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Heavy duty 10A 240VAC Motor Speed Controller 18  Silicon Chip T HIS NEW SPEED CONTROLLER can be used with power tools rated up to 10 amps and will give smooth control from zero to full speed. Use it to control the speed of electric drills, routers, circular saws, lawn edgers and other appliances with universal brush-type motors. Design by JOHN CLARKE Our last Drill Speed Controller, published in September & November 1992, has been extremely popular and has been used in a host of applications, some of them far beyond what we ever envis­aged. But while it is still a valid design, it does have shortcomings. The first of these is that the maximum speed attainable from the motor is considerably reduced. So for an electric drill which normally runs at say 3000 rpm, the maximum speed might be reduced to around 2200 rpm. This is inevitable with an SCR (silicon controlled rectifier) since the controller circuit effectively half-wave rectifies the 240VAC mains sinewave to give a maximum output voltage of around 160 volts RMS. Result: reduced speed and power capability. The second drawback has to do with low speed control. While the 1992 circuit does allow your drill or other appliance to run at quite low speeds, the result leaves much to be desired. There isn’t much torque available and the speed regulation is poor. This means that if you’re operating your drill at a low speed and you put a reasonable load on it, its speed will drop right away or it may stall completely. Worse, the motor will tend to “cog”. This is caused by erratic firing of the SCR (Triac) so that the motor gets inter­mittent bursts of power. An electric drill that is cogging badly is virtually useless and the only cure is to increase the speed setting which rather defeats the purpose if you want to operate at low speed. The new SILICON CHIP Motor Speed Controller overcomes these drawbacks. The design does away with traditional phase control circuitry and uses switchmode power supply techniques to produce an outstanding controller for universal brush-type motors. By the way, before we go further we should point out that virtually all power tools and appliances use so-called universal motors. These are series wound motors with brushes. We’ll have more to say on this point later in the article. Why use a speed control anyway? Well, why not? Most power tools will do a better job if they have a speed control. For example, electric drills should be slowed down when using larger drill bits; they make a cleaner cut. Similarly, it is useful to be able Features •  Control from zero to maximum speed •  Good speed regulation under load •  Smooth low speed operation •  Freedom from cogging •  Can power appliances rated up to 2400W •  Overcurrent limiting •  Fuse protection •  Earthed diecast case •  Interference suppression included What Motors Can Be Controlled? We’ve noted elsewhere in this article that virtually all power tools and appliances use so-called universal motors. These are series wound motors with brushes. But how do you make sure that your power tool or appliance is a universal motor and not an induction motor. Induction motors must not be used with this speed controller. In many power tools you can easily identify that the motor has brushes and a commutator – you see sparking from the brushes and that settles the matter. But if you can’t see the brushes, you can also get a clue from the nameplate or the in­struction booklet. OK, so how do you identify an induction motor? Most induc­tion motors used in domestic appliances will be 2-pole or 4-pole and always operate at a fixed speed which is typically 2850 rpm for a 2-pole or 1440 rpm for a 4-pole unit. The speed will on the name plate. Bench grinders typically use 2-pole induction motors. November 1997  19 ciples. Having said that, we had better explain what we mean by phase control before we can illustrate the benefits of the new circuitry. Phase control Fig.1: these waveforms illustrate the operation of a typical phase-controlled SCR when a motor is driven at a slow speed. The full sinewave is the 50Hz AC mains voltage, while the chopped waveform is the voltage applied to the motor. Its RMS value is 147V. Fig.2: chopped waveforms from an SCR speed control at high and low settings. At the high setting (lower trace) the motor has 164V applied to it while at the low setting (upper trace) the motor has 144V applied. If the motor is to run at full speed, it would need to be fed with both the positive and negative halfcycles of the 50Hz mains waveform. to slow down routers, jigsaws and even circular saws when cutting some