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Measure and control temperature over a wide range with this . . . By JOHN CLARKE High-Temperature Thermometer/Thermostat Need to measure or control temperature over a very wide range? Now you can do it with this compact unit which hooks up to a K-type thermocouple. It drives a relay which can be used to precisely control the temperature in ovens, kilns, autoclaves, solder baths or at the cold end of the spectrum, fridges and freezers. It is based on an Analog Devices AD8495 precision instrumentation amplifier with thermocouple cold junction compensation. N OW WE KNOW that some digital multimeters can measure temperatures with a K-type thermocouple but that’s all they can do; they cannot control the temperature in an oven etc. In other words, they do not provide an adjustable thermostat function. In all the above examples, our new High-Temperature Thermometer/ Thermostat can be used to measure 40  Silicon Chip and control the temperature at the same time. That’s because it has a relay output that opens or closes at a preset temperature. The switched output can be used directly or in conjunction with a higherrated relay to control power to the element of a heater or the compressor of a refrigerator. For heating, the power can be switched on when the tempera- ture is below the preset temperature and switched off when it is above. Alternatively, for cooling, power can be switched on when the temperature goes above the preset and off when it goes below. The preset temperature for this thermostat action can be adjusted between -50°C and 1200°C. It is important that the thermostat function does not cause rapid on and siliconchip.com.au Features & Specifications Main Features • • • • • • • • • K-Type thermocouple probe Ground referenced or insulated probe can be used Measures -50°C to 1200°C (depending on probe) Pre-calibrated temperature measurement Optional calibration of span and offset adjustment Thermostat switching at a preset temperature with adjustable hysteresis High to low or low to high thermostat threshold Relay output for thermostatic control Relay contacts rated at 10A (30V AC/DC maximum recommended switching voltage) Specifications Power supply: 12V <at> 100mA Measurement range: -50°C to 1200°C (probe dependent) Initial accuracy: ±4°C for -25°C to 400°C measurements (ambient between 0°C and 50°C) Optional calibration adjustment for span: -4%, +5.27% Optional calibration adjustment for offset: ±6.2mV equivalent to >±1°C off switching of the heater, compressor or whatever is being temperature-controlled. Hence the design incorporates adjustable hysteresis. This allows a preset temperature difference to apply between switching power on and off. The hysteresis is adjustable from less than 1°C to more than 9°C. The temperature is displayed on a 3½ digit LCD and while the unit can display a temperature from -50°C to 1200°C the actual measurement range will depend on the particular probe. Some K-type probes will operate from -50°C to 250°C, while others will operate from -40°C to 1200°C. The High-Temperature Thermometer/Thermostat is housed in a small instrument case and controls on the front include a power switch and a switch to select between measured temperature and the preset thermostat temperature. A LED indicator is for power indication and a second LED shows when the thermostat relay has switched on. At the rear of the case is the power input socket for a 12V DC supply and a socket for the K-type thermocouple. Additionally, there is a terminal connector inside the case for connection to the thermostat relay contacts. The common (C), normally open (NO) and normally closed (NC) contacts are available for connection. Inside the case there are jumper siliconchip.com.au Thermostat set point range: adjustable from -50°C to 1200°C Thermostat hysteresis: adjustable from <1°C to >9°C Cold junction compensation: optimised for 0-50°C ambient temperatures links to select whether the thermostat relay switches on above or below the preset temperature for the thermostat. There are also jumper selections to select whether the Thermometer/ Thermostat is built pre-calibrated or where the temperature calibration can be accurately adjusted. K-type thermocouple As mentioned above, this design uses a K-type thermocouple which comprises a junction of two dissimilar wires; in this case it uses an alloy of chrome and nickel (called Chromel) for one wire and an alloy of aluminium, manganese, silicon and nickel (called Alumel) for the second. These two wires are insulated and make contact at the temperature probe end only. The other end of the wires are usually connected to a 2-pin plug. Basically, a thermocouple’s operation relies on the principle that the junction of two dissimilar metals produces a voltage that is dependent on temperature. A K-type thermocouple produces a voltage output that typically changes by 40.44µV/°C. This change in output per is called the Seebeck coefficient and it refers to the output change that occurs due to the temperature difference between the probe end and the plug end of the thermocouple. In practice, the