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Stop that
dangerous
kick-back . . .
Soft Starter
for Power Tools
by
NICHOLAS
VINEN
Does your electric saw, router or other large mains-powered hand
tool kick like the proverbial mule when you squeeze the trigger? No
matter how firmly you hold it, it will still kick and that can be enough
to throw you off a carefully lined up cut. This can be bad enough
when you are trying to start an accurate cut with a circular saw but it
can damage the job if you are using a tool like a large plunge router.
But now you can stop that kick with our Soft Starter for power tools.
O
ur Soft Starter project from core drill bit hard against the wall or to oppose the applied mains voltage
April 2012, which tames floor and then press the trigger. The and the resulting surge current can
easily be ten times the rated current
switch-on current surges pri- resulting torque kick can easily jerk the
of the motor with full load.
marily in equipment with switch- whole tool out of your hands! And you
Elsewhere in this article we show
mode supplies, has been very popular. can be injured in the process!
some scope grabs depicting these masBut readers started asking “what about
Why does it kick?
sive currents which luckily die away
something similar for power tools?”
The reason for that enormous initial to much lower values within less than
Many of the smaller mains power
tools these days have speed controllers torque is the very high surge current half a second. It is those massive curbuilt into the trigger, so they are very pulled by a universal (series wound, rents which cause the lights to flicker
brush) motor when power is first ap- when you switch on a big power tool;
controllable when you turn them on.
But larger power tools such as circu- plied. Because the motor is not rotat- the mains voltage sags noticeably.
lar saws, plunge routers, angle grinders ing, it is not generating any back-EMF
The cure
and worst of all, large electric
Our solution is simple: When
drills for concrete core drillFeatures & Specifications
you squeeze the trigger switch
ing, have a simple trigger or
<20A
on the power tool, current imthumb switch which applies Inrush current limiting:
Minimum
load
power:
~100W
mediately starts to flow but is
full power to the motor. Core
10A
limited to a reasonable value
drilling is particularly danger- Maximum load current:
with a big power resistor. Then,
ous, as you have to brace the Minimum tool restart interval: 60s recommended
22 Silicon Chip
siliconchip.com.au
Shown here with two of the hand tools most
likely to be used with the Soft Stater, an
electric hand saw and plunge router. The unit
is housed in the Jiffy Box in front. If used on a
building site or other “rough” environments, it
could be housed in an aluminium diecast box.
after about half a second, we use a
relay to short out the resistor and full
power is applied to the motor. By that
time, the motor is already spinning at
high speed so the big peak current is
avoided. The basic scheme is shown
in the block diagram of Fig.1.
In this case though, we have not
used a big power resistor, simply because a suitable value with sufficient
rating would be large and expensive.
Instead we have used two large negative temperature coefficient (NTC)
resistors in series with the Neutral side
of the load (ie, the power tool). These
thermistors have a relatively high
initial resistance of about 10Ω each
and so they limit the surge current to
about 11.5A (230VAC÷20Ω).
Now while these thermistors are
relatively small, they normally become very hot as their resistance
drops. However, we don’t give them a
chance to get really hot because they
are switched out of the circuit after a
short delay.
So how do we know when to short
out the thermistors? Referring to Fig.1,
you will see that there is a current
sense resistor in series with the thermistor. This sense resistor has a value
of 10 milliohms (0.01Ω) so that the
voltage loss across it is quite low. We
siliconchip.com.au
use this shunt resistor to sense when
current starts to flow, immediately
after the power tool trigger switch has
been pressed.
The sense resistor is connected to a
comparator, which works by comparing the instantaneous load current to
a reference threshold.
When you turn the power tool on,
it will draw a lot of current at first,
well above this threshold. Once this is
detected by the comparator, it begins
charging a capacitor and after half a
second, it operates the relay. From that
point on, the tool is effectively connected directly to the 230VAC mains
and operates as if the Soft Starter isn’t
even there.
When the job is finished and you
release the trigger switch, the current
stops flowing and the circuit resets
itself, ready to go again.
As long as the tool continues to draw
at least 100W (and virtually all do), the
relay stays closed. When you switch
the tool off, the load current drops to
A
F1 10A
POWER
TOOL
RELAY1
TRIGGER
SWITCH
COMPARATOR
AND DELAY
CURRENT
SENSING
RESISTOR
CURRENT
LIMITING
THERMISTOR
N
0.01
Fig.1: the Soft Starter block diagram. Initially, mains current passes
through fuse F1, the power tool motor, a current-limiting thermistor
and current sense resistor. A short time after the motor is started, the
control circuitry energises Relay1, shorting out the thermistor so the
motor gets full power. We actually use two thermistors in series but the
principle is the same.
