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Into powered models? Get into this If you’re into fuel-powered model aircraft, boats or cars we might just have solved that age-old problem: how to heat the glowplug to its required temperature from a car or gell battery. by ROSS TESTER Glowplugs come in many shapes and forms but they all have one function to perform: to provide a source of combustion inside a model engine to allow it to start when given a quick turn. Without the glowplug to start things “cooking” inside the engine, the engine would normally refuse to start. It’s a similar process to a diesel engine (which, by the way, also normally have glowplugs). When the engine is cold there simply isn’t enough energy to force the fuel to combust. So the glowplug supplies this energy by heating the fuel vapour to its combustion point while starting. Once started, the engine relies on its own heat and the fact that there is a lot more energy being generated in the compression process – simply because the engine is running fast. To make the glowplug operate it must be connected to a heavy-duty but low voltage (1-2VDC) power source. This brings about a couple of wrinkles. First of all, finding a battery of that capacity and second, matching it to the type of glowplug. Those who remember the old manual telephones used in “the bush” until about the early ’70s may recall they were powered by large 1.5V cells, capable of delivering many amps. If the phone didn't work, the chances were the cells had been purloined by a model aircraft enthusiast for their glowplugs. (Honest, mum, it wasn’t me!). Aah, the good old days . . . While many enthusiasts now make up battery packs to suit their models, most dream of being able to use the battery they take with them everywhere – their car battery. MUFFLER GLOW PLUG HEAD IDLE ADJUST SCREW NEEDLE VALVE IDLE MIXTURE SCREW THROTTLE ARM PROP SHAFT There are many different types of glowplug but they all have one purpose: to ignite the fuel and get the motor started. 72  Silicon Chip CARBURETTOR A typical 2-stroke model engine with various parts identified. The glowplug screws into the head – clearly seen at the top of this photograph. But car batteries are 12V and would make short work of most model glowplugs. The usual answer is to drop the voltage via some high wattage resistors – not only wasteful but also a bit hit-and-miss. Many enthusiasts have also tried gell cells (6 or 12V) but the problems are much the same. Another problem with using a standard battery for a power source is that different glowplugs will glow at different brightness levels. Some glowplugs operate barely red hot, while others are made to work much brighter. As brightness (and therefore heat) equates to the amount of power being delivered to the glowplug, if you are running various model engines it doesn’t take much to realise that a variable supply is required. Another advantage of being able to increase the heat of the plug is that a flooded engine can be started more easily. The ideal brightness level for most glowplugs is a bright orange that can be seen in normal daylight conditions. There are however some glowplugs that are normally used at lower brightness levels. An example of this is the ENYA number 3. It has a very thick element and is normally operated at lower levels of brightness. At the other end of the scale is the OS number 8. It is normally operated at much higher brightness levels. The circuit described here is capable of powering the vast majority of glowplugs in use today to their correct brightness, simply by varying one control. This control can be a preset potentiometer if you only run one model – or it can be changed to a standard pot with a pointer and markings to indicate various heat settings. For general model use the glowplug would be operated at a brightness level that can clearly be seen during daylight conditions. To set this level, the glowplug would be removed from the engine and connected to the circuit described here. With power applied the required brightness level is set. Once this level has been established the glowplug would be disconnected then installed back in the engine. The process could be repeated for as many glowplugs or engines that you want, with each marked on a scale. It would be a simple matter of “dialling up” the required heat, connecting the Housed in a disposals case (which is actually much larger than needed!) and with a suitable front panel, the Glowplug Driver is ready for action . . . unit – and flying! A felt pen marker would then be used to mark the front panel indicating the ideal position for the control knob. After any variations of plug heat it would then be a simple matter to return the knob to the previously set position. Circuit operation. NAND gate IC1a in conjunction with its surrounding components The circuit is quite simple: one IC, one MOSFET and a handful of parts. March 2000  73 Compare the PC board component overlay above to the larger-than-life photo at left. Note that we did not use PC stakes (though these are recommended) nor did we connect the external meter in this photo. forms a variable duty cycle oscillator with a frequency around 3kHz. The frequency, though, is unimportant. What is important is the on time to off time ratio at the output of the oscillator. When power is first applied, capacitor C2 discharged so the inputs to IC1a are low. Therefore the output is high. This provides a charging voltage for C2 via D2, R2 and VR1. When it reaches the threshold voltage of IC1a, the output goes low again, discharging the capacitor via R1 and D1. When its voltage reaches the lower threshold of IC1a, the output goes high, starting the process over again. This continues as long as power is applied. Even if C2 was still charged from the last time power was applied, the same process happens. The IC output would be low, so C2 would discharge until IC1a’s lower threshold was reach-ed, when the output would go high, charging the capacitor, etc etc. The ratio of charging time to discharging time, or the duty cycle, is