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Words by Leo Simpson Design by Branko Justic* *Oatley Electronics High-power reversible DC motor speed controller Here’s a 12-32V 30A speed controller that’s easy to build. It’s available in two versions – reversible and non-reversible – and features soft start, relay switching of motor direction and PWM speed control. T HIS REVERSIBLE DC Motor Speed Controller uses a pair of highpower Mosfets connected in parallel to drive the motor and a unique relay changeover circuit to make it reversible. It can operate from 12-32V batteries at currents up to 30A. Logic control of the relay changeover circuit means that it can only change direction when the motor is stopped. The unit comes in two kit versions. The first is the basic speed control with two paralleled Mosfets and a dual op amp to provide pulse width modulation (PWM). The second version adds the relay changeover circuit 26  Silicon Chip and its logic control. If you don’t need a reversing feature, you only need buy the basic kit. Either way, the speed control can be via an onboard trimpot, via an external 5kΩ potentiometer, or via a motorcycle throttle based on a Hall Effect sensor. This could be ideal for a wheelchair controller or an electric bike. Circuit description Refer now to Fig.3 which shows both sections of the circuit. The lefthand side is the basic speed controller while on the righthand side are the relays and associated logic control. First, let’s focus on IC1 (the LM358 dual op amp) and the 5kΩ potentiometer. Op amp IC1a and its associated components comprise a triangle wave oscillator. Its frequency is around 300Hz and its output amplitude is around 1V peak-to-peak. The mean DC level of this triangle waveform can be lifted up or down, dependent on the setting of the 5kΩ speed control potentiometer. This output waveform is connected to the non-inverting input of IC1b, pin 5. IC1b is connected as a comparator and it compares the triangle waveform with the 3.5V fixed reference at its pin siliconchip.com.au Fig.1: the scope grab illustrates the basic operation. The triangle wave from the oscillator is compared to a 3.5V reference (pink trace) and when it exceeds this reference, a corresponding motor drive pulse (blue trace) is produced. 6. When the speed control is advanced so that peaks of the triangular waveform at pin 5 exceed the 3.5V reference voltage at pin 6, the output at pin 7 goes high and this turns on two power Mosfets, Q6 & Q7. This means that the Mosfets are pulsed on whenever the triangle waveform peaks go above 3.5V. Advancing the speed control increases the duty cycle of the pulses. The circuit operation is demonstrated above in the two scope screen grabs of Fig.1 & Fig.2. In each case, the green trace shows the triangle waveform while the pink trace shows the 3.5V reference which is fixed. As you can see, each time a portion of the triangle waveform intersects the pink trace and is above it, there is a corresponding pulse to the Mosfet gates, as shown by the blue trace. The voltage across the motor, between the positive supply line and Fig.2: this scope screen grab shows the operation at higher throttle settings. The triangle waveform now exceeds the reference voltage for a greater proportion of the time and so the pulses fed to the motor are much wider. the Mosfet drains, is shown in the yellow trace. Fig.1 shows the operation at a very low throttle setting and so the pulses fed to the motor are very narrow and its speed will be low. By contrast, Fig.2 shows the operation at higher throttle settings. As can be seen, the corresponding pulses fed to the motor are much wider. When the throttle control is fully advanced, this results in the triangle waveform being wholly above 3.5V. This means that pin 7 of IC1 remains high permanently and so Mosfets Q6 & Q7 are turned on continuously. Soft start When power is first applied to the controller circuit, the 100µF capacitor on pin 6 of IC1b is discharged which means that pin 6 will be high at about +7V. The capacitor then begins to charge via the 39kΩ resistors on pin 6, thus pulling the voltage at pin 6 