Full-wave Universal
Motor Speed Controller
Want exceptionally smooth speed control over the entire range for
your power tool? You want our new Full-wave Universal Motor Speed
Controller. It is ideal for use with mains-powered electric drills, lawn
edgers, strimmer / whipper snippers, circular saws, routers or any other
appliance with universal (ie, brush-type) motors, rated up to 10A.
O
By JOHN CLARKE
ur latest Full-wave Universal Motor Speed
appliance is switched on, the motor speed is slowly and auController is an upgrade on the one we published in tomatically brought up to the set speed. Without it, power to
July 2019. That one worked very well, but we iden- the motor is suddenly applied, and the motor can kick back.
tified several upgrades and improved features that could be
Soft start is essential when using the Controller with a
made to the design.
high-powered router or circular saw. For smaller appliancOne of the main drawbacks of the previous design was es, and when the motor is switched on and off often, you
that the feedback gain control was located inside the Con- might find that it limits how fast you can work, as you wait
troller’s housing. That control set the amount of compensa- for the motor to come up to speed.
tion for maintaining motor speed under load.
That would be the case when used with a strimmer / whipOnce set, the Controller was only suitable for the appli- per snipper and some hand drills. So we made it so you can
ance being used, since the feedback control would require easily switch the soft start feature off. While we were making
changing for different motors.
those changes, we took the opportunity to improve its ability
This control is now exterto maintain motor speed
nally adjustable via a conunder load, especially at
trol knob, making it easy to
low speed settings and
use the Controller across
for low-power appliances.
This Speed Controller operates directly from the 230V AC mains
a range of different power
The Full-wave Unisupply and contact with any live component is potentially lethal.
tools and other devices.
versal Motor Speed ConDo not build it unless you know what you are doing.
DO NOT TOUCH ANY PART OF THE CIRCUIT WHILE IT IS
We have also added the
troller can be used with
PLUGGED INTO A MAINS OUTLET and never operate it outside its
ability to switch the softmains supplies over the
earthed metal case or without the lid attached.
start feature off, also via an
range of 220-250V AC at
This circuit is not suitable for use with induction motors and must
external switch. Soft start
50Hz or 60Hz. This means
only be used with universal ‘brush type’ (series-wound) motors or
is useful when the Speed
that it can be used in
shaded pole (fan) motors with nameplate ratings up to 10A. For more
Controller is set at a cermany different countries,
information, see the section titled ‘What motors can be controlled’.
tain speed and the motor
although it is not suitable
Power tools with inbuilt fans must not be operated at low speeds for
is switched on and off at
for use with 100-120V AC
extended periods; otherwise, they could overheat.
the appliance. When the
mains supplies.
WARNING!
16
Practical Electronics | June | 2022
The Controller is mounted in a relatively low-profile
diecast aluminium case with mains plug and socket
leads attached to one end, through cable glands. A panel fuse is also provided on the same end of the case.
The speed control and feedback gain potentiometers, and soft start switch are mounted on the lid.
Features
* For universal and shaded-pole motors rated up to 10A
* Runs from 220-250V AC at 50Hz or 60Hz
* Full-wave motor speed control
* Full speed range (from nearly zero to close to 100%)
* Current feedback for maintaining speed under load
* Feedback gain adjustment
* Optional soft start from zero speed and at power-up
* Optimised control for inductive loads such as motors
Why do you need speed control?
Most power tools will do a better job if they have speed
control. For example, electric drills should be slowed
down when using larger drill bits as they make a cleaner
cut. Similarly, it is useful to be able to slow down routers, jigsaws and even circular saws when cutting some
materials, particularly plastics, as many will melt rather
than be cut if the speed is too high. The same comments apply
to sanding and polishing tools, and even electric lawn trimmers; they are less likely to snap their lines when slowed down.
typically 2850 RPM for a 2-pole unit or 1440 RPM for a 4-pole
unit. The speed will be on the nameplate. Bench grinders
typically use two-pole induction motors.
If you do need to control the speed of this type of motor,
use the 1.5kW Induction Motor Controller published in January and Februay 2019.
What motors can be controlled?
This Controller suits the vast majority of power tools and
appliances. These generally use universal motors which are
series-wound motors with brushes. They’re called universal motors because they can operate on both AC and DC.
You cannot control the speed of any universal motor which
already has an electronic speed control built in, whether
part of the trigger mechanism or with a separate speed dial.
