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50 Amp Mode Controller ­ – W Are you into large or powerful radio-controlled electricpowered models? The ones where battery life is measured in minutes, not hours? Here’s a controller which can handle motor currents of up to 50A and is compatible with your existing radio control equipment. Article by ROSS TESTER – Design by BRANCO JUSTIC* T he obvious question is who could possibly want a control ler capable of such huge currents? After all, your typical radio controlled model, say a race car or buggy, has only a 7.2V battery and a 75W motor – ergo, 10A. For this project, we’re not talking your typical off-the-shelf radio controlled car or buggy. We’re talking industrial-strength models powered by, say, 12V motorcycle or even car batteries. Large boats, electric-pow- ered planes and big cars and trucks, for example. At the opposite end of the scale are competition boats, cars and planes which may not be very big but have very powerful motors demanding a lot of electrical power. They might draw 10 or 20A or more on load and therefore need significantly more in the controller department. But 10 or 20A is a far cry short of 50A. Why the brute strength? Couldn’t we make it a bit simpler and save a few bob? Yes . . . and no! The problem lies not so much in the typical load current of the motor, nor even the start-up current (which can be high). It lies in the stall current. A motor loafing along at 10A might draw ten times as much if locked up – for example, when the car it is pushing hits an obstacle and before the wheels start slipping. Another scenario is when a boat runs through some underwater greenery and gets its prop snagged. Housed in a tiny plastic case the motor speed controller is small enough to fit into the vast majority of models. The 3-wire rainbow cable on the left connects to the radio control receiver servo output while the wires on the right connect to a battery and the motor. You will probably need thicker cables. Note the six MOSFET tabs emerging through the case lid. 78  Silicon Chip el Motor Speed With Brake Sometimes it will cut or power its way through; other times it will be locked up. (We make no comment about what happens when an electric plane’s prop is locked up...) That’s when you need a controller capable of significantly larger peak currents than you would otherwise think were necessary. Sure, you could take the risk and hope that you can cut power before any damage occurs – but that damage might occur in a few milliseconds and for the sake of a few extra MOSFETs valued at less than $10.00 total, why would you? Of course, if your application says that the motor can never be locked up, you can get away with fewer MOSFETs. Five MOSFETs handle 50A so it follows we’re rating each at about 10A. But we’ll look at this in more detail shortly. This controller is significantly cheaper than commercial units available and is also nice and small. Overall the assembled PC board is only about Three DC motors which would be ideal for this controller: the top one is one we had in the “junk box”and is rated at 12V and draws about 5A off load, rising quickly to 1520A under load. The two smaller motors are both from Oatley Electronics, the middle one rated at 4-8V DC while the bottom one is 12V DC. These motors sell for $8 each. For more info visit www. oatleyelectronics.com/ motors.html 25 x 35 x 60mm; even in its specified case it’s only 80 x 40 x 27mm, including mounting feet. So it should fit in the vast majority of models. Radio control compatibility This controller is compatible with typical radio control equipment which has servo outputs; ie, 99.99% of commercial radio controls. The radio control servo output (only one output – there are three wires but two of them are for power) gives a pulse train between 1ms and 2ms long, depending on the position of the radio control “stick”, on a frame Fig.1: the circuit uses a ZN409 servo driver IC not to drive a servo but to drive MOSFETs which control the motor speed. May 2000  79 Fig.2: the current standard for radio controls uses these wave-forms to achieve forward, stop and reverse in the servos. The 20ms repetition rate is usually not at all critical but the pulse width is. rate (or time between pulses) of about 20ms or so. At centre, or rest (in a ±stick), the pulses are 1.5ms long. This pulse stream results (or should result) in the servo adopting the centre or zero position. By the way, the frame rate isn’t usually at all critical but the pulse width is. We’ve seen frame rates of up to 50ms and they work fine. However, 20ms is the current “standard” for radio control systems so we’ll use that. Push the stick in the positive direction and the pulses lengthen – up to 2ms at maximum travel which should have the model’s servo in full forward position. Push the stick in the negative direction and the pulses shorten – 1ms pulses at full travel will have the servo in the full reverse position. Fig.2 shows the waveforms of these various pulse trains. In practice, a small amount of Fig.3: the component overlay, reproduced same size. Note the lengths of tinned copper wire soldered to the tracks under the MOSFETs to increase current capacity. Compare this diagram to the larger-thanlife photograph of the completed PC board below. Here you can also see the external connection wires are soldered to the back of the board, not the front. “trim” is usually required to achieve the correct positions – the trim tabs on the transmitter adjust the pulse width slightly to make sure the servo behaves as you intend (not as it sometimes wants to!). Note that no provision for reverse direction is made in this simple controller. It has only zero to maximum (or 1.5ms to 2ms) capability. However, moving the stick to the normal reverse direction actuates the controller’s braking circuit. No radio control? You’re one step ahead of us (or per80  Silicon Chip haps we’re one step ahead of you!). Because the controller’s input demands are relatively simple, a square-wave oscillator capable of producing a pulse between 1ms and 2ms every 20ms will give full-range (zero to maximum) control of the controller plus braking. Such an oscillator is quite simple to make with either discrete components or, say, a couple of 555 timer ICs. However, simple oscillators usually drift a little with temperature so this needs to be taken into account. A suitable oscillator which simulates a radio control receiver output is shown later in this article. This oscillator can not only be used as a “wired” controller but can also be used to set your controller up. The controller We’ve already discussed the reasons for the number of MOSFETs in the output but we haven’t yet explained what they do – or how they get the information they need. That information is all taken care of by IC1, a ZN409 servo driver IC. There are no servos in this circuit, of course, but this IC is ideal because it decodes the radio control receiver “servo” pulse signal described above. The ZN409 has its own reference oscillator, producing 1.5ms pulses every 20ms. The precise length of these pulses can be varied slightly by VR1. Incoming pulses from the receiver are fed to pin 14 and are compared to this reference. If the pulses are longer than the reference the pin 9 output is taken high and the pin 5 output is taken low. Pulses shorter than the reference have pin 9 low and pin 5 high. Pulses equal to the reference have both pin 9 and pin 5 high. Remember that all this is happening every 20ms or so. Pin 5 is connected to three Schmitt NAND gates wired as inverters in series, so a low on pin 5 will result in a high on the gates of parallel connected MOSFETs Q1-Q5 (and vice versa). Pin 9 controls the “brake” MOSFET, Q6, via another Schmitt NAND gate/ inverter and transistor Q7. A low on pin 9 will result in Q7 being turned fully on, turning on Q6 which is wired directly across the motor. This effectively shorts the motor terminals which in turn acts as a brake on the motor armature. If you don’t believe how effective this is, try spinning the shaft of a small, permanent-magnet DC motor with your fingers, then short the terminals together and try spinning it again. Notice the difference? The length of time that pin 9 or 5 is held low is in direct proportion to the difference between the incoming (receiver) and reference pulses. A pulse width equal to, or very close to, the reference will result in an extremely short “low” time on pin 5, so the MOSFETs will effectively be turned off. Increasing this incoming pulse width results in a longer and longer “low” time until the point is reached where at 2ms pulse width, pin 5 is low for almost all of the 20ms cycle, thus turning the MOSFETs fully on for virtually all of the cycle. Pin 9 operates in a similar manner except that it controls the brake MOSFET. When the pulse length is between 1.0 and 1.5ms pin 9 goes low, and the output of inverter IC2a (pin 3) goes high. This turns on Q7 which connects the Q6 gate to ground, turning it on. As the pulse length approaches 1.5ms, Q6 on time becomes shorter and shorter until at 1.5ms (centre stick) the brake MOSFET is fully off. MOSFET ratings We’ve mentioned that the output of the speed controller is handled by five N-channel power MOSFETs, all The completed project with the disassembled case in the background. The case “lid” is actually the larger piece – note the cut-out in the case lid for the MOSFETs. If space is a real