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IMPROVED UNIVE MOTOR SPEED CO This latest speed controller can be used with power tools and appliances rated up to 10A. It is suitable for use with brush-type universal motors such as those in small lathes, electric drills, grass trimmers, circular saws, routers, nibblers and jigsaws. By John Clarke A Speed Controller published in October Because this speed controller does lthough there are countless 2002. It is housed a larger diecast case, not apply full power to the motor at battery-powered and 230VAC not only making it easier to build but any of its settings, it cannot provide power tools with inbuilt speed also providing for the increased heat speed control up to full speed. That controls these days, there is still a need dissipation which comes from its is why we have incorporated a 10A for a stand-alone speed controller. uprating to suit 10A universal motors. bypass switch, to enable full speed Apart from power tools, many appliwithout unplugging the appliance ances need to have their motor speed Speed control range from the speed control. reduced. The speed controller will enable Some power tools and appliances In fact, we are constantly being suryou to set the motor speed over a wide don’t run smoothly at very low speeds prised by the range of uses that readers range, from about 80% of full speed when run from this type of phase have for this type of speed control. at low loads, down to a very slow control circuit. They sometimes disApart from drills and circular saws, rate, depending on the motor and its play a behaviour known as “cogging” a speed control is particularly useful gearing. whereby they run in short bursts. for controlling routers and jigsaws So the practical miniwhen cutting materials such as plastics mum speed for any apthat will melt pliance motor depends when cutting at WARNING! VAC mains 230 the from ctly on its freedom from high speed. dire s rate ope uit (1) This Speed Controller circ know exactly what you ss unle cogging. This will deOther items it d buil not Do al. leth lly supply and is potentia CIRCUIT WHILE IT IS THE pend on the design of that can benefit OF T PAR ANY CH TOU NOT DO you are doing. its ide outs uit circ the rate the particular motor but from speed conope not do PLUGGED INTO A MAINS OUTLET and . case the onto in general we can state trol are hobby wed scre lid the earthed metal case or without that the cheaper the aplathes that use motors or shaded pole pliance, the less likely sewing machine ction indu with use for ble suita not is (2) This circuit d”. rolle cont it will run smoothly at motors, food mixbe can ors motors used in fans – see “What mot very low speeds. ers when the intype” (series wound) sh “bru l Another factor that built speed conersa univ with used be (3) This circuit must only limits the minimum trol has failed and . 10A to up gs ratin te epla motors with nam speed at which an apgrass trimmers pliance can be run is that constantly be operated at low speeds for long Power tools with inbuilt fans must not (4) age. dam r that most universal break the Nylon suffe and heat over may periods, otherwise they motors have an inbuilt line when used at tric elec or ps lam to er fan for cooling. full speed. pow the rol cont (5) This circuit must not be used to Energy NSW the of ns Below certain speeds The circuit is latio regu the ne rave radiators. To do so would cont that fan is largely inefa revised version es. oriti auth gy ener Authority and other state fective, so there is no of the popular 5A 40  Silicon Chip siliconchip.com.au ERSAL ONTROLLER (MkII) Our new Universal Motor Speed Controller, shown here with a typical application – an older power drill which doesn’t have its own speed controller – will handle nameplate ratings up to 10A and has good low-end performance. cooling at all. This should be considered if you want to use an electric drill as a power screwdriver with this control. By all means, use it as a screwdriver but only for short periods – or run the risk of overheating and burning out the motor. At mid settings of the speed control, the circuit gives good speed regulation. This means that the circuit slightly increases the applied voltage to com- pensate if the motor is loaded down. Basic circuit operation The speed controller circuit is very similar in principle to the simple SCR speed controls developed in the past, 10A FUSE A A 10k 5W SCR D2 230V AC INPUT SPEED A 4.7 F 630V VR1 2k K A G K A SBS1 2N4992 G K 47nF VR2 10k 1k N Fig.1: this simple SCR controller, used extenFIG.1 sively in the past, has a number of drawbacks, including poor low-speed performance. N A1 GPO N A 1k A MOTOR TRIAC BTA41-600D E D1 1N4004 D1 siliconchip.com.au D2 1N4004 VR1 10k LIN A K A2 150k 