Fullscreen HTML5Med Res (??MB)HTML4

High-Power Reversible DC Motor Speed Controller Words by Leo Simpson Design by Branko Justic* *Oatley Electronics This reversible DC motor speed controller uses a switchmode Mosfet bridge circuit that drives the motor. It can be controlled by a 1-2ms pulse train from a radio control system or by a single potentiometer to give forward/reverse throttle control. It can operate from 12V or 24V batteries at currents up to 20A with just four Mosfets in the bridge circuit. O VER THE YEARS, motor speed controls have always been popular and this one is a beauty. Its Mos­ fet bridge circuit can be used for speed control in an R/C system using standard 1-2ms pulse control or you can simply connect a 10kW (linear) potentiometer or joystick to give single-handed forward/reverse control. As such, it would be suitable for a golf buggy, electric wheelchair, go-kart or whatever motor control application you have in mind. The bridge driver circuit employs 80A N-channel Mosfets that have an on-resistance of just five milliohms 26  Silicon Chip (5mW) and are suitable for 10-30V operation. In practice, that will mean operation from 12V or 24V batteries. When tested with a loaded 24V motor at a continuous 10A the MOSFETs became just slightly warm. No additional heatsinking would be required for operation at 20A. This test was conducted with four MOSFETs in the output bridge but there is provision for another four MOSFETs to be paralleled with the existing ones in the output bridge driver. This would result in each of the paralleled MOSFETs having one quarter of the power dissipation when compared to the original single devices! In a 24V system, there would be no problem powering motors with a power rating of up to 1kW. The complete circuit of the Speed Control For DC Motors is shown in Fig.1. With a total of four op amps, four comparators and four Mosfets, it may look fairly complicated but we can break it down into two sections in order to understand how it works. Bridge circuit operation First, let’s have a look at the bridge output circuit which drives the motor. You first need to understand how a siliconchip.com.au This view shows the top side of the assembled PC board. Be careful not to get the two ICs mixed up and take care to ensure that all polarised parts (ICs, diodes, zener diodes & electrolytic capacitors) go in the right way around. The power Mosfets are mounted on the underside of the board (see below). The surface-mount Mosfets are soldered to the underside of the PC board while the external connections are run via crimped eyelet assemblies which are fastened in place using M3 machine screws and nuts. Mosfet bridge circuit drives the motor. Only two Mosfets turn on to drive the motor at any one time. The motor is connected to the terminals marked “Motor 1” and “Motor 2”. For example, to drive the motor in the forward direction, Q7 and Q6 would be “on” while Q5 & Q8 would be “off”. This would mean that current would flow from the positive rail VPOS (10-30V), through Q7, through the motor and then Q6 to the 0V (GND) rail. To drive the motor in the reverse direction, Q5 & Q8 would be “on” while Q7 and Q6 would be “off”. Both the above forward and reverse siliconchip.com.au conditions imply full speed operation with the respective Mosfets being turned on all the time. But this speed control is fully variable and the voltage to the motor is switched on and off rapidly at about 300Hz. For low speed, the turn-on pulses to the gates of the relevant Mosfets are quite short and for the high speeds they become progressively longer until at full speed the relevant gates are pulled high continuously. OK. So we know that only two Mosfets in the bridge circuit are turned on at any one time to drive the motor in forward or reverse but an extra wrinkle in this circuit is that all four Mosfet are N-channel devices. In order to switch on the top Mosfet (Q5 or Q7), we need a gate voltage which is about 8V higher than the main (motor) supply voltage (VPOS). How do we manage that? What we need first is a higher voltage supply to provide those high voltage gate signals Q5 & Q7. This is provided by op amp IC1b, complementary transistors Q3 & Q4 and the capacitors associated with D2-D7. Op amp IC1b is connected to operate as a square wave oscillator at a frequency of 4kHz. Its output is about 6V peak-peak. This is coupled to the April 2007  27 Fig.1: the circuit uses four Mosfets in a bridge configuration to drive the motor and these are pulse width modulated by sawtooth oscillator IC1a and comparators IC2a-IC2d. IC1c & IC1d provide an interface for a standard 1-2ms R/C control. IC1b, transistors Q3 & Q4 and diodes D2-D7 provide a high gate voltage for Mosfets Q5 & Q7. bases of transistors Q3 & Q4 which are connected as complementary emitter followers to provide a buffered output from the op amp. This combination produces