down to 3.5V. Therefore, at the instant when the power is applied the motor cannot run, even if the throttle is fully advanced. This gives the circuit a “soft start” feature and the motor cannot start with a lurch at initial power-up. Two regulators There are two transistor regulator Where To Buy Kits Kits for this project are available from Oatley Electronics Pty Ltd, PO Box 89, Oatley, NSW 2223. Phone (02) 9584 3563. Website: oatleyelectronics.com The reversible version (Cat. K275) costs $39 plus p&p, while the basic non-reversible version is $24 plus p&p. Are all oscilloscopes created equal? "Cleverscope is still the best out there, keep up the good work !!!!!" Karl, USA Signal: Video color burst, as presented to an ADC. Task: check DC Ours: We have proper DC offset and 10, 12 or levels, noise, and Ours spectral leakage. 14 bit resolution. We digitize over the range 1.2 www.cleverscope.com siliconchip.com.au to 1.5V. With the 10 bit ADC the resolution is 0.3/1000 = 300 uV - with 14 bit ADC it’s 18uV! You see all the detail. The spectral response has good SNR. Theirs: They don’t have DC offset, and only 8 bits. They have to digitize over -2V to +2V to capture this signal. The resolution is 4000/256 = 16 mV - 52x worse than ours. You don’t see all the detail, and the spectral response has poor SNR. August 2010  27 +7V BAT+ Q2 BDX37 +7V C E A B 4.7k 1k 100nF 100 µF 100 µF E 39k Q3 BD140 HALL EFFECT THROTTLE λ LED1 K B RED C +5V K 1 E ZD1 15V 1M 2 IC1a 1 10k D 8 +3.5V 6 22Ω 7 IC1b Q6 IRF2804 G S 4 Q1 1M E A D1 1k 47nF 1M 5 C 1V P-P 4.7nF OR 5.6nF D 39k 22Ω Q7 IRF2804 G K S * USE EITHER TRIMPOT VR1 OR EXTERNAL 5k POT OR HALL EFFECT THROTTLE SC  2010 MOT– A IC1: LM358 B 100 µF 63V K 3 ZERO VR2 2k 2.2k B 47nF 220k A C A +4V TO +5V 5k POT (ALTERNATIVE TO HALL EFFECT THROTTLE) 4.7k Q5 BDX37 D7 VR1* 5k 2 D2 SR1060 (USE WIRE LINK FOR 12V OPERATION) R1 4 3 K 4.7k 3.3k GREEN BLACK 470Ω DC MOTOR SPEED CONTROL BAT– D6: 1N4004 A D1, D3-D5, D7: 1N4148 A K ZD1 A K K Fig.3: the circuit uses op amp IC1a to generate a 300Hz triangle wave. This is DC level shifted using the throttle and fed to comparator IC1b which then generates the PWM square-wave pulses to drive Mosfets Q6 & Q7 and the motor. Relays RLY1, RLY1a, RLY2 & RLY2a and their associated control circuit (IC2a-IC2d) provide the reversing feature. circuits in the controller. The first regulator, comprising transistors Q2 & Q3 and red LED1, provides the +7V rail. It works like this: LED1 provides a 1.8V reference at the base of Q3 and the resulting 1.1V at Q3’s emitter causes 2.34mA to flow in its 470Ω emitter resistor and through the 3.3kΩ resistor at its collector. This provides +7.7V at Q4’s base and so +7V appears at Q4’s emitter. This sets the voltage conditions for the throttle and the triangle wave generator based on IC1a. The second regulator is based on zener diode ZD1 and transistor Q5. ZD1 provides a 15V reference and is bypassed by a 47nF capacitor to the base of Q5 which operates as an emitter follower. Interestingly, for battery 28  Silicon Chip voltages of less than about 16V, ZD1 will not be biased on (ie, no zener current will flow) and therefore Q5 will act as a simple capacitance multiplier filter. It provides the supply rail to IC1 and thereby ensures that the gates of the Mosfets are driven with more than 10V, provided the battery voltage is at least 12V. This is desirable to ensure that the Mosfets are turned on fully to minimise their voltage loss and power dissipation. For higher battery voltages, up to 32V, ZD1 and Q5 ensure that the gate voltage delivered by IC1b is limited to about 13V. Note that the circuit shows three alternative throttle arrangements. The first is via trimpot VR1 which can be installed on the PC board. The second is for an external 5kΩ speed control and the third is a twist grip Hall Effect throttle. Only one of these options can be used at any one time. Trimpot VR2 is a zero control. This is adjusted so that no voltage is applied to the motor at the minimum setting of the speed control. Relay switching Having a speed control on a motor is all very well but in many applications you need to run the