That does not include tools such as electric drills which
have a two-position mechanical speed switch. In that case,
you can use our Speed Controller with the mechanical switch
set to fast or slow. The slow selection usually drives the motor with a half-wave voltage.
Induction motors (except shaded-pole types, which are
often found in fans and such) must not be used with this
Speed Controller. How do you make sure that your power
tool or appliance is a universal motor and not an induction
motor? One clue is that most universal motors are quite
noisy compared to induction motors. However, this is only
a guide, and it’s certainly not foolproof.
In many power tools, you can see that the motor has brushes and a commutator (usually through the cooling vents) and
you can see sparks from the brushes during operation. That
indicates that the motor is a universal type. But if you can’t
see the brushes, you can also get a clue from the nameplate
or the instruction booklet.
Most induction motors used in domestic appliances will
be 2-pole or 4-pole types which operate at a fixed speed,
Phase control
The AC mains voltage closely follows a sinewave. It starts at
0V, rises to a peak, falls back to 0V, then does the same thing
in the opposite direction. This repeats 50 times per second
for 50Hz mains, or 60 times per second for 60Hz mains.
A motor connected to the mains makes full use of the energy from each cycle so that it runs at its maximum speed.
So, if supplied only a portion of this sinewave to the motor, with less energy available to power it, the motor would
not run so fast.
Varying the time during each half-cycle when voltage is
applied to the motor gives speed control. This is the basis
of phase control: start feeding power very early in the cycle, and the motor runs fast; delay power until much later
in the cycle, and it runs more slowly.
The term ‘phase control’ comes about because the timing
of the trigger pulses is varied with respect to the phase of
the mains sinewave. Several devices can be used to switch
the mains voltage; here, we are using a Triac. That device
can be used to switch both the positive and negative voltage excursions of the mains waveform.
The oscilloscope traces show phase control varying the
power to an incandescent light bulb, as this shows phase
control in its pure form, without the extra hash caused by
driving a motor.
Scope1: the output waveform (live voltage, in cyan) at a
higher speed setting with a resistive load (a light bulb). You
can see that the output voltage matches the input voltage
most of the time, so the attached load will receive almost
full power and, if a motor, will run at high speed.
Scope2: by triggering the Triac later in each mains halfcycle, the output voltage (cyan) is zero most of the time,
and the load power is greatly reduced. This will cause an
attached motor to spin quite slowly, as the average applied
voltage will be low.
Practical Electronics | June | 2022
17
Specifications
* Power: 230V AC sinewave up to 10A
* Operating frequency: any fixed frequency
between 40Hz and 70Hz
* Soft start rate: two seconds from start to full speed
* Triac gate drive: 68mA
* Triac gate pulses, phase angle <90°:
40µs gate pulses repeated at 200µs intervals
Scope1 shows the chopped waveform from the phase
control circuit when the incandescent light bulb is driven at
high brightness. This is equivalent to driving a motor at a fast
speed. Here, the Triac is triggered 2.5ms after the zero-crossing (the point where the mains waveform passes through 0V).
The voltage applied to the load is the cyan trace, and
measures 200V RMS. That is less than the 219V RMS mains
waveform shown by the yellow trace.
Scope2 shows the waveform from the phase controller
driving a light bulb at a lower setting, with the Triac triggered later in the cycle. The voltage applied to the load is
now much less at 87.9V RMS.
Scope3 shows the waveform when driving a motor. The
lower blue trace is the voltage applied to the motor, with
the input mains shown on the top (yellow) trace. Note the
extra hash on the lower trace due to the motor being an inductive load.
Speed control
For a motor to have good low-speed performance, the Controller needs to compensate for any drop in motor speed as
the load increases. Many phase-based speed controllers rely
upon the fact that a motor can be used as a generator when
it is spinning with no power applied. When the motor is
loaded and the motor speed slows, the back-EMF (electromotive force) produced by the motor drops, and the circuit
compensates by providing more of the mains voltage cycle
to the motor, triggering the Triac earlier in the mains cycle.
However, in practice, the-back EMF generated by most series motors while the Triac is not conducting is either very low
or non-existent. This is partly because there is no field current,
and the generation of voltage is only due to remnant magnetism in the motor core. If there is any back-EMF produced,
it is too late after the end of each half-cycle to have a worthwhile effect on the circuit triggering in the next half-cycle.
