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Throttle Interface For The DC Motor Speed Controller Words by Leo Simpson Design by Branko Justic* *Oatley Electronics Last month, we presented the High-Power Reversible DC Motor Speed Controller. Here is a companion controller which works with a motorcycle-style throttle control. It also features a forward/ reverse switch to control the motor direction. L AST MONTH’S DC Motor Speed Controller was designed to work from a joystick or a potentiometer that is normally centred. This has the virtue of simplicity but having the motor speed and direction under the control of a single potentiometer can be a problem in some applications. Therefore, this companion design was produced to allow a potentiometer to control only the motor speed while the motor direction is controlled by a forward/reverse toggle switch. Furthermore, the potentiometer can be substituted with a spring-loaded motorcycle-style throttle control which could be just the ideal solution for applications like electric scooters, Go-karts, etc. This throttle control 92  Silicon Chip is shown in the photographs in this article. The motorcycle-style throttle is based on a magnet and a Hall-effect IC to derive a control voltage. The more you rotate the throttle against its spring tension, the higher the control voltage fed to the input of the Throttle Interface circuit. Circuit details In effect, the interface circuit emulates the effect of the 10kW speed control pot used in last month’s circuit. The DC voltage from the wiper of that 10kW speed pot determines the motor speed and direction. When the potentiometer is centred, the wiper voltage is +4.4V; when set for the maximum forward speed, this voltage is +6.4V and when set for maximum reverse speed it is +2V. In order for the motor to go in the forward direction only, the control voltage range must be from +4.2V (motor stationary) to +6.4V (maximum speed). Similarly, for the reverse direction, the control voltage has to vary from +4.2V (stationary) to +2V for maximum speed. Therefore, the voltage range needed in both directions is around 2.2V. So the Throttle Interface circuit of Fig.1 has to produce this voltage range. In essence, the circuit of Fig.1 uses two op amps as a voltage level translator and some CMOS analog gates to provide the forward-reverse function. siliconchip.com.au This Hall-effect throttle works just like the spring-loaded throttle on a motorcycle. It’s ideal for use on electric scooters and Go-karts, etc. Let’s have a look at how it works. Consider a 10kW potentiometer connected to the input of the circuit so that its wiper is point C. If the potentiometer (or the alternative Hall-effect throttle control) is rotated over its full range, the voltage at the C input can vary between +1.6V and +6.8V. The attenuator comprising the 56kW and 68kW resistors reduces this range to between +0.65V and +2.8V. This is almost exactly the required control voltage range of around 2.2V. This voltage is fed to op amp IC1a which is connected as a unity gain buffer (ie, the input and output voltages will be the same). So its output range will still be between +0.65V and +2.8V. Transistor Q1, in conjunction with diodes D1 & D2, is connected as a constant current source with delivers 3mA into the output of IC1a via a 1.2kW resistor (a buffer can act as a sink or a source and in this case we are forcing IC1a to “swallow” 3mA while maintaining a constant output voltage). The result is that the output voltage range at the collector of Q1 is exactly 3.6V above the voltage at pin 1 of IC1a. And guess what? That means the voltage range at the collector of Q1 will be between +4.25V and +6.4V. This is exactly the voltage range of Fig.1: this is the interface circuit for the Hall-effect throttle. Op amps IC1a & IC1b provide voltage level translation, while CMOS gates IC2a-IC2d provide the necessary switching for the forward-reverse function. siliconchip.com.au May 2007  93 Parts List 1 PC board, code K244, 50 x 67mm 2 2-way 5mm screw terminal blocks 2 3-way 5mm screw terminal blocks 1 20kW trimpot (VR1) 1 100W trimpot (VR2) Semiconductors 1 M5223P low-voltage dual op amp (IC1) 1 4066 quad analog switch (IC2) 1 C8550 PNP transistor 2 1N4148 signal diodes (D1, D2) 1 3mm green LED (LED1) Fig.2: follow this parts layout to build the PC board. Make sure that all polarised parts are correctly oriented (ie, the ICs, diodes, Q1, LED1 and the electrolytic capacitors). Capacitors 2 10mF 35V PC electrolytic 2 15nF (.015mF) ceramic or metallised polyester (greencap) Resistors (0.25W, 1% or 5%) 2 100kW 1 1.8kW 4 56kW 1 1.2kW 2 39kW 2 1kW 1 22kW 1 120W 1 2.7kW Kit availability This Throttle Interface project was produced by Oatley Electronics who own the design copyright. Kits (Cat. K244) can be purchased from Oatley Electronics Pty Ltd, PO Box 89, Oatley, NSW 2223. The kit includes the PC board and all on-board components only.The Hall-effect throttle (Cat.Throt2) can be purchased separately from Oatley Electronics. See their website at: http://www.oatleyelectronics.com This full-size view shows the fully-assembled PC board. Refer to the wiring diagram (Fig.3) for the external wiring connections. the speed control pot in last month’s circuit. So that’s the forward control voltage range provided for. What about the reverse control voltage range? This is provided by op amp IC1b which is configured as a unity gain inverting buffer with a reference voltage of +4.4V connected to its noninverting input (pin 5). By inverting the voltage appearing at the collector of Q1, it produces the required reverse control range of +4.4V to +2V. Now that we have the required voltage ranges for forward and reverse motor control of last month’s circuit, we only need some CMOS gates to select the correct output from the collector of Q1 or the output of IC1b. This function is provided by the analog gates in IC2, under the control of switch S1. This works as follows. First, consider that switch S1 is open. This allows the control inputs of IC2a, IC2b & IC2c to be pulled “high” Table 1: Resistor