an AC output voltage of 4.8V peak-peak. This AC output voltage is used to drive a Cockroft-Walton voltage multiplier made up of diodes D2-D7 and their associated 10mF capacitors. The DC output voltage from this multiplier is about 7-8V higher than the main supply voltage VPOS. The VPOS + 8V supply is coupled to the gates of Q5 & Q7 via 6.8kW resistors and these connect, in turn, to the outputs of comparators IC2a & IC2b. Note that this high voltage does not harm IC2 because it is an LM339 quad comparator with open-collector outputs. This means that its outputs 28  Silicon Chip are essentially the collectors of NPN transistors which can withstand any voltage up to +36V. In our circuit, the collector outputs of the four comparators are tied to VPOS + 8V via 6.8kW resistors for IC2a & IC2b and to VPOS via 4.7kW resistors for IC2c & IC2d. Switchmode operation For the following explanation, let’s assume that the 10kW potentiometer connected to terminals B, C, & D has its wiper initially centred. Op amp IC1a and its associated parts form an oscillator which produces a 300Hz sawtooth waveform of about 1.2V peak-peak. This sawtooth voltage is applied to the non-inverting input (pin 11) of IC2d and to the inverting input (pin 8) of IC2c. The 39kW, 15kW and 33kW resistors form a voltage divider from the regulated +8V supply in order to bias pin 10 of IC2d at +4.4V and pin 9 of IC2c at +3V. Since the swing of the sawtooth waveform is actually sitting between the upper and lower threshold voltages, both comparators (ie, IC2c & IC2d) have an output of 0V – ie, there is no pulse output from the comparators and the motor is stationary. Rotating the 10kW potentiometer so the voltage at its wiper is higher effectively raises the level of the sawtooth so that part of it intersects the 4.4V threshold for IC2d. This causes the output of IC2d to go high whenever the peaks of the sawtooth are above the +4.4V threshold. The output pulses from IC2d are buffered by IC2a. This means that gate pulses are delivered to Q6 & Q7 which siliconchip.com.au Table 1: Resistor Colour Codes o o o o o o o o o o o o o o o siliconchip.com.au No. 3 2 5 2 1 1 2 2 1 1 2 3 5 1 Value 1MW 220kW 120kW 68kW 39kW 33kW 15kW 12kW 10kW 8.2kW 6.8kW 4.7kW 2.2kW 220W 4-Band Code (1%) brown black green brown red red yellow brown brown red yellow brown blue grey orange brown orange white orange brown orange orange orange brown brown green orange brown brown red orange brown brown black orange brown grey red red brown blue grey red brown yellow violet red brown red red red brown red red brown brown 5-Band Code (1%) brown black black yellow brown red red black orange brown brown red black orange brown blue grey black red brown orange white black red brown orange orange black red brown brown green black red brown brown red black red brown brown black black red brown grey red black brown brown blue grey black brown brown yellow violet black brown brown red red black brown brown red red black black brown April 2007  29 Parts List 1 PC board coded OE-K243, 115 x 71mm 4 3mm screws 4 3mm nuts 8 3mm washers 2 14-pin IC Sockets 1 3-way 5mm screw terminal block 2 2-way 5mm screw terminal blocks 4 crimp eye terminals (for supply and motor connections) 1 10kW (lin) potentiometer 1 2kW trimpot (VR1) 1 100kW trimpot (VR2) Semiconductors 1 LM324 quad op amp (IC1) 1 LM339 quad comparator (IC2) 1 7808 8V voltage regulator (REG1) 1 C8050 NPN transistor (Q3) 1 C8550 PNP transistor (Q4) 1 1N4148 signal diode (D1) 6 1N5819 Schottky diodes (D2-D7) 4 18V 400mW zener diodes (ZD1-ZD4) 4 SDB85N03L N-channel surface-mount Mosfets (see text) Capacitors 4 100mF 35V electrolytic 6 10mF 35V electrolytic 2 1mF 16V electrolytic 1 4.7nF metallised polyester (greencap) 1 1nF metallised polyester (greencap) Resistors (0.25W, 1% or 5%) 3 1MW 2 12kW 2 220kW 1 10kW 5 120kW 1 8.2kW 2 68kW 2 6.8kW 1 39kW 3 4.7kW 1 33kW 5 2.2kW 2 15kW 1 220W Kit availability This project was produced by Oatley Electronics who own the design copyright. Kits (Cat. K243) can be purchased from Oatley Electronics Pty Ltd, PO Box 89, Oatley, NSW 2223. Phone: (02) 9584 3563 Fax: (02) 9584 3561 http://www.oatleyelectronics.com 30  Silicon Chip Fig.2: follow this parts layout diagram carefully when assembling the PC board. Eight surface-mount Mosfets are shown here but the “A” devices are all optional – see text. Note that Q3 and Q4 have different type numbers. drive the motor in one direction. Rotating the 10kW potentiometer in the opposite direction, so that the voltage at its wiper is lower, effectively lowers the level of the sawtooth so that part of it intersects the +3V threshold for IC2c. This causes the output of IC2c to go high whenever the troughs of the sawtooth are below the +3V threshold. The output pulses from IC2c are buffered by IC2b. This means that gate pulses are delivered to Q5 & Q8 which drive the motor in the other direction. The