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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 that power is dissipated as heat. But Fig.1: AC is applied to the motor but the diode bridge ensures that Mosfet Q1 only sees DC. 70 Silicon Chip Australia's electronics magazine 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. To prevent reverse current flow siliconchip.com.au 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. 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 Active voltage is more positive than the Neutral, current (i1) flows from Active through the motor, diode Da, Mosfet Q1, then diode Dc to Neutral. When the Active is more negative than Neutral, current (i2) flows from Neutral through diode Dd, Mosfet Q1, diode Db and the fan motor to Active. 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 siliconchip.com.au 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 Australia's electronics magazine 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. May 2024  71 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. 72 Silicon Chip 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 Australia's electronics magazine partial conduction 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. For each Mosfet to be used safely, we siliconchip.com.au 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 D2 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 should a fault occur, such as a broken wire short-circuiting against the siliconchip.com.au 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. enclosure. Both power switch S1 and over-temperature switch TH1 must be closed for the Active 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 Active 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 Active. 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 2014 issue (siliconchip.au/ Article/7595). However, the capacitor Make sure to use plenty of cable ties to secure the wiring, and heatshrink at the ends. Australia's electronics magazine May 2024  73 Fig.4: the overlay diagram for the Fan Speed Controller. is somewhat 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). Three different diecast aluminium enclosures can be used to house the Fan Speed Controller: an IP65 diecast box measuring 115 × 90 × 55mm (Jaycar HB5042), an IP66 diecast box measuring 114 × 90 × 55mm (Altronics H0423) or an economy diecast box measuring 119 × 94 × 57mm (Jaycar HB5064). The PCB is shaped so that it fits within the contours of the Jaycar HB5042, allowing it to be mounted horizontally on the enclosure’s integral lands. For the other two enclosures, 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 74 Silicon Chip Australia's electronics magazine siliconchip.com.au 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 siliconchip.au/Shop/19/6928 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 siliconchip.com.au Parts List – Fan Speed Controller Mk2 1 double-sided PCB coded 10104241, 94 × 79mm 1 115 × 90 × 55mm IP65 diecast box [Jaycar HB5042] OR 1 114 × 90 × 55mm IP66 diecast box [Altronics H0423] OR 1 119 × 94 × 57mm economy diecast box [Jaycar HB5064] 1 panel label (see text) 1 10kW dual-gang 24mm PCB-mount linear potentiometer (VR1) [Jaycar RP3510] 1 plastic knob to suit VR1 1 6-way 15A 300V terminal barrier strip, 8.25mm pin spacing (CON1) [Altronics P2106] 1 SPST 10A 250V AC rocker switch with integrated neon lamp (S1) [Altronics S3228] 1 normally-closed 10A 50°C thermal switch (TH1) [element14 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 (element14 2453436), AOT10N60 (SC4571) or IRF740 (Altronics Z1539)] 1 15V 1W zener diode (ZD1) [1N4744] 1 400V 6A PW04 diode bridge rectifier (BR1) [Altronics Z0082] 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 [element14 1306258] 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 [Altronics H1825A, Jaycar PT4714] 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 [Jaycar RP3500] 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) ● only for Altronics H0423 or Jaycar HB5064 case Extra parts for the standalone version, for pedestal and box fans 1 surface-mount GPO side-entry mains socket (GPO1) [Altronics P8241, Jaycar PS4094] 1 fused IEC mains input connector [Altronics P8324, Jaycar PP4004] 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 [Altronics S5992, Jaycar SZ2028] 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) Australia's electronics magazine May 2024  75 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. 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 76 Silicon Chip 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. Australia's electronics magazine Alternatively, 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 siliconchip.com.au 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. siliconchip.com.au 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. Australia's electronics magazine For the Altronics case, there is insufficient 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 May 2024  77 Fig.8: how to mount Mosfet Q1 to the case. The finished PCB for the Fan Speed Controller. can be mounted against that. In this 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: siliconchip.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 78 Silicon Chip into the hole in the case by angling 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: 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. Australia's electronics magazine siliconchip.com.au brown for Active, blue for Neutral and green/yellow striped for the Earth. Active 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. 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 driving the fan at intermediate speeds. This Fig.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 siliconchip.au/ Shop/19/6928 siliconchip.com.au Australia's electronics magazine 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. The recommended 105 × 25.5 × 55mm fan-type heatsink is available from Altronics (Cat H0520) or Jaycar (Cat HH8570). 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. SC May 2024  79