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A very efficient electronic thermostat By JIM ROWE TEMPMASTER Mk.2 Want to convert an old chest-type freezer into an energyefficient fridge? Or convert a spare standard fridge into an excellent wine cooler? These are just two of the jobs this lowcost and easy-to-build electronic thermostat has been designed to do. It can also be used to control 12V fridges or freezers, as well as heaters in hatcheries and fish tanks. It controls the fridge/freezer or heater directly via their power cables, so there’s no need to modify their internal wiring. 22  Silicon Chip siliconchip.com.au The switched IEC connector is snap-fitted to an aluminium plate and this assembly is then secured to one end of the case using Nylon screws & nuts. The other end of the case carries access holes for the sensor jack plug, trimpot adjustment and DC power supply. B ACK IN THE JUNE 2005 issue of SILICON CHIP, we described an electronic thermostat intended mainly for converting an old fridge into a wine cooler or a chest-type freezer into an energy-efficient fridge. Dubbed the “Coolmaster”, it turned out to be a very popular project, especially with people wanting to reduce their power bill and reduce their “carbon footprint”. Converting a chest freezer into a “chest fridge” results in much lower energy consumption than a normal “vertical” fridge of the same internal capacity, because cold air doesn’t fall out every time you open the door and siliconchip.com.au chest freezers tend to be better insulated anyway. The project became even more popular when the people in Jaycar’s kit department came out with a slightly modified version which could be used to control heating elements as well as fridges and freezers. This modified version was called the “Tempmaster”, to describe its expanded capabilities. Unfortunately, some constructors did experience problems with the project. In most cases, this seems to have been due to spurious triggering of the control Triac due to inductive spikes fed back from the motor in the compressor of the fridge/freezer, caus- ing noisy and/or hesitant switch-on or switch-off. This problem was solved in most cases by fitting a mains filter circuit between the Tempmaster and the motor but it did point to one shortcoming in the project’s use of a Triac for power control of motors. Of course, a Triac can only be used for controlling AC in any case, and this meant that the first Tempmaster could not be used to control fridges, freezers or heaters which run from 12V DC – shortcoming number two. We also received criticism from energy conservationist Dr Tom Chalko, who complained that the Coolmaster/ Tempmaster was mediocre in terms of energy efficiency. This was because of its own quiescent energy consumption and it would pose problems for those using electronic inverters to produce 240VAC from a solar or wind generating system, by preventing the inverters from ever being able to switch into “sleep” mode. Dr Chalko claimed that our Tempmaster had a continuous quiescent energy consumption of 60 watt-hours per day, equating to a power consumption of 2.5W. I’m not sure how he arrived at this figure, because my calculations gave a figure of only 0.25W or 6Wh/day with a 90% efficient plugpack supply, or perhaps 10Wh/day with a plugpack which was only 50% efficient. Added to the other shortcomings of the original Coolmaster/Tempmaster, though, Dr Chalko’s criticism did prompt us to produce a new and improved Mk.2 version of the project. So that’s the story behind the new version described here. There are two main differences between this new Tempmaster and the original. First, it now uses a 240VACrated relay to switch the load power instead of a Triac. This has three main advantages: no problems with noise triggering, the ability to switch DC just as easily as AC and lower quiescent energy consumption because there is now no snubber circuit or bias filter circuitry associated the Triac. The second main difference is that we have used a more efficient voltage regulator circuit, an LM723, to power the thermostat’s control circuitry. This has lowered the quiescent power consumption to below 48mW (0.048W) – equating to just 1.15Wh/day. How it works There’s very little in the thermostat February 2009  23 +5V