motor in forward or reverse. In order to do this on a DC motor, you need to swap the connections to the motor. In small motor circuits that could be done by a double-pole changeover switch but siliconchip.com.au +7V BAT+ +12V 100 µF 63V E RLY2,2A GREEN LINKS FOR 12V OPERATION C RLY1,1A K D6 1N4004 RED LINK FOR 24V OPERATION D A NOTE: REPLACE RELAY LINK WITH 82 Ω 2W RESISTOR FOR 32V OPERATION RELAY LINK B +7V MOT– 2.7k D3 K 47k 100 µF 5 A IC2b 100 µF 4 6 1M A D5 1M K 39k MOTOR IC2: 4093B 1 12V–32V BATTERY IC2a 3 A 9 K 12 10 IC2c D4 39k IC2d 13 2 HOLD DOWN FOR REVERSE 14 8 S1 47nF 11 7 A 2.7k LED2 47nF 2.7k λ A K λ LED3 B K BAT– LEDS K A * NOTE: CORRECT FOR D2: SR1060* THE SR1060 DIODES USED IN THIS KIT, BUT NOT THE STANDARD K SR1060 PINOUTS A when heavy currents are involved, relays are required. In the simplest arrangement, this can be done with a single large doublepole double-throw (DPDT) relay or it could be done with two single-pole double-throw (SPDT) relays being switched simultaneously. This circuit is a little novel in that uses four SPDT relays with the relays used as paralleled pairs to substantially increase the switch contact rating. But there is a further refinement in that the heavy motor currents are never actually broken by the relay contacts. Instead, the relays are only operated when the voltage across the load is zero and therefore no current is flowing. This means that there will not be siliconchip.com.au B C any contact arcing and accompanying contact erosion. Relay logic controller The relay logic controller is based on a 4093 quad 2-input Schmitt trigger NAND gate package. Gates IC2c & IC2d are connected as an RS flipflop which can be set or reset by having one of its inputs at pins 8 & 13 pulled low. Pin 10 controls the relay switching transistor Q4, so when this output is high, the relays are on and this provides the reverse direction for the motor. Pins 1 & 2 of IC2a are normally pulled high by the series-connected 1MΩ and 39kΩ resistors but when pushbutton switch S1 is pressed, the inputs are pulled low. When the motor C IRF2804 D G B E Q4 BD681 E BD140, BD681 BDX37 C8050 C E D S is running, the drains of the Mosfets (Q6 & Q7) are being pulsed low and this repeatedly pulls the negative side of the 100µF capacitor connected to pins 5 & 6 of IC2b low, via diode D3. Hence the output of IC2b is high and this pulls pins 8 & 13 high via diodes D5 & D4 respectively, so the RS flipflop cannot be toggled. Therefore motor direction cannot be changed while ever it is running. When the Mosfets are off, the motor stops running and pins 5 & 6 of IC2b are pulled high via the associated 47kΩ resistor, the 100µF capacitor being discharged. The motor direction cannot be changed during this discharge time which is around four seconds. This feature prevents sudden changes in the August 2010  29 USE ONE OR THE OTHER + – 5k POT TO BATTERY (12V– 32V) HALL EFFECT THROTTLE LINK B & C AND LINK D & E FOR 12V OPERATION LINK C & D FOR 24V OR 32V OPERATION BD140 BDX37 SPEED + Q2 LED1 Q3 2.2k 15V 470 4.7k 4.7k* * 3.3k MOT– 47k 4148 1M 39k 2.7k 2.7k 1M D5 MOT– D4 4148 39k BAT– RLY1 + Q6 BAT– FWD/REV 47nF S1 + + LED2 LED3 IC2 4093B RLY1A P4 100 F VR1* 5k Q5 BDX37 47nF REV 47nF RLY2 P3 1k 4.7nF P2 RED WIRE D7 IC1 LM358 39k 22 22 220k 39k 4.7k FWD BAT+ ZD1 K275A R1 100 F+ A SR1060 D2 K +7V BAT+ TO MOTOR +7V RLY2A P1 GREEN WIRE 100 F Q4 RELAY LINK* * * D6 100  F 4004 BD681 100 F 2.7k D B E C 4148 D3 100 F 100nF Q7 Q1 C8050 D1 © oatleyelectronics.com ZERO VR2 2k 1k 4148 1M 1M 1M 4148 10k + BLACK WIRE 47nF * DELETE TRIMPOT VR1 IF EXTERNAL THROTTLE USED * * REPLACE WITH WIRE LINK FOR 12V OPERATION * * * REPLACE RELAY LINK WITH 82  2W RESISTOR FOR 32V OPERATION LEADS TO MOTOR Fig.4: follow this parts layout diagram to build both versions of the controller (the non-reversible version uses only those parts to the left of the red dotted line). Note that some of the parts and linking options vary, depending on on whether the controller is to be powered from 12V, 24V or 32V. motor direction and this 4s period of time can be lengthened or shortened by respectively increasing or decreasing the value of