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This gutsy little speed controller has a wide range of applications and is simple to build and use. There are no software settings to fiddle with but it does have some really useful features such as low-battery protection, soft start and adjustable pulse frequency. It can run from 12V or 24V batteries at currents up to 20A. By JOHN CLARKE 20A 12/24V DC Motor Speed Controller Mk.2 Upgraded version of our very popular speed controller T HE MOTOR SPEED Controller described in the June 1997 issue of SILICON CHIP has to be one of the most popular projects we have presented; many thousands have been built. But as popular as it’s been, readers have often requested simple modifications to suit myriad applications. So we have come up with a revised design which should cope with virtually every possible variation that readers are likely to envisage. That’s a brave statement but it is based on literally hundreds of emails and letters we have answered on the original project in the 14 years since it was published. The original design is still OK but we strongly recommend this new ver28  Silicon Chip sion since it has more capabilities and is easier to build and connect. In fact, if you have an unassembled version of the old kit, we suggest you toss the old PCB and buy the new PCB plus a few extra bits to make up the new design; it will be worth it. New features First, the new PCB has provision for an on-board speed control trimpot (VR1) or as most builders seem to want, an off-board potentiometer. For ease of wiring, we have added heavy-duty screw terminals to the PCB for the power supply and motor connections. As well, the in-line fuse and fast recovery power diode are now mounted on the PCB and the power Mosfets and power diode have small heatsinks fitted. The circuit now provides full range speed control from zero to full power; the older design did not allow full speed. Apart from letting the motor operate at full power it also eliminates switching noise caused by the controller (at the full speed setting). Variable pulse frequency is another new feature. Because the speed controller works by pulse width modulation, with the pulse width varying the voltage fed to the motor, this can produce more noise from the motor. This is due to magnetostriction of the core laminations and rattling of the siliconchip.com.au OUTPUT CONTROL Vcc 13 6 Rt INSIDE THE TL494 OSCILLATOR 5 8 D DEADTIME COMPARATOR Ct Q Q1 FLIP FLOP 0.12V CK 0.7V 9 11 Q Q2 10 DEADTIME 4 CONTROL PWM COMPARATOR 0.7mA ERROR AMP 1 Vcc 12 UV LOCKOUT ERROR AMP 2 4.9V 5V REFERENCE REGULATOR 3.5V 1 3 2 FEEDBACK PWM COMPARATOR INPUT 15 16 Fig.1: the circuit is based on a TL494 Switchmode Pulse Width Modulation (PWM) Control IC. External timing components RT & CT on pins 5 & 6 set the PWM frequency, while output transistors Q1 & Q2 can be configured for either push-pull or single-ended operation. armature windings. You can often reduce this lamination noise by changing the pulse frequency and there is a trimpot (VR3) on the PCB to provide this feature. As mentioned above, we have also provided low-battery protection. This is mainly to prevent damage when the circuit is powered from 12V sealed lead-acid (SLA) batteries which will fail completely if they are discharged below 11V. Soft start is also included. This will bring the motor smoothly up to speed each time power is connected to the circuit, regardless of the speed setting. If soft start is not required, it can be disabled by removing a jumper link. What’s not in the new version? The answer is speed regulation. That is where the circuit reacts to an increase in load on the motor by increasing the pulse width, thereby better maintaining the preset speed. The June 1997 version of this circuit did have a form of speed regulation in that there was feedback from the negative side of the motor to one of the error amplifiers. However, since it did not monitor the motor’s back-EMF by itself, it could not really provide full speed regulation. Nor could it provide full speed operation which this latest version does. The new 12-24V DC Motor Speed Controller is presented as a bare PCB. siliconchip.com.au 7 14 GND REF OUTPUT This can be mounted within an existing enclosure using the four mounting holes with stand-offs and screws. Alternatively, the PCB can