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|
Constructional Project
John Clarke’s Mk2
Fan Speed
Controller
On a hot night, a gentle cooling breeze from a fan
can keep you cool and help you to sleep. This new
Fan Speed Controller is an effective, noise-free,
low-speed fan controller. It works with ceiling,
pedestal and box fans.
M
ost fans include speed control, but
many run too fast, even on their
slowest setting, and can be pretty noisy.
If you want to use the fan to keep cool
while sleeping, you don’t need a fast
breeze but just gentle air movement.
You also don’t want the fan blades or
the motor to make any noise that will
keep you awake.
Whether a fan makes noise at a slow
speed depends upon the type of speed
control. Of the methods used for controlling fan speed, phase control causes
the most motor noise. This type of
control is where just a portion of the
full mains sinewave is applied to the
fan motor.
Because just a part of the mains waveform is applied, it produces a rapid
change in voltage as the waveform is
switched on and off. That can produce
vibration in the motor windings and
bearings, causing a buzzing sound.
Other fan speed controllers use a
switch that selects from one of several different capacitors or inductors.
While they don’t generally make the
fan motor noisy, they only provide a
few fixed speeds and the lowest speed
is usually not that slow.
Our Fan Speed Controller does not
use phase control; instead, it introduces
resistance in series with the fan motor
to adjust the fan speed. The mains
sinewave is simply reduced in voltage without changing the wave shape.
Applying a sinusoidal voltage to the
motor ensures the fan makes minimal
noise. It also provides continuous adjustment from stopped to full speed or
anywhere in between.
This does have the disadvantage
Fig.1: AC is applied to
the motor but the diode
bridge ensures that
Mosfet Q1 only
sees DC.
58
that power is dissipated as heat. But
considering that most fans will draw
a maximum of 60W at full speed and
less as speed is reduced, the heat produced is modest and can be dissipated
by the aluminium diecast box, which
acts as a heatsink.
We don’t need to dissipate anywhere
near 60W because, at full speed, the dissipation in the controller is relatively
low since the resistance of the controller is low. At lower speeds, where the
controller resistance is higher, dissipation increases. But because the motor
is running slower, the overall power
drawn by the fan is much less than at
full speed.
An over-temperature thermostat will
switch the power off should there be
excess heat buildup. This precaution
prevents the speed controller from
overheating and possibly causing skin
burns if touched.
For the resistance element, we use
a Mosfet with a drain-to-source resistance that can be controlled by adjusting the gate voltage. The Mosfet can
behave like a very low resistance for
full-speed operation or a higher resistance under partial conduction for
slower speeds.
A single Mosfet cannot directly control the mains AC voltage. While it operates as a resistance element when the
current flows in one direction, in the
other direction, it is shunted by an intrinsic diode that’s part of the Mosfet
structure.
Practical Electronics | April | 2025
Fan Speed Controller Mk2
Fan Speed Controller, Mk2
Features & Specifications
» Quiet fan speed control
» Suitable for 230V AC shadedpole fan motors
» Full control of motor speed from
stopped to maximum
» Over-current limiting
» Over-temperature cutout
» Fuse protection against faults
» Rugged aluminium case
» Fan power: 80W maximum
» Fuse: 1A, 230V AC
» Current limiting: 235mA at low
speed, up to 940mA at high speed
» Over-temperature cutout:
triggers with case at 50°C
(resumes at 45°C)
This photo shows the completed Fan Speed Controller PCB mounted in the
case without any of the wiring.
To prevent reverse current flow
through the Mosfet, the Mosfet is placed
within a full wave bridge rectifier. That
way, it only handles current in one
direction, but an alternating current
(and voltage) is still applied to the fan.
Fig.1 shows the general arrangement.
The Mosfet (Q1) is between the positive
and negative terminals of the bridge
rectifier. When the mains Live voltage is more positive than the Neutral,
current (i1) flows from Live through
the motor, diode Da, Mosfet Q1, then
diode Dc to Neutral.
When the Live is more negative than
Neutral, current (i2) flows from Neutral
through diode Dd, Mosfet Q1, diode
Db and the fan motor to Live. In both
cases, the current through Mosfet Q1
is always from its drain to its source
and never in the reverse direction, so
the current never flows through the
body diode.