1W K D3 R250M (6A/600V) K A E FIG.2 Fig.2: this more recent design is signficantly better in the performance department but one of its key components, the Silicon Bilateral Switch, is now quite hard to obtain. February 2009  41 This waveform shows the Speed Controller set for maximum output when driving a 1kW resistive load. Note that the waveform is essentially a half-wave rectified sinewave with an RMS value of 161V (70% of 230V) and a peak value of 341V. The same waveform superimposed on the 230VAC 50Hz input (blue). Notice that there is a small voltage loss across the Triac. The “flat-topping of both waveforms is due to fluorescent and gas discharge lamps and switchmode power supplies. early in each AC half cycle, the power an SCR except that it conducts for both such as that shown in Fig.1. fed to the motor will be relatively high. positive and negative cycles of the AC It is based around an SCR (Silicon Conversely, if the SCR is turned on waveform. This would provide the full Controller Rectifier). When conductlate in each AC half cycle the power range in speed control. ing, an SCR is just like a diode; curfed to the motor will be relatively low In fact, we are using a Triac in our rerent can flow in one direction but not and hence the motor will run slowly. vised circuit but it still only provides the other. The difference between an The trigger voltage for the SCR conduction during one half cycle. The SCR and a diode is that the SCR will comes from VR1, a 2kΩ potentiometer reason why we do not provide fullblock current in both directions unconnected in series with a 10kΩ resiswave control (ie, conduction on both less driven into conduction with a tor and diode D1. The potentiometer positive and negative half cycles) is gate signal. is fed with half-wave rectified AC that speed regulation would be lost. Once it starts conducting, current that is partly smoothed by the 4.7μF The simple SCR circuit (and our will flow from anode (A) to cathode capacitor across it. The resulting ramp revised version) gives speed regula(K) and it will stay conducting until voltage from the wiper is fed to the tion by monitoring the back-EMF from the load current drops to zero. The gate of the SCR via diode D2. the motor. Back-EMF is the voltage circuit must be used with AC voltage developed by a motor that opposes or half-wave pulsed DC for the SCR to Speed regulation the current through it. It is directly be switched off. If the gate is triggered Now you might be thinking we proportional to speed, so at high motor then the SCR will again conduct on should use a Triac. This is similar to speeds the back- EMF will be higher. the next voltage cycle. Without the This circuit monigate triggered the SCR will remain off. tors the back-EMF in Because the SCR is the following way. a switching device, it What motors can be controlled? appliances use ed power tools and small One side of the motor and can be used as a very efr tato mu com a Virtually all mains operat h wit ” motors is connected directly re ficient power controller, “universal” motors. These are “series wound atu arm to the fact that the motor to the SCR’s cathode run carrying large amounts brushes. The “series wound” term refers be to tor and this allows the mo while the other side of current while itself and field windings are connected in series l”. is connected to the do dissipating relatively from AC or DC, hence the term “universa how So . ller tro con d with this speed use be not st mu cathode of diode D1 s ion little power. tor uct mo ind Induction versal motor and not an and to the mains The circuit of Fig.1 you make sure that your appliance is a uni Neutral wire. s controls the AC power motor? she bru has tor the mo can easily determine that This means that to the motor by triggering tles set t tha In many power tools you and s she from the bru the gate-to-cathode the the SCR into conduction and a commutator – you can see sparks m fro s, you can also get a clue voltage applied to at some point in each the matter. But if you can’t see the brushe the SCR is the difpositive half-cycle of the nameplate or the instruction booklet. uction motors used induction motor? Most ind ference between the 230VAC waveform. The rs, ste bla ter So how do you identify an wa , nes, fridges d wiper voltage from fixe SCR does not conduct at in domestic appliances (eg, washing machi a at e 4-pole and always operat VR1 and the backole all during the negative AC swimming pool pumps) will be 2-pole or 4-p a for 1440 RPM 0 RPM for a 2-pole unit or 285 is EMF generated by this half cycles. lly ica Typ ed. spe induction motors. the motor, if we If the SCR is turned on unit. Bench grinders typically use 2-poIe 42  Silicon Chip siliconchip.com.au This waveform shows the Speed