materials, particularly plastics. The same applies to sanding and polishing tools and even electric 20  Silicon Chip whipper snippers are less likely to snap their lines when slowed down. As mentioned above, the new design does not use phase con­trolled circuitry but uses switchmode prin- Phase control refers to a method of triggering a Triac or SCR (silicon controlled rectifier) at various times during each half-cycle of the 240VAC mains waveform. If the Triac is trig­ gered early in each half-cycle, the power applied to the load is high and if it is triggered late in each half-cycle, the power level is low. The term “phase control” comes about because the timing of the trigger pulses is varied with respect to the phase of the mains sinewave. The oscilloscope waveform of Fig.1 shows the chopped wave­form from a phase controlled SCR when a motor is driven at a slow speed. The full sinewave is the 50Hz AC mains voltage, while the chopped waveform is the voltage applied to the motor. Its RMS value is 147V. Fig.2 shows the chopped waveform from an SCR speed control at high and low settings. At the high setting (lower trace) the motor has 164V applied to it while at the low setting (upper trace) the motor has 144V applied. Note that these examples show only the positive half of the mains waveform being used, as is the normal case with a phase controlled SCR circuit. If the motor is to run at full speed, it would need to be fed with both the positive and negative half-cycles of the 50Hz mains waveform. Normally this is not possible with an SCR circuit and while it is possible with a Triac, it is difficult to achieve without a complex circuit. (We should note that full-wave control circuits are used in some washing machines using the Plessey TDA1085 power control IC. This uses tachometric feedback for a wide range of speeds from a series-wound motor.) Another big problem with conventional phase controlled circuits is that the trigger pulse applied to the Triac or SCR is very short and if this corresponds with the time when the brushes hit an open-circuit portion of the commutator, no current will flow and consequently, the motor will miss out on a whole cycle of the mains waveform. This problem is more critical at low speed settings and is one of the reasons for the “cogging” behaviour referred to earlier. Speed regulation In theory, most phase controlled SCR speed control circuits incorporate a form of feedback which is designed to maintain the speed of the motor under load. When the motor is loaded, the back EMF (electromotive force) produced by the motor drops and the circuit compensates by triggering the SCR earlier in the mains cycle. This helps to drive the motor at the original speed. In practice though, the back-EMF generated by most series motors when the SCR is not conducting is low or nonexistent or it is produced too late after the end of each half-cycle to have a worthwhile effect on the circuit triggering in the next half-cycle. So while the theory says good motor speed regulation should be obtained, in practice, it doesn’t happen in many cases. Pulse width modulation The new SILICON CHIP speed control circuit uses Pulse Width Modulation (PWM) and a different feedback method for speed regu­lation which solves the above problems associated with phase control. Fig.3 and Fig.4 shows the voltage waveforms applied to the motor at high and low speed settings. What happens is that we rectify the mains voltage and then chop it up with a high voltage IGBT (Insulated Gate Bipolar Transistor) at a switching rate of about 1.2kHz. For the high speed setting the pulses applied to the motor are relatively wide (Fig.3) while at the low speed setting, the pulses are very narrow (Fig.4). Note that there are 12 pulses during each and every mains half-cycle so that the motor does not miss out on large blocks of current because of erratic triggering. This means that the motor operates very smoothly over the whole of its speed range. The speed regulation does not rely upon motor back-EMF. Instead it monitors the current through the motor and adjusts the pulse width to maintain the motor speed. Block diagram Fig.5 shows the basic circuit arrangement of the Motor Speed Controller. The 240VAC input waveform is fed through a filter and full wave Fig.3 (top) and Fig.4 (above) show the voltage waveforms applied to the motor at high and low speed settings. The rectified mains voltage is chopped up with a high voltage IGBT (Insulated Gate Bipolar Transistor) at a switching rate of about 1.2kHz. For the high speed setting the pulses applied to the motor are relatively wide (Fig.3) while at the low speed setting, the