July 2012 23
Fig.2: the mains current (yellow) and voltage (green) when
starting a 1500W router. The peak current is in excess of
60A, hence the “kick”. Current drops as the motor comes
up to speed and it develops more back-EMF, opposing
the mains voltage and thus limiting the current. Note the
triangular shape of the current waveform which is almost
in phase with the mains voltage.
Fig.3: with the Soft Starter in circuit, the current at start-up
is much lower, initially just 10A peak. This increases slowly
over the first 200ms or so as the NTC thermistors warm up,
then for the next 400ms the current draw drops as the motor
comes up to speed. You can see the slight increase in current
as the relay kicks in after 600ms and then the current drops
further as the motor approaches full speed.
zero and the capacitor discharges. After about half a second, the relay opens and the unit is ready to be used again.
Note that if you start and stop the tool multiple times in
quick succession, the thermistors won’t have time to cool
down properly and the starting current on the second and
subsequent starts will be higher than the first and so the
tool kick-back will be higher. Even though the thermistors
only conduct briefly before being shorted out, they still get
quite hot in that short time; quick multiple starts means
they getter hotter, their resistance is lower and so the surge
currents are higher.
So the strategy is clear: to minimise switch-on kick back,
don’t stop and start the tool repeatedly in a short time. Wait
about ten seconds or so between each cut, or whatever.
While this is primarily intended to be used with power
tools, there are some other types of load for which may be
suitable. For example, it may work with some larger power
amplifiers and these could then be switched on using the
front panel or remote control rather than having to turn them
on and off at the wall, for the Soft Starter to be effective.
But there are some caveats. The main restriction is that
the load must have a relatively sinusoidal current waveform
and draw at least 100W when on.
Some devices with switch-mode supplies or with
transformers feeding bridge rectifiers will not be suitable.
Switch-mode supplies with Active Power Factor Correction
(Active PFC) should be OK.
The reason is that if the load current is drawn over a
narrow part of the mains cycle (ie, near the peaks), the duration of the portion which is above the detection threshold
may be too short for the comparator to detect and so the
relay will never activate. Active PFC spreads the current
out over the full mains waveform, overcoming this issue.
However, the only sure way of knowing whether a given
device can be successfully used with this Soft Starter is
to try it and check that the relay reliably switches in after
the load is turned on. If not, the Soft Starter is clearly not
suitable for that particular load.
24 Silicon Chip
Circuit description
Refer now to Fig.5, the circuit diagram. The mains input
and output sockets have their active terminals joined via
a 10A fuse, protecting both the Soft Starter and the load.
The earths are joined, possibly using pin 2 of CON1 as a
convenient anchor point. This is vital for safety.
The neutral connection is where the soft start action occurs. Initially, the Neutral input (from the mains) and the
Neutral connection to the PCB are joined via two series
NTC thermistors, TH1 and TH2.
Two thermistors provide better in-rush current limiting
than one and also reduce the required cool-down time
somewhat.
Also in series with these thermistors is a 10mΩ (0.01Ω)
surface-mount resistor which monitors the load current. Its
resistance is so low that it has no effect on the load current
and dissipates little power (<1W).
When the contacts of RELAY1 close, they short out
both thermistors. This has two advantages; the tool gets
full power soon after it’s switched on and it allows the
thermistors to immediately begin cooling down. The relay
is rated at 240VAC/16A, which suits loads up to 4000VA.
15A is the highest continuous current available from
“large earth pin” power outlets (10A is the maximum from
standard outlets) so we don’t see any problem with the
current limitation.
The rest of the circuit monitors the voltage across the
10mΩ resistor and turns on RELAY1 when appropriate. It
is based around two active devices, quad precision comparator IC1 and PNP transistor Q1.
Window comparator
IC1a and IC1b are connected so that if the voltage across
siliconchip.com.au
Fig.4: start-up current of a 1750W circular saw without
the Soft Starter. This is quite similar to the 1500W router
waveform opposite but the peak current is a little higher.
