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By JOHN CLARKE THERMO FOR CAR Has the thermostat failed on your electric radiator fan? This electronic thermostat can r eplace it and can be adjusted to switch on at a temperature to suit your particular vehicle. Electric radiator fans are excellent for cooling th e w ater in a car radiator. Unlike most belt driven fans , they are on ly sw itched on when cooling is required and so they are very energy efficient. If used to replace an engine driven fan , the resu lt is an increase in available engine p ower, reduced fuel consumption an d lower engine n oise. Oth er benefits include increased water pump bearing an d fan belt life. Given these facts, it's n o won der that many peop le with older cars h ave replaced th eir original engin e-driven fa n w ith an electric fan. Most late42 SILICON CHIP model vehicles now use electric fans as standard , as do all cars with eastw est engines. If air-conditioning is fitted to the vehicle, a second electric fan is usually fitted in front of the radiator and this operates whenever the air-conditioning is turned on. So why would you want to build this Thermostat Switch? There are two reasons. First, you may have installed an electric fan cooling system in your vehicle and the thermostat has subsequently gone faulty. Perhaps you've overcome this problem by shorting out the thermostat, since obtaining a replacement for many brands is virtually impossible. The problem with this scheme is that the fan now runs all the time and so provides too much cooling. That means less performance, longer warmup times and increased fuel consumption. The SILICON CHIP Thermostat Switch will ensure that the fan cuts in and out only as required, so that the engine always operates at the correct temperature for peak efficiency. The second reason for building this project is so that it can be used with a secondhand fan obtained from a wrecker. Often, the thermostats associated with such units are missing or faulty. An advantage of this unit is that it operates with greater accuracy than conventional thermostats and it can easily be adjusted to cut in at a temperature to suit the engine. The SILICON CHIP Thermostat Switch uses a commercially available temperature switch sender unit (available from K-Mart) which is mounted in the radiator. This sensor changes its resistance with the radiator fluid temperature. When th e resistance drops below a critical point, the Thermostat Switch switches on the fan. It then turns it off again when the sensor resistance increases (ie, when the coolant temperature goes down) but has a degree of inbuilt hysteresis to ensure that the fan does not "hunt". Circuit details Refer now to Fig.1 for the circuit details. An interesting aspect of the STATIC SWITCH RADIATOR FANS +12V F R O M O - - - - - - i l - - - - - - - - -IGNITION - -- - F1 RATING TO SUIT FAN +12V----c.-...c>----, - - - - - - - ~ D1 1N4002 ---------1 - 390ll 390\l ,w ,con 5W OR OUT 410n 7 IC1 5 COMP MC3334P INPUT POWER SENSE GND GND 4.7k 27Dn SE~SE B R1 510ll TEMPERATURE SENDER AUTO PACE TS6178 OR SIMILAR 0.1 0.1 1oon 0.1 0.1 5W ,oon FAN ON TEMPERATURE VR1 20011 0.1 5W 0 0 B GNDTO~-+-------~t----tl-----e-----t1---- VEHICLE CHASSIS - -- ---------~ DIVIDER VIEWED FROM BELOW CURRENT SENSE THERMOSTAT SWITCH FOR AUTOMOTIVE ELECTRIC FANS Fig.1: the circuit is based on a commercial temperature sensor (TS6178) & an MC3334P ignition chip. When the radiator temperature increases, the sensor pulls the base ofQ2 low via Q1 which is wired as a diode. Q2's collector tp.us goes high & triggers ICl which switches its pin 7 output high & turns on the fan motor via Q3. design is that it's based on a Motorola MC3334P automotive ignition chip (IC1). Apart from that, it uses only three transistors and a few minor components to complete the circuit. ICl was used for several reasons. First, this IC is rated for operation up to 125°C which makes it suitable for automotive applications. Second, the IC has a comparator input which switches the output at pin 7 from a high to a low and a low to a high over a precise voltage range. IC1 also has a sense input which can be used to monitor the current drawn by the fan. If the fan current becomes excessive, then ICl starts turning transistor Q3 off to limit the current through it to a safe value. Finally, the MC3334P is designed to withstand the high voltage transients found on automotive supply lines. The end result is a very reliable circuit. Sensor input The temperature sensor is a standard commerciai unit designed for radiator mounting. It has a negative temperature coefficient which means that its resistance decreases with increasing temperature. At about 100°C, its resistance is about 40Q and this increases to about zson at 2s c. One side of the sensor is connected to the vehicle chassis, while the signal output is connected to the emitter of transistor Ql. Zener diode ZD1 provides a 4. 