pulses are very narrow (Fig.4). Note that there are 12 pulses during each and every mains half-cycle so that the motor does not miss out on large blocks of current because of erratic triggering. This means that the motor operates very smoothly over the whole of its speed range. rectified. The resulting positive-going waveform is fed to one side of the motor, while the other motor terminal is switched on and off via transistor Q1. A triangle (ramp) waveform is generated using IC1b and this is ap- plied to comparator IC1a where it is compared with the voltage level from VR1, the speed control potentiometer. If the speed voltage is high relative to the triangle wave­ form, then the comparator will produce wide pulses November 1997  21 Fig.5: the basic circuit arrangement of the Motor Speed Controll­er. The 240VAC input is full-wave rectified and fed to one side of the motor, while the other motor terminal is switched on and off via IGBT Q1. Q1 is controlled by a conventional PWM circuit involving IC1, IC2 & IC3. at its output; a lower speed voltage will reduce the pulse width. This can be seen in the scope waveforms of Fig.6. The triangle waveform at the top is compared to the speed voltage, the horizontal voltage intersecting the triangle wave. The resulting lower trace is the pulse width modulation signal from the comparator. The comparator output is fed to the gate driver (IC2) which then drives the high voltage IGBT (Q1). Diode D1 is a fast recovery diode to conduct the motor current when Q1 is switched off while a snubber across Q1 prev­ents excessive voltage excursions on Q1. Resistor R1 monitors the current flow through the motor when Q1 is on and the resulting voltage generated is sampled by IC4, whenever Q1 is on. IC3a amplifies the voltage from R1 and applies it to the speed pot. Thus an increase in motor current, as the motor slows down, leads to an increase in the output from IC3a to increase the speed setting from VR1 and this results in an increase in the voltage applied to the motor. Yes, this is a positive feedback system and too much positive feedback is not good so the amount of feedback is fairly critical to optimum circuit operation. IC3b also monitors the voltage produced from R1 via IC4 and compares it against a reference voltage. If the voltage from R1 exceeds the reference threshold, IC3b’s output goes low and reduces the speed pot voltage via diode D2. This reduces the voltage applied to the motor and provides current limiting. Circuit description Fig.6: These waveforms show the interaction of the triangle waveform and the speed voltage. The triangle waveform at the top is compared to the speed voltage, the horizontal voltage inter­secting the triangle wave. The resulting lower trace is the pulse width modulation signal from the comparator. The comparator output is fed to the gate driver IC2 which then drives the high voltage IGBT. 22  Silicon Chip The circuit for the Motor Speed Controller is shown in Fig.7. It comprises four ICs, several diodes, resistors and capacitors plus the high voltage IGBT, Q1. IC1b is the triangle waveform generator and it is essen­tially an oscillator whereby the .018µF capacitor at pin 5 is charged and discharged via the 33kΩ resistor connected to the output at pin 12. The triangle or ramp waveform across the ca­pacitor has an amplitude of about 5V peak-to-peak. Comparator IC1a compares the triangle waveform at pin 10 with the speed voltage at pin 9, as set by VR1. VR1 is the centre portion of a voltage divider with a 1kΩ resistor connect­ing Fig.7: the circuit uses a 32A 1200V avalanche-protected IGBT (insulated gate bipolar transistor) as the switching element to the load. It is switched at 1.2kHz; ie, 12 times in each half-cycle of the 50Hz 240VAC mains supply. to the +15V rail and an 8.2kΩ resistor to 0V. The speed voltage from VR1 is filtered with a 47µF capacitor to prevent any sudden changes in level and this voltage is monitored by the inverting input (pin 9) of IC1a via a 1kΩ resistor. The 1MΩ resistor between pin 9 and the pin 7 output provides positive feed­back to give a small amount of hysteresis in the comparator action. This is to prevent “hunting” in the comparator output when changing levels. The pin 7 output of IC1a drives buffers IC2a and IC2e. IC2a drives three paralleled buffers, IC2b, 2c & 2d, which provide a high current capability to charge and discharge the gate of the high voltage IGBT Q1. The gate is protected from excessive Warning! (1) The entire circuit of this motor speed controller floats at 240VAC and is potentially lethal. Do not build it unless you know exactly what you are doing. DO NOT TOUCH ANY