REG 2.7k 5.6k LM393 COMPARATOR VSENSOR 2 1.2k VREF 3 1 IC1a COMPARATOR OUT 10M SET TEMPERATURE TS1 LM335Z – 8 4 3.3k TEMP SENSOR + +12V VR1 500 COMPARATOR CONFIGURATION – COOLING CONTROL Fig.1: the circuit is based on remote temperature sensor TS1. It’s output is fed to the inverting input of comparator IC1a where it is compared with a preset reference voltage (VREF) derived from a regulated +5V rail. 2.92 2.91 2.90 LM335Z SENSOR VOLTAGE 2.89 2.88 2.87 2.86 2.85 2.84 2.83 2.82 2.81 2.80 2.79 2.78 2.77 2.76 2.75 3 2 275K 4 5 6 7 8 280K 9 10 11 12 13 14 285K 15 16 17 18 290K 19 TEMPERATURE – DEGREES CELSIUS (KELVIN) Fig.2: the graph plots the output voltage of the LM335Z as a function of temperature. It rises linearly by 10mV for every 1°C increase. circuit and its operation is straightforward. Fig.1 shows the basic details. The heart of the circuit is the remote temperature sensor TS1, which is an LM335Z device specifically designed for temperature sensing. The LM335Z acts like a special kind of zener diode, in which its voltage drop is not fixed but varies linearly and quite accurately with its temperature. In fact, its voltage drop is directly proportional to absolute temperature, having a value of 0V at 0 Kelvin (-273°C) and rising linearly by 10mV for every Kelvin (or °C) rise in temperature. This is shown in the graph of Fig.2. 24  Silicon Chip At a temperature of 2°C (275K), the voltage drop of the LM335Z is very close to 2.75V. Similarly, at 19°C (292K), it rises to 2.92V. It’s this change in voltage that we use to control the temperature of our fridge/freezer or heater, by comparing the sensor’s voltage with a preset reference voltage. The actual comparison is made by IC1a, one section of an LM393 dual comparator (the other section is not used). Sensor TS1 connects to the inverting input (pin 2) of IC1a which compares it with a reference voltage at its non-inverting input (pin 3). Now remember that the lower the temperature being measured by sensor TS1, the lower will be its output voltage. So when the sensor voltage is below the reference voltage, the output of the comparator will be high and this means that nothing will be switched on by it. When the voltage from the sensor rises, corresponding to an increase in measured temperature, at some point it will go above the reference voltage at pin 3. This will cause the output of the comparator to go low and it will then switch on transistor Q1 and the relay which we will come to later on. Fig.1 also shows the sensor connection details and the means of deriving the reference voltage from the +5V supply. As can be seen, sensor TS1 is connected to the +5V rail via a 5.6kΩ resistor, which is used to provide the sensor with a small bias current. The reference voltage at pin 3 is derived from the +5V rail via a voltage divider formed by the 2.7kΩ resistor and the 3.3kΩ resistor in series with VR1, a 500Ω multi-turn trimpot. As a result, when VR1 is adjusted over its range this varies the reference voltage between 2.75V and 2.92V. These happen to be the LM335Z sensor voltages at 2°C and 19°C respectively. VR1 therefore becomes the thermostat’s “set temperature” control. OK, the foregoing description of IC1a applies to when the Tempmaster is in cooling mode. Now have a look at the complete circuit of Fig.3. This shows IC1a connected so that it can provide either heating or cooling control. Sensor TS1 plugs into socket CON2 which in turn connects to test point TP2 and one end of links LK1 and LK2. The reference voltage is fed to test point TP1 as well as the two other pins of LK1 and LK2. The two inputs of IC1a are connected to the centre pins of LK1 and LK2. This allows us to set the Tempmaster for either cooling or heating control, simply by moving the jumpers on LK1 and LK2 from one end to the other. For example, when LK1 connects IC1a’s pin 3 to the reference voltage (TP1) and LK2 connects pin 2 to TP2 and temperature sensor TS1, this configures the Tempmaster for cooling control (ie, control of a fridge or freezer). Conversely when LK1 connects pin 3 to TP2 and TS1, and LK2 connects pin 2 to TP1 and the reference voltage, this configures the Tempmaster for heating control. The siliconchip.com.au Fig.3: the complete circuit of the Tempmaster Mk.2. Links LK1 & LK2 allow comparator IC1a