problem the PC board could be simply insulated in heatshrink plastic and shoe-horned into a suitable area within the model. May 2000  81 Parts List 1 PC board, 60 x 33mm, with chamfered corners to fit case Semiconductors 1 ZN409 servo driver (IC1) 1 4093 quad NAND gate (IC2) 5 IRFZ44 N-channel Power MOSFETs (Q1-Q5) 1 MTP2955 P-channel Power MOSFET (Q6) 1 C8050 NPN transistor (Q7) Capacitors 2 10µF 25VW electrolytic 2 1µF 25VW electrolytic 4 0.1µF MKT polyester 1 .022µF MKT polyester Resistors (0.25W, 1%) 1 68kΩ   1 47kΩ 1 33kΩ 1 4.7kΩ   3 1kΩ Miscellaneous Suitable case (if required) Heavy duty hook-up wire (see text) Fuseholder and fuse to suit Short lengths heavy tinned copper wire connected in parallel. Like all semiconductors, MOSFETs have a variety of ratings but there are only a few which really concern us in this application. Of course, we must ensure that the voltage rating is sufficient for not only the battery voltage but also any back-emf generated by the motor. And this can be substantial. The IRFZ44 MOS-FETs specified have a VDS (ie, drain-source voltage rating) of 55V. Likewise, the current rating of the MOSFET must be considered. In fact, there are two ratings – a continuous current rating (ID cont) which is 41A and the pulsed current rating (IDM) which is significantly higher (160A). We are using the MOSFETS in a pulsed mode but the limiting factor in this speed control circuit is the heat dissipation in the MOSFETs. Most important of all, though, is the MOSFET’s “on” resistance. When turned on as hard as possible (ie, any increase in drive to the gate results in no further drain/source current) the MOSFETs still offer some resistance to current flow. It is tiny – MOSFETs are significantly better than bipolar transistors in this regard but even then, the Speed controller rating. . . should it be 200A? We have rated this speed controller at 50A and this is a continuous rating, to suit the very high current motors used in today’s electric flight models, as well as those used in high performance model cars and boats. As noted in the text, we base this rating on the drain-source resistance of the specified IRFZ44 Mosfets. This gives rise to two limitations in the speed control circuit: voltage drop and power dissipation. For a 50A load, the circuit would have a likely voltage drop of 240mV and that means not much loss in speed compared to running the motor directly off the battery. Secondly, the power dissipation for a 50A load would be around 2.4W for each Mosfet or a total of 12W. That is quite a significant amount of power to be dissipated in such a small package and it is going need good ventilation which is often difficult to provide inside the fuselage or body of the model. But if you purchased an equivalent 82  Silicon Chip speed control from your local model shop it would be rated at 200A or higher. This is based on the peak current ratings of the Mosfets. Could a speed control such as this withstand 200A? The answer is yes but only for a second or two, as the likely total dissipation of around 50W in such a small package would not only blow the Mosfets but would melt the solder off the back of the PC board. We should also note that some motors that are likely to draw around 50A continuous could also draw as much as 200A or more, at initial start and if the motor is accidentally stalled. Under those conditions, a speed control like this one could survive the very high current, provided the overload condition did not last any more than a second or two. So when you see those 200A speed controllers in model shops, remember that, at best, it is only an instantaneous rating. The continuous or “real” rating is likely to be 50A or less. small amount of resistance has to be considered. In fact, there are two important considerations: one is heat dissipation, the other voltage loss. The IRFZ44 has an on resistance (RDS (on)) of just 0.024Ω. But as you know, passing a current through any resistor causes that resistor to heat up. So it is with the “resistance” in the MOSFET. Our maximum current is about 10A per device, which equates to a dissipation of some 2.4W. (P = I2 x R). Even though well within the device ratings that’s a significant amount of heat for any component to get rid of and we have five of these devices all wired cheek-by-jowl. The second problem any significant resistance causes is voltage loss. Passing a current through a resistor causes a voltage to develop across that resistor – voltage which is then not available to the load. If for a moment we assumed a single MOSFET could handle the total 50A load, we would be losing almost 1.2V across it (E = I x R). That’s an intolerable loss from a 12V supply and will make the motor run significantly