be clipped into a standard UB3 plastic case measuring 130 x 68 x 44mm. Pulse width modulation The circuit for the 12-24V DC Motor Speed Controller is based on a TL494 Switchmode Pulse Width Modulation (PWM) Control IC. Its block diagram is shown in Fig.1. An internal sawtooth oscillator sets the PWM frequency, as determined by external timing components RT and CT. The oscillator frequency is fed to two comparators (dead-time and Main Features • • • • • • • • 20A current rating 12V or 24V operation On-board trimpot or external potentiometer for speed adjustment Optional soft start 0-100% speed control range Efficient PWM control PWM frequency adjustment Low battery protection PWM) and the resulting PWM signal gated through a 4-input OR gate to a flipflop and thence to the steering control logic for transistors Q1 & Q2. Q1 & Q2 can be configured to provide push-pull or a single-ended output, as set by the output control input (pin 13). Our circuit ties pin 13 Specifications Supply Voltage ............................................................................................ 12-30VDC Supply Current ......................................................................................20A maximum Output Current ......................................................................................20A maximum Standby Current ................................................................................................. 20mA Control Range ................................................................................................. 0-100% Low Voltage Cut-out ....................................... typically set for 11.5V (for 12V battery) or 23V for a 24V battery Pulse Frequency Adjustment ..............................~100Hz to 1.1kHz (129Hz to 1.28kHz measured on prototype) Soft Start ...............................................from 0-100% (or to set speed) over about 1s Mosfet gate rise and fall times .......................................... 1.5μs & 1.6μs respectively June 2011  29 Parts List 1 PCB, code 11106111, 106 x 60mm 1 UB3 plastic box, 130 x 68 x 44mm (optional) 1 4-way PC-mount screw terminal block with barriers (9.5mm spacing) (Jaycar HM-3162 or equivalent) (CON1) 1 3-way screw terminal block with 5.08mm pin spacing (CON2) 3 TO-220 tapped, finned heat­ sinks, 16 x 22 x 16mm (Jaycar HH-8516) 2 3AG PC-mount fuse clips 1 20A 3AG fast-blow fuse (F1) 1 DIP16 IC socket (optional) 1 TO-220 silicone insulating washer and insulating bush 2 M4 x 15mm screws 2 M4 nuts 4 M3 x 10mm screws 4 M3 x 6mm screws (optional) 1 M3 nut 1 6mm M3 tapped standoffs (optional) 1 2-way pin header for LK1 (2.54mm pin spacing) 1 jumper shunt (LK1) 5 PC stakes (TP1-TP5) 1 2N5484 or 2N5485 N-channel JFET (Q4) 1 MBR20100CT dual 10A 100V Schottky diode (D1) 5 1N4148 switching diodes (D2-D6) 2 15V 1W zener diodes (1N4744) (ZD1,ZD2) Semiconductors 1 TL494N Switchmode Pulse Width Modulation Control Circuit (IC1) 1 LM2940CT-12 12V low dropout regulator (REG1) 2 IRF1405 55V 169A N-Channel Mosfets (Q1,Q2) 1 BC327 PNP transistor (Q3) Resistors (0.25W, 1%) 3 100kΩ 1 1kΩ 2 10kΩ 2 47Ω 2 2.2kΩ low to select single-ended operation, with Q1 & Q2 driven together up to a possible 100% duty cycle, ie, full on. Dead-time normally refers to pushpull operation and is the time between Q1 switching off and Q2 turning on. But we are not using push-pull operation in this circuit so the only time it comes into play is when the “soft start” feature is enabled. In this case, the dead-time comparator increases the PWM duty cycle as the voltage to the dead-time input, pin 4, slowly drops in voltage after power is applied. The TL494 includes a 5V reference regulator and we use it here as a bias source for the two error amplifiers. Error amplifier 1 is used for the speed 30  Silicon Chip Capacitors 1 22µF 16V low-ESR PC electrolytic 4 10µF 16V PC electrolytic 1 1µF monolithic ceramic 1 470nF MKT polyester 3 100nF 63V or 100V MKT polyester (one required across motor terminals) 1 56nF MKT polyester Trimpots 1 10kΩ miniature horizontalmount trimpot or 1 10kΩ linear potentiometer (VR1) 1 10kΩ top-adjust multi-turn trimpot (3296W style) (VR2) 2 100kΩ top adjust multi-turn trimpots (3296W style) (VR3,VR4) Resistors for testing 1 1kΩ 0.5W resistor (for 12V supply) or 1 2.2kΩ 0.5W resistor (for 24V supply) control function while error amplifier 