Note how the mains voltage (green, top) sags quite
markedly for the first few cycles after switch-on due to
the huge initial current. With the Soft Starter, the result is
similar to the router (see Fig.2).
the 10mΩ shunt exceeds about 3.3mV (ie, a peak load current of 330mA), their common output at pins 1 and 2 goes
low. One end of the 10mΩ shunt is connected to ground
and the other to pin 6 of IC1b and, via a 1kΩ series resistor, pin 5 of IC1a.
Since the current waveform is AC, the voltage at these
pins can be above or below ground, so IC1b checks to see
whether it goes above +3.3mV while IC1a does the same
below -3.3mV.
These references voltages are derived from the forward
voltage of D3 and D4 (around 0.6V each) using 180kΩ/1kΩ
voltage dividers, ie, 0.6V x 1kΩ ÷ (180kΩ + 1kΩ) = 3.3mV.
Diodes D3 and D4 are fed from the +12V and -12V rails
respectively via 22kΩ current-limiting resistors. Their
forward voltages are reasonably stable over a wide range
of supply voltages and the expected operating temperature
range. The 22kΩ resistors set the current through each to
(12V – 0.6V) ÷ 22kΩ = 0.5mA. A small amount of this
current flows through the parallel resistors.
Now consider the operation of comparator IC1b. The
shunt is connected directly to its inverting input while
the 3.3mV reference voltage is applied to its pin 7 noninverting input. The open-collector output pin 1 goes low
when the voltage at pin 6 exceeds that at pin 7. This will
occur when the voltage across the shunt is above +3.3mV.
Hysteresis
When the shunt voltage is between -3.3mV and +3.3mV,
IC1b’s output (pin 1) is pulled up to +12V by a 100kΩ
resistor. There is a 10MΩ resistor between this output
and the non-inverting input (pin 7) which provides some
hysteresis, so that the output does not vacillate when the
threshold is crossed.
This resistor works as a voltage divider in combination
with the resistors connected to pin 7, which provide the
+3.3mV reference voltage. When the output is high, the
siliconchip.com.au
10MΩ resistor is effectively in parallel with the 22kΩ and
180kΩ resistors at the anode of D3.
This allows an extra 12V ÷ (10MΩ+ 100kΩ) = 1.2µA to
flow through the 1kΩ resistor, adding 1.2mV to the reference voltage, ie, it becomes +4.5mV.
But when the output of IC1b is low (-12V), the 10MΩ
resistor sinks a similar amount of current from this point,
lowering the reference voltage to around 3.3mV – 1.2mV
= 2.1mV. It is the 2.4mV difference between the positivegoing threshold (4.5mV) and the negative-going threshold
(2.1mV) which provides the hysteresis.
In other words, once the shunt voltage goes above 4.5mV
and the comparator output goes low, it must drop below
2.1mV before the comparator output will go high again.
The 3.3mV level is just a nominal voltage and does not
actually occur in the circuit.
The operation of IC1a is similar but since it its inputs
must be swapped to allow it to act as the other half of the
“window”, the voltage hysteresis is applied to the feedback
from the shunt, rather than the reference voltage.
The 10MΩ and 1kΩ resistors form a divider which has a
virtually identical effect on this sense voltage as described
above, ie, it raises or lowers it by 1.2mV depending on the
output state.
The minimum ±2.1mV thresholds have been selected
based on the precision of the LM339A. This has a 2mV
maximum input offset voltage with a 5V supply, at 25°C.
Unfortunately, the data sheet is coy about just how this
varies with supply voltage and temperature but under our
operating conditions, it should normally be below 2.1mV.
This is why we have chosen the LM339A rather than
the more common LM339 variant; if the input offset voltage exceeded the window comparator thresholds, either
the relay would switch on with no load or it would never
switch off once the load current ceases.
(Remember, power is still applied to the Soft Starter even
after you’ve let go the tool’s trigger).
Time delay
When the load current is above the stated threshold and
the outputs of IC1a and IC1b are low, this charges a 220nF
capacitor via the 2.2MΩ resistor and when the outputs are
high, it is discharged in the same manner.
Comparators IC3c and IC3d are wired up in parallel and
the capacitor voltage is applied to their non-inverting inputs
(pins 9 and 11) via a 3.3MΩ resistor.
When the relay is off, the outputs of these comparators
(pins 13 and 14) are at around +11.4V, since there is little
voltage across the relay coil and one diode drop across Q1’s
base-emitter junction (~0.6V).
