Fullscreen HTML5Med Res (??MB)HTML4

SILICON CHIP Mini Projects #003 – by Tim Blythman Thermal Fan Control This project demonstrates how two modules can be combined with a bit of extra circuitry to do a useful job. The result is a circuit that will power a load, like a fan, when the ambient temperature exceeds a set threshold. T he fans in practically all modern desktop PCs are thermally controlled. This means that they are only turned on when needed, usually when the PC’s internal temperature gets too high. The fans can turn on when the temperature rises, moving hot air and replacing it with cooler air. Some PCs can even run the fans at different speeds, depending on the temperature. Older PCs always had their fans running at full speed. Being able to control them means that noise is kept down and the wear and tear on the fans is minimised. This project has a similar function; it provides automatic control of a fan based on temperature and can be adjusted to work at different temperatures, but it doesn’t require a microcontroller. It could be useful, for example, to power a ventilation fan in a room if the temperature inside that room gets too high. We use the Jaycar XC4494 Temperature Sensor Module to sense the ambient temperature and the Jaycar XC4488 Mosfet Module to switch the fan (or other low-voltage DC load) on and off. The Temperature Sensor Module produces an analog voltage that depends on the temperature. We apply that voltage to a simple comparator chip that produces a high or low level output, depending on whether the analog voltage is above or below a set level. Circuit details Fig.1 shows the resulting circuit. Note the two boxes that correspond to the two modules. The circuitry Parts List – Thermal Fan Control (JMP003) 1 Temperature Sensor Module [Jaycar XC4494] 1 Mosfet Module [Jaycar XC4488] 1 12V DC fan [Jaycar YX2512 or similar] 1 12V 500mA plug pack or other 12V power source [Jaycar MP3011] 1 17-row breadboard or protoboard [Jaycar PB8820 or HP9570] 1 2.1mm DC socket [Jaycar PS0526 or PA3713] 1 10kW potentiometer (VR1) [Jaycar RP7510] 1 LM311 comparator IC, DIP-8 (IC1) [Jaycar ZL3311] 1 100nF 100V MKT capacitor (C1) [Jaycar RM7125] 1 100μF 25V electrolytic capacitor (C2) [Jaycar RE6140] 1 220W 1/2W axial resistor (R1) [Jaycar RR0556] 1 1MW 1/2W axial resistor (R2) [Jaycar RR0644] 1 8-pin DIL IC socket (optional, for IC1) [Jaycar PI6500 or PI6452] Assorted breadboard wire/jumper wires [Jaycar PB8850 or WC6027] 62 Silicon Chip Australia's electronics magazine and components in those boxes come pre-soldered to the module when you buy it. One advantage of this analog approach is that we can run all the circuitry from 12V DC rather than needing to generate a lower voltage to run a microcontroller. That reduces the necessary parts and simplifies the design. The parts we’re using will happily run from 5V up to 18V. IC1 is a comparator. In simple terms, when the voltage at + pin (pin 2) is higher than the – pin (pin 3), output pin 7 ‘floats’ and is pulled up to 12V by current from the 220W resistor. The rest of the time, when the + voltage is lower than the – voltage, IC1 internally connects pin 7 to pin 1, where pin 1 is at 0V (ground). That means the output is 12V when the + voltage is higher than the other, or at 0V when the + voltage is lower. By connecting the pin 7 output to the SIG line of the Mosfet Module, the Mosfet switches on when the voltage at pin 7 is 12V, and when the Mosfet is on, it powers the fan. VR1 is a potentiometer that provides our voltage/temperature setpoint; the wiper voltage can be adjusted between 0V and 12V by rotating the shaft on top. This adjustable voltage is applied to pin 2 of IC1. Thus, IC1 compares the VR1 setting to the voltage from the Temperature Sensor Module, so siliconchip.com.au have delayed the hysteresis, making it much less effective, as we found in one of our early prototypes! By reversing the connections to the trimpot and Temperature Sensor module and swapping those inputs, the output of IC1 behaves the same. However, the hysteresis problem is solved, as no capacitor is connected to the trimpot wiper. The remaining components are 100nF and 100μF supply bypass capacitors that stabilise the circuit by smoothing out any changes to the incoming supply voltage. Top left: the small component with the black bead is a 10kW negative temperature coefficient (NTC) thermistor. That means its resistance is close to 10kW at 25°C, decreasing as the temperature rises. Jaycar’s RN3440 is an NTC thermistor similar to the module’s onboard one. Top centre: the Mosfet Module consists of the components shown in the right-hand box in Fig.1 and can be replaced by their equivalents if you want to build a version without modules. Jaycar’s ZT2468 (IRF1405 Mosfet) is similar to the IRF520. Right: any 12V brushless DC fan will do for this project. We have used the Jaycar YX2512. adjusting VR1’s screw lets you set the temperature at which the output will switch. Hang on – isn’t it backwards? While the S pin of the Temperature Sensor Module connects to pin 3 of IC1, its V pin is connected to ground (0V), and the G pin is connected to the 12V supply. That might seem backwards, but the Temperature Sensor module is just a group of passive