the 47kΩ resistor. Now, when the motor voltage (and current) is zero, the RS flipflop can be toggled. So to change from forward to reverse direction, you press pushbutton switch S1. This not only pulls IC2a’s inputs low but also pulls pin 8 of IC2c low and this sets the flipflop so that it turns on transistor Q4 and energises the two relay coils. Note that as soon as you release pushbutton switch S1, it will immediately allow pins 1 & 2 of IC2a to go high again and this will cause pin 3 to go low. This will then reset the RS flipflop, thereby turning off transistor Q4 and lighting LED2, which indicates forward direction. Hence, for reverse operation, you need to keep the pushbutton pressed. This makes sense if you are making this speed control for an electric bi­ cycle and you only want reverse engaged for very limited time. However, if the speed control needs to be in reverse mode for much longer periods, the pushbutton switch is not practical and you will need to substitute a standard SPST toggle switch. Building it Both versions of the DC Motor Speed Control are available as kits from Oatley Electronics (see parts list). The reversible version is built on a double-sided PC board coded K275 (138 x 70mm), while the non- Table 1: Linking Options Supply Rail Relay Configuration Relay Link Resistor R1 12V 24V Link B & C, Link D & E Wire Link Wire Link Link C & D Wire Link 4.7kΩ 32V Link C & D 82Ω 2W Resistor 4.7kΩ 30  Silicon Chip reversible version (without the relays) uses a double-sided PC board coded K275A (60 x 70mm). Note that the parts layout on the latter is identical to the corresponding section on the fully-reversible version. Fig.4 shows the assembly details. If you are building the non-reversible version, just follow the layout to the left of the red dotted line. Conversely, for the reversible version, you will need to assemble the entire board. Begin by installing the resistors and diodes. Table 2 shows the resistor colour codes but check each resistor using a DMM before installing it. Note that resistor R1 (4.7kΩ, near LED1) should be replaced with a wire link for 12V operation. Conversely, you will need to install the resistor is you intend operating the controller from 24V or 32V. Be sure to install the correct diode type at each location and check that they are all correctly orientated. Diode D2 (SR1060) goes in with its metal tab adjacent to the edge of the PC board. Once these parts are in, install the siliconchip.com.au This view shows the reversible version. Note that you must fit M3 x 10mm screws to the BAT+, M-, GND and MOTOR positions to carry the high currents. It’s also a good idea to run a layer of solder over the high-current copper lands for currents above 15A – see text. capacitors and IC sockets. Check that the electrolytics are all correctly orientated and make sure that the sockets go in with their notched ends positioned as shown. Now for the transistors. These should all be pushed down onto the PC board as far as they will comfortably go before soldering their leads. Use the correct type at each location and take care with their orientation – the metal faces of Q2 & Q3 face the 100µF capacitor and LED1 respectively, while Q5’s metal side faces the adjacent 4.7kΩ resistor. Q4 goes in with its metal face towards the edge of the PC board. The two power Mosfets (Q6 & Q7) should now be loosely attached to their U-shaped heatsinks using M3 x VR1 (5kΩ) but you must leave this part out if you are using an external throttle to control motor speed. VR1 is installed only if you are using the controller to set a fixed motor speed (ie, no external throttle). The two ICs can now be plugged into their sockets (note: they face in opposite directions) and the relays installed. These relays will only fit 10mm machine screws, washers and nuts. That done, install each assembly in position and push it down until the bottom edge of its heatsink rests against the PC board. The heatsink tabs should go through the holes in the board and these should be bent using pliers to hold the assemblies in position while you solder the device leads. Bending the heatsink tabs will also make