The 10MΩ/3.3MΩ feedback voltage divider across the
comparators means that when the capacitor is charged beyond 15.8V (ie, its bottom end goes below -3.8V), the voltage
at the comparator non-inverting inputs drops below 0V.
We confirm this by performing the calculation for this
voltage divider, ie, (-3.8V x 10MΩ + 11.4V x 3.3MΩ) ÷
13.3MΩ = -0.03V.
The inverting inputs, pins 8 and 10, are connected to
ground so once the capacitor has sufficient charge, the
outputs of IC1c and IC1d go low and pull the base of PNP
transistor Q1 to -12V. Q1 is an emitter follower and so
in this case, it sinks current through the coil of RELAY1,
turning it on.
July 2012 25
0.01
TH2 SL32 10015
TH1 SL32 10015
–12V
A
A
WARNING
VIEWED
FROM
FRONT
E
N
A
OUTPUT
SOCKET
SOFT STARTER FOR POWER TOOLS
SC
22k
–0.6V
K
ZD2
12V
1W
K
220F
16V
D2
1N4004
Nout
4
Nin
3
2
ALL COMPONENTS
AND WIRING IN THIS
PROJECT MAY BE AT
230V POTENTIAL IN OPERATION.
CONTACT COULD BE FATAL!
2012
K
A
A
10M
180k
–3.3mV
D4
1N4148
K
1W
470
10M 1W
A
1
CON1
330nF X2
F1
10A
E
VIEWED
FROM
FRONT
K
1N4004
1N4148
K
12
2
IC1a
4
5
1k
A
K
A
D1
1N4004
K
A
N
230V PLUG
A
ZD1, ZD2
13
11
10
IC1c
8
9
IC1: LM339AN
3.3M
1k
+3.3mV
D3
1N4148
220F
16V
A
K
ZD1
12V
1W
A
180k
+0.6V
22k
1k
6
7
IC1b
3
1
220nF
2.2M
220nF
100k
10M
IC1d
14
10M
D5
1N4004
K
A
B
–12V
E
Fig.5: NTC thermistors TH1 and TH2 are connected between the neutral terminals of
the input & output mains sockets. A 0.01Ω resistor is used to monitor the neutral current
and shortly after it rises, RELAY1 is energised, shorting out the thermistors and allowing the tool to
run at full power. The relay is switched off shortly after the tool is, so the unit is ready to go again.
B
C
BC557
C
E
Q1
BC557
RELAY1
+12V
+12V
26 Silicon Chip
The voltage at the non-inverting
inputs them becomes (-3.8V x 10MΩ
+ -12V x 3.3MΩ) ÷ 13.3MΩ = -5.8V.
This is the hysteresis for this stage
and the capacitor must discharge by
this additional amount before the relay
turns off.
This allows the relay to stay on
through brief dips in the load current.
Diode D5 protects transistor Q1 from
any voltage spike created when the
relay turns off.
Power supply
The ±12V rails are derived from the
mains Active line via a 330nF X2 series
capacitor, 470Ω current-limiting resistor and dual half-wave rectifier formed
by diodes D1 & D2. These diodes charge
the 220µF capacitors alternately with
each mains half-cycle, to provide the
positive and negative rails. 12V zener
diodes ZD1 and ZD2 limit the voltage
across these capacitors to about 11.5V.
The 330nF capacitor and 470Ω resistor
limit the current and thus dissipation
in ZD1 and ZD2 to well below their
rated 1W.
If you ignore the X2 capacitor and
two 1W resistors, this is a traditional
AC-to-DC voltage doubler supply. The
X2 capacitor has an impedance at 50Hz
of around 9.65kΩ which limits the
mains current to about 230V ÷ 9.65kΩ
= 24mA. It’s a bit more complicated
than this calculation implies but that’s
a reasonable approximation.
We could have used a wirewound
resistor of a similar value but it would
then dissipate 0.024A2 x 9.65kΩ =
5.5W. The capacitor dissipates virtually no power.
The parallel 10MΩ resistor discharges the X2 capacitor once power is
removed while the 470Ω series resistor
limits the inrush current when power
is first applied.
For more details on how this type of
supply works, see the description in
the original Soft Starter article (April
2012).
The specified relay has a nominal
coil resistance of 1.1kΩ. This means
with a 24V supply it will draw around
22mA. As stated earlier, the X2 capacitor limits the supply current to about
24mA; less due to the series 470Ω
resistor and other factors.