2 is used for the low-voltage cut-out function. The outputs of the two error amplifiers are ORed together by internal diodes and the commoned output used to control the PWM comparator as well as being made available at pin 3. Two under-voltage (UV) lock-out Schmitt trigger comparators monitor the reference regulator output and the supply voltage. These comparators switch off the PWM output when the reference regulator drops below about 3.5V (eg, if it is shorted) or if the supply voltage drops below 4.9V. But just to confuse the issue, we don’t use these comparators for the low-battery pro- tection; instead, we use error amplifier 2, as mentioned above. Circuit details The full circuit of the DC Motor Speed Controller is shown in Fig.2. The motor speed is adjusted using onboard trimpot VR1 or an external potentiometer connected to CON2. This varies the voltage applied to the IN1+ input (pin 1) of internal error amplifier 1 in the TL494. This is configured as a unity-gain amplifier to buffer the input voltage from the speed-control pot. Trimpot VR2 is connected in series with VR1 to adjust the voltage range for VR1. With VR2 adjusted correctly, the full rotation of VR1 will give the full speed control from 0-100% PWM duty cycle. In this case, 100% duty cycle means that the output Mosfets are fully turned on and so there is no pulse width modulation; the motor is fed with smooth DC. As already noted, pin 13 of IC1 is tied low for single-ended operation. The collectors (C1 & C2) of the internal transistors are tied together to the Vcc supply while the common emitters (E1 & E2) at pins 9 & 10 are tied to ground via a 2.2kΩ resistor. When the internal transistors are switched on, the gates of Mosfets Q1 & Q2 are driven high via diode D2 and their 47Ω gate resistors. 15V zener diodes ZD1 & ZD2 protect the gates from positive transient voltages above 15V and also from voltages below ground (clamped to -0.7V) When the internal transistors are switched off, the 2.2kΩ resistor on pin 10 pulls the base of transistor Q3 low and this in turn discharges the gate capacitances of Q1 & Q2 to rapidly switch them off, within less than 2μs. The drains of Mosfets Q1 & Q2 connect to the M- motor terminal and they act as a “low side” switch, pulling one side of the motor low while the other side of the motor connects to the full supply voltage. Fast recovery diode D1 clamps the transient spike voltages generated each time the Mosfets switch off to about 0.7V above the battery supply. Soft start As noted above, the dead-time control input is pin 4. Normally this pin should be at 0V so that the PWM duty cycle is set by trimpot VR1 or the external potentiometer at CON2. However, when power is first applied to the circuit, a 10µF capacitor consiliconchip.com.au +12-30V REG1 LM2940CT-12 22 F 16V GND 470nF 100k TP3 OUT IN 10 F 16V LOW ESR 100nF 16 15 K LOW VOLTS VR4 CUTOUT 100k VR2 10k C2 TP5 SPEED VR1 10k 11 D2 A +IN2 E1 –IN2 E2 CUT THESE TRACKS ON THE PCB TO USE AN EXTERNAL POT 2 1 9 10 E B 1k 2.2k IC1 TL494 47 –IN1 +5V +IN1 IRF1405 S K ZD1 15V 1W SOFT START D Q1 G A LK1 6 Rt Ct 10k VR3 100k Q4 2N5485 G 5 GND 7 100nF OUTPUT 13 A 100k D S D4 56nF K TP4 A 2.2k K D3 10 F D5 A B K E 2N5485 12-24V 20A DC MOTOR CONTROLLER S G LM2940 BC327 ZD1, ZD2 A –0.3V K A K D2–D5: 1N4148 2011 A FB PWM DEAD 4 TIME SC  S K C MMC FREQUENCY ADJUST Q2 IRF1405 G ZD2 15V 1W Q3 BC327 1 F TP1 D 47 K 10 F CON2 A2 A1 8 100k 3 EXT SPEED POT CON1 D1 MBR20100CT C1 10k D6 A M+ 12 Vcc 14 REF OUT 10 F M– K TP2 +5V 0V F1 20A D IN C MBR20100 A1 K GND OUT K IRF1405 G A2 GND D D S Fig.2: the complete circuit for the 12-24V DC Motor Controller. IC1 is configured for single-ended operation and its common emitter outputs at pins 9 & 10 drive parallel Mosfets Q1 & Q2 via diode D2 and their 47Ω gate resistors. Q3 ensures that the Mosfets switch off quickly when the internal transistors switch off. nected between the 5V reference and pin 4, initially holds pin 4 at +5V. This voltage gradually drops to 0V as the capacitor charges via the 100kΩ charge resistor. While ever the voltage at pin 4 is above about +2.8V, it sets the duty cycle at 0%, ie, no voltage is applied to the motor. As the voltage falls below 2.8V, the duty cycle progressively ramps up to that set by VR1. The maximum duty available when the dead-time input is at 0V is about 92%. This restriction