Full circuit description
The circuit for the Fan Speed Controller is shown in Fig.2. It comprises
just one IC, several diodes, the high
voltage Mosfet, Q1, plus some resistors and capacitors. Power for the circuit is derived directly from the 230V
AC mains.
The entire circuit floats at mains
potential, including circuit ground,
which is not connected to mains Earth.
The critical part of the circuit comprises potentiometer VR1b, op amp
IC1a and Mosfet Q1. This part of the
Practical Electronics | April | 2025
circuit allows the user to adjust the average voltage across Mosfet Q1 using
potentiometer VR1b. As VR1b is rotated clockwise, the voltage applied to
pin 2 of IC1a reduces. IC1a reacts by
increasing the gate voltage of Mosfet
Q1 to reduce the average voltage across
its channel.
That might seem backward, rotating clockwise to reduce the voltage.
However, Q1 is in series with the fan
motor, so the fan gets more voltage
when the voltage between Q1’s drain
and source is lower.
So when VR1b is fully anticlockwise, the average voltage across Q1 is
at a maximum, and the applied voltage to the fan is at a minimum. As
VR1b is rotated clockwise, the voltage
across Q1 decreases, and the voltage
applied to the fan increases, allowing
it to speed up.
At the same time, IC1b monitors
the current through Q1 and provides
current limiting to prevent excessive
current flow that could overheat and
damage Q1. That usually should not
happen, but it depends on what is
plugged into the outlet. Perhaps someone will plug in a fan that’s too large
or a different load, in which case IC1b
will activate to protect Q1.
In more detail
Op amp IC1a, which drives the gate
of Mosfet Q1, is connected in a feedback control loop that monitors a divided version of the voltage across Q1’s
channel (drain to source) and the voltage from the wiper of speed potentiometer VR1b. IC1a adjusts its output
voltage so the divided Mosfet channel
voltage matches that set by the speed
potentiometer.
The divider is formed by a 220kW
1W resistor and a 5.1kW 1/4W resistor.
The voltage from this divider is filtered
with a 10μF capacitor, providing a DC
voltage proportional to the average of
the full-wave rectified voltage.
The resistive divider is there to produce a voltage suitable for monitoring by IC1a. When monitoring up to
230V AC (325V DC peak), the divider
output is around 7.4V peak that averages to 4.7VDC after filtering. This average voltage is 63.7% of the waveform
peak voltage and well within the input
range for IC1a when powered from a
15V supply.
As the resistance of Q1 decreases
and the fan speeds up, there is more
voltage across the fan motor and less
voltage across the Mosfet. The voltage from the divider therefore also
reduces.
The Mosfet source also has a 1W
series resistor that connects it to circuit ground for current monitoring.
This increases the voltage applied to
the divider by about 1V, depending on
the fan motor current, but this does
not affect the output from the voltage
divider much. That’s because 1V is a
small fraction of the hundreds of volts
that can be across the Mosfet.
59
Constructional Project
GPO
TO FAN
N
E
TH1 50°C
L
L
q
NC
1
2
3
5
4
6
C O N1
N
E
FUSED IEC MAINS
CONNECTOR
PW04
BR1
PW04
S1
D1
1N4004
~
K
NEON
~
+
–
22kW
5W
G
+15V
K
470mF
25V
A
Z D1
15V
100nF
1W
VR 1 b
10kW
10kW
D
2
1kW
3
1 kW
1
IC1a
150W
G
4
1 MW
5
10mF
SC
S
S
220kW
1W
6
IC1b
7
A
K
1N4148
A
100nF
FAN SPEED CONTROLLER
10mF
5.1kW
5W
K
ZD1
A
K
1N4004
Ó2024
Q1
FQP11N40C
1W
D2 1N4148
8
1MW
3.3kW
D
5.1kW
200kW
CURRENT
LIMIT
D
1kW
+15V
VR1a
10kW
~
10mF NP
IC1: LM358
S PEED
~
+
Q1
~
22kW
–
~
+
A
–
1A
A
K
ALL COMPONENTS AND WIRING IN THIS
CIRCUIT OPERATE AT MAINS POTENTIAL.
DO NOT OPERATE WITH CASE OPEN –
ANY CONTACT COULD BE FATAL!
Fig.2: the circuit diagram for the Fan Speed Controller Mk2. Op amp IC1a controls the resistance of Mosfet Q1 to
regulate the fan speed while op amp IC1b prevents the fan from drawing too much current.