When the relay is turned on, the
X2 capacitor and 470Ω resistor form a
voltage divider with the coil resistance.
The supply rails then drop to about
±6V and the two zener diodes cease
siliconchip.com.au
470 1W
D2 4004
ZD2
D1
4004
220F
16V
22k
22k
CON1
18 0 k
D5
1k
220nF
1k
Q1 BC557
0.01
3.3M
10M
RELAY1
2.2M
TH2 SL32 10015
TH1 SL32 10015
4004
COIL
18 0 k
10107121
D4
4148
12V
4148
D3
220F
16V
10M 1W
+
EARTH
OUT
121
7010IN1NEUTRAL
ACTIVE
Warning: 230VAC!
ZD1
12V
+
330nF X2
IC1 LM339A
220nF
100k
10M
10M
1k
10107121
COMPONENT SIDE OF BOARD
UNDER SIDE OF BOARD
Fig.6: use these overlay diagrams and the photograph below as a guide when building the Soft Starter. Just one
component, the 0.01Ω SMD resistor, goes on the underside. The diodes, electrolytic capacitors and IC1 must be installed
with the orientations shown here. Multiple pads are provided to suit differently sized X2 capacitors. Secure CON1 with a
machine screw at each end before soldering its pins.
conducting, since most of the input
current flows through the relay coil.
The relay gets close to the full 24V
across its coil initially to turn it on but
the 220µF capacitors then partially
discharge. The reduced coil voltage is
sufficient to keep it energised and the
rest of the circuit will run happily with
±6V or less. When the relay turns off,
the 220µF capacitors charge back up
to their original level.
PCB layout
While various components in the
circuit are shown connected to ground,
the main reference point is the “Nin”
(Neutral In) terminal of CON1. This is
the potential which the shunt sense
voltage is relative to. Because this is
very low (just a few mV), it’s critical
that the ±3.3mV references track this
ground potential accurately or the unit
won’t work properly.
Therefore, the connection between
the cathode of D3, the anode of D4 and
pin 3 of CON1 is separate from other
ground paths.
This way, current flowing through
ZD1, ZD2, the 220µF capacitors and
other components to ground does
not interfere with the comparator’s
operation.
As is typical with a circuit which
runs directly from mains, the PCB has
a high voltage section at 230VAC and
a low voltage section of ±12V (relative
to the neutral potential).
Since the only components connected to active are the 10MΩ 1W
resistor and 330nF X2 capacitor, all
other tracks are clear of those pins.
There can also be a fairly high voltage
across TH1 and TH2 when they are
conducting so their terminals are kept
clear of other tracks.
Construction
The Soft Starter for Power Tools
is built on a PCB coded 10107121,
measuring 59 x 80.5mm. It is a doublesided PCB with tracks on the top side,
paralleling the high-current paths on
the bottom to improve its currenthandling capability. All components
Here’s a view inside the box, fairly close to life-size. You can clearly see the way the wiring is connected to the terminal
block on the left end of the PCB – follow this along with the diagram above when wiring it up. If placed inside a metal
box, the earth wires must instead be firmly anchored to the box – see text for more details.
siliconchip.com.au
July 2012 27
Parts list – Power Tool Soft Starter
1 PCB, code 10107121, 59 x 80.5mm (available from SILICON CHIP for $10 + P&P)
1 6-position, 4-way PCB-mount terminal barrier (CON1) (Jaycar HM3162, Altronics
P2103)
2 Ametherm SL32 10015 NTC thermistors (Element14 1653459)
1 250VAC 16A SPST relay, 24V DC coil (Element14 1891740 or similar)
1 UB3 jiffy box or 1 diecast IP65 aluminium case (eg, Jaycar HB5046)
4 tapped M3 spacers, 5-6mm long (required only for diecast case)
4 M3 x 15mm Nylon machine screws
4 M3 nuts
4 M3 shakeproof washers
1 chassis-mount M205 safety fuse holder
1 10A M205 fuse
2 M3 x 15mm machine screws and nuts (to attach terminal block to PCB)
2 cord-grip grommets to suit 7.4-8.2mm cable (Jaycar HP0716, Altronics H4270)
1 100mm length brown mains-rated heavy duty (10A) insulated wire
1 50mm length 2.5mm diameter heatshrink tubing
1 short (~1m or so) 10A mains extension cord
Semiconductors
1 LM339A quad precision comparator (IC1) (do not substitute LM339)
(Element14 9755969)
1 BC557 100mA PNP transistor (Q1)
2 12V 1W zener diodes (ZD1, ZD2)
3 1N4004 1A diodes (D1, D2, D5)
2 1N4148 small signal diodes (D4, D4)
Capacitors
2 220µF 16V PCB-mount electrolytics
1 330nF X2 capacitor (Element14 1215460, Altronics R3129)
2 220nF MKT
Resistors (0.25W, 5% unless otherwise stated)
3 10MΩ
1 3.3MΩ
1 2.2MΩ
2 180kΩ
1 100kΩ
2 22kΩ
3 1kΩ
1 10MΩ 1W
1 470Ω 1W
1 10mΩ 2W/3W SMD resistor, 6331/2512 package (Element14 1100058)
(NB: that is 10 milliohms, not 10 Megohms!)