in duty cycle is absolutely necessary when the TL494 is used in the push-pull configuration, where the output transistors siliconchip.com.au are switched on and off alternately. However, we are using this circuit in single-ended mode and we don’t need it; we want to be able to provide a 100% duty cycle, ie, full on. The restriction in duty cycle to 92% is set by a 0.12V offset applied to the dead-time comparator input from pin 4. This is shown on Fig.1. To negate the effect of this 120mV offset, we need to generate a small negative voltage to cancel it. This is something the chip designers probably never envisaged but we have come up with a devious scheme. It involves feeding the sawtooth oscillator signal at pin 5 to the gate of junction FET (JFET) Q4 which is connected as a source follower. It is used to drive a diode pump consisting of diodes D3 & D4, together with the 56nF and 10µF capacitors. Diode D5 prevents the negative voltage going beyond about -0.3V. The reason it clamps to -0.3V rather than the typical -0.6-0.7V is due to the very low current flow through D5. This negative voltage is then fed to pin 4 via a 100kΩ resistor and this cancels the 120mV offset. Is that sneaky or not? Pulse frequency variation As mentioned earlier, we have made provision to vary the PWM switchJune 2011  31 D6 100nF F1 20A MAX. 2.2k 100k D2 D1 H1 Q1 47 M+ +M TP3 22 F LOW ESR REG1 TP1 H3 15V 2.2k CON1 D3 10 F 11160111 4148 10 F TP4 15V VR3 4148 100nF D4 VR1 4148 D5 Q3 D E EP S R O T O M A 0 2 Q2 ZD2 47 10k 100k x Q4 H2 10 F 1k VR2 4148 CON2 IC1 TL494 100k 10 F TP2 LK1 1 F 56nF TO EXTERNAL SPEED POT VR4 0V +12-30V V21+ V0 4148 M-M TP5 10k ZD1 470nF Fig.3: follow this layout diagram to install the parts on the PCB but leave VR1 out if you are using an external speed control pot. Note that diode D1 (but not Q1 or Q2) must be insulated from its heatsink – see Fig.4. M3 TAPPED HOLE HEATSINK SILICONE WASHER M3 x 10mm SCREW INSULATING BUSH x D1 PCB Fig.4 (above): this mounting arrangement shows how diode D1 is insulated from its heatsink using an insulating bush and silicone washer. Fig.5 (right): cut the tracks indicated here if you install trimpot VR1 but later decided to use an external speed control pot (see text). ing frequency because it allows you to use a setting which produces the minimum “singing” noise from the motor laminations. Hence, the oscillator frequency is set by varying the resistance from pin 6 to the 0V line using 100kΩ multiturn trimpot VR3. This provides a frequency range of adjustment between about 120Hz and 1.2kHz. Although the input voltage can be anywhere from 12-30V or a little more, to cope with 12V or 24V lead acid batteries, the TL494 is run from a 12V low-dropout regulator REG1 (LM2940CT-12). This can provide a 12V output with an input voltage that is only 0.5V above 12V. As the input voltage drops below this, the regulator’s output will also drop in value but the circuit will continue to function until the supply drops below the preset low-voltage cut-out which we will come to in a moment. 32  Silicon Chip CUT THESE TRACKS WHEN USING AN EXTERNAL SPEED POT (UNDERSIDE OF PCB) REG1 can cope with supply voltage spikes up to 45V which is important if the circuit is run from a 24V battery; in a vehicle, this can range up above 29V and motor spikes will add to that. The IN2- input, pin 15, which monitors the battery voltage is connected via a 100kΩ resistor and is protected by diode D6, so reverse voltage will cause no problems there. For the rest of the circuit, if the battery supply is reverse connected, heavy current will flow through the integral diodes within Q1 & Q2, via forward biased diode D1 and fuse F1 which will blow and prevent any damage. Low-battery protection We already mentioned that error amplifier 2 provides this function and the low voltage setting is provided by trimpot VR4. You can monitor the voltage setting at test point TP5. The set-up procedure is described later in this article. However, there is a little more to the story because we can’t simply have the circuit cutting off when the battery voltage drops below 11.5V (for a 12V lead-acid battery). What would happen is that when the circuit stops operating, the battery voltage will inevitably bounce back up again because the current drain suddenly drops. So if the battery voltage goes back up, the circuit