
is reduced but so is the bandwidth.
For example, if you use 10nF instead
of 1nF, bandwidth drops to 8kHz and
noise to ~20mV (200mA) peak-topeak. If you use 100nF then bandwidth
drops to 1kHz and noise to ~10mV
(100mA) peak-to-peak. If unsure, stick
with the recommended value of 1nF.
IC3’s output is at pin 7 and sits at
half supply (about +2.5V) when there
is no current flow. This is buffered by
IC4a, half of an LM358 dual low-power
op amp. Its is biased into Class-A operation with a 10kΩ resistor from its
output pin 1 to ground (The LM358
data sheet explains why this is neces-
sary). A 100Ω series resistor prevents
instability that may occur due to output cable capacitance and the signal
is available at the “+” output of CON2.
Ideally, we want 0V across CON2
when no current is flowing, rather than
2.5V, so we generate a half-supply rail
at around +2.5V and connect that to
the negative output terminal of CON2,
so there is no voltage across it in the
quiescent condition.
This is achieved using a voltage divider consisting of two 10kΩ resistors
and 500Ω trimpot VR1. The voltage at
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August 2012 71
72 Silicon Chip
siliconchip.com.au
D1
1N5819
47k
K
A
10M
100nF
47k
D2
3.3M
1M
47nF
K
A
11
10
9
12
RS
Rtc
Ctc
MR
D5
14
13
15
1
2
8
Vss
O4
O5
O6
O7
7
5
4
6
O9
IC2
4060B O8 14
O10
O12
O13
3
7
IC1c
IC1d
O14
9
8
13
12
10k
16
Vdd
3.3M
100nF
A
10
11
D7
K
6
5
2
1
N
IC1b
IC1a
IC1: 4093B
100nF
E
A
K
A
B
OUT
IN
N
E
A
K
A
K
A
(IN S1)
D4
E
N
4
3
2
1
IP–
IP–
IP+
IP+
LED1
K
A
D3
6.8k
OUTPUT
SOCKET
1
2
3
4
CON1
470nF
VIA CON1,
TERMINAL 4
D6
C
VIA CON1, TERMINAL 3
INPUT
PLUG
4
3
22k
E
Q1 BC559
GND
5
OUT
VIout
FILTER
6
7
100nF
GND
IC3
ACS712
8
Vcc
IN
REG1
LP2950ACZ-5.0
1nF
10k
100nF
K
A
K
1N5819
A
D2-D7: 1N4148
10k
VR1
500
+5V
100 F
+5V
+8.7VSW
IC4a
8
6
5
8
1
1
4
10k
7
10k
ACS712
4
IC4b
IC4: LM358
2
3
100nF
E
IN
B
OUT
C
BC559
GND
LP2950ACZ-5.0
100
–
CON2
+
100
OUTPUT
TO SCOPE
OR DMM
Fig.1: the full circuit of the Current Adaptor. Connections are shown for measuring the mains current but it can also be used to measure low-voltage AC or
DC current. Current flows through IC3’s internal shunt and a proportional voltage appears at its VIout terminal (pin 7). Op amp IC4 buffers this voltage and
a half-supply rail to provide differential output voltages at CON2. IC3’s 5V rail is derived from a 9V battery via low-dropout regulator REG1 and switched
by transistor Q1, which is controlled by a flipflop formed by IC1a & IC1b. The unit is turned on by a short press from momentary pushbutton S1 and turned
off by a long press or after 15 minutes by timer IC2. This prevents the battery from being discharged if the unit is accidentally left on; the timer can be reset
with a brief press of S1.