Controller set for maximum output when driving an electric drill. Notice that there is considerable hash at the beginning of each positive half-cycle, caused by interaction between the drill’s commutator and the Triac. ignore the voltage drop across diode D2. Actually, in so-called universal motors (AC/DC series motors with commutators and brushes), there are two back-EMFs. The first is a function of motor speed and the remanent magnetism of the field coils. It is generated during the time when the SCR is not conducting, ie, during the negative half cycles of the AC waveform and during the first portion of the positive half cycles before the SCR conducts. The second back-EMF is generated during the time when the SCR is conducting and there will now be current flowing in the field coils (and also in the armature). This back-EMF will be higher than the first. Now set for a lower speed from the electric drill, the Triac is on for a shorter time and the RMS value of the waveform is considerably reduced to 45V. Note the frequency error which is caused by hash on the waveform and the fact that the Triac triggering is more erratic. We are only concerned with the back-EMF generated while the SCR is not conducting since it is this voltage which determines how late or early in each positive half cycle that the SCR begins conduction. In our circuit, the back-EMF from the motor applies negative feedback to the gate of the SCR. Say a particular motor speed is set by VR1and then the motor speed tends to drop because of an increase in loading. This reduces the motor back-EMF and therefore increases the voltage at the gate the SCR. More correctly, it means that the trigger voltage for the SCR gate will exceed the voltage at the SCR cathode earlier in the positive half-cycle and hence more power will be applied to the motor. This will tend to correct the drop in motor speed. Speed regulation is not perfect but it’s better than having no speed regulation at all. Better circuit The basic circuit of Fig.1 has a number of drawbacks. First, the total power dissipation through the 10kΩ resistor is about 2W which means that it gets rather hot. Second, even though the current through the 10kΩ resistor and VR1 is relatively high, it is not sufficient for reliable triggering of higher power SCRs. And third, the circuit is not particularly good at very low speed settings. A much better circuit is shown in Fig.2 which was published in SILICON CHIP in September 1992. Instead of End-shots of the Speed Controller case showing the fused IEC mains input connector (left), while the shot at right shows the output socket and the controlled/full speed switch. Case and lid drilling details are shown on page 48. siliconchip.com.au February 2009  43 A F1 10A IEC MALE SOCKET 100k 1W SPEED VR1 CONTROL 10k LIN D2 1N4004 A SCR1 BT169D (MCR100) G A1 (BOX) S1 A CONTROLLED N E (BOX) 1k 47nF CAUTION! K A ALL COMPONENTS AND WIRING IN THIS CIRCUIT OPERATE AT MAINS POTENTIAL. CONTACT COULD BE LETHAL. K D3 STTH3012W (30A/1200V) D1 1N4004 A BT169D 1N4004 STTH3012W G K A SC 10A  TRIAC1 BTA41-600BRG (40A/600V) GPO 10A FULL SPEED 2.2k A N 2009 G 100 K A 47k VR2 10k E K A2 K UNIVERSAL MOTOR SPEED CONTROLLER K A BTA41-600BRG K A1 A2 G Fig.3: here’s our new Universal Motor Speed Controller which has good low-end speed performance and speed regulation but is based on components that are easy to obtain. As a bonus, its rating has doubled to a nameplate rating of 10A. an SCR, it uses a Triac and instead of feeding the gate directly from VR1 we have used a trigger circuit consisting of a silicon bilateral switch (SBS1) and a 47nF capacitor. As mentioned above, while the Triac is capable of conducting on both positive and negative half-cycles of the 230V AC 50Hz waveform, this circuit only enables it to trigger on positive half cycles, because of the rectifier action of diode Dl. The SBS is a voltage break-over device and at voltages below its breakover point it is essentially open circuit but once the break-over voltage is reached, it conducts. The 47nF capacitor charges up from VR1 via diode D2 until it reaches the break-over voltage of about 8V. At this point the SBS dumps the capacitor’s charge into the Triac’s gate to trigger it into conduction and the cycle repeats for the next positive half cycle of the mains AC waveform. The energy stored in the capacitor is quite enough to trigger even insensitive Triacs; hence we were able to use a high power 40A device in this circuit. In this circuit, the motor back-EMF acts to reduce the charging voltage to the 47nF capacitor rather than reducing the SCR gate voltage as in Fig.1. Although the circuit arrangement is a little different, the speed regulation is just as good as Fig.1. The circuit efficiency is