PART OF THE CIRCUIT WHILE IT IS PLUGGED INTO A MAINS OUTLET and do not operate the circuit outside its metal case. (2) This circuit is not suitable for induction motors or shaded pole motors used in fans – see panel. drive voltage with ZD2, a 15V zener diode. Normally the circuit should have no way of providing excessive gate drive however we blew a number of devices during the development process when attempting to monitor gate drive levels with an oscilloscope. So the 15V zener has been included for insurance. Three circuit features combine to ensure that the IGBT can safely switch high levels of current through the motor load. First, there is a snubber network comprising an 82Ω resistor and .01µF capacitor connected in series across the IGBT’s source and drain and second, there is the fast recovery diode D1. Third, there is a 275VAC metal oxide varistor (MOV) connected across the output of the bridge rectifier. These measures combine to damp any spike voltages which would otherwise occur every time the IGBT switched off. Finally, the specified IGBT is a November 1997  23 The lid of the case must be independently earthed by running an extra lead from a solder lug to the earth terminal on the mains socket – see Fig.8. Fit the earth solder lug mounting screws with washers and locknuts so that they cannot possibly come adrift. Siemens BUP213 1200V 32A avalanche-protected device. We do not recommend substitution of lower rated devices. During the development of this project we ended up with quite a graveyard of IGBTs and Mosfets which should have been up to the task but were found wanting. Current monitoring R1 is a used to monitor the current flow through the motor and IGBT Q1. The voltage developed across R1 is fed through a low pass filter consisting of a 10kΩ resistor and .001µF capacitor to one side of a 4066 analog switch, IC4. This is the sample and hold cir24  Silicon Chip cuit and IC4 is switched on to sample the voltage across R1 each time the IGBT is switched on. Hence, IC4’s gate signal comes from comparator IC1a and is buffered by IC2e. The sampled signal from R1 is held in the .047µF capacitor at pin 4 of IC4. The sampled voltage from IC4 is fed to two op amps, IC3a & IC3b. IC3a amplifies the voltage by about 53 when VR1 is set to maximum and 3.2 when set to minimum. IC3a acts to vary the DC level fed to comparator IC1a from VR1 and thereby compensates for speed variations in the motor. IC3b acts as a comparator, comparing the sampled voltage from R1 with a reference voltage at its pin 3. If the current through R1 is excessive, the output of IC3b goes low and pulls pin 9 of IC1a low via diode D2 and a 470Ω resistor. This has the effect of greatly reducing the motor drive voltage. Power for the circuit is derived directly from the 240VAC mains. Fuse F1 protects against shorts while the .01µF capacitor in conjunction with L1 & L2 prevents switching artefacts from the IGBT and motor being radiated by the mains wiring. BR1 is a bridge rectifier with a 600V 35A rating. BR1 provides the circuit with the positive full-wave rectified mains voltage and this is lightly filtered using a 0.1µF 250VAC capaci­tor. Power for the low voltage circuitry is derived via two series 4.7kΩ 5W resistors, diode D3 and the 15V zener diode ZD1. A 22µF capacitor across Table 1: Capacitor Codes ❏ ❏ ❏ ❏ ❏ ❏ Value IEC Code EIA Code 0.1µF   100n   104 .047µF   47n  473 .018µF   18n  183 .01µF   10n  103 .001µF   1n0  102 the 15V zener smooths the DC while diode D3 prevents the capacitor from discharging when the mains voltage falls to below 15V every half cycle. The result is a regulated 15V supply. Construction The Motor Speed Controller is constructed on a PC board which is coded 10311971 and measures 112 x 144mm. It is housed in a diecast case measuring 171 x 121 x 55mm. The PC board has circular cutouts to suit the case. By the way, we do not recom­mend a sheet metal case for this project. Since all the circuitry inside is at 240VAC mains potential, it is important that the case is strong and rigid. The complete wiring diagram is shown in Fig.8. THE EARTHING DETAILS OF THE CASE ARE MOST IMPORTANT SINCE THE IGBT, FAST RECOVERY DIODE D1 AND POTENTIOMETER VR1 ARE ALL AT MAINS POTENTIAL YET ARE ATTACHED TO THE CASE. If the mica washers or the insulation of the potentiometer were to break down, the case would be live (ie, at 240VAC) if it was not properly earthed. For this reason, the case lid must also be sepa­rately earthed, as shown in Fig.8 because otherwise the lid could be live if the potentiometer broke down and the lid was not actually attached to the case. Begin construction by checking the PC board