to be connected so that it can provide either heating or cooling control. IC1a drives transistor Q1 which in turn controls relay RLY1 to switch power through to the GPO. “C” and “H” at each end of LK1 and LK2 indicate this. Cooling control Now consider that LK1 and LK2 are set for cooling mode, as shown by the two red links on the circuit diagram (Fig.3). This means that while ever the temperature of TS1 inside the fridge or freezer remains lower than the set temperature level, the voltage drop across TS1 (applied to input pin 2 of IC1a) will be lower than the reference voltage applied to pin 3 via LK1 and the 1.2kΩ resistor. As a result, the open-collector output at pin 1 will not draw any current from the +12V rail and transistor Q1 will not be able to conduct to turn siliconchip.com.au on LED1 or relay RLY1. So no output power will be delivered to the GPO. On the other hand, if the temperature inside the fridge/freezer rises to just above the reference voltage (set temperature) level, the voltage drop across TS1 (fed to pin 2 of IC1) will just rise above the reference voltage on pin 3. The comparator output will switch low to pull current through the 4.7kΩ resistor and hence turn on Q1. This will operate LED1 and energise the relay coil. This will provide power to the compressor in the fridge/freezer, causing it to cool things down again. Of course when the fridge/freezer temperature drops below the set level again, the voltage from TS1 will drop below the voltage on pin 3 of the comparator and the comparator will switch back off again, turning off Q1, the LED and the relay once again. It runs the compressor only long enough to bring the temperature just below the set level. Heating control If links LK1 and LK2 are swapped to their “H” ends, this reverses the way the comparator controls the power fed to the Tempmaster’s GPO in response to changes in TS1’s voltage. Since TS1’s voltage is now fed to pin 3 of IC1a and the reference voltage to pin 2, the comparator’s output will remain high and not draw any current while ever TS1’s voltage is higher than the reference voltage. February 2009  25 Fig.4: install the parts on the PC board and complete the wiring as shown here. Note that all connections to the GPO, the IEC connector and the relay contacts (1) must be run using mains-rated cable. Be sure to secure this wiring using cable ties, as shown in the photos. As a result, Q1 and the relay will remain off and no power will be fed to the GPO or any heating element connected to it. However, if the temperature inside the hothouse or fish tank falls just below the set temperature, TS1’s voltage will drop below the reference voltage. The comparator’s output will thus switch low, drawing current and turning on Q1 and LED1 and energising relay RLY1. As a result, power will be switched through to the heating element to warm things up again. Then when the temperature rises above the set level again, TS1’s voltage will rise above the reference voltage and the comparator’s output will switch high again. This will turn off Q1, LED1 and the relay, removing power from the heater. of positive feedback. This is arranged by the 10MΩ resistor between pins 1 and 3 of IC1a and the 1.2kΩ resistor connecting pin 3 to LK1. This lowers the voltage at pin 3 slightly when the comparator is switched “on” (pin 1 low and Q1 energising RLY1) and raises it slightly when the comparator is “off”. In cooling mode, this means that in the input voltage from TS1 at pin 2 must drop down to a level at pin 3 that is slightly lower than the reference voltage, before the comparator will turn off again. Conversely, it must rise to a level slightly higher than the reference voltage before the comparator will turn on. In other words, we give the comparator a small amount of hysteresis. Positive feedback All the low-voltage part of the circuit operates from a nominal 12V DC supply, which is derived from an external DC plugpack or battery via CON1 and protection diode D1. The 470μF capacitor provides a reservoir Regardless of whether the circuit is working in cooling or heating modes, we need to prevent the comparator from oscillating back and forth (or hunting) by applying a small amount 26  Silicon Chip Low power drain for the additional current needed when the relay is