improved as well, with only 200mW being dissipated in the 150kΩ resistor that feeds VR1. This resistor has a rating of 1W to ensure 44  Silicon Chip that it has an adequate voltage rating to withstand the full 230VAC. The functions of the three diodes in the circuit need to be explained. Diode D1 is there to reduce the power dissipation of the series resistor string and to ensure half-wave operation of the circuit. D2 is there to protect the gate of the Triac when it is in the conducting state – terminal A1 can be above the potential of the gate. Diode D3 has been included as a flyback diode to quench the large inductive spike generated by the motor at the end of each positive half cycle. While the voltage spike does not cause any damage to the circuit, it does have the effect of disrupting the back-EMF monitoring system described above. Trimpot VR2, connected in series with VR1, is there to provide a minimum speed setting for the circuit. One question we have not answered so far is why we specified a high current Triac instead of an equivalently rated SCR. The reason is quite simple. The Triac is half the price of an equivalent SCR. The 600V 40A rating is so that it can withstand the “locked rotor” current of any power tool with a nameplate rating of up to 10A. Note that a “locked rotor” condition – eg, when a drill or power saw jams or stalls in the work – will probably blow the 10A fuse but the 40A Triac should not be damaged. Another reason for using the 600V 40A Triac is that it is an isolated tab device. This means that it can be attached directly to the metal case without any need for a mica washer or other means of insulation. Revised circuit The circuit of our new 10A Speed Controller is shown in Fig.3. As already mentioned, this is a revised version of the design we featured in the September & November 1992 issues and later in the October 2002 issue. Our new circuit replaces the SBS with a sensitive-gate SCR (SCR1) and this provides the same capacitor dump function as the SBS. The SCR was chosen instead of the SBS because the SBS is now difficult to obtain. The 47kΩ and 2.2kΩ resistors form a voltage divider between the anode and cathode of the SCR, with the divided voltage applied to the gate. The SCR conducts when the gate voltage reaches 0.6V and is triggered by a mere 200μA of gate current. Because of the resistive divider, the voltage across SCR1 must rise to some 13.4V before the gate reaches the 0.6V sufficient to trigger the SCR. When the SCR fires, the charge on the 47nF capacitor is dumped, via the 100Ω resistor, into the gate of the Triac to fire it. Switch S1 bypasses the Triac so that the motor gets the full 230V AC applied to it. Note that the switch must be a changeover type to select either Active or the Triac A1 output rather than just using a single switch across the Triac. In the latter case, there would be a short circuit when diode siliconchip.com.au VR2 VR2 K K A A N N NEUTRAL OUT SCR1 SCR1 100 100 TOM TNCORC ORTOM TNRO LORLO A1 A1 A2 A2 G G D3 D3 1k 1k 47k47k TO POT TO POT 2.2k 2.2k 40044004 D2D2 D1D1 NEUTRAL N IN NEUTRAL N IN 40044004 100k 1W 100k 1W 47nF 47nF A A TRIAC1 (UNDER BOARD: TRIAC1 SEE BELOW) (UNDER BOARD: SEE BELOW) 1901290012001101 ACTIVEAIN ACTIVEAIN TO FULL/CONTROLLED SWITCH TO FULL/CONTROLLED SWITCH NEUTRAL OUT Fig.4 (top): the PC board component overlay. Note that the Triac mounts under the board, flat side down, with its legs bent up 90° through the board. ANODE LEAD ANODE LEAD SOLDER SOLDER D3 D3 M3 x 10mm SCREW M3 x 10mm SCREW M3 NUT M3 NUT PC BOARD PC BOARD STAR LOCK WASHERS STAR LOCK WASHERSBOX BOX SOLDER SOLDER 6mm 6mm Fig.5 shows the mounting arrangement for both the Triac and the power diode. Only the anode of the diode solders to the PC board. D3 conducts on negative half-cycles of the 240V AC mains. Construction All the components of the 10A Speed Controller are mounted on a PC board coded 10102091 and measuring 79 x 38mm. It is housed in a diecast box measuring 120 x 92 x 57mm. Begin the construction of the 10A Speed Controller by checking the PC board against the published pattern. There should not be any shorts or breaks between tracks. If there are, repair these as necessary. Use the overlay diagram of Fig.4 as a guide when assembling the PC board and Fig. 5 to complete the wiring inside the case. TRIAC1 TRIAC1 M3 NUT M3 NUT M3 x 10mm SCREW M3 x 10mm SCREW Start assembly by soldering in the PC stakes to the external wiring connection points on the PC board (shown as black dots on the overlay). Then insert the resistors, using the table on P47 as a guide to the values. It’s always a good idea to also check their values with a digital multimeter as some colour bands, particularly oranges, browns and reds, can sometimes be mistaken for