So we use a different method for speed regulation, by
monitoring the current through the motor. When a motor
is unloaded, it draws a certain amount of current to keep
itself running.
When the motor is loaded, the motor speed drops and the
current draw increases. The motor controller senses this,
and compensates for this speed drop by increasing the voltage to the motor.
This might sound like positive feedback, where the detection of more current drawn will increase the voltage and so
allow the motor to draw more current. It’s true that this can
happen if the amount of compensation is too high, which
is why we include a feedback control knob, to adjust the
compensation gain.
With the right setting, the speed regulation is very impressive, but too much feedback will have the motor increasing in speed with increased load instead of maintaining the set speed.
Controlling a Triac with an inductive load
One major problem when using a Triac for full-wave control
of a motor is the way a Triac switches off and the nature of
18
Scope3: the same speed setting as shown in Scope2, but this
time with a motor attached. The inductance of the motor
windings causes the Triac to switch off after the zero-crossing
due to the output current phase shift from its reactance.
the motor load. A Triac is usually switched on by applying a current to its gate. If the current flowing between the
Triac’s main terminals is greater than its holding current,
the Triac will remain switched on for the remainder of the
mains cycle.
A Triac will only switch off when the gate is not being
driven and the Triac current drops below its holding current. Since a motor is not a purely resistive load, but instead has a significant inductance, the motor current lags
the voltage. That means a Triac driving a motor will not
necessarily switch off at the zero-crossing; motor current
can continue to flow for sometime after.
Our circuit uses a microcontroller to produce the required
gate pulses to correctly drive an inductive load like a motor
using a Triac. It feeds a series of gate pulses to the Triac to
provide for the full range of phase control.
Circuit description
The Speed Controller circuit is shown in Fig.1. Its key components are Triac Q1 and PIC12F617 microcontroller IC1.
IC1 monitors the speed potentiometer (VR1) at its analogue input AN1 (pin 6) and the feedback gain potentiometer (VR2) at AN0 (pin 7). It also monitors the motor current
at analogue input AN3 (pin 3), with that signal originating
at current transformer T1 and passing through a full-wave
rectifier based around IC2. The mains voltage waveform is
monitored for zero crossings at pin 5, via a 330kΩ resistor.
In response to all those parameters, IC1 produces a series
of pulses at its digital output GP5 (pin 2), and these drive
the base of NPN transistor Q2 which, in turn, sinks current
from the gate of Triac Q1. The Triac gate current flows via
the 47Ω resistor connected between the 5.1V supply and the
Triac’s A1 terminal, then out through the gate and to circuit
ground via Q1 (ie, the gate current is negative).
This method of connection places the 47Ω resistor between the 230V AC mains supply and the 5.1V supply which
runs the PIC microcontroller. This avoids Triac switching
noise getting into the 5.1V supply, which can cause the microcontroller to latch-up.
Snubber
The snubber network comprises two 220Ω 1W resistors in
series and a 220nF 275V AC X2-rated capacitor connected
between the A1 and A2 terminals of the Triac. This network
prevents rapid changes in voltage from being applied to Triac Q1, which would otherwise cause it to turn on (due to
dV/dt switching) when it is supposed to be off.
Practical Electronics | June | 2022
Scope4: the first stage of the precision full-wave rectifier
works as a half-wave rectifier with an output voltage
half that of the input. Both signals (original and clipped/
attenuated) are fed into the second stage and combined to
produce the output shown in Scope5.
Scope5: the final output waveform of the precision full-wave
rectifier is in cyan. It is identical to the yellow trace, except
that the negative portions have become positive voltages, so
that it can be fed to a single-ended ADC for measurement.
These rapid changes in voltage can occur when power
is first applied, or can come from voltage transients generated by the inductance of the motor being controlled each
time the Triac turns off. The snubber network acts to damp
transients and reduce their amplitude.
The DC supply for the microcontroller is derived directly from the 230V AC mains supply via a 470nF 275VAC
X2-rated capacitor in series with a 1kΩ 5W resistor. The capacitor’s impedance limits the average current drawn from
the mains, while the 1kΩ resistor limits the surge current
when power is first applied.
When the neutral line is positive with respect to live,
current flows via the 470nF capacitor, diode D1 and 47Ω
resistor to the 1000μF capacitor to charge it up. On negative half-cycles, the current through the 470nF capacitor is
reversed and flows through diode D2, discharging the capacitor back into the mains.