set by VR1. With VR1 at the lower end of its resistance, charging time is very short and discharge time longer. The charge time increases as VR1 is increased but of course can never equal or exceed the discharge time because of the much higher resistance of R1. With the values shown, the duty cycle varies from 17% on in the minimum position to about 60% in the maximum position. The remaining gates, IC1b, c and d square up the variable duty cycle waveform with the resulting waveform at R3 effectively being only high or only low – the transition between the two states is very fast. This high and low waveform is then used to switch MOSFET Q1 on and off. When the voltage at Q1’s gate is high, Q1 turns on. If it stayed this way it would apply almost the full supply voltage to the glowplug and the glowplug would quickly burn out. But Q1 doesn’t stay on for long: it turns on and off rapidly, the period depending on the setting of VR1. With Q1 “off” most These waveforms show the operation of the Glowplug Driver. The upper trace is the gate voltage of the Mosfet while the lower trace is the waveform across the Glowplug. Note that it is set to produce an average voltage of 2V from a 12V input. 74  Silicon Chip of the time, the glowplug is powered only a fraction of the time. The average power is within the heat range of the glowplug. The very low value resistor (R5 – 0.1Ω) is in series with the supply to the glowplug. The average voltage across this resistor is proportional to the current flowing through it. By connecting a moving needle meter (eg, an analog multimeter – but not a digital multimeter) across this resistor we can get an indication of current flowing through the glowplug. This can be useful as a blown glowplug cannot be detected unless it is removed from the engine. Why not a digital multimeter? Simply because the moving needle (or more correctly, moving coil) multi-meter is not capable of responding to the rapid changes in voltage across the resistor as Q1 turns on and off. Instead, it produces an average reading of the voltage – exactly what we want. The reading on a typical digital voltmeter would depend on precisely when the meter sampled the voltage In this case, the Glowplug Driver is operating from 6V and the duty cycle has been increased by adjusting trimpot VR1 (ie, for longer pulse times) so that the output is maintained at 2V. Note that while the frequency has increased, that is not important. and in all likelihood would produce completely meaningless readings. Construction All components are mounted on a single PC board, with the possible exception of VR1. As previously mentioned, “serious” modellers may care to make VR1 a standard, as distinct from preset, potentiometer and mount it off the board with a scale indicating various glowplug brightnesses. That we’ll leave up to you – however, a preset pot will normally be supplied in the Oatley Electronics kit. After giving the PC board the usual inspection for defects, solder the low-profile components in first (resistors and diodes) followed by the 5W resistor, LED, electrolytic capacitor and finally the IC and MOSFET. Take care with the polarity of all components which matter – diodes, electrolytic, MOSFET and IC. Given the very fast rise and fall times and modest current through it, the MOSFET should not need a heatsink. However, you could fit a small one to it if you wish. There are four connections to the board – power (+V and 0V) and of course the glowplug. These should be Parts List 1 PC board 80 x 41mm 1 case to suit Semiconductors 1 4093B quad NAND gate (IC1) 1 BUK453 N-channel Power MOSFET (Q1) 1 5mm LED (any colour) (LED1) 2 GIG or 1N4004 power diodes (D3, D4) 2 1N914 small signal silicon diodes (D1, D2) Resistors 1 47kΩ 1 6.8kΩ 1 2.2kΩ 1 22Ω 1 0.1Ω 5W 1 10kΩ potentiometer (see text) Capacitors 1 100µF 16VW electrolytic 1 0.01µF polyester Miscellaneous 6 PC stakes 6 lengths insulated hookup wire (including red and black) The PC board mounts upside-down on the assembly pillars in this disposals case from Oatley Electronics. No extra screws are needed. made via PC stakes for convenience but there is nothing to stop you soldering the connecting wires direct to the PC board, as we have done. If you are going to use a moving-coil meter (or multimeter) you’ll also need to solder two wires in for that. The prototype was housed in a small disposals-type case which we understand will be available with the kit if required. The PC board mounts upside down in this case, with the two mounting holes drilled out to be a snug fit on the recesses in the case assembly pillars. No screws are needed – the board sits in position when the case halves are assembled. Testing There is no need to connect a glowplug or anything else to the unit to test it. Simply connect power and ensure the LED lights. Varying the pot to its maximum and minimum should vary the brightness of the LED somewhat (but certainly not from full on to full off!). If this works, you can be reasonably confident your Glowplug Driver is working correctly. Now for the acid test. You may care to remove the glowplug from the motor for this part! Connect the glowplug to the glowplug leads (they’re not polarised so can go either way around) and turn the pot to its minimum. Apply 12V DC power and note the colour of the glowplug. As you wind the pot up, the glowplug should glow brighter and brighter – if you go too far it might say “enough” and give up the ghost. Leave the pot at the point where the brightness is at the required level. If fitting an external pot, make sure you mark the position on a scale of some type so you can return to that setting. And that’s just about all there is to this simple project. Happy flying (or SC boating, or car racing, or . . .) Where to get the kit: This project design and PC board are copyright (C) Oatley Electronics. They will have a complete kit available, including case & label, for $14.95 Contact Oatley Electronics on (02) 9584 3563, fax (02) 9584 3561; email sales<at>oatleyelectronics.com; website www.oatleyelectronics.com March 2000  75