Potentiometer VR1b is used to set
the fan speed. It connects in series between a 22kW resistor from the +15V
supply and a 150W resistor to the 0V
supply. With this resistor string, the
voltage range for VR1a’s wiper is 5V
to 0.07V. The lower voltage for VR1b
is deliberately made to be slightly
above 0V as IC1b would oscillate if it
were set to 0V.
Another reason for keeping the lower
limit at 70mV is to avoid the Mosfet
operating outside its safe operating
area, but more about that later.
If VR1b is set to produce 2V DC at its
wiper, IC1a adjusts its drive to the gate
of Q1 so that the voltage monitored at
the resistive divider junction is also 2V
DC. 2V on the divider means that there
is 88V average across Q1, equivalent
to 97.5V RMS. If the mains voltage is
230V AC, the voltage across the fan
is 230V minus 97.5V or 132.5V RMS.
60
The feedback control ensures that
the voltage across the Mosfet is strictly
maintained to prevent changes in the
motor speed. Without the feedback
control, just applying a fixed voltage
to the gate of Q1, the fan would slow
quite markedly as the Mosfet heats
up. That’s because the Mosfet drainto-source resistance increases with
temperature.
Apart from adjusting the speed control (VR1b), the only other factor that
can alter the fan speed is if the mains
voltage changes. Typically, the mains
voltage is reasonably constant, fluctuating by less than 5%.
Current limiting
Current limiting for this circuit is
necessary since we are operating the
Mosfet in a linear mode for speed control. Linear operation has the Mosfet
operating in a region of partial conduc-
tion where it is neither fully conducting (with minimal on-resistance) nor
fully off. This differs from a switching circuit where the Mosfet is either
fully on or off.
Linear operation sees the Mosfet
dissipating significant amounts of
power, so the Mosfet must be kept
within the safe region of its drain current (Id) versus drain to source voltage (Vds) over the entire voltage range.
The manufacturer’s safe operating area
(SOA) graph for the Mosfet shows the
region of operation.
Fig.3 shows the DC SOA curves
for three different Mosfets that can
be used in this circuit. SOA graphs
also show the pulsed region of operation, but since we are not switching
the Mosfet on and off, we have only
included the DC SOA curves. These
keep the Mosfet semiconductor junction below 150°C.
Practical Electronics | April | 2025
Fan Speed Controller Mk2
For each Mosfet to be used safely, we
need to keep the curve in the operating
region below the DC SOA curve. If the
Mosfet is operated above the curve, it
will likely fail due to melting (maybe
not immediately, but eventually).
The red line indicates our circuit’s
current limit to safeguard the Mosfet.
We restrict the maximum current to
around 1A up to about 20V Vds. Up
to 20V, the fan will run fast. The Vds
will be higher at lower fan speed settings, so we reduce the current limit
to prevent it from encroaching on the
SOA curve. For the slowest speeds
and highest Vds, the current is limited
to around 230mA.
That does not mean the Mosfet will
be operating near this curve. It is just
an overload threshold where the Mosfet
is protected from damage, should conditions cause the Mosfet operating
point to otherwise go above the current limit curve.
IC1b monitors the voltage across the
1W 5W resistor in series with Q1 for
current limiting. This resistor converts
the fan current to a voltage; eg, at 1A,
it has 1V across it.
IC1b is connected as an amplifier
with a level shift due to VR1a. As the
voltage across the 1W resistor exceeds
the voltage at the wiper of VR1a, IC1b’s
output goes high and drives the pin 2
input of IC1a high via diode D1 and the
1kW series resistor. This overrides the
motor speed setting of VR1b, slowing
the fan speed to reduce the current.
If the voltage across the 1W resistor is less than the voltage set at the
wiper of VR1a, IC1b’s output is low
and has no effect on IC1a, as D2 is
reverse-biased.
VR1a is connected across the 15V
supply similarly to VR1b, but the
padder resistors have different values.
The 200kW and 3.3kW resistors set
VR1a’s wiper range to 235-940mV.
VR1a and VR1b share the same shaft,
so adjusting the fan speed will automatically adjust the current limit.
Note that VR1b’s wiper produces a
lower voltage as the potentiometer is
rotated clockwise, while VR1a’s wiper
voltage increases as it is rotated clockwise. That’s so that the current limit is
higher for faster fan speeds.