against the published pattern in Fig.11. There should not be any shorts or breaks between tracks. If there are, repair these as necessary. If the cutouts in the sides of the PC board have not been made, they should be done before any components are soldered on. A large semicircular cutout is required on both the long sides of the board, as well as notches to clear the vertical slot channels in the sides of the case. Also you will need to round off the corners of the board. Make sure Parts List 1 PC board, code 10311971, 112 x 144mm 1 metal diecast case, 171 x 121 x 55mm 1 front panel label, 100 x 70mm 1 Neosid iron powdered core, 17742-22 (L1,L2) 1 GPO mains power point (Clipsal NO.16N or equivalent) 1 10A mains cord and plug 1 cordgrip grommet 3 solder lugs 1 10kΩ linear potentiometer (VR1) 1 500kΩ horizontal trimpot (VR2) 1 knob 2 3AG (or 2AG) PC mount fuse clips 1 10A 3AG fast blow fuse (or 2AG), (F1) 2 3mm x 10mm screws, nuts & star washers 4 4mm x 15mm screws, nuts and star washers plus two locknuts 7 small cable ties 2 TO-218 mica insulating washers OR 1 SIL-PAD 400 washer 2 TO-220 mica insulating washers OR 1 SIL-PAD 400 washer 2 insulating bushes 1 500mm length of blue 10A mains wire 1 150mm length of brown 10A mains wire 1 1.5m length of 1mm enamelled copper wire 1 1m length of 0.8mm enamelled copper wire 1 140mm length of 0.8mm tinned copper wire 1 26mm length of 15mm ID heatshrink tubing 9 PC stakes the PC board fits into the case before starting assembly. You can start the board assembly by inserting the PC stakes and the links now and then the resistors, using the accompanying table for the colour codes. The two 5W resistors should be in­serted so that they stand several millimetres above the PC board to allow cooling since each will be dissipating about 2.7W and will run hot. When inserting diode D2 and the zeners, take care with their orientation and be sure to place each type in its correct place. Install the ICs, taking Semiconductors 1 LM319 dual comparator (IC1) 1 4050 hex CMOS buffers (IC2) 1 LM358 dual op amp (IC3) 1 4066 quad CMOS analog switch (IC4) 1 Siemens BUP213 32A 1200V IGBT (Q1) 1 STTA3006P SOD93 30A 600V fast recovery diode (D1) 1 1N914, 1N4148 signal diode (D2) 1 1N4004 1A 400V diode (D3) 1 15V 1W zener diodes (ZD1) 1 15V 400mW zener diode (ZD2) 1 36MB60A 35A 600V bridge rectifier (BR1) 1 S14K275 275VAC metal oxide varistor (MOV) Capacitors 1 47µF 16VW PC electrolytic 1 22µF 16VW PC electrolytic 1 10µF 16VW PC electrolytic 2 0.1µF 63V MKT polyester 1 0.1µF 250VAC X2 class MKT polyester 1 .047µF 63V MKT polyester 1 .018µF 63V MKT polyester 2 .01µF 250VAC X2 class MKT polyester 1 .001µF 63V MKT polyester Resistors (0.25W, 1%) 1 2.2MΩ 2 4.7kΩ 1 1MΩ 2 4.7kΩ 5W 1 470kΩ 1W 2 1kΩ 4 100kΩ 1 470Ω 1 33kΩ 1 390Ω 1 22kΩ 1 82Ω 1W 4 10kΩ 1 10Ω 1 8.2kΩ care to orient them as shown on Fig.8. D1 and Q1 are oriented with the metal flange towards the edge of the PC board and are located as high as possible with their leads extending about 1mm below the PC board. The capacitors can be installed next. The accompanying capacitor table shows the various codes which may be used to indicate the capacitance values. The electrolytic capacitors must be oriented with the correct polarity. L1 & L2 are wound on a single Neosid toroidal core as shown in Fig.9. Make sure that there are an equal November 1997  25 Fig.8: the complete wiring diagram of the Motor Speed Controller. Note that the case and lid must be separately earthed, as shown here. Note also that all parts of the circuit, including the terminals of VR1, float at 240VAC. number of turns on each winding and that they are wound in the directions as shown. Insert the wire ends into the PC board holes and secure the toroid with two cable ties. The wire ends can be soldered to the PC board using a hot soldering iron to strip the self-fluxing insulation on the wire. 26  Silicon Chip The current monitoring resistor is made from a 1m length of 0.8mm enamelled copper wire which is wound onto a 10mm former (3/8"). This may be a drill bit, pen or a wooden dowel. Wind on about 26 turns then remove the former and secure the coil with insulation tape so that each winding touches the adjacent one. Bend the wire ends outward and place a 26mm length of heatshrink tubing over the coil and shrink it down with a hot air gun. Re-bend the wire ends and secure in place into the PC board mount­ing holes. The bridge rectifier (BR1) is attached Fig.10: mounting details for the IGBT (Q1) and the fast recovery diode (D1). Fig.9: winding details for the input filter choke. Note that L1 and L2 are wound so that their flux cancels in the toroid core. must