Zener diode ZD1 limits the voltage across the 1000µF capacitor to 5.1V. This is the supply for microcontroller IC1,
op amps IC2a and IC2b, and for the gate current of Triac
Q1. IC1’s 5.1V supply is bypassed with a 100nF capacitor
while IC2 is bypassed with 100µF.
Switch S1 allows the soft-start feature to be enabled or
disabled. This switch controls the input level of the GP3
input (pin 4). When S1 is open, the GP3 input is held high
at 5.1V via a 47kΩ resistor, so soft start is disabled. When
switch S1 is closed, GP3 is pulled low, and the program
runs the soft-start routine.
S1 pulls GP3 low via a 100Ω resistor, which is included
to protect the input from current transients that could cause
latch-up in the IC. The 100nF capacitor provides a low impedance to transients, preventing incorrect detection of the
GP3 input when S1 is open due to transients or interference.
VR1 is the speed potentiometer, and it is connected across
the 5.1V supply. IC1 converts the voltage from VR1’s wiper into a digital value using its internal analogue-to-digital converter (ADC). The 100kΩ resistor from the wiper to
ground holds the AN1 input at 0V, setting the motor speed
to zero should VR1’s wiper go open-circuit.
Potentiometer VR2 is connected similarly. Its wiper voltage sets the feedback gain to maintain motor speed under
load. It is also converted to a digital value within IC1. The
capacitors at the wiper of VR1 and VR2 provide a low source
impedance to IC1’s ADC, and to filter out supply ripple.
Both VR1 and VR2 are connected to IC1 via screw connectors. CON2 provides the common +5V and 0V connections for VR1 and VR2, while VR1’s wiper also connects
to CON2. CON3 provides the wiper connection for VR2,
with switch S1 utilising the remaining two connections
in CON3.
Practical Electronics | June | 2022
Mains synchronisation
The timing of the Triac’s trigger pulses is critical to its correct operation. IC1 monitors the mains voltage at its pin 5,
with the 330kΩ resistor connecting to neutral plus a 4.7nF
low-pass filter capacitor.
An interrupt routine is triggered in IC1 whenever the
voltage at pin 5 changes from a high to a low level, or vice
versa. The interrupt tells IC1 that the mains voltage has just
passed through 0V, so it can synchronise its gate triggering
with the mains waveform.
The phase lag introduced by the 4.7nF capacitor is compensated for within IC1’s software, as is the asymmetry of
the triggering due to the 5V difference between low and
high levels.
Current feedback
T1 is a current transformer comprising a ferrite toroid with
a two-turn primary winding in series with the Triac. The
secondary winding has 1000 turns, and it is loaded with a
510Ω resistor. With this loading, the transformer produces
800mV per amp of load current at the secondary output.
This is proportional to the current through the motor being controlled.
Its output signal is applied to a precision full-wave rectifier comprising IC2a and IC2b. This configuration is unusual in that it does not use any diodes. Most precision
rectifiers with diodes require a negative supply for the op
amps. While we could have incorporated a negative supply,
it would increase the circuit complexity and cost.
The full-wave rectifier operation relies on op amps that
have specific characteristics. The first is that the op amp
output has to swing fully to the negative supply rail (ie, all
the way down to 0V). Also, this 0V output must be maintained when the input to the op amp drops below 0V. The
LMC6482 op amp (IC2a and IC2b in the circuit) has these
characteristics, as well as a low supply current.
We have labelled several points in the circuit and shown
the expected waveforms to help explain how this section
works. The signal from the transformer secondary appears at
point A. This signal swings above and below 0V as shown.
The signal flows along two paths from here. One is through
the 20kΩ resistor to point D, and the other through the two
series-connected 100kΩ resistors to 0V.