Power supply
Mains power is applied to the controller via fuse F1, which is within the
IEC input connector. This protects the
circuit against excessive current flow
Practical Electronics | April | 2025
Fig.3: the DC SOA (safe
operating area) for three
different Mosfets. The
current limiting curve is
well within all three. Two
of the curves are limited
by the minimum Mosfet
on-resistance at lower
voltages, so even if the red
line was extended to lower
Vds values, it couldn’t
cross them.
should a fault occur, such as a broken
wire short-circuiting against the enclosure. Both power switch S1 and
over-temperature switch TH1 must be
closed for the Live mains conductor
to be connected.
Switch S1 includes a Neon indicator
that lights when the unit is switched on.
TH1 monitors the temperature of
the fan speed controller enclosure
and switches off power if it reaches
50°C. It will reconnect power once
the temperature drops to 45°C. This
5°C temperature hysteresis prevents
the controller from switching on and
off rapidly since it will take
some time to cool by 5°C.
The Neon indicator within
S1 will be unlit whenever
TH1 is open.
The AC terminals of
bridge rectifier BR1 connect between the Neutral of
the incoming mains supply
and the Neutral of the general purpose outlet (GPO)
for the fan motor. When the
fan is connected, it is connected to mains Live via
the GPO from switch S1.
BR1 is a 6A, 400V
bridge rectifier. As
mentioned earlier,
the bridge keeps
the polarity of the
voltage applied to
the Mosfet consistent while the
fan motor receives
AC.
A 15V supply
to power the remainder of the circuit
is obtained using a 22kW dropping
resistor via diode D1 directly from
the 230V AC mains switched Live. A
470μF capacitor filters the rectified
waveform to produce a smoothed DC
voltage clamped to 15V by zener diode
ZD1. This 15V supply powers dual op
amp IC1, Mosfet Q1 and the associated
diodes, resistors and capacitors.
Using an X2 capacitor instead of a
22kW resistor would be slightly more
efficient, like the previous design from
the May 2015 issue.
However, the capacitor is somewhat
Make sure to use
plenty of cable ties
to secure the wiring,
and heatshrink at the ends.
61
Constructional Project
Fig.4: the overlay
diagram for the Fan
Speed Controller.
expensive and bulky, and requires
other support components like a
second bridge rectifier. We decided
it was not worth the size, expense
or complexity for a slight increase in
efficiency.
Enclosure & mounting options
Fig.5: the
cutting and
drilling guide
for the diecast
aluminium
case.
Depending
on the
application, the
potentiometer
can project
from the side
of the case or
the lid, so read
the text before
making any
holes. The red
circle shows
the hole for
the shaft when
mounting the
pot on the lid
(which is only
possible if the
GPO is not
used).
While the
prototype has
an Australian
mains outlet,
this cut-out has
been adjusted
to suit the UK
socket specified
in the parts list.
Both countries
use a nominal
230V AC so the
socket is the
only change
required.
ALL DIMENSIONS IN MILLIMETRES
15
15
4.0mm diameter
4.5mm diameter
ALTERNATIVE
9.5mm HOLE IF
SPEED CONTROL
IS MOUNTED
ON LID
CUTOUT FOR
PANEL-MOUNTING
GPO IF MOUNTED
ON LID
30
13
28
30
50
43
16
34
4.5mm diameter
LID
END
16
Mounting holes
3mm diameter
20
31
4.0mm
diameter
1 7 .5
25
15.5
27
30
36
62
CL
Three different diecast aluminium
enclosures can be used to house the
Fan Speed Controller: an IP65/IP66 diecast box measuring 115 × 90 × 55mm
(Gainta BS25MF, available from TME)
or an economy diecast box measuring 119 × 94 × 57mm (Bud Industries
CU-234), which is stocked by Farnell,
DigiKey and Mouser.
The PCB is shaped so that it fits
within the contours of the Gainta
case, allowing it to be mounted horizontally on the enclosure’s integral
lands. For the Bud Industries enclosure, there are minimal internal
contours to avoid but also no integral PCB-mounting lands, so the PCB
needs to mount using four 9mm Nylon
standoffs, attached via holes drilled
in the base.
The Fan Speed Controller can be built
as a standalone controller that plugs
into a mains socket for power and has
a general purpose outlet (GPO) that the
fan plugs into. This version is suitable
for pedestal and box fans.