energised, while the 10Ω resistor and zener diode ZD1 provide protection against over voltage damage. The regulated +5V supply needed for TS1 and the reference voltage divider is derived from the nominal +12V rail via REG1, an LM723C regulator. We have used the LM723C here because it has a very low quiescent current. As a result, the maximum total quiescent current drawn from the 12V supply (via CON1) is less than 3.8mA. This is when the relay is not energised, of course. When the relay is energised, the current rises to about 79mA. So in a typical freezer-to-fridge conversion application where the relay will be off for most of the time, the Tempmaster’s average power consumption will be only around 50mW and its energy consumption around 1.2 watt-hours per day. Construction Most of the components used in the Tempmaster circuit are mounted on a siliconchip.com.au PC board measuring 151 x 109mm and coded 10202091. The board has rounded cut-outs at one end so it mounts inside a sealed polycarbonate enclosure measuring 171 x 121 x 55mm, sitting on the tapped pillars moulded into the bottom of the enclosure. Relay RLY1 is mounted on the board at lower centre, as shown in the internal photos. The connectors for the 12V DC input and remote temperature sensor TS1 are mounted on the righthand end of the board, being accessed via matching holes at that end of the enclosure. The “set temperature” trimpot VR1 is mounted centrally at the same end of the board and is accessed via a small hole in one end of the enclosure. The only components not mounted on the PC board in the Tempmaster itself are the fused and switched IEC mains input connector and the 3-pin mains outlet or GPO. The latter is mounted in a cut-out at upper left on the enclosure’s lid, while the former mounts on the lefthand end of the enclosure. Since the IEC connector is a snapin type that’s suitable for panels with a maximum wall thickness of 1mm (much thinner than the enclosure walls), it’s first fitted to a small metal plate of 1mm-thick sheet steel or aluminium. The resulting assembly is then fastened inside the enclosure behind the connector’s cut-out, using four M3 x 10mm Nylon screws and eight M3 Nylon hex nuts (two on each screw, for safety). This arrangement gives maximum safety combined with neatness, as the front surface of the IEC connector is virtually flush with the outside of the enclosure wall. Wiring up the board and in fact the Inside the completed Tempmaster Mk.2 – note how the mains wiring is firmly secured using cable ties, so that it’s impossible for the leads to come adrift and contact low-voltage wiring. Table 1: Resistor Colour Codes o o o o o o o o o o o siliconchip.com.au No.   1   1   1   1   1   2   2   1   1   2 Value 10MΩ 22kΩ 11kΩ 6.8kΩ 5.6kΩ 4.7kΩ 3.3kΩ 2.7kΩ 1.2kΩ 10Ω 4-Band Code (1%) brown black blue brown red red orange brown brown brown orange brown blue grey red brown green blue red brown yellow violet red brown orange orange red brown red violet red brown brown red red brown brown black black brown 5-Band Code (1%) brown black black green brown red red black red brown brown brown black red brown blue grey black brown brown green blue black brown brown yellow violet black brown brown orange orange black brown brown red violet black brown brown brown red black brown brown brown black black gold brown February 2009  27 LM335Z (FLAT SIDE DOWN) CUT ADJ LEAD SHORT BROWN WIRE TO THIS LEAD RED WIRE TO CENTRE LEAD 2 x 25mm LENGTHS OF 2.5mm HEATSHRINK 30mm LENGTH OF 5mm DIA HEATSHRINK 3-METRE LENGTH OF 2-CORE RIBBON CABLE 1 SOLDER RIBBON CABLE WIRES TO TEMP SENSOR LEADS 2 SLIDE HEATSHRINK SLEEVES UP AND HEAT TO SHRINK 3 FIT LARGER SLEEVE AND HEAT TO SHRINK OVER ALL LEADS M3 x 9mm LONG COUNTERSINK HEAD SCREWS WITH STAR LOCKWASHERS AND M3 NUTS 4 CLAMP SENSOR ASSEMBLY TO 25 x 50mm ALUMINIUM HEATSINK PLATE 5 FIT 3.5mm JACK PLUG TO OTHER END OF RIBBON CABLE (RED WIRE TO TIP) Fig.5: follow this 5-step procedure to make the temperature sensor assembly. As shown, the sensor is clamped to a 25 x 50mm aluminium heatsink plate. colour codes but it’s also a good idea to check each one with a DMM just before it’s fitted to the board. Once they are in, fit trimpot VR1 – this goes at centre right, between CON1 and CON2. The two non-polarised ceramic capacitors can be fitted