slower. But as you also know, when you connect resistors in parallel the resistance drops. We’re connecting five of these MOSFET “resistors” in parallel so the equivalent resistance is just .0048 ohms. Using Ohm’s law again, 50A x .0048 is just 0.24V loss – a much better proposition. Remember that’s the worst case; at say 20A the loss is only going to be about 50mV. The MTP3055 P-channel power MOSFET used as the brake doesn’t have to handle very high currents. That’s fortunate, because P-channel devices generally have a higher RDS than N-channel devices (in this case 0.3Ω). Its 60V, 12A rating should be more than adequate for this application. Construction All components are mounted on a small PC board, nominally 60 x 33mm. Before commencing construction, make the usual checks for defects in etching. Also, if you are not building this from the Oatley Electronics kit, you will need to file the corners off the board – to about 5mm in each direc- tion – so that it will fit in the specified case. The Oatley kit, by the way, includes the case, the wiring loom pictured including fuseholder and fuse and, of course, the PC board and components. After checking that the board fits the case, commence assembly with the smallest components first. Note that most of the resistors mount on end. Our prototype used sockets for both ICs but this is left up to you. Use two of the resistor lead cut-offs to form the two links required on the board – both under where the MOSFETs mount. The final components to be mounted should be the MOSFETs. Note particularly their orientation – all go the same way but they must be the right way around – and also the location of Q6, the P-channel MOSFET. It mounts closest to the BAT + and MOTOR terminals. To keep the MOSFETs straight and in position we lined them up with a 3.2mm drill bit through all their holes and then soldered them in position. Because of the significant current drawn by the MOSFETs some short lengths of heavy tinned copper wire should be soldered along the appropriate PC board tracks (ie, under the MOSFETs) to increase the current carrying capability significantly. Just remember that Q6 is not in parallel with the rest of the MOSFETs! Speaking of current capability, the wiring used in the prototype for battery connection was certainly not rated at 50A! Our application called for only a fraction of this capacity so we used standard 10A hookup wire and a 4A in-line fuse. If you are powering anything larger, not only will greater capacity cabling be needed but you will also have to think seriously about connections to the PC board – soldered connections may be inadequate. Some form of busbar may be required. Some model shops sell silicone-coated hookup wire which is specifically intended for high-current applications such as this. It could be worth a look. Fitting to the case The final step is to mount the complete assembly in its case. The case is in two sections with the larger section of the case actually the “lid”. The PC board mounts upside-down in the Where do you get it? This project, including the circuit and PC board pattern, is copyright © 2000 to Oatley Electronics. They can supply a complete kit of parts, including the case, for $35.00 They will also shortly have available a simulator (see next page) suitable for use with this circuit. Contact Oatley Electronics on (02) 9584 3561, fax (02) 9584 3563, by email at sales<at>oatleyelectronics.com, via mail at PO Box 89, Oatley NSW 2233, or via their website www.oatleyelectronics.com * Branco Justic is the Manager of Oatley Electronics. “lid” so that when you turn it over it’s the right way up. Double dutch? Not really, but the photos might give a better idea of what we’re saying. No screws are necessary to hold the PC board in place – it’s held captive by its leads and the MOSFETs. A hole needs to be cut through the top of the lid for the MOSFETs – ours was 27 x 10mm, centred 5mm from one edge – and also a small hole drilled to allow VR1 to be adjusted from outside the case with a fine screwdriver. A 3mm hole would be about right, lined up with VR1 underneath. With the external leads soldered to the underside of the PC board (ie, direct to the tracks) they emerge from the assembled case through the cable- ways provided. Significantly larger leads will of course need larger holes cut. One feature of the specified case worth noting is that no extra screws are required to hold it together. The two portions snap together and then the same screws which mount the case prevent it from coming apart. The centres for the mounting screws are 73mm apart. Testing You will need a radio control transmitter and a matching receiver with servo outputs, a suitable DC motor and a DC supply or battery equal