For ceiling fans, the Fan Speed Controller can be built to intercept the
fan wiring at the wall switch. In this
case, it will need to be installed by a
licensed electrician. The speed control adjustment potentiometer can be
placed at one end of the enclosure, like
the standalone version, or on the lid,
which may be more convenient if the
enclosure is wall-mounted.
Construction
The Fan Speed Controller is built on
a PCB coded 10104241 that measures
94 × 79mm. To assemble it, follow the
overlay diagram, Fig.4. Begin by soldering in the resistors, using the table
for the colour codes in the parts list,
but leave the 5W resistors off for the
moment. Diodes D1, D2 and ZD1 can
be fitted next, taking care to orientate
them correctly (and don’t get the three
different types mixed up).
You can use an IC socket for IC1,
or it can be directly soldered in. The
latter should give better long-term reliability. Either way, be sure to install
the socket and the IC correctly, with
the notch facing the direction shown
Practical Electronics | April | 2025
Fan Speed Controller Mk2
on the overlay. Then mount the two
5W resistors, slightly raised from the
PCB surface, to aid in cooling.
Install the capacitors next. The 100nF
capacitor may be labelled as 104. The
electrolytic capacitors have their value
directly marked and must be orientated correctly, with the longer leads
through the holes marked with a +
symbol. However, the larger 10μF capacitor is non-polarised (NP) and can
be mounted either way around.
Fit diode bridge BR1 now, taking care
that the cut corner is towards the top
left of the board and placed adjacent
to the + symbol.
Before installing VR1, its shaft may
need to be cut to length to suit its knob.
Do not install the potentiometer on
the board if it is to be mounted on the
lid. The six-way screw terminal strip
(CON1) can be fitted now.
Q1 is mounted by kinking the outer
two leads outward so that they will fit
into the more widely spaced holes in
the PCB. This wider spacing provides
a 2.54mm clearance between the Q1
mounting pads and prevents possible
arcing between the leads with peak
voltages approaching 400V. Keep the
Mosfet as high as possible above the
PCB, with about 1mm of the leads protruding below the PCB.
Final assembly
The cutting and drilling guide (Fig.5)
should help you to make the required
cutouts in the case. You can download
that as a PDF, along with the panel label
artwork, from our website at https://
pemag.au/Shop/11/6928
Fig.5 shows the locations, sizes and
shapes of the IEC connector and GPO
cutouts, which are suitable for all three
enclosure options.
For the version that mounts on a
wall for controlling ceiling fans, you
don’t need to make the IEC connector
hole or the one for the lid-mounted
GPO. Just fashion the cutouts for the
switch, potentiometer and Earthing
points.
As mentioned earlier, in the wallmount application, the potentiometer
can be mounted either on the PCB for
end-mounted speed adjustment or on
the lid. Regardless, the box must be
Earthed. Access holes to fit grommets
for the wiring can be made in the base
of the box so that the fan wiring can
be concealed in the wall.