only part of the circuit which remains to be explained is that comprising op amps IC1c & IC1d and associated components. This takes the standard 1-2ms pulse from a radio control decoder and converts it to a varying DC level to control the sawtooth oscillator of IC1a. It does this in the following way. The pulse signal is first fed to IC1c which is connected as a comparator to buffer and “limit” the signal before it is fed to diode D1 and filtered by the 1mF capacitor. The resulting DC level represents the width of the input pulses. Short pulses give a low level while long pulses give a higher level. This is amplified and level-shifted by op amp IC1d and then fed to terminal A on the connector strip. This is linked to terminal C on the connector strip and fed via the 220kW resistor to IC1a to level-shift Table 2: Capacitor Codes Value 4.7nF 1nF mF code IEC Code    EIA Code .0047mF 4n7 472 .001mF 1n0 102 the sawtooth waveform and hence control motor speed and direction as described above. It is important to note that if you are using the 10kW potentiometer to control speed and direction, then terminals A & C must not be linked. Conversely, if you are using 1-2ms pulse control, then terminals A & C must be linked and the 10kW potentiometer must be omitted. Note that transistors Q1 & Q2 are missing from the circuit and PC board. These were present in an earlier prototype but have been designed out the circuit. Construction All the components of the Speed Control, with the exception of the 10kW potentiometer, are mounted on a PC board measuring 115 x 71mm. Assembly is best started with the SDB85N03L surface-mount Mosfets. Solder the legs of the Mosfets first and then solder the metal tag of each Mosfet to the PC board. A wooden clothes peg can be used to hold each Mosfet in place while it is soldered. Note that you will need a larger than normal siliconchip.com.au WARNING! Fig.3: here’s how to connect the speed pot and run the external wiring connections. The supply and motor connections are fastened to underside of the PC board (see photo). soldering iron to do this because most temperature-controlled irons will not have enough power to do the job. Make sure you place and solder each Mosfet in the correct location, so as to leave room for the additional Mosfets if they need to be fitted as well. With the Mosfets installed, you can then solder in all the smaller components. Make sure that the diodes, transistors, ICs and voltage regulator (REG1) are correctly located and oriented. Mistakes here can cause major damage if not discovered before power is applied. C O N T R O L S       The supply polarity is crucial. Reversed polarity may destroy the unit. In particular, note that Q3 and Q4 are different. Q3 is a C8050, while Q4 is a C8550. Don’t mix them up. Check each resistor’s value with your digital multimeter, before it is installed. Finally, make sure that you install each electrolytic capacitor with the correct polarity. nect a 12V battery or DC power supply. Do not connect the motor yet. Now check that +8V is present at the output of voltage regulator REG1 and on pin 4 of IC1. +12V should be present at pin 3 of IC2. That done, check that the voltage multiplier is working by measuring the voltage at the cathode end (white band) of diode D7. It should be about +20V or thereabouts. With the 10kW potentiometer centred (ie, for zero motor speed in either direction), the voltages at pins 1, 2, 13 & 14 of IC2 should all be low (ie, less than about 100mV) and similarly, the voltages at the Motor1 and Motor2 outputs should also be close to 0V. Now try rotating the 10kW pot in one direction and then other. You should find a proportional increase in the voltage at the Motor1 or Motor2 terminals. If all these checks are OK, you should be able to then connect the motor and control its speed. Note that as its speed is increased, the motor will produce a more or less musical tone. That is due to the 300Hz switching frequency. Next month, we will describe a companion interface board which provides a hand throttle control and has a toggle SC switch for motor direction. Testing When assembly is complete, check all your work very carefully. As noted above, any mistake in component placement or polarity may cause damage when the supply is connected. When everything checks OK, con- CC16: A professional, quality controller at an affordable price! Ready to use. No soldering, no extra circuitry required 16 industrial strength input/output points Easy to program Huge 25K program memory, 250 bytes RAM Floating point, multitasking, ModBus communications Expandable Ma de i nA us tra lia From $98 incl GST (OEM pack, board + connectors) Developer’s kit $148.50 includes programming cable & programming software -v alu ed wo rld - Save $56! Visit splat-sc.com for a special offer exclusive to SILICON CHIP readers siliconchip.com.au wi de April 2007  31