to the task. If you don’t have a radio control you may wish to build the radio control servo pulse simulator described at the end of this article. We will assume you are using a radio control receiver but if not, simply connect the wires to the simulator the same way around. Connect the three servo wires to the radio control receiver output. The red and black wires go to + and - on the output while the brown wire goes to the data output – usually the middle pin and on “real” servos, usually coloured yellow. Set the radio control transmitter stick to either minimum if it is a single direction controller or to centre (off) in a dual-direction controller, with trimtabs set to the centre as well, and turn both transmitter and receiver on. Apply power to the controller. You’ll almost certainly find the motor starts to turn (be careful of the starting kick on a large motor if it is not secured in some way!) but when you adjust VR1 you should be able to stop the motor completely. If so, move the stick on the radio control transmitter and you should find the motor turns with its speed proportional to the stick position. Full stick should give you full motor speed, or very close to it. What if it doesn’t work? Obviously, there is an error somewhere. Perhaps as a starting point, eliminate the radio control transmitter and receiver by connecting a real servo to the receiver and make sure it works properly. That ensures you have the right sort of waveform coming from the receiver. If it works, check your wiring and component placement again – more than 95% of faults in kits are due to one or two wrongly placed or reversed components or poor soldering. Check that you have +5V coming to the ZN409 supply rail from the radio control unit. If you have an oscilloscope, view the waveforms at pins 14, 5 and 9 of IC1. If you get what looks like a correct waveform, look further along. Otherwise the error is somewhere around that IC. There should be a positive-going waveform at approx. 50Hz from pin 11 of IC2, its width varying with either the input signal or the position of VR2. Also check the inversion of signal between pins 1/2 and 3, 5/6 and 4, 8/9 and 10 and finally 12/13 and 11. May 2000  83 Manual motor control via a simulator Earlier we referred to the waveform from a radio control receiver – a square wave of 50Hz with a duty cycle dependent on the setting of the radio control transmitter stick. At rest the pulse should be 1.5ms wide and full forward it should be 2ms wide. It follows then that if a waveform of this type was fed into the input the system would operate as if it was attached to a radio control receiver. All we need do is simulate that waveform. Fortunately, that is quite simple to do. Two suitable circuits are shown below. The first consists of two 555 timers (actually a 556 which is two 555s in one package) – one connected as an astable oscillator running at 50Hz (ie, producing continuous 20ms-wide pulses). This triggers the second 555 wired as a monostable which has its pulse width variable from less than 1ms to more than 84  Silicon Chip 2ms by adjusting VR1. The output from pin 3 of IC2 then is a series of pulses, 20ms apart, which vary in length from less than one to greater than two milliseconds. Now where have we heard that before? A similar circuit was first described in SILICON CHIP in May 1994. It produced 30ms pulses – which should work fine – but we’ve adjusted the values to give approximately 20ms, just to be consistent. The second circuit, from the same issue, is even simpler and contains just one 4001 quad NOR gate and a few other components. Its drawback was that due to its simplicity the frame rate changed with the pulse width but apparently that didn’t cause any problems. For a full description of these circuits, refer to the May 1994 issue. Copies of that issue are still available from SILICON CHIP Publications for $7 each including postage & packing ($7.70 after July 1). We believe either could be used but we must say that we haven’t tried either with this circuit. PC board patterns are shown for both but as they are so simple these could just as easily be built on a small piece of Veroboard to save the cost of a PC board. You can use these simulators to either set up your controller in the absence of a radio control system or you can use it to “hard wire” control an electric motor (low voltage DC only!). That’s up to you. Note that you will have to arrange a 5V supply for both the simulator and the ZN409 circuitry in the speed controller. This could most easily be done with a 7805 regulator taking its input from the 12V supply. (When used with a radio control receiver the speed controller takes its 5V supply from the servo output of the SC receiver). T