components, none of which care about polarity, so we’re free to connect it this way. When wired this way, an increasing temperature causes a decreasing voltage at the S pin. VR1 is also wired ‘backwards’, so that turning the screw clockwise reduces the wiper voltage, to match the behaviour of the Temperature Sensor Module. The reason for doing it this way is to allow us to provide predictable hysteresis. That is the purpose of the 1MW resistor. When IC1’s pin 7 is high, some current flows through the 1MW resistor, raising the potentiometer wiper voltage slightly. Since VR1 is wired backwards, this is the same as reducing the setpoint slightly, meaning that the temperature has to drop a little after the fan switches on before it switches off. That stops it from ‘juddering’ on and off rapidly when the ambient temperature is hovering near the switching setpoint. If we had wired the circuit up the ‘normal’ way, the hysteresis current would have to be applied to the input connecting to the S terminal of the Temperature Sensor Module. The filter capacitor in that module would Construction We built our prototype on a PB8820 solderless breadboard. Still, the design is well-suited to the HP9570 prototyping board, which has an identical layout and will provide a more robust and permanent result. We soldered short lengths of wire to the modules to make for a neat layout. If you prefer not to solder, the circuit will work with jumper wires but may not be as tidy. In our photos, all red wires connect to the 12V supply and all black wires go to 0V. Use the photos and circuit diagram to wire yours up like ours. We placed the 1MW resistor on top of IC1, as it connects between pins 2 & 7. Also, the 220W resistor has had its lead bent by 180° so that it can be wired to two adjacent rows, connecting to both pins 7 and 8 of IC1. We used a soldered DC socket to supply power, but you could use the Jaycar PA3713 screw terminal version if you prefer. Leave the fan off for testing, since the Mosfet Module has an indicator LED that shows whether it is on or off. We used a 12V DC plugpack for Fig.1: this simple circuit uses a comparator (IC1) to compare a setpoint (from VR1) with the voltage from a Temperature Sensor Module (on the left). The 1MW resistor feeds some voltage back from the comparator’s output, providing hysteresis that stops the fan from turning on and off rapidly if the temperature is near the setpoint. siliconchip.com.au Australia's electronics magazine May 2024  63 Silicon Chip Binders REAL VALUE AT Pin 1 $21.50* PLUS P&P Are your copies of Silicon Chip getting damaged or dog-eared just lying around in a cupboard or on a shelf? Can you quickly find a particular issue that you need to refer to? Keep your copies safe, secure and always available with these handy binders These binders will protect your copies of S ilicon C hip . They feature heavy-board covers, hold 12 issues & will look great on your bookshelf. H 80mm internal width H Silicon Chip logo printed in goldcoloured lettering on spine & cover Silicon Chip Publications PO Box 194 Matraville NSW 2036 Order online from www. siliconchip.com.au/Shop/4 or call (02) 9939 3295 and quote your credit card number. *see website for delivery prices. 64 Silicon Chip The potentiometer at upper left sets a voltage that is compared to the voltage from the Temperature Sensor Module. If the temperature is higher than that set by the potentiometer, the fan is switched on by activating the Mosfet Module to supply 12V. We have included some close-ups of the wiring. power, but a 9V battery (connected using a PH9251 battery snap to 2.1mm plug) should be fine for initial testing. If you start with VR1 fully anti-­ clockwise, the LED should be on initially. It should go off at some point as you rotate VR1 clockwise. If the LED works in reverse or isn’t switching on and off as VR1 is adjusted, check your wiring before connecting the fan. Turn VR1 anti-clockwise until the LED is on, then turn it back until it just goes off. If you now touch the Temperature Sensor Module’s thermistor, the LED should switch on as the thermistor registers a higher temperature (assuming the ambient temperature is lower than your body temperature!). After a while (depending on the settings and ambient temperature), the LED will switch off. In that case, all is well, and you can connect the fan and adjust VR1 for a suitable switch-on threshold. Australia's electronics magazine That would be easiest to do if the thermistor were exposed to a temperature close to your desired threshold, eg, by heating a bit of metal and then holding it against the thermistor. Let it stabilise, then adjust VR1 until the fan just switches on at that temperature. If a DC motor is connected to the output (rather than a BLDC fan), a back-EMF quenching diode needs to be connected across it to avoid damaging the Mosfet at switch-off. Summary The comparator was one of the first integrated circuits, appearing around 60 years ago. Even modern microcontrollers often include one or more among their internal peripherals. This project is a great example of how a simple chip like a comparator can interface to analog and digital modules, and perform a role often delegated to more complex devices. SC siliconchip.com.au