7V reference voltage which is derived 0 from the +12V rail via two parallel 390Q resistors. The thermal coefficient of this zener diode is close to zero which means that the voltage across it remains relatively constant over a wide temperature range. This fixed 4.7V rail is the supply for the collector resistors of both Ql & QZ. Ql is actually connected as a diode and is included to provide temperature compensation for QZ. In operation, Ql 's base-emitter voltage matches that of QZ's. If Ql's baseemitter voltage increases (due to an increase in temperature), then QZ's similarly increases and the voltage between their emitters remains the same. This temperature compensation is important if we are to obtain reliable MARCH 1992 43 <at> 03 l0.1 5W 0.1 5W ~ 0 0 Q ~-- SHAKE-PROOF WASHER TO CHASSIS Fig.2: install Qt & Q2 so that they are touching each other & mount the 5W resistors slightly proud of the board so that the air can circulate around it. All wiring from the board should be run using 4mm auto cable to ensure an adequate current rating. switching of the fan at a given radiator temperature. It eliminates changes to the trigger point due to ambient temperature variations. In operation, the current through the 4700 resistor, Ql and the temperature sensor sets the voltage at the base of transistor QZ. This voltage is then amplified by QZ which operates as a common emitter amplifier. VRl allows adjustment of the quiescent voltage on QZ's collector and thus sets the trigger point. When the resistance of the temperature sensor goes down (ie, when the water temperature goes up) , it pulls the voltage on QZ's base down and thus its collector voltage goes up . This voltage is fed to the pin 5 comparator input of ICl via a 22kQ resistor and decoupled with a 0. lµF capacitor. The threshold voltage of the comparator is 1.8V on a rising voltage and 1.5V on a falling voltage. This means that the output at pin 7 of ICl goes high when pin 5 rises above 1.8V and low when pin 5 drops below 1.5V. ICl thus provides the necessary hysteresis so that the fan doesn't continually switch on and off at a single trigger point. Pin 7 of ICl is an open collector output and thus has a lO0Q pullup resistor which also supplies current to the base of transistor Q3, a power Darlington. When pin 7 is low, the base of Q3 is pulled directly to ground and Q3 and the fan are off. Conversely, when pin 7 goes high, Q3 turns on and switches on the fan motor, either directly or via relay RLYl. I __ soLDER LUG I ~ CASE ~ Q -._INSULATING BUSHES / ' I SHAKE-PROOF ~--•-WASHERS ~ ., ~---NUTS Fig.3: the MJ10012 Darlington power transistor (Q3) must be isolated from the case using insulating bushes & a mica washer. Smear both sides of the mica washer with heatsink compound before bolting the assembly together. Current limiting is provided by using the pin 8 sense input to monitor the current through Q3. It does this by sampling the voltage developed across the two parallel 0. lQ 5W resistors (via a voltage divider network). This sense input has a nominal threshold of 150mV but it can be anywhere in the range from 120-190mV. When the current through Q3 produces a voltage on pin 8 that exceeds its threshold value, the voltage on the pin 7 output ofICl is reduced. This in turn "throttles" back Q3 to limit the current through it to a safe value. The gain of this control is set by the 4. 7kQ RESISTOR COLOUR CODES 0 0 0 0 0 0 0 0 0 0 0 44 No. Value 4-Band Code (1%) 5-Band Code (1%) 1 22kQ 4.7kQ 1.SkQ 510Q 470Q 390Q 270Q 100Q 100Q SW 0.1Q5W red red orange brown yellow violet orange brown brown green orange brown green brown brown brown yellow violet brown brown orange white brown brown red violet brown brown brown black brown brown not applicable not applicable red red black red brown yellow violet black red brown brown green black red brown green brown black black brown yellow violet black black brown orange white black black brown red violet black black brown brown black black black brown not applicable not applicable 1 3 2 2 SILICON CHIP ol SC05204921 0 Fig.4: check your PC board carefully against this full-size pattern before mounting any of the parts, to ensure that there are no defects. resistor between pins 7 & 8. The range over which this current limit can occur is from 5.65-9.57 A, depending on the actual threshold of the pin 8 input. If required, the current threshold can be increased by removing Rl (510Q) ifit is at the lower limit of about 6A but more on this later. The current limit feature is important because the normal starting current for the fan motor can exceed Q3's 10A rating. With current limiting in circuit, the fan motor will start more slowly and take slightly longer to get up to speed before then running normally (assuming direct drive). · Whether you use a relay to drive the fan or drive the