the assemblies more secure, particularly if the board will later be subject to vibration. Once everything is in place, tighten the screws that secure the Mosfet tabs to the heatsinks. Trimpot VR2 (2kΩ) can be installed and the board has been designed to accept either a horizontal or vertical trimpot. The same goes for trimpot Table 3: Capacitor Codes Value 100nF 47nF 4.7nF µF Value IEC Code EIA Code 0.1µF 100n 104 .047µF   47n 473 .0047µF   4n7 472 Table 2: Resistor Colour Codes o o o o o o o o o o o o o siliconchip.com.au No.   3   1   1   4   1   3   1   3   1   2   1   2 Value 1MΩ 220kΩ 47kΩ 39kΩ 10kΩ 4.7kΩ 3.3kΩ 2.7kΩ 2.2kΩ 1kΩ 470Ω 22Ω 4-Band Code (1%) brown black green brown red red yellow brown yellow violet orange brown orange white orange brown brown black orange brown yellow violet red brown orange orange red brown red violet red brown red red red brown brown black red brown yellow violet brown brown red red black brown 5-Band Code (1%) brown black black yellow brown red red black orange brown yellow violet black red brown orange white black red brown brown black black red brown yellow violet black brown brown orange orange black brown brown red violet black brown brown red red black brown brown brown black black brown brown yellow violet black black brown red red black gold brown August 2010  31 The basic non-reversible version is shown here, together with the optional handle-bar type (Hall effect) throttle. Be sure to omit trimpot VR1 from the board if you intend using an external throttle – see text. Parts List: Kit K275A 1 PC board, code K275A, 60 x 70mm 2 PC-mount 2-way screw terminal blocks 1 5kΩ trimpot (VR1) 1 2kΩ trimpot (VR2) 1 8-pin DIP IC socket 5 M3 x 10mm machine screws 5 M3 nuts 5 M3 washers 2 heatsinks Tinned copper wire for links Semiconductors 1 LM358 dual op amp (IC1) 1 C8050 NPN transistor (Q1) 2 BDX37 NPN transistor (Q2,Q5) 1 BD140 NPN transistor (Q3) 2 IRF2804 Mosfets (Q6,Q7) 1 15V zener diode (ZD1) 2 1N4148 small-signal diodes (D1,D7) 1 SR1060 Schottky diode (D2) 1 red 3mm LED (LED1) Capacitors 1 100µF 63V electrolytic 2 100µF 16V electrolytic 1 100nF monolithic 2 47nF monolithic 1 4.7nF greencap Resistors (0.25W, 5%) 3 1MΩ 1 3.3kΩ 1 220kΩ 1 2.2kΩ 2 39kΩ 2 1kΩ 1 10kΩ 1 470Ω 3 4.7kΩ 2 22Ω 32  Silicon Chip one way and you should use generous amounts of solder on their contact pins since they can carry quite high currents. Pushbutton switch S1 is the Forward/Reverse switch. This should only be installed on the board if you want a switch that you hold down for reverse operation (ie, if you only want reverse for a short time). However, as stated above, you will need to substitute a standard SPST toggle switch if you want reverse for extended periods of time. In that case, just connect the switch contacts to the appropriate pads on the PC board using flying leads. operation, depending on the supply voltage. For 12V operation, use separate wire links to connect points B & C together and points D & E together. Alternatively, for 24V or 32V operation, connect points C & D together (don’t forget to replace the Relay Link with an 82Ω 2W resistor for 32V operation – see above). Note that the two links installed for 12V operation overlap each other. Be sure to position them so that they cannot short together (or sleeve them with heatshrink, or use insulated wire). The final option concerns one of the 4.7kΩ resistors (R1) in series with LED1. As stated previously, this must be replaced with a wire link for 12V operation. Linking options High-current connections There are several linking options and component changes, depending on whether you are operating the controller from 12V, 24V or 32V. Table 1 shows the details. First, on the reversible version, you will need install the “Relay Link” at the top of the board (above the relays). This is simply a wire link for operation up to 24V but this must be replaced with an 82Ω 2W resistor (not supplied with the kit) for 32V operation. Similarly, you also need to link the relay coils for either parallel or series All connections to the motor and battery must be run via crimped eyelet connectors which are attached to the