be bent so that the metal flange of each device is in contact with the case sides. Remove the PC board and drill out these holes plus holes for the cordgrip grommet and the earth lug screw. Deburr the holes for D1 and Q1 must be deburred with a larger drill to prevent punch-through of the insulating washers. Attach the PC board to the case with the supplied screws (yes, they do come with the case) and secure D1 and Q1 to the case with a screw, nut, insulating washer and insulating bush. The arrangement for this is shown in Fig.10. If you use mica washers apply a smear of heatsink compound to the mating surfaces before assembly and use two for each device, to prevent flash-over. Silicone heatsink washers do not require heat­sink compound and if the 3.5kV-rated SIL-PAD 400 types are used, one is enough for each device. to the PC board with the (-) and adjacent AC terminal sitting over and soldered to PC stakes. The other AC terminal and the positive (+) terminal are wired to the PC board pins using 10A 250VAC-rated hookup wire. Fuse F1 is mounted in fuse clips which attach to the PC board as shown. We have catered for both 2AG and 3AG sizes. Clip the fuse into the clips first, insert them into the PC board and solder in position. Mounting the hardware Insert the PC board into the case and mark the mounting hole positions for diode D1, IGBT Q1 and bridge rectifier BR1. Note that the leads for D1 and Q1 After mounting, check that the metal tabs of the devices are indeed isolated from the case by measuring the resistance with a multi­meter. The bridge rectifier (BR1) is secured to the case with a 4mm screw, nut and star washer. It does not require an insulating washer between its body and the case. Mark out and drill the case lid for the mains socket and potentiometer. Attach the mains socket with the 4mm screws and nuts and secure the pot after the stick-on front panel label has been affixed. Solder the Active and Neutral wires of the power cord to the stakes on the PC board and secure the cord with a cordgrip grommet. The earth connection on the mains socket should be run to a solder lug using green/yellow mains wire. Similarly, solder the earth wire from Table 2: Resistor Colour Codes ❏ No. ❏  1 ❏  1 ❏  1 ❏  4 ❏  1 ❏  1 ❏  4 ❏  1 ❏  2 ❏  2 ❏  1 ❏  1 ❏  1 ❏  1 ❏  1 Value 2.2MΩ 1MΩ 470kΩ 100kΩ 33kΩ 22kΩ 10kΩ 8.2kΩ 4.7kΩ 1kΩ 470Ω 390Ω 82Ω 10Ω 1Ω 4-Band Code (1%) red red green brown brown black green brown yellow violet yellow brown brown black yellow brown orange orange orange brown red red orange brown black red orange brown grey red red brown yellow violet red brown brown black red brown yellow violet brown brown orange white brown brown grey red black black brown black black brown brown black gold gold 5-Band Code (1%) red red black yellow brown brown black black yellow brown yellow violet black orange brown brown black black orange brown orange orange black red brown red red black red brown black black red brown grey red black brown brown yellow violet black brown brown brown black black brown brown yellow violet black black brown orange white black black brown n/a brown black black gold brown brown black black silver brown November 1997  27 the mains cord to a solder lug and connect both solder lugs to the case using a screw, nut and star washer. An additional locknut should then be fitted so that the earth lugs can not possibly come loose. Note that the case lid should also be earthed, via a third solder lug, with a wire connected to the earth terminal on the mains socket. Wire up the potentiometer using 250VAC-rated hookup wire. Secure the wiring with cable ties. Testing Fig.11: check your PC board by comparing it with this full-size etching pattern before installing any of the parts. MOTOR SPEED CONTROLLER WARNING! Internal circuit floats at 240VAC SLOW FAST SUITABLE FOR SERIES MOTORS RATED UP TO 10A <at> 240VAC OR 2400W. Fig.12: this full-size front panel artwork can be used as a drilling template for the front-panel speed control. 