are through-hole types which mount
on the top with the exception of the
10mΩ resistor which is an SMD.
Refer to the overlay diagram, Fig.6.
Start by soldering the chip resistor in
place. First, add some solder to one
of its two pads using a hot iron. Place
the resistor near the pads with its
labelled side up, then heat the solder
and slide it into place. Remove the
iron and check that it is centred over
its pads. If not, re-heat the solder and
nudge it again.
Once it’s in the correct position,
solder the other pad. Add a little extra
solder to the first one, to re-flow it and
ensure a good joint.
You can then fit the smaller throughhole resistors, checking each value
with a DMM to ensure they go in the
right locations. Follow with the seven
diodes, orientating them as shown on
the overlay diagram. There are three
28 Silicon Chip
different types; use the overlay diagram as a guide to which goes where
(if you mix them up it won’t work!).
Fit the two 1W resistors next, then
solder IC1 in place. While used a
socket on our prototype (for development reasons) you shouldn’t. Ensure
IC1’s pin 1 notch or dot goes towards
the bottom left as shown in the overlay diagram. You can then mount Q1,
bending its leads with small pliers to
suit the pad spacings. Its flat face is
orientated as shown.
The two MKT capacitors go in next,
followed by the electrolytic capacitors, with their longer (positive) leads
through the holes marked “+”.
There are multiple pads to suit different sized X2 capacitors; solder it in
place with one pin in the right-most
position and the other through the
appropriate left-hand hole.
Now you can fit the relay and ther-
Fig.7: the correct
cut-out to make sure
cord-grip grommets
do grip! Don’t be
tempted to simply
drill a 16mm hole!
Suits
7.4-8.2mm
cable
15.9mm
14mm
mistors (pushed as far down as they
will go). Attach the terminal barrier
using the 15mm M3 machine screws,
with a star washer under each head
and nut. Do them up tight, make
sure it’s straight and then solder the
four pins. The PCB assembly is then
complete.
Housing
We housed our prototype in a UB3
jiffy box, which the PCB is designed to
fit in. It is pushed down to the bottom
of the box, so the taller components
will clear the lid.
Even though it is a tight fit, to ensure
it cannot move around it is fixed to the
bottom of the box using Nylon screws
(the nuts inside can be Nylon or metal).
If this unit is to be used on construction sites or in other rough situations
where it’s likely to be knocked around
a bit, it should be housed in a larger,
sturdier ABS plastic or (preferably) a
diecast aluminium case.
If you want to do this, fit four tapped
spacers to the mounting holes on the
PCB and then drill four corresponding
holes in the box. If the box is plastic, be
sure to use Nylon spacers and screws
(metal is OK on the inside) so that you
don’t breech the insulation barrier.
If you use a diecast aluminium box,
the two mains earth wires must have
crimp eyelet connectors fitted (use a
ratcheting crimping tool), both terminated on a machine screw through the
case which is fitted with star washers
and two nuts. This earths the case so
that an internal wiring fault can’t create a lethal situation.
Whichever housing you use, the first
step is to drill three holes; two 14mm
holes for the cordgrip grommets which
the mains cables pass through and one
11-12mm hole for the chassis-mount
fuse holder. The fuse holder can go
alongside the entry for the mains
supply lead.
Use needle files to expand the
grommet holes to the correct profile
(see Fig.7). The requirements for fuse
holders varies but they also often require the hole to be profiled; refer to
the supplier or manufacturer data for
the correct shape.
siliconchip.com.au
Solder a short length of brown
mains-rated wire to one of the fuseholder terminals and heatshrink the
joint. Fit the fuseholder to the box and
position the completed PCB inside it.