ISOLATED HIGH-CURRENT ADAPTOR FOR SCOPES & DMMS
100nF
9V
BATTERY
2012
SC
A
K
POWER
S1
+8.7V
Parts List: Isolated High-Current Adaptor
1 PCB, code 04108121,
60 x 107mm
1 UB3 jiffy box
1 right-angle PCB-mount tactile
pushbutton with blue LED (S1)
(Altronics S1181)
1 500Ω mini sealed horizontal
trimpot
1 9V battery holder, PCB-mount
1 9V battery (alkaline or lithium
recommended)
1 4-way PCB-mount (screw fix)
terminal barrier (CON1)
(Jaycar HM3162)
1 2-way polarised header, 2.54mm
pitch (CON2)
1 2-way polarised header
connector, 2.54mm pitch
1 female BNC panel-mount socket
(Jaycar PS0658, Altronics
P0516)
1 100mm length of light duty
figure-8 cable or ribbon cable
3 M2 x 6mm machine screws
2 M3 x 15mm machine screws
4 M3 nuts
2 M3 flat washers
VR1’s wiper is filtered with a 100nF
capacitor and buffered by op amp IC4b,
the other half of the LM358. VR1 is adjusted so there is 0V across CON2 with
no current through the shunt. CON2 is
normally wired to a BNC socket with
the negative pin side to its shell.
IC4, the LM358, runs off the +8.7V
(nominal) switched rail from the battery so that both outputs have a full
0-5V swing. However, note that once
the battery has dropped below 6.5V
(when it’s quite flat), the full swing
may no longer be available. This could
result in low readings towards the end
of the battery’s life.
To improve performance in this
respect, an LMC6482 rail-to-rail op
amp can be used in place of the LM358
and this will operate normally with a
battery voltage down to 5V. However,
the LMC6482 draws slightly more supply current; about 1.5mA compared to
0.5mA for the LM358, so the battery
life will be slightly less.
Power supply
The ACS712 isolated shunt IC (IC3)
runs from a regulated 5V rail, drawing
about 10mA. This is controlled using
momentary pushbutton S1 which also
siliconchip.com.au
2 M3 star washers
2 M3 x 10mm tapped Nylon
spacers
1 M3 x 15mm tapped Nylon spacer*
3 M3 x 6mm Nylon machine
screws
1 sheet of Presspahn insulation,
70 x 30mm*
1 mains extension cord with
moulded plug and in-line
socket*
2 cord-grip grommets to suit 7.48.2mm cable (Jaycar HP0716,
Altronics H4270)*
5 small cable ties*
Semiconductors
1 4093 CMOS quad Schmitt
trigger NAND gate (IC1)
1 4060 CMOS oscillator/counter
(IC2)
1 ACS712ELCTR-20A-T
(Element14 1329624) OR
1 ACS712ELCTR-30A-T
(Element14 1651975)
1 LM358 dual op amp (IC4)
1 BC559 PNP transistor (Q1)
has an integrated blue LED. This LED
lights up when the unit is on. When
on, pressing S1 briefly resets the autooff timer while holding it down for a
second or two turns the unit off.
The power on/off control and autooff timer functions are provided by
IC1, a 4093B quad CMOS Schmitt
trigger NAND gate IC and IC2, a 4060B
CMOS oscillator/counter. Both these
ICs are permanently powered by the
battery but being static CMOS logic,
only draw a tiny amount of current,
typically <1µA. This is probably lower
than the battery’s self-discharge current so it will last many years with the
unit switched off. Schottky diode D1
provides reverse polarity protection.
NAND gates IC1a and IC1b are
configured as an RS-flipflop which
controls power to IC3 and IC4. When
the unit is off, output pin 3 of IC1a is
low and output pin 4 of IC1b is high.
Therefore, PNP transistor Q1 has no
base drive and so no current can flow
through its collector-emitter junction
and into the rest of the circuit.
The high output from pin 4 in this
state also forward biases diode D6,
pulling pin 12 of IC2 (MR or master
reset) high. This prevents IC2’s oscil-
1 LP2950CZ-5.0 low dropout, low
quiescent current 5V regulator
(REG1) (Jaycar ZV-1645,
Element14 1262363)
1 1N5819 1A Schottky diode (D1)
6 1N4148 small signal diodes
(D2-D7)
Capacitors
1 100µF 16V electrolytic
1 470nF MKT
7 100nF MKT
1 47nF MKT
1 1nF MKT
Resistors (0.25W, 1%)
1 10MΩ
1 22kΩ
2 3.3MΩ
5 10kΩ
1 1MΩ
1 6.8kΩ
2 47kΩ
2 100Ω
* For measuring mains current,
substitute different parts for DC or
low-voltage AC current measurement.