28  Silicon Chip Before you power up the circuit, set trimpot VR2 to the mid-position – this setting should give good performance with most motors. This done, check all of your wiring very carefully against the circuit of Fig.7 and the wiring dia­ gram of Fig.8. Use your multimeter to check that there is no leakage between the Active and Neutral wires of the power cord and the case. Also check that the case and lid are connected to the earth pin of the power cord. The lid should be screwed to the case. The safest and best way to test the circuit operation is to connect a load. This may be an ordinary incandescent lamp with a rating of between (say) 40W and 100W. Apply power and check that you can vary the brightness of the lamp from zero up to full brilliance. If that checks out OK, connect up a drill or other power tool and check that you can vary its speed over the full range. If so, your project is complete but some motors may require adjustment of VR2 for best speed regulation. In practice, if VR2 is adjusted too far anticlockwise, the motor will tend to be overcompensated when loaded and will actually speed up. It may even hunt back and forth between a fast and slow speed. Back off the adjustment for VR2 for best results. This must be done on a trial and error basis, with the plug removed from the mains outlet before each adjustment. Replace the lid before reapplying power. If you are using a drill for example, at fairly low speed, the motor should not slow down by much as you put a reasonable load on it. Troubleshooting If the speed controller did not work when you applied power, it’s time to don your troubleshooting hat. Note that all of the circuit is connected to the 240VAC mains supply Silicon Chip Binders REAL VALUE AT $11.95 PLUS P &P These binders will protect your copies of SILICON CHIP. They feature heavy-board covers & are made from a dis­ tinctive 2-tone green vinyl. They hold up to 14 issues & will look great on your bookshelf. The holes in the side of the case for D1 and Q1 must be deburred using an oversize drill to prevent punch-through of the insulating washers. After the devices have been mounted, use your multimeter (set to a low ohms range) to confirm that their metal tabs are indeed correctly isolated from the case. should be able to vary the voltage at pin 7 of IC1a by winding the speed pot up and down. The same effect should be observed at the gate of the IGBT. If you have an oscilloscope you should be able to observe the waveforms shown in Fig.6. Should you wish to monitor any of the other waveforms il­lustrated in this article, the circuit will need to be powered from 240VAC again and will then be completely live. If you connect an oscilloscope under these conditions, you cannot con­nect the earth terminal of the probe to any part of the circuit. In fact, the only really safe way to monitor waveforms in the circuit when it is powered is to use an oscilloscope with fully floating differential inputs. Two final points: if you are using this controller with a high power tool such as a large circular saw or 2HP router, it will not give the same kick when starting. Because of the current limiting, the motor will take a few seconds to come up to full speed. To use the appliance at full speed, it is better not connect the Speed Controller at all. Finally, note that this unit is not suitable for use with devices such as 2400W heaters which will draw 10A SC continuously. ★  80mm internal width ★ SILICON CHIP logo printed in gold-coloured lettering on spine & cover Price: $A11.95 plus $A3 p&p in Australia; or $A11.95 plus $A8 p&p in NZ & PNG. Not available elsewhere. Silicon Chip Publications PO Box 139 Collaroy Beach 2097 Or fax (02) 9979 6503; or ring (02) 9979 5644 & quote your credit card number. Use this handy form  and is potentially lethal. This includes the tabs of D1 and Q1, the terminals of potentiometer VR1 and all other parts. Do not touch any part of the circuit when it is plugged into a mains outlet. Always remove the plug from the mains outlet before touching any part of the circuit. If you wish to work on or measure voltages in any part of the circuit, connect it via an isolating trans­former. Failing that, you can at least check that there is ap­proximately 15V present in the circuit by connecting a multimeter across the zener diode ZD1. If you wish to check the circuit operation in detail, you should power it from a low voltage power supply set to provide 14V. At 15V, you run the risk of blowing zener diode ZD1. Note that the unit must not be plugged into 240VAC if the low voltage part of the circuit is to be separately powered. Assuming that you are powering the unit from a 14V power supply, you can use your multimeter to check that +14V is present at pin 11 of IC1, pin 1 of IC2, pin 8 of IC3 and pin 14 of IC4. You can also check the circuit operation by measuring the average DC levels around the circuit. For example, if the circuit is working correctly, you ★  Hold up to 14 issues Enclosed is my cheque/money order for $________ or please debit my ❏ Bankcard  ❏  Visa   ❏ Mastercard Card No: ________________________________ Card Expiry Date ____/____ Signature ________________________ Name ___________________________ Address__________________________ __________________ P/code_______ November 1997  29