You can then cut the extension cord
in half and strip a 50mm length of the
outer insulation from both free ends.
Also strip back 6-8mm of insulation
from each of the three inner wires of
the two cables.
Feed the cables through cordgrip
grommets, squeeze the grommet
halves together and push them into
place through the holes you made
earlier.
If you are lucky enough to have a
tool for inserting cordgrip grommets
use that, otherwise some sturdy pliers
will do. The grommets are hard to take
out once they’re in so check that you
have fed through an appropriate length
of cable so that the individual wires
will reach the terminals on the PCB.
Keep in mind that the brown (active)
wire from the plug end of the cable
must reach the fuseholder.
Slip some heatshrink tubing over
that Active wire (plug end) and solder
it to the free tab on the fuseholder. Slip
the tubing down and shrink it over the
joint. Secure the five remaining wires
into the PCB terminal barrier as shown
in the photo on page 27.
Make sure there are no stray copper
strands and that the terminal screws
are done up very tightly so nothing
can come loose.
As mentioned earlier, if you are using a metal box (eg, diecast aluminium)
you will need to make the earth connections to a chassis earth point rather
than on the PCB.
Testing
Because the X2 capacitor limits the
circuit current, it can be quite safely
tested from mains – but don’t put your
fingers anywhere near the PCB.
o
Here’s the complete project, ready to use. There are no controls on the box . . .
because there are no controls! If used in a rough environment, we’d suggest a
diecast box – even if a little larger (eg, Jaycar cat HB5046).
First, check your wiring. Then put
the lid on the box and install a fuse.
Use a DMM to check for continuity
between the Earth terminals of the
plug and socket. The resistance must
be low (<1Ω).
Do the same check with the two
Active terminals and two Neutrals.
The resistance between the two Actives should also be low (<1Ω) while
between the two Neutrals should be
around 20-30Ω (the cold resistance of
the NTC thermistors).
Also measure the resistance between each combination of Active,
Earth and Neutral on each plug. You
should get >10MΩ resistance between
Earth/Neutral and Earth/Active at
both plug and socket. The resistance
between Active and Neutral should be
around 10MΩ at each end (it may read
lower initially due to the capacitors
charging).
Connect a 100W or greater 230V
lamp (eg, a portable PAR38 floodlight
– incandescent, not LED!) to the output
socket. While keeping your eye on the
Resistor Colour Codes
No. Value
4-Band Code (1%)
o
4a 10MΩ
brown black blue brown
o
1 3.3MΩ orange orange green brown
o
1 2.2MΩ red red green brown
o
2 180kΩ brown grey yellow brown
o
1 100kΩ brown black yellow brown
o
2
22kΩ
red red orange brown
o
3
1kΩ
brown black red brown
o
1b 470Ω
yellow violet brown brown
a 1 of the 10MΩ is 1W b1W
siliconchip.com.au
5-Band Code (1%)
brown black black green brown
orange orange black yellow brown
red red black yellow brown
brown grey black orange brown
brown black black orange brown
red red black red brown
brown black black brown brown
yellow violet black black brown
lamp, plug the power cord into the
wall outlet and switch it on.
Check that the lamp switches on
properly – for all intents and purposes, it should appear pretty normal
in brightness.
But about one second after this, you
should hear the relay click and the
lamp will get slightly brighter. Switch
the lamp off and check that the relay
clicks off after about a second.
If it doesn’t work, switch off at the
wall, unplug both ends, open the
box and remove the PCB. Check for
components which are swapped or
incorrectly orientated. If you don’t
see any component problems, check
the solder joints and ensure that there
are no breaks or short circuits between
the tracks or pads.
(Kit suppliers tell us that around
50% of problems with kits are mistakes
in component placement. Most other
problems are bad solder joints [or
components not soldered in!]).
Assuming all is well, you can then
do a full test with a power tool to check
that it is working as expected.
Remember that if you start the tool
multiple times in quick succession, the
second and later starts will not have
as effective current limiting due to the
thermistors heating up.
SC
Capacitor Codes
Value µF Value IEC Code EIA Code
330nF* 0.33µF 330n 334
220nF 0.22µF 220n 224
* must be X2 type
July 2012 29
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