starts operating again and then it goes off again and so on. The result is that the motor will get rapid bursts of power as it stutters on an off; not good. We get around that problem by adding hysteresis to the low-voltage cut-out function. So instead of simply biasing the +IN2 input, pin 16, from the +5V output at pin 14, we also connect it to the PWM input at pin 3 via a 100kΩ resistor. Now when the speed controller is working normally, the voltage at pin 3 will vary between 2.5V at 0% duty cycle and 0.7V for 100% duty cycle and this causes the voltage at the IN2+ input to vary between +4.61V and +4.77V. However, when a low-battery condition is detected, the PWM comparator output at pin 3 is forced high to nearly +5V and this means that the +IN2 input at pin 16 is now very close to +5V (instead of between +4.61V and +4.77V). Hence, for normal operation to resume, the -IN2 input at pin 15 must rise above +5V and that effectively means that the battery voltage has to siliconchip.com.au increase by about 0.8V, a fairly big increase. By the way, if you need more hysteresis, just reduce the 100kΩ resistor, eg, to 91kΩ or 82kΩ. This view shows the completed unit, wired with an external speed control pot. Note the insulating bush and silicone washer used to isolate diode D1 from its heatsink. The complete board can be clipped into a standard UB3 utility box. Construction The 20A 12/24V DC Motor Controller is built on a PCB coded 11106111 and measuring 106 x 60mm. Fig.3 shows the assembly details. Begin by checking the PC board for breaks in the tracks or shorts between tracks and pads. That done, check that the hole sizes are correct by test fitting the larger parts (fuse clips, screw terminal blocks, Mosfets Q1 & Q2, etc). The four corner holes should each be drilled to 3mm. Start the assembly by installing the resistors, followed by diodes D2-D6 and zener diodes ZD1 & ZD2. Table 1 shows the resistor colour codes but you should also check each resistor using a digital multimeter (DMM) before installing it. Take care with the diodes and zener diodes – they must be orientated exactly as shown on Fig.3. Once these parts are in place, install a socket for IC1. Alternatively, this IC (TL494) can be soldered directly to the board. Make sure it is orientated correctly. The capacitors can then go in and again the electrolytic types must be oriented correctly. Follow with the trimpots but leave trimpot VR1 out if you intend using an external potentiometer for speed adjustment. Trimpots VR2-VR4 are all multi-turn types and should be orientated as shown. Note that VR2 is a 10kΩ unit while VR3 & VR4 are both 100kΩ trimpots. Don’t mix them up. The 3-way screw terminal block is next on the list. Make sure it is correctly seated against the board and that its openings face outwards before soldering its pins. The 4-way terminal strip can then go in. It’s secured to the board at either end using two M4 x 15mm screws and M4 nuts. Tighten the mounting screws firmly before soldering its leads to the PCB. The two fuse clips are next. Note that these must both be orientated with their end stops towards the outside. If you get them the wrong way around, you will not be able to install the fuse afterwards. Don’t be tempted to solder the fuse clips with the fuse in place. If you do, the heat may partially melt the solder used to secure the fuse wire to the end Table 2: Capacitor Codes Value 1µF 470nF 100nF 56nF µF Value 1µF 0.47µF 0.1µF 0.056µF IEC Code 1u0 470n 100n 56n EIA Code 105 474 104 563 Table 1: Resistor Colour Codes o o o o o o siliconchip.com.au No.   3   2   2   1   2 Value 100kΩ 10kΩ 2.2kΩ 1kΩ 47Ω 4-Band Code (1%) brown black yellow brown brown black orange brown red red red brown brown black red brown yellow violet black brown 5-Band Code (1%) brown black black orange brown brown black black red brown red red black brown brown brown black black brown brown yellow violet black gold brown June 2011  33 Fig.5: this scope grab shows the controller’s operation at a low setting, ie, a duty cycle of 15%. The top (yellow) trace is the signal applied to the gate of Mosfet Q1 and has an amplitude of 11.9V. Each positive gate pulse turns on the Mosfets and pulls the motor’s M- terminal low, as shown by the green trace. The blue trace shows the battery voltage at the motor’s M+ terminal. Each time the gate voltage