19
Parts list – F ull- wave Motor Speed Controller
1 double-sided PCB coded 10102211, 103 x 81mm
1 diecast box, 119 x 94 x 34mm [Jaycar HB5067]
2 linear 50k 24mm potentiometers (VR1,VR2)
2 plastic knobs to suit VR1 and VR2
1 SPST mini rocker switch (S1) [Jaycar SK0984 or Altronics S3210]
1 Talema AX-1000 10A current transformer (T1) [RS Components 775-4928]
1 M205 10A safety panel-mount fuse holder (F1) [Altronics S5992]
1 M205 10A fast-blow fuse
1 4-way PCB-mount screw terminal (CON1) [Jaycar HM-3162]
2 3-way PCB-mount screw terminals, 5.08mm pitch (CON2,CON3)
2 GP9 cable glands for 4-8mm diameter cable
1 8-pin DIL IC socket (for IC1)
1 2m-long 10A mains extension cord
3 chassis lugs with 4mm eyelets
4 6.3mm-long M3 tapped nylon spacers
3 M4 x 10mm panhead or countersunk machine screws (for mounting Q1; earthing)
2 4mm inner diameter star washers
3 M4 nuts
8 M3 x 5mm panhead or countersunk machine screws
4 stick-on rubber feet
1 20mm length of 12mm diameter heatshrink tubing
1 80mm length of 3mm diameter heatshrink tubing
1 600mm length of 7.5A mains-rated wire (for VR1, VR2 and S1)
4 100mm-long cable ties
Semiconductors
1 PIC12F617-I/P 8-bit microcontroller programmed with 1010221A.hex, DIP-8 (IC1)
(1010221A.hex software available from the ‘Downloads’ section of the PE shop)
1 LMC6482AIN dual CMOS op amp, DIP-8 (IC2)
1 BTA41-600B 40A 600V insulated tab Triac, TOP3 (Q1)
1 BC337 500mA NPN transistor, TO-92 (Q2)
1 5.1V 1W (1N4733) zener diode (ZD1)
2 1N4004 400V 1A diodes (D1,D2)
Capacitors
1 1000µF 16V PC electrolytic
1 100µF 16V PC electrolytic
1 10µF 16V PC electrolytic
1 2.2µF 16V (or higher) PC electrolytic
1 470nF 275VAC X2-class metallised polypropylene
1 220nF 275VAC X2-class metallised polypropylene
3 100nF 63/100V MKT polyester
1 4.7nF 63/100V MKT polyester
Resistors (all 0.25W, 1% unless otherwise stated)
1 330k 5 1W car on film
3 1 k
1 10k
1 4.7k
1 510
1 470
1 100
2 47
(value printed on body)
(value printed on body)
(code 103 or 100n)
(code 470 or 4n7)
1 47k 2 20k
1 1k 10% 5W wire wound
2 220 5 1W car on film
Miscellaneous
Super Glue (cyanoacrylate), thermal paste, solder
IC2b is connected as a unity-gain
buffer. The op amp’s internal diode
will clamp any voltage at the non-inverting input (pin 5) below −0.3V. Its
output (pin 7) will be at 0V whenever
its input is 0V or less.
The operation of this part of the circuit is best explained by describing the
signal flow for the negative and positive
excursions of the waveform separately.
Negative portion
When the voltage at point A is negative, the voltage at point B is clamped
to −0.3V by the internal protection diode at the pin 5 input of IC2b. The out20
put of IC2b at pin 7 (point C) is therefore at 0V, and so is the non-inverting
input to IC2a.
As a result, IC2a acts as an inverting amplifier with a gain of −1. This is
set by the input 20kΩ resistor and the
20kΩ feedback resistor from the pin 1
output to the inverting input at pin 2.
So IC2a will produce a positive voltage
at its output pin 1, proportional to the
negative voltage at point A.
To understand how this works, consider that the op amp operates to keep
the voltages at its inputs equal. As the
non-inverting input is held at 0V, with
equal-value resistors in the feedback
Reproduced by arrangement with
SILICON CHIP magazine 2022.
www.siliconchip.com.au
+5.1V
47kW
CON3
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START
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path forming a 1:1 divider, the output
voltage (E) must have equal magnitude
and opposite polarity compared to the
input voltage (A) for the inverting input voltage (D) to be at 0V.
So for example, when point A is at
−1V, point E will be +1V, so point D
will be at 0V, equal to C.
Note that the 10kΩ resistor at point D
does not have any effect in this example, since pin 2 is at 0V, and therefore
there is no voltage across that resistor.
It has a function only during positive
signal excursions.
Positive portion
For positive voltages at point A, the
voltage at point B will be half the voltage of point A due to the 100kΩ/100kΩ
resistive divider. Point C and the
non-inverting input to IC2a will also
be half the applied voltage at A, as IC2b
is acting as a buffer.
Remember that usually, the inverting input voltage will be the same as
the non-inverting input. The op amp
will ensure this by adjusting its output
so it can maintain that voltage via the
feedback resistor.