next, followed by the two electrolytics. Take special care with the latter as they are polarised. Make sure you follow the diagram carefully for their orientation or you’ll strike trouble later. Take the same care with the semiconductors. These can be fitted now, starting with diodes D1 and D2 and zener diode ZD1. Follow these with transistor Q1 and LED1. The latter should be mounted vertically, with the bottom of its body about 12mm above the board. Make sure the LED is orientated with its flat (cathode) side as shown, then fit IC1 and REG1, soldering these into place if you’re not using IC sockets. Now you can bolt relay RLY1 to the board at lower centre. It’s attached to the board using two M4 x 10mm machine screws, flat washers, lockwashers and hex nuts. Make sure that you mount the relay with its coil connection spade connectors to the right and its contact connectors to the left, as shown in the wiring diagram and photos. Also make sure that you fit the relay mounting screws with their heads under the board, and their nuts and washers above the relay mounting flanges. Preparing the enclosure This view shows the completed temperature sensor unit. It connects to the main Tempmaster circuit via a 3.5mm mono jack plug. thermostat as a whole should be very easy if you follow the internal photos and the wiring/overlay diagram carefully. Begin wiring up the PC board by fitting the three terminal pins (used to provide test points). These go at centre right on the board. Follow these with DC input connector CON1, the temperature sensor socket CON2 and the two-way terminal block CON3. 28  Silicon Chip If you are using sockets for IC1 and REG1 these can now be fitted as well. You can also fit the two 3-way SIL headers for LK1 and LK2, which are located just to the left of CON2. It’s also a good idea to now fit the wire link which goes just to the left of the LK1 header. Next, fit the various resistors, making sure you fit each one in its correct position. Table 1 shows the resistor Your board assembly should now be complete and you can place it aside while you drill and cut the various holes needed in the enclosure and its lid (note: you probably won’t have to do this if you’re building it from a kit, as it will very likely come with the enclosure and lid fully prepared for you). Use the enclosure cutting diagram shown in Fig.6 as a guide to the size and location of all holes. Fig.6 also shows the details for the metal mounting plate for the IEC connector. Once the enclosure has been prepared, slip the PC board assembly down into it and screw it into place using the four M3 x 6mm machine screws, which mate with the metal nuts moulded into the standoffs in the bottom of the enclosure. That done, clip the IEC mains connector into its metal mounting plate and fit this siliconchip.com.au assembly into the matching cut-out in the lefthand end of the enclosure, from the inside. You’ll find that the flange of the IEC connector slips snugly inside the cutout and the mounting plate is flush against the inside of the enclosure wall. You can then fasten the assembly in place using four M3 x 10mm Nylon screws and nuts. It’s a good idea to then install an additional Nylon nut on each mounting screw. These will firmly lock the first nuts into position and ensure that the assembly can not come loose. Next, mount the mains outlet socket in its matching lid cut-out. This is done by undoing the screw in the centre of the socket to separate the front and rear sections, then screwing them back together with the lid sandwiched between the two sections. Fitting the mains leads After the outlet socket is fitted you can then prepare the various mains connection wires which link it to the IEC mains connector and the contacts siliconchip.com.au of RLY1 – see Fig.4. Note that these leads must all be rated at 250VAC. The blue (Neutral) and green/yellow (Earth) leads from the outlet socket each have a 4.8mm insulated spade connector crimped firmly to their far ends. By contrast, the brown (Active) wire from the GPO is fitted with a 6.4mm insulated spade connector to connect to one of the relay contact terminals. The brown (Active) lead between the IEC connector and the relay is fitted with a 4.8mm insulated spade connector at one end and a 6.4mm connector at the