drops to zero (ie, at the end of each positive gate pulse), the Mosfets turn off and the motor voltage rises to a spike above the blue (battery voltage) trace. Schottky diode D1 stops it rising a great deal higher. Fig.6: this scope grab shows the operation at a much higher setting, with a duty cycle of 80.3%. In this case, the positive gate pulses (yellow trace) are much longer, at 1.83ms. Now, each time the Mosfets turn off, they generate an even higher spike voltage. caps and you could get an open circuit or dry joint. Hint: you can use sticky tape to hold the fuse clips (and other parts) in place while you solder them. Follow by installing PC stakes at test points TP1-TP5 and the 2-way header for LK1. A shorting jumper can then be fitted to this header to enable the soft start feature. Installing the semiconductors Transistors Q3 (BC327) and Q4 (2N5485) can now be fitted, followed by regulator REG1 which is mounted horizontally on the board. The latter is installed by first 34  Silicon Chip bending its leads down at right angles so that they pass through their mounting holes. REG1’s tab is then secured to the PCB using an M3 x 6mm machine screw and nut, after which the leads are soldered. Don’t solder REG1’s leads before securing its tab. If you do, you could crack the board tracks as the mounting screw is tightened down. Mosfets Q1 & Q2 and Schottky diode D1 are each mounted vertically and fastened to separate small heatsinks. The three heatsinks are installed first, by soldering their locating pins to the relevant PCB pads. Make sure that the heatsinks are properly seated against the PCB before soldering them in place. That done, slide Q1 & Q2 into their mounting holes and fasten them directly to their respective heatsinks using M3 x 10mm machine screws (the heatsinks come pre-tapped). Tighten the screws firmly, then solder their leads. Diode D1 is mounted in a similar way except that it requires an insulating bush and silicone washer to insulate its tab from the heatsink. Fig.4 shows how this is done. As before, tighten the screw firmly before soldering its leads. Finally, use your multimeter to confirm that D1’s metal tab is indeed isolated from its heatsink (and from the metal tabs of Q1 & Q2). Testing Before moving on to the test procedure, note that a 100nF MKT capacitor should be connected directly across the motor’s terminals. This is necessary to reduce electromagnetic radiation from the motor. If you are using an external 10kΩ potentiometer for the speed control, connect this up now. Conversely, if you are using trimpot VR1 instead, this should be installed on the PCB. If you do install VR1 but later decide that you want to use an external pot, then you must cut the PCB tracks running to the top of the this trimpot and to its wiper. This is necessary to prevent the trimpot and the external potentiometer acting in parallel Fig.5 shows which tracks to cut. These tracks have been deliberately thinned at the indicated locations and can be cut using a sharp hobby knife. If necessary, they can later be rejoined using solder bridges (ie, if you want to revert to using the trimpot). Alternatively, you can leave the tracks intact and remove the trimpot instead. The completed unit can now be tested by following this step-by-step procedure (without the motor connected): (1) Connect a 1kΩ 0.5W resistor between the M+ and Mterminals and apply 12-15V DC to the supply terminals (ie, to the +12-30V and 0V terminals). Watch the polarity. Note that a 24V DC supply can also be used but in that case, you should connect a 2.2kΩ 0.5W resistor between the M+ and M- terminals. (2) Connect a digital multimeter (set to volts) between test points TP1 (ground, bottom left) and TP3 (above REG1). This lets you check the regulator voltage. You should get a reading on TP3 of somewhere between 11.4V and 12.6V, provided the supply voltage is above 13.6V. TP3’s voltage may be slightly lower if the supply voltage is less than 13.6V. If TP3’s voltage is below the expected range, check for incorrectly oriented components (eg, IC1 and the electrolytic capacitors) and for short circuits between tracks. (3) Check that the reference voltage on TP2 is between siliconchip.com.au 4.75V and 5.25V. If not, check for a short circuit from pin 14 of IC1 to 0V. (4) Assuming all is correct, adjust VR1 (or the external pot) fully