each other. The 47nF capacitor can be installed next. Neither the resistors nor capacitor are polarised but the diodes certainly are, so when inserting them, take care with their orientation. D1 and D2 mount in the conventional way but D3 is a larger (TO-220 case) type which is mounted quite differently. Parts List - 10A Universal Motor Speed Controller 1 PC board coded 10102091, 79 x 38mm 1 diecast box, 115 x 90 x 57mm (Jaycar HB-5064 or equivalent) 1 panel label, 119 x 56mm 1 flush-mount mains socket (Jaycar PS-4090 or equivalent) 1 IEC male input socket with integral 10A M205 fast blow fuse 1 IEC mains lead (moulded 3-pin plug to IEC socket) 1 DPDT 10A 250V rocker switch (S1) (Jaycar SK-0981 or equivalent) 1 10kΩ linear 24mm potentiometer, 250VAC rated (VR1) 1 knob for potentiometer 2 crimp eyelets or solder lugs for earth connection 4 6mm Nylon spacers 2 M3 x 15mm screws 1 M3 x 10mm screw 1 M3 x 10mm csk head screw 8 M3 x 6mm screws 4 3mm star washers 3 M3 nuts 4 stick-on rubber feet 1 200mm length of blue 10A 250VAC wire 1 200mm length of brown 10A 250VAC wire 7 100mm long cable ties 1 120mm length of 5mm heatshrink tubing 1 40mm length of 8mm heatshrink tubing 7 PC stakes Semiconductors 1 BTA41-600BRG Triac (TRIAC1) 1 BT169D or MCR100 sensitive gate SCR (SCR1 ) 2 1N4004 1A 400V diodes (D1,D2) 1 STTH1512D 15A 1200V diode (D3) Capacitors 1 47nF 63V MKT polyester (code 473 or 47n) The case before the label was applied. Obviously a countersunk-head screw would be a better choice for the lid earth termination. siliconchip.com.au Resistors (0.25W 1%) 1 100kΩ 1W 5% 1 47kΩ 1 2.2kΩ 1 1kΩ 1 100Ω 1 10kΩ horizontal 5mm trimpot (code 103) (VR2) (Jaycar RT4360 or equivalent) February 2009  45 Only the anode (A) lead solders to the PC board; the cathode connection is made using the metal tab to physically connect to the PC board track using a screw, nut and star washer as shown in Fig.5. In fact, you should cut off the cathode (K) lead on the left of the package before mounting it. Make sure you insert SCR1 into the PC board correctly, otherwise the circuit won’t work. Note that in a kit you may be supplied with a BT169D or MCR100 – pinouts and mounting are the same. VR2 can also be installed at this stage. The Triac is mounted on the underside of the PC board with its leads protruding up through the holes in the PC board. Bend the leads up 90° so that the copper side of the PC board is 6mm away from the back of the Triac body, as shown in Fig.5. Short (~3mm) lengths of Triac leads should emerge from the top of the PC board. Bend these back down flat onto the PC board. Putting it together Temporarily place the PC board into the case and mark out the positions for the mounting holes for the four standoffs and for the Triac and the earth lug screw. Check these against Fig.8 – this diagram shows the hole positions for both the PC board and Triac. Because the positioning of the Triac could easily be a couple of millimetres different to our prototype, it is probably best to mark and drill out the four corner holes for the PC board first. Then position the PC board within the box and mark out the hole position for the Triac. The hole for the Triac must be de-burred with a larger drill or countersinking bit before it is secured in place. Fig.8 also shows the holes and locations for the fused IEC socket, mains output socket (GPO) and DPDT switch on the ends of the case and for the lid-mounted components. These This photo matches the component overlay at right. Follow the photo and diagram exactly – especially the earth wiring, heatshrink tubing and cable ties. These are all very important for your safety. must be cut out accurately to avoid any “slop” in these components. This is particularly important for the DPDT switch (S1), which is a snap-in type. Only one side, or pole, of switch S1 is used. This is simply because we were unable to find a suitably-rated SPDT switch. Don’t be tempted to substitute a lower-rated switch for S1 because it has to be able to switch mains voltages at a high current. All hole locations have been positioned so as to ensure adequate clear- RESISTOR COLOUR CODES p p p p p No. Value 4-Band Code (1%) 5-Band Code (1%) 1 100kΩ brown black yellow gold (5%) 1 47kΩ yellow violet orange brown yellow violet black red brown 1 2.2kΩ red red red brown red red black brown brown 1 1kΩ brown black red brown brown black black brown brown 1 100Ω brown black brown brown brown black black black brown 46  Silicon Chip ance between the wire connections and component terminals and to the sides of the case. If using a different case, ensure you allow a 6mm clearance between any two Active or Neutral terminals and between the case and a PC board terminal and the case and switch terminals. For the same