Note: the PCB is available from
the SILICON CHIP Partshop.
lator from running, minimising its
power consumption. Less than 1µA
flows through the 10MΩ pull-down
resistor.
When pushbutton S1 is pressed, two
47kΩ resistors, a 100nF capacitor and
diode D2 provide a delay to debounce
the switch. The delay is around 28ms,
whether the button is being pressed or
released. Because IC1d has Schmitttrigger inputs (ie, inputs with hysteresis), the resulting slow rise and fall
times are not an issue.
When S1 is pressed, input pin 12
of NAND gate IC2d goes high and assuming pin 13 is high (more on this
later), its output pin 11 goes low. This
sets the RS-flipflop, sending pin 3 high
and pin 4 low, turning on Q1 and thus
the rest of the circuit.
Pin 13 of IC1d is driven by IC1c.
IC1c’s inputs (pins 8 & 9) are tied together so that it operates as an inverter.
It is fed from a further delayed version
of the pushbutton signal; the 3.3MΩ
resistor and 100nF capacitor form an
additional low-pass filter which adds
a delay of roughly two seconds. This
means that the input to IC1c is still low
when pin 12 of IC1d goes high; thus
pin 13 of IC1d is also high.
August 2012 73
5819
9V BATTERY
HOLDER
1nF
100nF
2
CAV 0 3 2
100nF
10k
M3 x 15MM
NYLON SPAC ER
AC S712
(UNDER)
1
100nF
IC 1 4093B
1M
3.3M
REG1
LP2950AC Z-5
C
47k
4148 D2
3.3M
47k
D5
4148
4148
D6
47nF
100 F
22k
470nF
10k
4148
D7
100nF
10k
BC 559
04108121
Q1
VR1
+
IC 4
LM358
10k
100nF
10M
500
100
S1
D4
4148
4148
100nF
D3
IC 2 4060B
6.8k
OUT
–
+
3
IC 3
AC S712
4
!R E G NA D
s M M D &OUT
sepo cINS rof rNotpadAE tnerru C
WARNING: LIVE 230V!
2102 C
C urrent Adaptor
TOP OF BOARD
If S1 is held down, after this two second delay, the second 100nF capacitor
charges up, bringing input pins 8 & 9 of
IC1c high. IC1c’s output therefore goes
low. Since IC1c also drives an input
of IC1d, IC1d’s output simultaneously
goes high. This condition, with input
pin 6 of IC1b low and input pin 1 of
IC1a high, resets the RS-flipflop, pulling the base of Q1 high and switching
the unit off.
When pushbutton S1 is released,
pin 12 of IC1d goes low before pins 8
and 9 of IC1c do, due to the different
time constants of the two low-pass
RC filters. This is important so that
the unit stays off when S1 is released.
Auto-off timer
Alternatively, if pushbutton S1 is
only pressed briefly while the unit is
on, the 3.3MΩ/100nF RC filter does not
have time to charge fully and so the
unit does not switch off. But diode D5
will still become forward-biased and
this pulls IC2’s MR pin high, resetting
the auto-off timer.
Once S1 has been pressed, the
timer (IC2) runs for about 15 minutes
and then switches the unit off. This
time is set by the timing capacitor
74 Silicon Chip
04108121
D1
12180140
DANGER!
1
Fig.2: the PCB overlay
diagram for the Current
Adaptor. IC3, the ACS712
hall-effect shunt monitor is
soldered to the underside
as shown. A slot in the
board prevents surface
contamination from
forming a leakage path
between the high and low
voltage sides of the IC. The
current to be measured
flows between the “IN” and
“OUT” terminals of the
terminal barrier at bottom
and the output voltage
appears across the 2-pin
polarised header at upper
left, just below the 9V
battery holder. Pushbutton
switch S1 at upper-right
provides on/off control,
timer reset and power
indication via its in-built
blue LED.
230VAC
C urrent Adaptor for Scopes & DMMs
C 2012
UNDERSIDE OF BOARD
and resistor (47nF and 1MΩ), which
give an oscillator frequency of around
8.5Hz. Output O14 (pin 3) goes high
after 213 = 8192 clocks and this gives
8192 ÷ 8.5Hz = 963 seconds or about
15 minutes.