other. Finally, the lead that’s used to link two terminals on the IEC connector has 4.8mm insulated spade connectors at both ends. Fig.4 shows what type of spade connector to fit to each wire. These spade connectors must all be fully insulated. If you are unable to obtain fully insulated 4.8mm connectors, then use non-insulated connectors but be sure to fully insulate them using 6mm-diameter heatshrink tubing after the leads are crimped. Points To Check (1) Be sure to use the specified ABS plastic case & note that Nylon screws must be used to secure the IEC connector plate to ensure safety. (2) Use mains-rated cable for all connections to the IEC socket, the GPO and the relay contacts. Secure this wiring using cable ties – see photos. (3) Use fully-insulated spade connectors to terminate the leads to the IEC connector and to the relay contacts. A ratchet-driven crimping tool is necessary to fit the spade connectors. (4) Do not touch any part of the 230VAC wiring while this device is plugged into the mains. Do NOT attempt to build this device unless you know what you are doing and are familiar with high-voltage wiring. Make sure you attach all of these space connectors very firmly using a ratchet-type crimp connector, so they will give reliable long-term connections. Two short wires are used to connect February 2009  29 (RIGHT-HAND END OF BOX) 19 15.5 9.0mm DIAMETER HOLE FOR 2.5mm DC CONNECTOR 19 14 A CL (LEFT-HAND END OF BOX) 10 A 5.5 27 47 10 A 13.5 A 5 18 50 A A CUTOUT FOR IEC CONNECTOR 6 30 5 A HOLES A: 3.0mm DIAMETER CORNER RADIUS 2.5 A 18 CL 72 25 IEC CONNECTOR MOUNTING PLATE: MATERIAL 1mm SHEET ALUMINIUM OR STEEL 5.5 A 26 6 40 18 38 33.5 16.75 12 9.0mm DIAMETER HOLE FOR 3.5mm JACK PLUG ENTRY (BOX LID) 14 10.9 4.5mm DIAM. 4.0 Fig.6: this diagram shows the cutout and drilling details for the GPO socket in the case lid, the access holes for the DC socket, the temperature sensor socket and the trimpot (righthand end), the IEC connector (lefthand end) and the metal mounting plate for the IEC connector. A large cutout can be made by drilling a series of small holes around the inside perimeter, then knocking out the centre piece and carefully filing the job to a smooth finish. 30  Silicon Chip siliconchip.com.au Parts List Non-insulated 4.8mm spade connectors can be used, provided they are fully insulated with 6mm heatshrink sleeving as shown here. the coil of RLY1 to terminal block CON3. These can be made up from medium-duty insulated hookup wire, with each one having a 4.8mm insulated spade connector crimped to one end. That completes the wiring of the Tempmaster but before you screw on the lid of the enclosure to finish it, fit plastic cable ties to the mains wiring as shown in the internal photos. These will ensure that the spade connectors cannot come loose and make it impossible for a mains lead to make contact with any of the low-voltage wiring. You can also fit one or two cable ties around the wires from the relay coil to CON3, to make sure these will also hold each other in place. Installing the jumper shunts Another job to do at this stage is to fit the jumper shunts to the two 3-way header strips on the top of the Tempmaster PC board, to provide links LK1 and LK2. Whichever positions you use for these two jumpers will depend on whether you’re going to be using the Tempmaster to control cooling or heating. They go in the lower “C” positions for cooling or the upper “H” positions for heating. Finally, fit the rubber sealing strip around the groove in the underside of the enclosure lid, and then screw the lid to the enclosure using the four screws provided. You should now be ready to make up the Tempmaster’s remote temperature sensor. Making the remote sensor Follow the step-by-step diagram shown in Fig.5 as a guide. The first step is to clip short the unwanted third lead off the LM335Z sensor and then solder the ends of the 2-core ribbon cable wires to the other two leads after slipping 25mm lengths of 2.5mm diameter heatshrink sleeving over each one. After the solder cools and you are happy that both joints are good, the sleeves are then moved up until siliconchip.com.au 1 PC board, code 10202091, 151 x 109mm 1 IP65 ABS sealed polycarbonate enclosure with clear lid, 171 x 121 x 55mm (Jaycar HB-6248 or equivalent) 1 2.5mm DC input