fan directly from Q3 depends on the current drawn by the fan. If the fan current is greater than about 6A, then you should activate the fan motor via a relay. This is covered later in the installation procedure. Power for IC1 is supplied via a 390Q resistor and decoupled using a 0. lµF capacitor. This RC network provides sufficient transient suppression to prevent damage to the IC. Construction The Thermostat Switch circuit is assembled on a PC board coded SC05204921 and measuring 100 x 55mm. This is housed in a metal diecast case which also provides the necessary heatsinking for the output transistor (Q3). Fig.2 shows the assembly details. Begin construction by installing PC stakes at the external wiring points , followed by the resistors, capacitors and the IC. Mount the 5W resistors about 2mm above the board surface to allow the air to circulate around them for cooling. Make sure that the IC is oriented correctly before soldering it into place. Install the zener diode (ZD1) and the trimpot next. Note that the zener diode should be mounted with a loop in its cathode lead to provide temperature and shock stress relief (see photo). Mount the trimpot with the adjusting screw positioned as shown on Fig.2. The two BC33 7 transistors are mounted facing one another. Bend the transistor leads slightly so that the top edges of the transistors touch each other. This will ensure that they are run at a similar temperature. Work can now begin on the diecast case. Begin by placing the PC board inside the case and marking out its four mounting holes. You also have to drill a hole in one end of the case for the cord grip grommet plus another hole in one side for the earth connection assembly. The switching transistor (Q3) is bolted to the other side of the case using a standard TO-3 insulating kit (mica washer plus insulating bushes). Use the mica washer to mark out the hole positions, then drill the holes and carefully deburr them so that the mounting surface is perfectly smooth. The PC board can now be mounted PARTS LIST 1 PC board, code SC05204921, 100 x 55mm 1 metal diecast case, 121 x 66 x 39mm 1 temperature switch sender unit (Auto Pace TS6178 from K-Mart, or similar) 1 tapered nut for sender unit, suitable for radiator mounting (available from radiator repair specialists) 1 200Q top adjust multi-turn trimpot (Bourns 3296W or equivalent) 1 cordgrip grommet 4 6mm plated brass standoffs 4 TO-3 transistor insulating bushes 1 TO-3 mica washer 1 TO-3 transistor cover 5 PC stakes 3 solder lugs 3 9 x 3mm machine screws 4 12 x 3mm machine screws 7 3mm nuts 7 3mm spring washers 7 3mm flat washers Semiconductors 1 MC3334P ignition chip (IC1) 2 BC337 NPN transistors (01 ,02) 1 MJ1001210A Darlington (03) 1 4.7V 1W zener diode (ZD1) Capacitors 4 0.1 µF 63VW metallised polyester Resistors (0.5W, 1%) 1 22kQ 2 390Q 1W 1 4.7kQ 1 270Q 1 1.5kQ 2 100Q 1 510Q 1 100Q 5W 1 470Q 2 0.1Q 5W 1 390Q Miscellaneous Heavy duty hookup wire, solder, heatsink compound. in the case on the 6mm standoffs and the earth connection assembly installed. Use spring or star washers under the nuts to prevent the screws from coming loose. This done, smear both sides of the mica washer with heatsink compound and bolt the power transistor to the case as shown M ARCH 1992 45 To mount the temperature sensor, you have to drill a hole in the bottom of the radiator tank to accept the mounting nut. This nut has a spigot & is soldered to the tank using 50/50 plumber's solder. The temperature sensor is then screwed into position & connected to the Thermostat Switch via a lead. in Fig.3. As before , use washers under the nuts and tighten the screws firmly to secure the transistor. Note that plastic bushes are used to insulate the emitter and collector leads from the case, as well as the mounting bolts. A solder lug is fitted to one of the mounting bolts and is used to terminate the lead from the fan motor (or from the relay). Once the transistor is mounted in position, use your multimeter to confirm that its collector is indeed isolated from the case. The various wiring connections can now be run using heavy-duty hookup wire. Don't use light-duty cable; it may not be able to handle the currents involved. The ground lead connects between the solder lug on the case and the ground terminal on the PC board. Make the three external leads long enough to reach the fan, the +12V automotive rail (via a fuse) and the temperature sensor. These three leads 46 SILICON CHIP should be sheathed together in plastic tubing and secured to the case by the cordgrip grommet. Installation The Thermostat Switch should be mounted in a convenient position in the engine bay where there is sufficient air circulation to provide a degree of cooling. We fitted the prototype with angle brackets so that it could be screwed to the side of the engine bay. The temperature sender unit is mounted at the bottom of the