PC board using M3 x 10mm machine screws, washers and nuts. In addition, if building the reversible version, you must also fit an M3 x 10mm screw, washer and nut to the MOT- hole position, immediately to the left of relay RLY1A (see Fig.4). That last step is important because the screw through the MOT- hole helps carry the heavy currents that flow through the motor and power Forward/reverse switch siliconchip.com.au Mosfets Q6 & Q7. The same goes for the machine screws that are used to terminate the eyelets for the battery and motor connections. In short, you must have machine screws running through the BAT+, BAT-, MOT- and TO MOTOR holes to carry the heavy currents involved. Don’t just rely on the through-hole plating of the board – it could “fuse” under high-current conditions. If you are building the smaller nonreversible version, use the alternative BAT-, MOT- & BAT+ connections shown on Fig.4. The motor is connected between MOT- and BAT+. As before, all connections must be made via crimped eyelet connectors which are attached using M3 x 10mm screws, washers and nuts. Make sure that the leads are adequately rated to carry the currents involved. Generally, this will involve using heavy-duty cabling rated at 15A or greater if required. Finally, for currents above about 15A, run a thick layer of solder over all the high-current copper lands on the PC board. This must be done on both sides of the board and involves the lands running to the power Mosfets, the motor and battery connections and the relay contacts. Testing When the assembly is complete, check your work very carefully. Any mistakes in component placement or polarity could result in damage when the power supply is connected. Supply polarity is also crucial – getting it wrong can seriously damage the unit. Parts List: Kit K275 (Reversible Version) 1 PC board, code K275, 138 x 70mm 1 PC-mount tactile switch (S1) 2 PC-mount 2-way screw terminal blocks 1 5kΩ trimpot (VR1) 1 2kΩ trimpot (VR2) 1 8-pin DIP IC socket 1 14-pin DIP IC socket 4 12V 30A relays 7 M3 x 10mm machine screws 7 M3 nuts 7 M3 washers 2 heatsinks Tinned copper wire for links Semiconductors 1 LM358 dual op amp (IC1) 1 4093 quad 2-input NAND gate (IC2) 2 red 3mm LEDs (LED1, LED3) 1 green 3mm LED (LED2) 1 C8050 NPN transistor (Q1) 2 BDX37 NPN transistor (Q2,Q5) If everything checks OK, connect a 12V battery (or other high-current DC power supply) but do not connect the motor yet. Now check that +7V is present on the emitter of transistor Q1. If it is, set the throttle (either an external pot or VR1) to minimum and monitor the voltage at pin 7 of IC1. This voltage should vary as you vary the throttle and if you have an oscilloscope, you can check that the PWM duty cycle varies as shown on the scope screen shots of Fig.1 & Fig.2. Presensitized PCB & associated products 1 BD140 NPN transistor (Q3) 1 BD681 NPN Darlington transistor (Q4) 2 IRF2804 Mosfets (Q6,Q7) 1 15V zener diode (ZD1) 5 1N4148 small signal diodes (D1,D3-D5,D7) 1 SR1060 Schottky diode (D2) 1 1N4004 1A diode (D6) Capacitors 2 100µF 63V electrolytic 4 100µF 16V electrolytic 1 100nF monolithic 4 47nF monolithic 1 4.7nF greencap Resistors (0.25W, 5%) 5 1MΩ 1 3.3kΩ 1 220kΩ 3 2.7kΩ 1 47kΩ 1 2.2kΩ 4 39kΩ 2 1kΩ 1 10kΩ 1 470Ω 3 4.7kΩ 2 22Ω Next, set the throttle to minimum, connect a motor and connect your DMM (set to volts) across the motor’s terminals. Adjust trimpot VR2 for a reading of 0V – this will zero the controller’s output when the throttle is at minimum. Alternatively, you can set it to give a minimum motor speed. Now try adjusting the throttle. The motor should start and respond to throttle adjustments and the DMM should indicate corresponding voltage SC variations. IN STOCK NOW! •Single Sided Presensitized PCBs •Double Sided Presensitized PCBs •Fibreglass & Phenolic •UV Light Boxes •DP50 Developer •PCB Etch Tanks, Heaters & Aerator Pumps •Thermometers •Ammonium Persulphate Etchant •PCB Drill Bits (HSS & Tungsten) For full range, pricing and to buy now online, visit 36 Years Quality Service siliconchip.com.au www.wiltronics.com.au Ph: (03) 5334 2513 Email: sales<at>wiltronics.com.au August 2010  33