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This new speed controller can be used with power tools rated up to 5 amps. Use it to control the speed of circular saws, electric drills, lawn edgers and other appliances with universal "brush type" motors. By LEO SIMPSON Heavy-Duty 5A Drill Speed Controller his new speed controller uses higher rated components to provide a higher current rating than similar circuits which have been around for quite a few years now. It will find many uses around the home and won't cost a bomb to put together. I've used a version of this circutt for years but I must admit I've rarely used it with my power tools. Where I have used it is with my electric lawn edger. This is a large unit and without the speed controller it is very noisy and breaks the Nylon line frequently. With the speed controller in use , the edger is much quieter and rarely breaks the line. No doubt, there are many other equally useful applications for a speed controller such as this since it gives a wide range of control with little tendency for "hunting" or "cogging". T How it works The circuit is well proven and one which many readers will be quite 28 SILICON CHIP familiar with except that in previous versions it would have used an SCR (silicon controlled rectifier) instead of a Triac, as shown here. In fact, for the purpose of understanding how the circuit works it is easier to think of the Triac as being an SCR. So now let's have a look at how a basic SCR speed control circuit works. Refer to Fig.1. This is just about the simplest speed control you can get. The SCR device conducts in one direction only and feeds half-wave rectified AC to the motor. Essentially, the SCR is a rectifier diode which only conducts when it receives a trigger voltage at its gate. Once it starts conducting, from anode (A) to cathode (K), it stays conducting until the load current drops to zero or the circuit voltage is reversed. Because the SCR is a switching device, it can be used as a very efficient power controller, and can vary large amounts of current while itself dissipating very little power. The circuit of Fig.1 controls the AC r l. A SCR 4.7 63VW 240VAC 1k MOTOR Fig.1: this basic circuit uses an SCR to feed half-wave rectified AC to the motor. The power fed to the motor is varied by triggering the SCR sooner or later during each positive half-cycle of the 240V AC waveform. power to the motor by triggering the SCR into conduction late or early in each positive half-cycle of the 240VAC waveform; the SCR does not conduct at all during the negative half cycles. If the SCR is turned on early in each AC half cycle, the power fed to the motor will be relatively high. Conversely, if the SCR is turned on late in each AC half cycle the power fed to the motor will be relatively low and hence the motor will run slowly. The trigger voltage for the SCR comes from VRl, a ZkQ potentiometer connected in series with a lOkQ resistor and diode Dl. This zkn potentiometer is fed with half-wave rectified AC which is partly smoothed by the 4. 7µF capacitor across it. The resulting ramp voltage from the wiper is fed to the gate of the SCR via diode DZ. F1 10A A 150k 1W SPEED VR1 10k LI N 02 1N4004 TRIAC1 BTA41600P Fig.2: the final circuit uses a silicon bilateral switch (SBSl) to trigger a Triac during the positive mains half cycles. This arrangement give a greater range of speed control than the circuit shown in Fig.1 & is more efficient. A2 240VAC SET MINIMUM SPEED VR2 5k 03 R250H 6A 01 1N4004 N Speed regulation Now you might ask: why use an SCR and allow conduction on only positive AC half cycles? Why not use a Triac which can be triggered into conduction on both positive and negative half cycles? The answer is that we could but then a fundamental advantage of this basic circuit would be lost. The advantage is speed regulation. A circuit with good speed regulation will maintain a selected motor speed regardless of variations in the load. If you are using a speed controller with an electric drill, you don't want the motor to bog down when you start to drill into the heavy stuff, do you? So how does the circuit give speed regulation? The answer is that the circuit monitors the back-EMF from the motor. Back-EMF can be defined as the voltage developed by a motor which opposes the supply voltage. The higher the speed of the motor, the higher the back-EMF. This circuit monitors the back-EMF in the following way. Notice that one side of the motor is connected directly to the SCR's cathode while the other side is connected to the cathode of diode Dl and to the mains Neutral wire. This means that the gate-to-cathode voltage applied to the SCR is the difference between the wiper voltage from VRl and the backEMF generated by the motor (disregard the voltage drop across DZ). Actually, in so-called universal motors (AC/DC series motors with commutators as used in most power tools and appliances), there are two back-EMFs generated. The first is a function of motor speed and the remnant magnetism of the field coils and 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 VIE WED FROM BELOW .