reason, you should carefully check the solder (copper) side of the PC board to ensure that all component leads have been clipped off very close to their solder joins. Attach the PC board to the case using Nylon standoffs and M3 x 15mm screws. Nylon standoffs are essential here, to preclude the possibility of arcing from the PC board tracks to the mounting screws. Be sure to use metal screws for the Triac and earth connections. Secure the Triac to the case with a metal M3 siliconchip.com.au N N IEC INPUT CONNECTOR (WITH FUSE) MOUNTED ON BOX END CRIMP EYELET SECURED TO BOX WITH M3 x 10mm SCREW, NUT AND STAR LOCKWASHER E A A GPO NYLON CABLE TIES HEATSHRINK SLEEVING A A S1 19020101 N A LORTNOC ROTOM E UNDER N N NYLON CABLE TIES PC BOARD HEATSHRINK SLEEVING VR1 CRIMP EYELET SECURED TO LID WITH M3 x 10mm SCREW, NUT AND STAR LOCKWASHER (BOX LID) Fig.6: the wiring diagram shows all components in place. While the IEC socket is shown here “flat” for clarity, it is mounted vertically on the box end. x 10mm screw and nut after applying a smear of heatsink compound on the mating surfaces. Note that the specified Triac is an insulated tab device and does not require an insulating washer. If you are using a different Triac – and we do not recommend that you do – check that the metal tab is isolated from the A1, A2 and gate pins. In addition check the data sheet for that Triac to ensure it is an isolated tab device. Using a photocopy of the front panel label as a guide to the positions, mark out and drill the front panel for the speed control pot (VR1) and earth screw. Note that it is important to drill Fig.7: full-size artwork for the front panel. A photocopy of this can also be used as a drilling template for VR1 – but don’t forget the 3mm locating hole. siliconchip.com.au the 3mm hole for the locking tab on the potentiometer to prevent it rotating inside the case should the pot nut work loose over time. Attach the mains input (IEC) and output (GPO) sockets. Attach the front panel label and solder the wiring between the PC board and the pot, leaving enough length to enable the lid to “fold back” for convenience. Wiring must be done using 10A, 250V AC-rated wire and heatshrink tubing should be used over all PC stake connections, the switch terminations and the IEC input socket. The earth connections are made using green/yellow mains wire. It is important to follow the diagram of Fig.6 and the photo alongside for your safety. You will note that two wires connect to the earth terminal on both the IEC input socket and the output socket. From the IEC input socket, one goes directly to the metal case and the other to the earth terminal of the output socket. The second wire from the output socket goes to the case lid. The earth wires which connect to the case and lid are crimped to eyelet lugs and are secured to the case using a metal screw, nut and star washer as shown in Fig.8. Even though it is unlikely that any of the wiring can break off or move inside the closed box, tie the wires together with cable ties to prevent them breaking from their terminations. SILICON CHIP www.siliconchip.com.au 230V INPUT 230V - 10A CONTROLLED MOTOR SPEED OUTPUT CONTROLLER Mk II For universal-type motors up to 10A nameplate rating O = CONTROLLED I = FULL SPEED Do NOT use on induction or shaded-pole motors SPEED Switch between modes ONLY when motor is not turning February 2009  47 10 4mm DIAMETER 7 28 10 27 22 7 6 20 DPDT SWITCH 30 3.5mm DIAMETER 15 3.5mm DIAMETER IEC SOCKET AND FUSE HOLDER 3-PIN OUTLET 15.5 37 33mm DIAMETER BOX END BOX END 3mm DIAM ALL HOLES 3mm DIAMETER 13 20 20 15 10 3mm DIAM 10mm DIAM 10 CL 70 10 10 30 10 12.5 BOX LID BOX BASE Fig.8: complete drilling and cutting details for the specified box and lid. These dimensions should be adhered to closely. Finally, attach rubber feet to the base of the case. Testing Note that all of the circuit is connected to the 230V AC mains supply and is potentially lethal. Do not touch any part of the circuit when it is plugged into a mains outlet. Always remove the plug from the mains before 48  Silicon Chip touching the circuit. In particular this applies to when making adjustments to trimpot VR2, which you may need to do to set the minimum speed. Screw the lid onto the case and plug in your favourite power tool. Switch to the “O” (controlled) position and note how it runs at the minimum setting of VR1. If it runs well (ie, no cogging) then VR2 could be set to make the motor run slower. Disconnect the mains power first before making an adjustment to VR2 and then try the motor again when the lid has been replaced. You may then want to try other power tools to get a compromise setting for the trimpot. SC IMPORTANT: Do not operate the circuit with the lid off the case. siliconchip.com.au