When O14 goes high, this forwardbiases diode D7 which charges the
100nF capacitor at pins 8 & 9 of IC1c
via a 10kΩ resistor, resetting the RSflipflop and switching the unit off.
Regulator
When Q1 is on, it supplies the
~8.7V from the battery to REG1, a
low-dropout, low quiescent current
5V linear regulator. This draws less
power from the battery than a 78L05
would and also allows the unit to
continue operating down to a lower
battery voltage.
The power LED integrated within
S1 is powered from the 8.7V rail
via two series 1N4148 diodes and a
6.8kΩ resistor to limit the current.
The two diodes cause the LED to dim
significantly as the battery voltage
drops below about 6V, since the LED
has a forward voltage of around 3.3V
and the two diodes add another 1.2V
to this. This gives a low battery indica-
tion before the voltage drops too low
for the device to function.
Construction
The unit is built on a PCB coded
04108121 and measuring 60 x 107mm.
This is available from the SILICON CHIP
Partshop. It’s designed as a singlesided PCB with one wire link although
we supply a double-sided PCB with
that link already present (as a track
on the top layer).
IC3, the ACS712, is a surface-mount
device (SMD) in an SOIC-8 package
and this goes on the underside. There
is a slot down the middle of its mounting position, to maximise electrical
isolation between the shunt and lowvoltage sides. If you have made your
own PCB, you should drill a series
of 1.2mm holes between the IC pads
where shown and file them into a slot.
The first job is to solder this IC in
place. It must go in with its pin 1 (indicated with a divot, dot or bevelled
edge) towards the bottom of the PCB,
as shown in the PCB overlay diagram
(Fig.2). The PCB indicates the correct
orientation too.
Put a small amount of solder on
one of the pads with the IC resting
siliconchip.com.au
Fig.3: the correct
cut-out to make
sure the cord-grip
grommets do grip.
Don’t be tempted
to simply drill a
16mm hole!
The completed PCB, without the two corner mounting posts. We used IC
sockets for our prototype but it’s better to solder the ICs to the PCB so they
can’t come loose if the unit is dropped. Once the wires have been connected
to the screw terminal block, the clear cover is clipped in place (not shown).
alongside, heat the solder and slide
the IC into place. If it isn’t aligned
properly on its pads, reheat the solder
and nudge it. Repeat until it is correctly aligned, then solder the rest of
the pins. Finally, re-solder the initial
pin, to ensure the solder has flowed
correctly, making a good joint.
Next, fit all the horizontally-mounted resistors, checking their values
with a DMM. You can also refer to
the resistor colour code table below.
Follow with the diodes, being careful to orientate them as shown on the
overlay diagram. Make sure that the
larger Schottky diode (D1) goes at
upper-right as shown.
Next, solder the DIP ICs in place. In
each case, the pin 1 notch or dot goes
towards the top of the board. Don’t
get the 4060 and 4093 mixed up. We
recommend you solder them directly
to the PCB so that they can’t come
loose and float around inside the box
(rather than using sockets).
Fit the MKT capacitors next. There
are four different values and they go
in the locations shown on the overlay
diagram. Then mount transistor Q1
and regulator REG1 which are both
in TO-92 plastic packages; check the
markings so you don’t get them mixed
up. You can then install the single electrolytic capacitor (longer lead toward +
symbol) and the polarised pin header,
followed by the remaining resistors
which go in vertically.
Trimpot VR1 can go in next, followed by pushbutton switch S1. You
may need to bend the latter’s leads
slightly to get them to fit the holes as
they are quite delicate and can easily
be bent out of shape during transport.
That done, use three short M2 machine
screws to attach the battery holder to
the board, then solder the leads.
Table 1: Resistor Colour Codes
o
o
o
o
o
o
o
o
o
siliconchip.com.au
No.