radiator tank. This job requires draining the radiator fluid and removing the radiator. A hole is then drilled in the bottom radiator tank to accept the nut for the temperature sender. This nut has a spigot which is inserted into the tank and soldered using 50/50 plumber's solder or preferably silver solder. If you do not feel confident about this procedure, a radiator repair shop will be able do it for you at reasonable cost. Once the job is done, re-install the radiator, screw in the temperature switch sensor and refill the radiator. Now for the final wiring. Whether you can drive the fan motor directly via Q3 or via a relay depends on the current drawn by the fan. Most fans used with 1.6-litre engines or smaller can be driven directly from Q3 without a relay. If two fans are used or the fan draws more than 5A continuous (check the fan rating), then a relay will be required (see Fig.1). Make sure that the relay contacts are rated to take the required current. Note that a diode should be placed across the relay coil to quench any voltage spikes when the coil is switched off. A fuse should be installed between the +12V supply rail and the fan motor regardless as to whether a relay is used or not. This will prevent a fire if the fan jams or if there is a short in the wiring. All wiring should be done using automotive connectors to ensure a professional job. The +12V supply to the Thermostat Switch should be derived via the ignition switch, while the supply to the fan motor (or relay) is derived from the unswitched side of the fusebox (again via a suitable fuse; eg, 10A or 15A). The lead to the temperature sender is terminated using an eyelet connector, nut and spring washer. You should also check that the earth side of the temperature sender is connected to chassis using your multimeter. If it isn't, you will have to earth the radia- . tor to the chassis. The Thermostat Switch is now ready for testing. Switch on the ignition without starting the engine and check that there is a +12V supply to the circuit. The fan should now start if you short the temperature sensor to chassis. Check that the fan runs in the correct direction, so that it blows air into the radiator. If the Thermostat Switch controls the fan directly rather than via a relay, you should now check that the current sensing circuit is set up correctly. To do this, connect your multimeter across one of the 0. H2 resistors and check the voltage across it when the fan is just starting and then when it is running at full speed. If the voltage is less than 300mV at Protect your valuable issues Silicon Chip Binders After mounting the Darlington power transistor, use your multimeter to confirm that its metal case has been correctly isolated from chassis. The transistor should be fitted with a plastic cover to prevent accidental short circuits. start-up and then decreases as the fan gets up to speed, remove Rl (510Q) from the circuit using a pair of side cutters. If the start-up voltage is greater than 300mV, then leave the resistor in circuit. If the fan subsequently does not run at full speed, then ICl is probably current limiting. In this case, you should use the Thermostat Switch to activate the fan motor via a relay. Now run the engine until it reaches its normal operating temperature and adjust VRl until the fan just switches on. At this point, turn VRl anticlockwise by one turn (the fan should turn off). The fan should now come on again when the engine has heated up to a temperature above normal. Now watch the temperature gauge and check that the fan switches off again just before the engine cools to normal temperature. After that, it's simply a matter of making further adjustments as necessary after driving the vehicle. Note that VRl must be set so that the fan always switches off slightly above the normal engine temperature. If you don't do this, the fan just runs continuously after the engine has warmed up. SC These beautifully-made binders will protect your copies of SILICON CHIP. They feature heavy-board covers & are made from a distinctive 2-tone green vinyl. They hold up to 14 issues & will look great on your bookshelf. * High quality * Hold up to 14 issues * 80mm internal width * SILICON CHIP logo printed in gold-coloured lettering on spine & cover Price: $A11 .95 plus $3 p&p each (NZ $6 p&p). Send your order to: Silicon Chip Publications PO Box 139 Collaroy Beach 2097 Or fax (02) 979 6503; or ring (02) 979 5644 & quote your credit card number. Use this handy form l ----------Enclosed is my cheque/money order for $_ _ _ or please debit my O Bankcard O Visa O Mastercard Card No: Card Expiry Date __/__ Signature _ _ _ _ _ __ _ __ Name _ _ _ __ _ _ _ __ _ Address _ _ _ __ _ _ _ __ The Thermostat Switch can be mounted in any convenient location in the engine bay. Don't forget the connection to chassis. . ___________. _ _ __ _ _ _ P/code_ __ MARCH 1992 47