For the standalone controller, first
mark the hole position for the IEC
Practical Electronics | April | 2025
Parts List – Fan Speed Controller Mk2
1 double-sided PCB coded 10104241, 94 × 79mm
1 115 × 90 × 55mm Gainta G113/BS25MF IP65/IP66 diecast aluminium box
(recommended) [TME G113 or BS-25MF] OR
1 119 × 94 × 57mm Bud Industries CU-234 economy diecast box [Farnell, Mouser, DigiKey]
1 panel label (see text)
1 10kW dual-gang 24mm PCB-mount linear potentiometer (VR1)
1 plastic knob to suit VR1
1 6-way 15A 300V terminal barrier strip, 8.25mm pin spacing (CON1)
1 SPST 10A 250V AC rocker switch with integrated neon lamp (S1)
1 normally-closed 10A 50°C thermal switch (TH1) [Farnell 1006842]
1 1A 250V AC M205 fuse (F1)
1 8-pin DIL IC socket (optional)
Semiconductors
1 LM358 dual single-supply op amp, DIP-8 (IC1)
1 400V 10A N-channel Mosfet, TO-220 (Q1) [FQP11N40C (Farnell 2453436)]
1 15V 1W zener diode (ZD1) [1N4744]
1 400V 6A PW04 diode bridge rectifier (BR1)
1 1N4004 1A 400V diode (D1)
1 1N4148 200mA 75V signal diode (D2)
Capacitors
1 470μF 25V 105°C PC electrolytic
2 10μF 16V 105°C PC electrolytic
1 10μF 50V 105°C non-polarised (NP) PC electrolytic
2 100nF 63V or 100V MKT polyester
Resistors (all ¼W, 1% axial unless specified)
2 1MW
1 22kW 5W
1 220kW 1W 5%
1 10kW
1 200kW
2 5.1kW
1 22kW
1 3.3kW
3 1kW
1 150W
1 1W 5W 5%
Hardware & cable
1 TO-220 mica insulating washer
1 TO-220 3mm screw hole insulating bush
4 5.3mm ID insulated quick connect crimp eyelets with 4-6mm wire diameter entry
1 200mm length of green/yellow striped 7.5A mains-rated wire
1 200mm length of brown 7.5A main-rated wire
1 200mm length of blue 7.5A mains-rated wire
1 160mm length of 5mm diameter heatshrink tubing
1 20mm length of 20mm diameter heatshrink tubing
2 M4 × 10mm panhead machine screws and hex nuts
2 4mm shakeproof (toothed) washers
3 M3 × 10mm panhead machine screws and hex nuts
3 extra 24mm potentiometer washers
10 100mm cable ties
2 M3.5 × 6mm screws (only for Jaycar HB5042 case)
4 M3 × 9mm Nylon spacers
4 M3 × 6mm panhead machine screws
4 M3 × 6mm countersunk head machine screws
1 small tube of thermal compound
1 2mm-thick piece of scrap aluminium sheet (if required; see text)
Extra parts for the standalone version, for pedestal and box fans
1 13A chassis-mount mains socket [eg, RS 500-0459]
1 fused IEC mains input connector
1 7.5A IEC mains plug lead
2 M3 × 10mm countersunk machine screws and hex nuts
4 small stick-on rubber or felt feet
Extra parts for the wall-mounted version, for ceiling fans
1 M205 10A 250VAC panel-mount safety fuse holder
1-2 grommets or cable glands for input and output wires
1 600mm length of brown 7.5A mains wire (if VR1 is mounted on the lid)
1 120mm length of 5mm diameter heatshrink tubing (if VR1 is mounted on the lid)
63
Constructional Project
Fig.6: the wiring
diagram for the Fan
Speed Controller with
the potentiometer
mounted on the
PCB and its shaft
projecting out the
side of the case.
GPO1
(LID)
S1
EARTH
M4 SCREW,
STAR LOCKWASHER
AND NUT
COVER EXPOSED TERMINALS
WITH HEATSHRINK
INSULATING WASHER
M3 SCREW
INSULATING BUSH
M3 NUT
Q1
EARTH
50 ° C
L
200kW
5.1kW
WASHERS
4148
D2
1k W
VR1
DUAL 10kW LINEAR
10kW
1MW
BR1
–
220kW 1W
22kW 5W
N
~
E
+
PW04
1kW
~
4004
D1
150W
1W 5W
COVER EXPOSED
CONNECTION
WITH SILICON
SEALANT
IEC
CONNECTOR
WITH FUSE
10m F
10 m F
NP
22kW
1MW
connector and Earth screw in the end
wall of the case. The IEC connector
mounts with a gap of about 4mm from
the base of the case to the bottom of
the IEC connector. The hole is made
by drilling a series of small holes
around the perimeter of the desired
shape, knocking out the piece and
filing it to shape.
Alternatively, use a Speedbore drill
64
470mF
ZD1
1W
1kW
10 m F
IC1
LM358
Fan Speed Controller
15V
SILICON CHIP
5.1kW
100nF
100nF
3.3kW
to make a larger round hole to remove
most of the required area, then file that
hole to the required shape. The Earth
screw hole is 4mm in diameter.
A hole is required for the potentiometer at the opposite end of the box.
Measure the height of the potentiometer
shaft above the base of the enclosure
and mark out the drilling position at
the end of the enclosure. Alternative-
ly, for the potentiometer mounted on
the lid, drill the hole in the centre of
the GPO cutout.
Note that the potentiometer can only
be installed on the lid for the ceiling
fan version that doesn’t require the
GPO socket.