anti-clockwise and check the voltage on the centre terminal of CON2. Adjust trimpot VR2 so that this voltage is the same as that previously measured at TP2 (ie, between 4.75V and 5.25V). (5) Check that the “dead-time” offset voltage between TP4 & TP1 is at about -0.3V. If this is a positive voltage, check the value of the 100kΩ resistor at D5’s cathode and that D3-D5 are orientated correctly. The 10µF capacitors across D5 and on pin 4 of IC1 should also be checked for correct polarity. (6) Adjust VR4 so that the voltage between TP5 & TP1 is above the TP2 voltage (if this is not done, the PWM drive will not operate due to low-voltage detection). (7) Connect a DMM set to read DC volts across the M+ and M- terminals. Adjust the speed control pot (or trimpot VR1) and check that the output voltage varies accordingly. With the speed pot fully anticlockwise, the measured voltage between M+ and M- should be 0V. As the pot is wound clockwise, this voltage should rise. The maximum level should be very close to the supply voltage. Fig.7: this shows an intermediate speed setting, with a duty cycle of 53.7% and a gate pulse width of 1.225ms. Note that when the Mosfets turn off, the M- voltage briefly rises above the battery voltage (M+). It then falls to a plateau value which represents the motor’s back-EMF. Note that there is also some hash on this waveform and this is due to brush and commutator hash. Control range At this stage in the adjustment procedure, the voltage between M+ and M- should reach its minimum well before the speed control pot is wound fully anticlockwise. You now need to readjust VR2 to broaden this range. To make this adjustment, wind the speed control pot fully anticlockwise and adjust VR2 clockwise so that the voltage between M+ and M- just starts to rise above 0V. That done, slowly adjust trimpot VR2 back anticlockwise until 0V is reached, then back it off slightly further by about a half turn. Low-voltage cut-out Trimpot VR4 sets the low-voltage cut-out. To set this at 11.5V, first measure the battery supply voltage and subtract 0.6V from this measurement. That done, multiply the result by 0.426, then adjust VR4 so that the voltage on TP5 measures this calculated value. For example, if the battery voltage is 12V, then (12 - 0.6) x 0.426 = 4.86V. VR4 is therefore adjusted to give a reading of 4.86V at TP5. When the battery voltage drops to 11.5V, TP5’s voltage will fall to 4.65V and the low-voltage cutout will activate at close to this voltage (ie, between 4.61V and 4.77V). The battery voltage required for the circuit to switch on again is 12.33V – ie, (5V ÷ 0.426) + 0.6V. If you have an adjustable power supply, the low-battery cut-out action can be tested. To do this, first set the speed pot to its mid-point, so that there is a voltage between the M+ and M- terminals. Now reduce the supply voltage until the voltage between the M+ and M- terminals suddenly drops to 0V. The supply voltage at which this occurs is the battery cut-out voltage and should be close to 11.5V. If necessary, adjust VR4 to give a more accurate cut-out voltage. For a 24V battery, the low-battery cut-out voltage can be set to 23V. In this case, measure the battery supply voltage and subtract 0.6V. Now multiply the result by 0.208 and adjust VR4 so that the voltage at TP5 equals the calculated value. siliconchip.com.au Fig.8: this scope shot illustrates the clamping action of the fast recovery diode (D1). Taken at a much faster scope horizontal sweep speed, it clearly shows the diode action. Note that there is quite a lot of ringing on both the battery supply line (M+ blue trace) and the M- line (green trace). So even with the ringing, the diode faithfully clamps the M- line just above the M+ line. It actually appears to clamp at about 1V above the supply line but the real value is less than 1V. That completes the adjustment procedure and you can remove the resistor that’s across the M+ and M- terminals. Tweaking the PWM frequency As stated, the motor may generate an audible noise due to the Mosfets switching on and off at the PWM frequency. VR3 can be adjusted to minimise this noise, although it may not be possible to completely silence it. A 100nF MKT polyester capacitor connected directly across the motor terminals can also help reduce motor noise (and reduce SC electromagnetic interference). June 2011  35