1
1
1
2
1
5
1
2
Value
10MΩ
3.3MΩ
1MΩ
47kΩ
22kΩ
10kΩ
6.8kΩ
100Ω
4-Band Code (1%)
brown black blue brown
orange orange green brown
brown black green brown
yellow violet orange brown
red red orange brown
brown black orange brown
blue grey red brown
brown black brown brown
Suits
7.4-8.2mm
cable
15.9mm
14mm
That just leaves the terminal barrier,
which is mounted using M3 screws
with flat washers under the heads and
star washers between the nuts and
PCB. Do up the screws tight, check that
it is parallel with the edge of the board
and then solder the pins, using a hot
iron and a generous amount of solder.
The PCB assembly can now be
completed by attaching three tapped
Nylon spacers. As shown in one of the
photos, the two M3 x 10mm spacers
are attached to the two corner holes
adjacent to the terminal strip (ie, on
the underside of the PCB) using M3 x
6mm Nylon screws.
The M3 x 15mm Nylon spacer goes
on the top of the board as shown in
Fig.2 and is also attached using an M3
x 6mm Nylon screw. It’s used to help
retain a Presspahn isolation barrier.
Testing
Check that the power supply works
by connecting the battery and pressing
the pushbutton. The blue LED should
light up. Hold down the pushbutton
for a couple of seconds and check that
it goes off. Then set the trimpot to its
mid-position, turn the unit back on
and measure the voltage across the
polarised pin header. It should be
Table 2: Capacitor Codes
Value
470nF
100nF
47nF
1nF
µF Value IEC Code EIA Code
0.47µF 470n
474
0.1µF
100n
104
.047µF 47n
473
.001µF 1n
102
5-Band Code (1%)
brown black black green brown
orange orange black yellow brown
brown black black yellow brown
yellow violet black red brown
red red black red brown
brown black black red brown
blue grey black brown brown
brown black black black brown
August 2012 75
The unit all wired up and ready to go. Note how the 2-wire ribbon cable for the output signal is clamped by the PCB.
There isn’t a lot of room for the output connector next to the battery so we had to trim its central solder pin. You can
also see how the Presspahn cover is held in place by the plastic case slots, terminal block and tapped spacer.
less than ±250mV. Adjust it as close
to zero as you can, using the trimpot,
then switch it off again.
Preparing the case
The next step is to drill a 5mmdiameter hole in the side of the case
for the on/off pushbutton. This hole is
positioned 22mm down from the top
lip of the case (ie, not including the
lid) and 47.5mm from the output end
(again as measured from the top lip).
You can then drop the PCB into the
case at an angle, to check that the hole
lines up correctly when the PCB snaps
into place. If not, enlarge it slightly.
Next, make the holes for the output
socket(s). We simply drilled a 9mm
diameter hole in the middle of the
end for the panel-mount BNC socket
but you could use a pair of binding
posts if you want. Keep in mind that
there is only about 11mm of clearance
from the battery to the end of the case
so whatever you use, it can’t intrude
very far. In fact, before installing the
BNC socket, we had to cut off most
of the central prong since it stuck out
too far (you only need a short section
to solder to).
Remove the PCB and fit the BNC
socket. Crimp and solder a 70mm
length of light-duty figure-8 cable
to the two polarised header pins,
then push the pins into the moulded
plastic housing. Solder the other end
of these leads to the rear of the BNC
socket, with the lead from pin 1 on the
polarised header (normally indicated
on the plastic housing) going to the
BNC shield while pin 2 goes to the
central pin.
Mains leads
Two M3 x 10mm tapped Nylon spacers are
fitted to one end of the PCB as supports.
76 Silicon Chip
If you are not planning on using
the adaptor with mains, you can use
binding posts or whatever you prefer
to make connections to the terminal
barrier. However this section will
describe the procedure for connecting
mains cables.
The first step is to cut the extension
lead in half. Strip away about 50mm
of outer insulation from both ends and
then expose 7-8mm of insulation from
each Active and Neutral wire and 1520mm for the Earth wires.
You will then need to make two
holes in the case, at the opposite end
to the BNC socket. These are spaced
25mm apart, on either side of the
centre of that end and have a diameter
of 14mm. Start with a smaller hole
(4-5mm say) and then enlarge using
a tapered reamer or stepped drill bit.
Make sure you don’t make the holes
too large since the cordgrip grommets
need to be a tight fit. Then profile the
holes to the shape shown in Fig.3,
using a file. Again, be careful not to
make the opening too large.