Insert the PCB into the case and note
that the leads for Q1 must be kinked
outward from the PCB a little so the
Practical Electronics | April | 2025
Fan Speed Controller Mk2
Fig.7: here's how
to wire up the Fan
Speed Controller
if you're mounting
the potentiometer
on the lid. This
is only practical
for hardwired
installations.
metal flange of the Mosfet sits in intimate contact with the side of the case.
You can then mark the mounting
hole position for Q1’s tab and drill it
to 3mm in diameter. Deburr this hole
on the inside of the case with a countersinking tool or larger drill to round
off the sharp edge of the hole. This is
to prevent punch-through of the insulating washer.
Practical Electronics | April | 2025
TH1 also mounts on the side of the
box adjacent to Q1. There is room in
the Jaycar HB5042 enclosure to mount
TH1 against the side of the enclosure
between two sets of protruding slots
intended for mounting PCBs vertically. The Jaycar HB5064 enclosure does
not have such slots, so there is plenty
of room for mounting TH1.
For the Altronics case, there is in-
sufficient room for TH1 to mount flat
against the side of the enclosure. One
solution is to grind away sufficient
protruding slot material so the thermostat’s body can sit flat.
The alternative is to make up an
aluminium packing piece that’s 19 ×
45 × 2mm. This can sit between the
protruding slots, and the thermostat
can be mounted against that. In this
65
Constructional Project
Fig.8: how to
mount Mosfet
Q1 to the case.
The finished
PCB for the
Fan Speed
Controller.
case, the top mounting hole should
be about 8mm down from the top edge
of the box.
Note that you will find it easier to
install TH1 if the M3 nuts are tack-
soldered to the thermostat mounting
bracket. To do this, place the screws
into the thermostat mounting bracket
(when it is out of the case) and screw
on the nuts, then solder them in place
and remove the screws.
For the standalone version, holes are
also required in the lid for the general
purpose outlet (GPO) mains socket,
the power switch and the Earth terminal. Four PCB mounting holes are
also needed if you are not using the
Jaycar HB5042 enclosure. The PCB is
positioned so the speed potentiometer
can protrude through the hole at the
end of the enclosure.
Labels
Panel labels (see Fig.9) can be downloaded as a PDF from our website using
the earlier link. Details on making
a front panel label can be found at:
https://pemag.au/Help/FrontPanels
The download includes two versions
of the front panel. Which one you use
depends on whether the control pot is
mounted on the lid or is at the end of
the enclosure.
If the potentiometer is PCB-mounted,
its locating lug must be bent backward
or snapped off, as we have not made
a hole for it. Then slip three washers
over the potentiometer shaft, insert it
into the hole in the case by angling
66
the board and drop the PCB onto the
mounting points.
For the Jaycar HB5042 enclosure,
secure the PCB to the case with the
two screws supplied with the case plus
two extra M3.5 × 6mm screws. For the
other enclosures, the PCB is mounted
using M3 × 6mm screws into M3-tapped
standoffs. Secure the PCB-mounted potentiometer by placing another washer
over the shaft on the outside of the case
and doing up the nut on top.
Attach Q1 to the case with an M3
machine screw and nut, with the mica
insulating washer and insulating bush
as per Fig.8. Apply a thin smear of heatsink compound on all mating surfaces
before assembly.
We use the mica washer in preference to silicone since mica has a higher
thermal conductivity (lower °C per
watt value), and the mounting screw
can be tightened more. That keeps the
Mosfet cooler compared to using a silicone washer.
After mounting Q1, check that the
metal tab of the device is isolated from
the case by measuring the resistance
between them with a multimeter. The
meter should show a very high resistance measurement (several megohms
or possibly “0L”) between the enclosure and Mosfet tab or the enclosure
and any of Q1’s leads. Check that it also
reads close to 0W between the enclosure and the mounting screw.
The complete wiring diagrams for
the two versions are shown in Figs.6
& 7. The Earthing details of the case
are most important since Q1 and the
potentiometer are all at mains potential, yet they are attached to the case.
If the insulating washer or the insulation of the potentiometer were to
break down, the case would be live
(at 230V AC) if it was not properly
Earthed.
The case lid must be independently
Earthed rather than relying on the lid
making contact with the base of the
enclosure.