The only way that can happen for
IC2a in this case is when the op amp
Practical Electronics | June | 2022
5
47W
+5.1V
+5.1V
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1000mF
100nF
+5.1V
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ZD1
5.1V
1W
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A
B
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C
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D1,D2: 1N4004
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4
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GP4/AN3
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470nF
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6
2
275VAC
100kW
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MAINS LINE PLUG
(VIEWED FROM
WIRING SIDE)
CURRENT
20kW
IC1
C12F617
-I/P
GP2
8
20kW
T1 TRANSFORMER
A
10kW
GP5
Vss
D
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510W
CON1
2
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F1
10A
FUSE
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BC337
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A2
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220W
1W
GATE DRIVE
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220W
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330kW
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SNUBBER
SC
Full-wave
Controller
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S L Speed
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Ó2021
Fig.1: the Full-wave Motor Speed Controller uses current-sense transformer T1 and op amps IC2a and IC2b (operating as
a full-wave precision rectifier) to sense the motor current. IC1 adjusts the gate pulses from its pin 2 output to the gate of
Triac Q1 to maintain a more-or-less constant motor speed under load
output at point E is the same as the signal input at point A. In that case, the
same voltage is applied to both 20kΩ
resistors and they are essentially in
parallel, forming an equivalent 10kΩ
resistor to point D. This forms a 1:1 divider with the 10kΩ resistor from point
D to ground, halving the voltage at this
point compared to points A and E.
So, to conclude. IC2a provides the
same positive voltage at its output E
as the input at A during positive excursions. Hoever, during negative excursions, IC2a instead inverts the voltage. So the output of IC2a is positive
for both negative and positive inputs
at point A. Thus, we have a diode-free
full-wave rectifier.
Its output is low-pass filtered using a 4.7kΩ resistor and 10µF capacitor for a smooth DC output that’s then
applied to the AN3 analogue input of
IC1, ready to be digitised.
Scope4 shows a sinewave signal at
point A (in yellow) and the lower blue
trace shows waveform C, the half-amplitude positive waveform output.
When waveform A goes below 0V,
waveform C stays at 0V.
Scope5 shows the same sinewave
signal at A in yellow, together with
Practical Electronics | June | 2022
the full-wave rectified output at E in
the lower blue trace.
Construction
Most components for the Full-wave
Universal Motor Speed Controller
are mounted on a double-sided, plated-through PCB coded 10102211, measuring 103 x 81mm and available from
the PE PCB Service. This is mounted
inside a diecast box measuring 119 x
94 x 34mm.
Follow the PCB overlay diagram,
Fig.2. Begin by installing the resistors
except for the 5W type – double-check
each resistor using a digital multimeter.
Following this, fit the diodes, which
must be oriented as shown. There are
two different diode types: 1N4004 for
D1 and D2, and zener diode ZD1 is a
5.1V 1W type (1N4733).
IC1 is mounted on an 8-pin DIL socket so install this socket now, taking care
to orient it correctly, with the notch facing towards the top of the PCB. Leave
IC1 out for the time being, though; we’ll
fit it later. IC2 can be installed on a socket or directly on the PCB. Q2 can also
be installed now.
Place the capacitors next. The MKT
and polypropylene types are usually
printed with a code indicating their
value. These are all shown in the
parts list.
By contrast, electrolytic capacitors
are almost always marked with their
value in μF, along with their polarity.
Typically, the negative lead is marked
with a stripe. They must be inserted
with the polarity shown.
The screw terminals are next. The
3-way terminal blocks for CON2 and
CON3 are installed with the lead entries facing each other, while CON1
does not have a specific orientation.
Then fit the 5W resistor about 1mm
above the PCB for improved cooling.
Finally (for now), install current
transformer T1. It does not matter
which way it is oriented. Triac Q1 will
be fitted later.
Cut the underside pigtail leads from
all components short to prevent contact with the base of the case.
Drilling the case
Fig.4 shows a template/guide for drilling the case. The lid requires 9.5mm
diameter holes for potentiometers VR1
and VR2, a 19mm x 10mm rectangular
cutout for switch S1 and a 4mm hole
for the earth screw.
21
SILICON CHIP
Fig.2: most of the components are mounted on the top of the board, with the main exception being Triac Q1. It mounts
on the inside of the case, under the PCB. Once you have finished the wiring, check it carefully against this diagram.
The earth screws and lugs must all make good contact, and use cable ties to bundle up the control wires, as shown.