Now place one of the mains leads
through one of the cord-grip grommets, with the bare leads towards the
narrower end. Squeeze the grommet
together hard using large pliers (or if
you’re lucky enough to have one, a
grommet insertion tool), so that only a
short length of the cable’s outer insulation protrudes from that narrow end.
Push the grommet into one of the
holes and it should snap into place.
If it won’t go, enlarge the hole very
slightly and then try again. Be careful
since once it’s in, it’s very hard to get it
out. Do the same with the other cable
and grommet into the other hole.
Now check that the two mains cords
are securely anchored. You must not
siliconchip.com.au
This close-up view shows how the Presspahn cover is held
in place by the plastic case slots, the mains terminal block
and the M3 x 15mm tapped spacer.
be able to pull the cords out of the case,
even if you exert considerable force.
That done, connect the two Active
wires to the terminals marked “IN”
and “OUT” on the PCB. For correct
output polarity, “IN” should go to the
plug and “OUT” to the socket (current
flowing from IN to OUT will give a
positive output voltage). Do these up
tightly, too.
Twist the two Neutral wires together
and screw them down tightly to one
of the spare the terminals on the PCB
(see photo). Do the same for the Earth
wires. Make sure both are secure. You
can then use several small cable ties to
hold the wiring in place. These must be
installed to prevent individual leads
from moving and contacting other
wiring if they come loose.
Once these are in place, clip the
clear cover on top of the screw terminal block.
Presspahn barrier
The next step is to fit a Presspahn
insulation barrier between the mains
terminal block and the low-voltage
section of the PCB. This insulation
barrier is retained by the adjacent
slots in the side of the case and must
be trimmed to exactly 63 x 25mm so
that it is a tight fit.
As shown in the accompanying
photo, this barrier is sandwiched between the screw terminal block and
the adjacent M3 x 15mm Nylon spacer.
If necessary, rotate the spacer slightly
so that one of its lobes presses the
Presspahn insulation firmly against
the screw terminal block.
siliconchip.com.au
The completed unit with the lid in place. Note how the
illuminated on/off pushbutton switch protrudes through
a hole in one side of the case.
Do not leave the Presspahn barrier
out – it makes it impossible for any of
the mains wiring to contact the lowvoltage section of the PCB and is an
important safety measure.
Note that once the lid is in place,
the Presspahn barrier is also clamped
between the lid and the PCB.
A BNC plug-to-banana
socket adaptor can be
fitted to the BNC output
socket if you want to
connect a DMM.
Final assembly
Plug in the polarised header and put
the lid on the box. Then use a DMM to
make some checks before connecting
the device up:
(1) Check that the Earth terminals on
the mains plug and socket have a very
low resistance between them (should
read zero or very close to it).
(2) Do the same check between the
Neutral terminals and then for Active.
(3) Check that there is no connection
between all three pairs of terminals on
the mains plug and then on the socket
(ie, many megohms; meter should
normally read “0L” or similar).
(4) Check that there is no connection
between both terminals of the BNC
socket and all the mains terminals;
again, the meter should read “0L”.
Now plug the unit into mains and,
without touching anything, switch on
and measure the AC voltage between
the BNC shield and Earth using a
DMM. It should be just a few volts. Do
the same check with the BNC centre
pin. Only when you have ensured that
there is no mains voltage on these two
conductors should you connect the
BNC output to an oscilloscope.
You can then do a functional test by
connecting an appliance with a known
current to the output. For example, if
you use a 1kW bar radiator, its current
should be about 2.4A, depending on
the actual value of the mains voltage.
You can then monitor the current with
a DMM or oscilloscope. Check that you
get a sensible reading.
Assuming all is well, disconnect
your test load and check the DC output level of the adaptor. It should be
close to zero. If not, disconnect all
mains cables, open the unit up, make
sure it is switched on and adjust the
trimpot again.
We found that the offset changed
slightly the first time we used the unit
to measure a high current, so you need
to do the final trimming at this stage
to guarantee a low offset.
That’s it; using the device is simply a
matter of plugging it in and switching
it on. Don’t forget to periodically reset
the timer if you are undertaking a long
SC
test or measurement session.
August 2012 77
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