socket, PC board mounting (CON1) 1 3.5mm jack socket, PC board mounting (CON2) 1 2-way terminal block, PC board mounting (CON3) 1 3.5mm stereo jack plug 1 10A flush mounting mains outlet socket with side wire entry 1 snap-fit fused male IEC connector with switch 1 chassis-mount 12V coil SPDT relay with 20A contacts (Jaycar SY-4042) 1 10A M205 slow blow fuse 3 PC board pins, 1mm diameter 1 14-pin machined IC socket 1 8-pin machined IC socket 1 300mm length of 10A brown mains wire 1 100mm length of 10A blue mains wire 1 100mm length of 10A green/ yellow mains wire 2 50mm lengths of medium duty insulated hookup wire 6 plastic cable ties 1 72 x 38 x 1mm sheet steel or aluminium (for IEC connector mounting plate) 2 3-pin SIL headers 2 jumper shunts 1 25 x 50 x 3mm aluminium sheet 1 30 x 10 x 1mm aluminium sheet 2 6.4mm fully-insulated spade connectors for 1mm2 wire 7 4.8mm fully-insulated spade connectors for 1mm2 wire 1 2m length of 2-conductor ribbon cable 2 25mm lengths of 2.5mm heatshrink sleeving 1 150mm length of 6mm heatshrink sleeving 4 M3 x 10mm Nylon screws, pan head 8 M3 Nylon hex nuts 2 M3 x 10mm machine screws, countersunk head 2 M3 hex nuts & star lockwashers 4 M3 x 6mm machine screws 2 M4 x 10mm machine screws 2 M4 hex nuts 2 M4 flat washers 2 M4 lockwashers 1 500Ω multi-turn cermet trimpot, horizontal adjust (VR1) 1 30mm-length tinned copper wire (for link) they butt hard against the body of the LM335Z, after which they are heated to shrink them in place (step 2). Then a 30mm-length of 5mm diameter heatshrink sleeving is slipped along the cable and over the other sleeves, and heated in turn to shrink it in place as well (step 3). Next, prepare the sensor’s heatsink assembly by drilling two 3.5mm diameter holes on the centre line of the 50 x 25mm aluminium plate, 18mm apart. Both holes should be countersunk to accept countersink-head screws installed from underneath. Next make the 30 x 10mm piece of 1mm aluminium into a clamp piece, by bending its central 8mm section into a half-round shape to fit snugly over the LM335Z’s body. After this, drill 3.5mm holes in the flat ends of this clamp piece, 18mm apart again to match the holes in the larger plate. You should then be able to assemble the probe with the LM335Z clamped to the top of the plate flat side down and Semiconductors 1 LM335Z temperature sensor (TS1) 1 LM393 dual op amp (IC1) 1 LM723C regulator (REG1) 1 BC327 or BC328 transistor (Q1) 1 16V 1W zener diode (ZD1) 1 5mm red LED (LED1) 2 1N4004 1A diodes (D1,D2) Capacitors 1 470μF 25V RB electrolytic 1 10μF 16V RB electrolytic 1 1nF disc ceramic 1 100pF disc ceramic Resistors (0.25W, 1%) 1 10MΩ 2 4.7kΩ 1 22kΩ 2 3.3kΩ 1 11kΩ 1 2.7kΩ 1 6.8kΩ 1 1.2kΩ 1 5.6kΩ 2 10Ω February 2009  31 1MM-THICK METAL PLATE NYLON SCREWS & NUTS NOTE CABLE TIES USED TO SECURE NEUTRAL & EARTH LEADS TO GPO This inside view clearly shows how the mains wiring is installed and secured. Note the Nylon screws & nuts used to secure the IEC connector/ bracket assembly. the screws tightened down using M3 nuts and star lockwashers (step 4). Complete the sensor assembly by fitting the 3.5mm mono jack plug to the other end of the 2-core ribbon cable, connecting the red wire to the “tip” lug and the brown wire to the “sleeve” lug (step 5). Initial checks Before doing anything else, use your multimeter (set to a low ohms range) to check between the earth pin of the IEC connector and the Earth outlet of the GPO. You should get a reading of zero ohms here (this checks the integrity of the earth connection). Having verified the earth connection, fit the 10A fuse to the fuseholder in the IEC socket. Note that this fuse should be a slow-blow type. Note also that we strongly advise against connecting this unit to mains 32  Silicon Chip power without the lid in place, to eliminate the risk of electric shock. Setting it up This is mainly a matter of adjusting trimpot VR1 to produce the reference voltage level at test point TP1 that corresponds to the average temperature you want the Tempmaster to maintain. This can be done by trial and error once the project is finished and working but if you have a DMM it can also be done before the case is closed up (but before the IEC mains connector is connected to the power, of course). If you want to do this, first plug the 12V DC