Colour Codes o o o o o o o o o o No.   2   4   2   1   1   1   1   2   1 94  Silicon Chip Value 100kW 56kW 39kW 22kW 2.7kW 1.8kW 1.2kW 1kW 120W 4-Band Code (1%) brown black yellow brown green blue orange brown orange white orange brown red red orange brown red violet red brown brown grey red brown brown red red brown brown black red brown brown red brown brown 5-Band Code (1%) brown black black orange brown green blue black red brown orange white black red brown red red black red brown red violet black brown brown brown grey black brown brown brown red black brown brown brown black black brown brown brown red black black brown siliconchip.com.au Fig.3: the interface board connects between the Hall-effect throttle and the DC Motor Speed Controller board as shown here. Alternatively, you can use a 10kW pot instead of the Hall-effect throttle. by the associated 100kW resistor and do the job of the 10kW speed potentherefore all these switches are “on”. tiometer from last month’s circuit. IC2b lights LED1, indicating a forward Well, that’s not quite right because we direction. The voltage at pin 2 of IC2a wanted to change the function but you is “low” and therefore switch IC2d is now have the picture. “off”. Since IC2c is “on”, the forward The only wrinkle to be added is control voltage from the collector of that the low output voltage required Q1 is applied to output terminal C2 from IC1a (ie, +0.65V or less) means via the 56kW resistor. The time delay that an ordinary dual op amp would provided by the 56kW and the 10mF not do the job. Instead, an M5223P capacitor is included to prevent any low-voltage dual op amp is specified sudden changes in speed. for this task. If switch S2 is now closed (ie, terminal D2 is grounded), the control Construction inputs of IC2a, IC2b & IC2c are pulled All the components for the Throtlow, switching those gates off and tle Interface, with the exception of IC2d “on”. This connects the revers- the Throttle Control itself (or 10kW ing voltage from the output of IC1b potentiometer VR3) and switch S1, to output C2 via the abovementioned are mounted on a PC board measuring 56kW resistor. 50 x 67mm. The component overlay is So there you are. That’s how to use shown in Fig.2. RF_SiliconChip_60x181mm.qxd 30/3/07 2:12 PM Page 1 two op amps and four CMOS gates to It is easiest to fit the components to the PC board in order of height. Start with signal diodes D1 & D2 and finish with the electrolytic capacitors, soldering and trimming the leads of each component as you go. Make sure the ICs and electrolytic capacitors go in the right way around. Testing & adjusting Before you can check the operation of the Throttle Interface, you need to have assembled and checked the operation of the DC Motor Speed Controller presented last month. In particular, you should connect it as shown last month, with a 10kW potentiometer connected to the B, C & D terminals of the PC board. You also need to check that +8V is available at the output terminal on the main PC board. With those checks verified, you can ELECTRO CHEMICALS Chemical Technology siliconchip.com.au • Dust Off • Freezing Spray • Electronic Cleaning Solvent No. 1 • Electronic Circuit Board Cleaner • Electrical Contact Cleaner Lubricant • Video Head Cleaner • Ultrasonic Bath Cleaner • Isopropyl Alcohol • Protek • Contact Treatment Grease • Contact Treatment Oil • Solvent Diluted Oil • Contact Cleaning Strip • Circuit Board Lacquer • Q43 – Silicon Grease Compound • Heat Sink Compound Contact us to find your nearest distributor: sales<at>rfoot.com.au Tel: 02 9979 8311 Fax: 02 9979 8098 Richard Foot Pty Ltd, 14/2 Apollo Street,Warriewood NSW 2102 May 2007  95 Switchmode H-bridge: How It Works As noted last month, the DC Motor Speed Controller relies on a switchmode H-bridge employing four Mosfets. The relevant part of the circuit is shown in Fig.4. Only two Mosfets are turned on at any one time. For example, to drive the motor in the forward direction, Q7 & Q6 would be “on” while Q5 & Q8 would be “off”. Similarly, to drive the motor in the reverse direction, Q5 & Q8 would be “on” while Q7 & Q6 would be “off”. Furthermore, to turn on the upper Mosfets (ie, Q5 or Q7), a much higher gate voltage is required, as explained last month. This is demonstrated in the accompanying scope screen shots. For example, Scope 1 shows the gate voltage signals needed to turn on Q5 & Q8 while keeping Q7 & Q6 “off”. The higher amplitude yellow trace is the gate signal to Q5 and the lower amplitude blue trace is the gate signal to Q8. The purple trace and hidden green trace are the 0V gate signals to Q6 & Q8, keeping them “off”. Scope 2 shows the gate signals for reverse operation, where Q7 & Fig.4: the switchmode H-bridge output stage of the DC Motor Speed Controller employs four power Mosfets but only two are turned on at any one time. Q7 & Q6 are turned on to drive the motor in one direction, while Q5 & Q8 are turned on to drive the motor in the other direction. Q6 are “on”. The higher amplitude purple trace is the gate signal to Q7 and the lower amplitude green trace in the gate signal to Q6. The yellow and hidden blue traces SCOPE 1 connect the Throttle Interface board to the main PC board, as shown in Fig.3. If you use a “Hall-effect” throttle (from Oatley Electronics), it has to be connected to the “T1”, “T2” & “C” terminals. If you use a standard 96  Silicon Chip show the 0V gate signals to Q5 & Q8, keeping them off. In both scope screen shots the duty cycle of the gate signals is about 60%, corresponding to about half-speed operation. SCOPE 2 potentiometer, it has to be connected to the “P1”, “P2” & “C” terminals. Do not connect both the 10kW pot and the Hall Effect throttle. A motor should be connected, to check overall operation. First, adjust VR1 so that the motor stops when the throttle (or pot.) is at its minimum speed setting and the switch is set for the Forward direction. That done, adjust VR2 so that the motor stops when the throttle (or pot.) is at its minimum speed setting and the switch is set for SC the Reverse direction. siliconchip.com.au