The PCB is mounted in the base of
the case using 6.3mm-long M3 tapped
spacers, which require mounting holes.
Use the PCB as a template, and note
that the CON1 screw terminal end sits
further away from the end of the box
compared to the other end. This allows
space for the cable gland nuts. With the
PCB in place, mark out the hole positions, remove it and drill them to 3mm.
Attach the 6.3mm-long spacers to
the PCB using short machine screws,
then bend the Triac leads up by 90°
4mm from its body. Insert the leads
into the PCB from the underside (see
Fig.2 and Fig.3).
Secure the PCB to the case with
screws from the underside and mark
the Triac mounting hole position on
the base of the case. Remove the PCB
again and drill this to 4mm. Clean
away any metal swarf and slightly
chamfer the hole edges, then reattach
the PCB and adjust the Triac lead
height, so the metal tab sits flush onto
the flat surface.
Close-up, same-size photo of the Speed Controller PCB. Because it is a mainspowered and mains-controlling device, your construction must be exemplary.
Don’t attempt this project if you’re not experienced with mains devices.
22
Secure the Triac tab to the case with
an M4 screw and nut. The metal tab
is internally isolated from the leads,
so it does not require any further insulation between its tab and the case.
Solder the Triac leads on the top of the
PCB and trim them close. Now remove
the screws to gain access to the underside of the PCB and solder the Triac leads
from the underside of the PCB as well.
Now is a good time to attach rubber
feet to the base of the case.
Panel preparation
As well as drilling the holes in the lid
mentioned above, you need to partially
drill a 4mm hole on the inside for the
pot location pin that prevents the pot
body from rotating. Drill it so that it
almost reaches the outside of the lid,
but doesn’t go all the way through.
If you use a countersunk-head earth
screw and countersink its hole appropriately, it can be mounted under the
panel label for a neater appearance.
Otherwise, you’ll need to cut a hole
in the panel label (with a sharp hobby knife) when the label is stuck on.
Fig.3: Triac Q1 mounts on the base of
the case, using it as a heatsink. A hole
in the PCB gives access to hold the nut
while you tighten the screw.
Practical Electronics | June | 2022
Fig.4: drill the three holes in
the lid as shown here, plus the
rectangular cut-out. It is most
easily made by drilling a series
of small holes inside the outline,
knocking the central piece out,
then carefully filing the edges
flat and to shape until the switch
snaps in. The three large holes in
the box end are for the two cable
glands and fuseholder,with a
small one (4mm) in the box side
for the earth screw.
SILICON CHIP
.
.
GAIN
.
www.siliconchip.com.au
10A
Fuse
For universal motors
rated up to 10A,
50/60Hz 230V AC.
Not suitable for
induction motors.
SOFT
START
OFF
ON
.
.
.
.
.. . .
.
.
..
. .
.
.
.
.
.
.
.
.
.
SPEED
Full Wave 10A
Motor Speed Controller
Fig.5: full-size front-panel artwork which can be copied or downloaded (from
the June 2022 page of the PE website) and printed. This is glued to the top of the
diecast box – and it can also be used as a template to drill the three panel holes
and cutout for the soft start switch.
Practical Electronics | June | 2022
The panel label file can be downloaded from the June 2022 page of the
PE website and printed. To produce
a front panel label, you have several options. For a more robust label,
print as a mirror image onto clear
overhead projector film (using film
suitable for your type of printer). Attach the label, printed side down, to
the lid with a light-coloured or clear
silicone sealant.
Alternatively, you can print onto a
synthetic sticky label that is suitable
for inkjet printers, or a sticky label for
laser printers. Then affix the label using the sticky back adhesive.
Wiring
Cut the 10A extension lead into two, to
provide one lead with a plug and another with a socket. Where you cut the
lead depends on how long you want
each section to be. You might prefer a
long plug cord and short socket lead,
so the appliance is located near the
Controller, or the lead can be cut into
two equal lengths.
23
Before cutting, make sure
you have sufficient length
to strip back the insulation
as detailed in the next two
paragraphs. Make sure the
two leads are fed through
the correct gland and wired,
as shown in the wiring diagram, Fig.2.
For the socket (output)
lead, you need a 100mm
length of earth wire (green/
yellow stripe) for the connection between the chassis and lid, so strip back the
outer insulating sheath by
about 200mm. Cut the live
(brown) and neutral (blue)
wires to about 50mm long
and keep the off-cuts.