~ FRONT PANEL A2 SPEED CONTROLLER the positive half cycles before the SCR conducts. The second back-EMF is generated during the time when the SCR is conducting and since there will now be current flowing in the field coils (and also in the armature), this back-EMF will be higher than in the first case. However, 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. Hence, the motor applies negative feedback to the gate of the SCR. This negative feedback enables the circuit to give good speed regulation. Say a particular motor speed is set by VRl and 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 of the SCR. More correctly, it means that the ramp voltage at 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. Better circuit Now the basic circuit presented in Fig. l will actually work and was the basis of most speed control circuits used about 20 years ago. However, it has a number of drawbacks. First, the power dissipation through the lOkQ resistor is about 2.4 watts which means Warning! II This Speed Controller circuit operates directly from the 240VAC mains supply and therefore must be regarded as a potentially lethal project. If you are not confident about working on mains-powered projects we advise you to leave this one strictly alone. The wiring conforms to SAA standards and if constructed according to the instructions in this article, it is a safe appliance to use. Notes (1) The Speed Controller must only be used with universal "brush type" motors" with nameplate ratings of up to 5 amps. (2) Power tools should not be operated at low speeds for long periods otherwise they may overheat & suffer damage. (3) The Speed Controller must not be used with induction motors. (4) The Speed Controller must not be used to control the power to lamps or electric radiators. To do so would contravene regulations of the NSW Energy Authority and affiliated state energy authorities. SEPTEMBER1992 29 r-0 MAINS CORO " Fig.4: check your PC board for defects against this full-size pattern before mounting any of the parts. The board measures 72 x 52mm. Fig.3: take care with component orientation during the PCB assembly & be sure to use mains-rated wire for the connections to the mains socket (GPO). All adjustments to the circuit should be made with the power off. that it gets rather hot. Second, even though the current through the 10kQ resistor and VR1 is relatively high, it won't be enough for reliable triggering of higher power SCRs. And third, the circuit is not particularly good at very low speed settings. Silicon bilateral switch This is where the circuit of Fig.2 comes into the picture. You can see the similarities between it and Fig, 1. Instead of an SCR, we have used a Triac and instead of feeding the gate directly from VR1 as in Fig.1, a trigger circuit consisting of a silicon bilateral switch (SBS1) and a .047µF capacitor has been used. While the Triac is capable of conducting on both positive and negative half cycles of the 240VAC 50Hz waveform, this circuit only enables it to trigger on positive half cycles, because of the rectifier action of diode DL A silicon bilateral switch (SBS) is a voltage breakover device; ie, at voltages below its breakover point it is essentially open circuit but once the breakover voltage is reached, it becomes a low value of negative re- This CRO photograph shows the waveform applied to the motor when the Triac is triggered relatively early during the positive half of the mains waveform. 30 SILICON CHIP sistance. Don't worry too much about the "negative resistance" bit. All you have to remember is that it is used in conjunction with the .047µF capacitor. This charges up from VR1 via diode DZ until it reaches the breakover voltage of 8V (nominal). At this point, the SBS dumps the capacitor's charge into the Triac's gate and then it reverts to an open circuit whereupon the sequence can repeat itself during the next positive half cycle of the mains AC waveform. The energy stored in the capacitor is quite enough to trigger even insensitive Triacs and hence we are able to use a high power 40-amp device in this circuit. In this circuit, the motor back-EMF acts to reduce the charging voltage to the .047µF capacitor rather than reducing the SCR gate voltage as in Fig.1. But although the circuit arrangement Triggering the Triac later in the mains half cycle reduces the power delivered to the motor & hence it runs more slowly than in the previous example. You can now afford a sate IIite TV system For many years you have probably looked at satellite TV systems and thought "one day". You can now purchase the following K-band system from only: $995.00 Here's what you get: * The PC board is mounted on the back of the mains socket using 10mm metal spacers & secured using the socket mounting screws. Note the clamp that's used to secure the mains cord. is a little different, the speed regulation is just as good. The circuit efficiency is improved too, with only 200m W being dissipated in the 150kQ resistor which feeds VR1. This resistor has a rating of 1W to ensure that it has an adequate voltage rating. 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. DZ is there to protect the gate of the Triac when it is in the conducting state - terminal A 1 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. VRZ, the 5kQ trimpot in series with VR1, is there to provide a minimum setting for the circuit. Why use a Triac? One question we have not answered so far is why we have specified a Triac instead of an equivalently rated SCR. The reason is quite simple. We did it to avoid the need for parts stockists to have to order in another device. We have specified a 600V 40A device so that it can withstand the "locked rotor" current of any power tool with a nameplate rating of up to 5A. Note that a "locked rotor" condition may well blow the 10A fuse but the 40A Triac should not be damaged. We have done this test! Another reason for using the 600V 40A Triac specified in the parts list is A 1.6-metre prime focus dish antenna, complete with all the mounting hardware. * One super low-noise LNB (1.4dB or better). * One Ku-band feedhorn and a magnetic