cable from your plugpack or battery supply into CON1 at the righthand end of the box. Now connect the leads of your DMM (set to a low DC voltage range) to TP1 and TPG. Read the voltage, which should be somewhere between 2.75V and 2.92V. Now all you have to do is look up the voltage level for the temperature you want from Fig.1 and adjust VR1 until the DMM reading changes to this value. The enclosure can then be closed up again. All that remains now is to mount the remote sensor inside the fridge or freezer cabinet, or inside the hothouse, fishtank or seed germinating cabinet, attaching the sensor’s heatsink plate to the side of the cabinet using two short lengths of gaffer tape. Then you can run its ribbon cable outside, holding it down with further strips of gaffer tape so it will pass neatly under the rubber door seal when the door is closed. If you mount the Tempmaster on the wall just behind the fridge/freezer or heater, the plug on the end of the ribbon cable can be plugged into CON2 on the righthand end of the enclosure to complete the job. siliconchip.com.au Connection Options For The Tempmaster There are at least three different ways that the Tempmaster Mk.2 can be connected up to control the temperature of a fridge, freezer or heater set-up. Which one you use will depend on whether your fridge/freezer/heater operates from 240V AC or 12V DC and also whether you will be running it from the AC mains or from a battery supply. The three main options are shown for your guidance in the diagram at right. Option A 240V WALL OUTLETS (GPOs) TEMPMASTER Mk2 12V DC PLUG PACK (OR CHARGER + 12V SLA BATTERY) A siliconchip.com.au 240V FRIDGE/FREEZER (OR HEATER) IEC MAINS CORD TEMPMASTER Mk2 (12V DC LEAD) WIND GENERATOR TEMPERATURE SENSOR + – CHARGING CONTROLLER BATTERY SOLAR PANEL B USE WITH SOLAR/WIND POWER, 240V FRIDGE/FREEZER LOW VOLTAGE PLUGS & SOCKETS Option C Now you can unplug the power cord of the fridge/freezer/heating cabinet from its original GPO and plug it instead into the GPO on the top of the Tempmaster. Then when you connect the Tempmaster’s own IEC mains connector to the original GPO via a suitable IEC mains cable, the complete USE WITH 240V FRIDGE/FREEZER, MAINS POWER 12V–240V INVERTER Option B Option (C) shows how to connect things up when the Tempmaster is to be used with a 12V fridge/freezer and a solar power system. Here the configuration is quite straightforward but you MUST replace both of the Tempmaster’s “mains” connectors with suitable low-voltage plugs and sockets – to make sure that they can’t be accidentally connected to 240V. TEMPERATURE SENSOR (12V DC LEAD) Option A shows the simplest arrangement, where a 240V fridge/freezer or heater is to be operated directly from the 240V AC mains supply. The 12V DC needed by the Tempmaster itself can be supplied either by a small plugpack DC supply or from a 12V SLA battery which is kept “topped up” by a suitable charger. The next option (B) shows how a 240V fridge/freezer or heater can be connected to a 12V/240V power inverter, in a home or building which relies on solar panels or wind-generated power. As you can see, the Tempmaster itself can be powered from the main battery, along with the power inverter used to operate the fridge/ freezer/heater. Because there is no current whatsoever drawn from the Tempmaster’s IEC mains input socket when the Tempmaster has switched off the power to the fridge/ freezer/heater, the inverter should drop back to “sleep” mode at these times. 240V FRIDGE/FREEZER (OR HEATER) IEC MAINS CORD 12V FRIDGE/FREEZER (12V DC LEAD) TEMPMASTER Mk2 WIND GENERATOR + – CHARGING CONTROLLER TEMPERATURE SENSOR BATTERY SOLAR PANEL C USE WITH SOLAR/WIND POWER & 12V FRIDGE/FREEZER system will begin working. If you want to make sure that the thermostat is holding the fridge/ freezer/heater to the temperature you want, this can be done quite easily using a thermometer placed inside the cabinet for a while. You can see when the Tempmaster is switching power to the compressor or heater simply by watching LED1. If you need to adjust the average temperature up or down, this is done quite easily by adjusting trimpot VR1 using a small screwdriver, through the small central hole in the righthand end SC of the enclosure. 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