All mains wiring must be done using
7.5A minimum mains-rated (230V
AC) wire. The IEC connector must be
wired using the correct wire colours:
brown for Live, blue for Neutral and
We used an
aluminium
packing piece
between the
thermal cutout
and the case
rather than
grinding the
rails down.
Note the
soldered nut
highlighted
in yellow.
Practical Electronics | April | 2025
Fan Speed Controller Mk2
green/yellow striped for the Earth.
Live and Neutral wires soldered to
the IEC connector must be insulated
with heatshrink tubing covering all
exposed metal.
Solder the Earth wire to the IEC
connector Earth pin, ensuring the
Earth terminal is heated sufficiently
so that the solder wets and adheres
properly to both the Earth terminal
and wire. After that, use a crimping
tool to secure the Earth wire into the
crimp eyelet. Make sure the wire is
securely crimped and can’t be pulled
out. The Earth wires from the Earth
point to the lid and the GPO are also
terminated with crimped eyelets.
Secure the Earthing eyelets with M4
machine screws, star washers and nuts.
A second nut should be tightened on
top of the first as a lock nut. The IEC
connector is secured to the case by
10mm M3 countersunk head screws
and nuts.
Finally, attach cable ties to hold the
wire bundles together as shown in the
wiring diagrams and the earlier photo
of the fully assembled unit. Remember to place the four rubber feet on the
bottom of the case.
Testing
As the whole circuit floats at mains
potential, everything on the board
should be considered unsafe to touch
whenever the circuit is connected to
the mains. That means the IEC mains
power lead must be unplugged every
time before opening the lid. Do not
be tempted to operate the fan speed
controller without the lid in place and
screwed in position.
Before you power up the device, set
VR1 fully anticlockwise. Also check all
of your wiring very carefully against
the overlay and wiring diagram. Verify
that the case, lid and potentiometer
are connected to the Earth pin of the
power socket using a multimeter on
its low ohms range.
If you are satisfied that all is correct, you are ready to screw the lid
onto the case.
Note that the IP65 and IP66 enclosures are supplied with a rubber seal
that goes between the enclosure base
and lid. We did not use that seal so
that heat from the case can transfer to
the lid more efficiently for better dissipation.
The easiest way to test the circuit
operation is to connect a fan. Apply
power and check that you can vary
its speed with VR1. Note that the fan
controller box will begin to run quite
warm with extended use when drivFig.9: this label
is for the Speed
Controller with
potentiometer
on the lid. The
other smaller
label is only used
if mounting the
pot to the end
of the case. All
labels (including
the alternative
lid label) are
available to
download from
https://pemag.au/
Shop/11/6928
Practical Electronics | April | 2025
ing the fan at intermediate speeds.
This temperature rise is normal. The
temperature rise should be lower if the
fan is set to a low speed.
Troubleshooting
If the speed controller does not work
when you apply power, it’s time to do
some troubleshooting. First, a reminder: all of the circuitry is at 230V AC
mains potential and can be lethal. That
includes any exposed metal parts on
components, except those tied to the
Earthed case. Do not touch any part
of the circuit when it is plugged into
a mains outlet.
Before going any further, give your
PCB another thorough check. Check
for incorrectly placed components,
incorrect component orientation or
bad solder joints (dry joints, missed
joints or bridges).
Optional heatsink
If the Fan Speed Controller works
but cycles on and off due to the thermal cutout activating, a fan heatsink
can be attached to the side of the enclosure where Q1 is mounted using
M3 screws and nuts. A 105 × 25.5
× 55mm fan-type heatsink is recommended in this role.
The mounting holes are placed along
the centre line of the heatsink. The
lower hole should be positioned high
enough not to foul the PCB when the
nut is on.
The heatsink is positioned with its
lower edge at the same level as the
bottom edge of the box.
The heatsink should be counter-
bored at the Q1 and TH1 mounting
screw positions. You can find where
these screws are located by temporarily securing the heatsink onto
the side of the case with the two M4
screws, with a thin layer of Blu-tack
pressed onto the heatsink in each
screw area. When the heatsink is removed, there will be an impression
of the screw heads.
Drill out those two locations to a
shallow depth using a larger
drill to allow for the screw
heads to sit inside the heatsink. Mount it with a smear
of heatsink compound over
the mating surfaces.
As an alternative, if countersunk screws are used for
TH1 and Q1, there will be
less counter-boring required
on the heatsink.
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