signal polariser. * 30 metres of low-loss coaxial cable with a single pair control line. * lnfrared remote control satellite PARTS LIST 1 plastic case, 130 x 70 x 40mm 1 20-gauge steel front panel to suit case 1 PC board, code SC10109921 , 72 x 52mm 1 flush-mount AC socket 1 3-core mains flex with moulded 3-pin plug 1 cable clamp to suit mains cord 1 solder lug 2 M205 fuse clips 1 10A M205 fuse 2 10mm metal spacers 1 10kQ linear potentiometer (VR1) 1 knob to suit pot 1 5kQ trimpot (VR2) 1 150kQ 1W resistor 1 1kQ 0.25W or 0.5W resistor 1 .047µF 63VW MKT capacitor Semiconductors 1 BTA41-600P, Q6040J7 40A 600V Triac 1 2N4992 silicon bilateral switch (8B81) 2 1N4004 silicon diodes (D1 ,D2) 1 6A 400V silicon diode (D3) Miscellaneous Screws, nuts , lockwashers, solder. receiver with selectable IF & audio bandwidth, polarity & digital readout. Your receiver is pre-programmed to the popular AUSSAT transponders via the internal EEPROM memory. This unit is also suitable for Cband applications. Call, fax or write to: AV-COMM PTY LTD PO BOX 386, NORTHBRIDGE NSW 2063. Phone (02) 949 7417 Fax (02) 949 7095 All items are available separately. Ask about our C-band LNBs, NTSCto-PAL converters, video time date generators & Pay TV hardware. ----------- I YES GARRY, please send me more in- I I formation on K-band satellite systems. I II Name ... .... .... .............. ..... ............... .. I I I Address ············ ······· ············ ··· ········· I I I I ........................... P/code .............. .. I I : Phone .......... .. .. .... ........... .. .............. . I I ACN 002174478 01 192 I . ___________ . SEPTEMBER 1992 31 the steel front panel, along with the flush mount mains socket. ~ Mount the lOkQ potentiometer first, having cut its shaft to a length to suit the knob - 12mm should be about right. This done, fit the solder lug as shown, usFOR BRUSH ing a screw, nut and lockwasher. MOTORS UP This terminates the Earth wire TOSAMPS from the mains cord and also the Earth wire to the mains socket. The 240VAC mains cord enMIN ters through a grommeted hole in the case and is clamped to the steel lid using a suitable cable DRILL SPEED CONTROLLER clamp. The Active (brown) and Neutral (blue) wires are then Fig.5: this artwork can be used as a drilling template for the front panel. Be sure to stripped and soldered to their use a steel panel as specified - aluminium or plastic will not be strong enough. relevant points on the PC board. You will need to strip the outer that it is an isolated tab device. This flush-mount mains socket and is se- insulation of the mains cord back and means that it can be attached to a cured using the socket's mounting trim the wires so that the Earth (green/ heatsink without any need for a mica screws. Note that the holes for the yellow) wire is about 120mm long, washer or other means of insulation. Triac and the 6A diode should be while the Active and Neutral wires drilled to 1mm. are about 80mm long. Construction No special order needs to be folYou will need to run three insulowed when assembling the board (see lated wires from the potentiometer We built the prototype into a standard plastic case and have specified a Fig.3) but there are a few points to (VR1) to the PC board. These wires flush mount mains socket which is watch. First, the silicon bilateral should have 250VAC insulation and mounted on the lid. Since the stand- switch is a bipolar device so it can go should be about 100mm long. ard plastic or thin aluminium lid sup- into circuit either way around. SecTwo wires remain to be connected. plied with these cases would not be ond, do not bend the leads of the 6A These are the Active and Neutral wires strong enough with the necessary diode too close to the body - use a from the PC board to the flush-mount 35mm diameter cutout, we are speci- pair of pliers when doing this job. mains socket. Again, these must be fying a lid made of 20-gauge steel. Third, the leads of the Triac need to run in 250VAC insulated wire (strip This lid also provides the limited be cranked 90° at 5mm from the body some from the mains cord) and should amount of heatsinking required for so that the mounting tab lines up with be about 100mm long. the Triac. the adjacent mounting hole in the PC With all the wiring complete, you All the components with the ex- board. are ready to mount both the mains ception of the lOkQ potentiometer are socket and the PC board. Two 1/8Mounting the PC board mounted on a PC board measuring 72 inch brass screws 40mm long are rex 52mm (code SC10109921). This When assembly of the PC board is quired. These screws pass through board is mounted on the back of the complete, it needs to be mounted on both the socket and the 10mm spacers and retain the PC board with a nut and lockwasher each. In addition, one of the mounting screws also retains the tab of the Triac. This screw also provides the heatsinking path for the Triac to the front panel. This is why brass screws are specified in the parts list. After checking all the wiring carefully against the circuit (Fig.2) and wiring diagram (Fig.3) , the unit may be tested on the mains using a power tool such as an electric drill. Note that the unit must be disconnected from the AC mains when adjusting VRZ to set the minimum motor speed. Do not ever work on the unit while it is open and connected This side view shows how the mains socket & PCB are secured to the front panel to the AC mains - it is a potentially using 40mm screws and 10mm spacers. Note that brass screws are necessary to ensure adequate heatsinking for the Triac. lethal device. SC ::/lll!lilJ- MAX 32 SILICON CHIP