The spare 150mm brown
wire can be used later, to
connect from the fuse to
CON1 via the transformer,
T1. Recall that this requires
two turns of the live wire
looped through the transformer hole.
The 100mm earth wire (green/yellow stripe), which is routed around the
edge of the PCB, and twisted together with the earth wire from the plug
(input) lead, needs to be crimped into
one of the earth lugs.
Strip the plug lead outer sheath
insulation back to expose 100mm of
wire. All three wires pass through the
cable gland and connect it as shown
in Fig.2. Cut the neutral wire to 50mm
and strip back the insulation before
connecting it to the terminal block.
Now mount the fuse holder in the
hole you made earlier and prepare
to solder the live (brown) wire to it,
as shown.
But before doing that, slide 10mm
diameter heatshrink tubing over the
live (brown) wire. After soldering that
wire, slide the tubing up and over the
fuse holder to cover the fuseholder
side terminal and shrink it.
Similarly, use 3mm-diameter heatshrink tubing to cover the fuse holder
end terminal after soldering that wire.
Now twist the ends of the input
earth (green/yellow stripe) wire and
the output earth wire together and
crimp both into one of the eyelet lugs.
Cut VR1 and VR2’s shafts to 12mm
long from the front of the pot bodies
and file the edges smooth. Then attach the three 100mm lengths of 7.5A
mains-rated wire to the three terminals
of VR1, plus a fourth 100mm wire to
the middle terminal of VR2. Use short
lengths of the same wire to connect the
two ends of VR2’s track to the same terminals on VR1. Cover all six terminals
with 3mm heatshrink tubing.
24
This ‘opened out’
photo matches the
PCB/wiring diagram
on P42. Of course,
we made sure that
the Controller was
not plugged into
mains power before
removing the lid!
Next, connect the free ends of these
wires to CON2 and CON3, making sure
you follow the layout in Fig.2. You now
need to wire up switch S1 in a similar
manner. It is simply wired to the two
remaining terminals in either order.
Now secure all these wires to the
PCB using a cable tie that feeds through
the holes provided in the PCB. Attach
VR1, VR2 and S1 to the lid of the case,
noting that the potentiometers must be
located as shown (ie, with their leads
emerging away from the edge of the
case). This is so that they will fit between the two mains-rated capacitors
on the PCB.
Add cable ties around the wire bundles closer to VR1, VR2 and S2 as well.
Fit the knobs now; you might need
to lift out the knob caps with a hobby
knife and re-orientate them so that the
pointers match the rotation marks on
the lid panel.
That 100mm length of earth wire
you cut off from the output lead can
now be crimped into two eyelet lugs,
which are screwed to the underside of
the box lid and the earth screw on the
side of the case using M4 screws, star
washers and nuts. Ensure that the nuts
are fully tightened.
Final assembly
Apply a smear of thermal paste to the
underside of the Triac tab before installing the PCB inside the case. As
mentioned, the tab of the Triac is insulated, so it can contact the case.
The last components to insert are
IC1 (taking care it is oriented correctly), the 10A fuse into its holder and
the cover for the barrier terminals
(CON1). This is simply pressed on to
cover the screw terminals. Finally,
rotate VR2 fully anticlockwise to initially disable feedback.
Now check your construction carefully. Verify that the earth wires (green/
yellow striped) connect together the
case, to the lid and the earth pins on
both the mains plug and socket. Check
this with a multimeter set to read low
ohms. You should get readings below
1Ω between all earth points.
The cable glands need to be tightened to hold the mains cords in place.
Because these are easily undone, apply
a drop of Super Glue to the thread of
the glands before tightening. That way,
the glands cannot be undone by children or other curious people.
Attach the lid, ensuring the wiring is
routed so that it fits around the higher
components on the PCB. Use the four
screws supplied with the case; don’t
be tempted to run the Speed Controller without the lid in place!
Testing
Now it’s test time! Connect up a
universal motor appliance (eg, a
mains-powered electric drill) to the
Controller, apply power and check
that the motor can be controlled when
adjusting the speed potentiometer.
VR2 may need adjustment to avoid
speed changes when under load. Rotate
it clockwise if the speed drops off too
markedly under load, and anticlockwise if the motor speeds up under load.
Check that the soft-start feature
works when enabled by switching the
power off, letting the tool spin down,
then switching it on again to verify that
it ramps up smoothly with S1 in the
correct position.
Practical Electronics | June | 2022