Seebeck coefficient for the K-type thermocouple varies with temperature and is not precisely 40.44µV but this is a good average value over the temperature range from 0°C to 1200°C. If we know the temperature at the plug end of the thermocouple, we can calculate the temperature at the probe since we also know the Seebeck coefficient. For example, if the plug end is held at 0°C, the output will increase by 40.44µV for every 1°C increase. Similarly, the output will decrease by 40.44µV for every 1°C drop in temperature. In practice, we do not keep the plug end of the thermocouple at 0°C; it’s not practical. Instead, we compensate the thermocouple output by measuring the temperature at the plug end and then adding 40.44µV for every 1°C that the thermocouple plug end is above 0°C or subtracting 40.44µV for every 1°C that the plug end is below 0°C. May 2012  41 THERMOSTAT PRESET REF1: 2.5V REFERENCE VR1 IC2d COMPARATOR (IC2a) A=3 +2.5V – IC1 AD8495 OUT 5mV/°C 2 S2 REF +2.5V 1.25V 1 NO COM NC K-TYPE THERMOCOUPLE + RELAY RELAY DRIVER (Q1, Q2) 1 = THERMOMETER 2 = THERMOSTAT 1/50 DIVIDER (VR4, LK3-4) A=1 ~ 1.25V 100 V/°C INHI INLO 3.5-DIGIT LCD PANEL METER (200mV FULL SCALE) BUFFER (IC2b, IC2c,VR3 Fig.1: block diagram of the High-Temperature Thermometer/Thermostat. IC1 processes and amplifies the thermocouple’s output and drives the LCD panel meter and comparator IC2a. Trimpot VR1 sets the thermostat temperature. For example, if the thermocouple plug is at 25°C, its output will be 1.011mV (ie, 25 x 40.44µV) lower than it would be if it were at 0°C. By adding an extra 1.011mV to the reading, we obtain the correct result without having to keep the plug end at 0°C. Note that there are several dissimilar metal junctions within the connections between the thermocouple plug and amplifier. These include the Chromel to copper junction and the Alumel to copper junction on the PCB itself. These do not contribute to the overall voltage reading provided they are all kept at the same temperature. As a result, the PCB has been designed to help maintain similar temperatures at these junctions by making the copper connections all the same size. Once the PCB is installed inside its case, the inside temperature should remain fairly constant for all these junctions. Note that if the thermocouple lead needs to be extended, it’s necessary to use the same K-type thermocouple wire for the whole length between the probe and plug. Signal processing Refer now to Fig.1 which shows the block diagram of the High-Temperature Thermometer/Thermostat. As shown, the thermocouple signal is processed using the Analog Devices AD8495 IC. This is a precision instrumentation amplifier with K-type thermocouple 42  Silicon Chip cold junction compensation. Its output is 5mV/°C. The amplifier within the AD8495 is laser trimmed for a gain of 122.4. This gain effectively converts the 40.44µV/°C output of the thermocouple to 4.95mV/°C. The output is optimised for a 25°C measurement where a gain of 122.4 gives a result of 123.75mV. Within the AD8495, a 1.25mV offset is added to the amplified value, giving a 125mV output at 25°C. For temperatures other than 25°C, the combination of the variation in the Seebeck coefficient over temperature, the 122.4 gain and the 1.25mV offset provides an accurate 5mV/°C output over the range of -25°C to 400°C. For this range, the output is within 2°C. Note that the specification panel shows that the accuracy is ±4°C for ambient between 0°C and 50°C and -25°C to 400°C measurements. This is different to the 2°C error for the AD8495 because the display is showing a reading via a voltage divider that is prone to extra tolerance errors. It’s possible to calibrate the measurement to a finer accuracy if this is required. Table 1 shows the expected output from the AD8495 over a wide range of temperatures and compares this with the ideal 5mV/°C output. How it works Returning now to the block diagram of Fig.1, the K-type thermocouple con- nects directly to the AD8495 (IC1) at the IN+ and IN- terminals. The resulting 5mV/°C output signal from IC1 is then fed to the non-inverting input of comparator IC2a and also to position 1 (Temperature) on switch S2. S2 selects between the Temperature and Thermostat modes of operation. In order to allow for negative temperature measurements, the output from the AD8495 is offset by approximately 1.25V. This offset is derived by a voltage divider connected across a 2.5V reference (REF1) and buffered using op amps IC2b and IC2c. The buffered 1.25V signal is then applied to the AD8495’s REF (reference) input. This effectively “jacks up” the AD8495’s output by 1.25V. As a result, a -50°C measurement now gives an output that’s theoretically 250mV below (-5mV x 50) the 1.25V reference offset (ie, 1V). Without this offset, the AD8495 would not be able to handle negative temperature measurements since its output cannot go below 0V. Although the offset only needs to be 250mV to allow for a -50°C measurement, a value of 1.25V is used because of the LCD panel meter that’s used to measure the voltage. This meter requires an input that’s at least 1V above the 0V supply for correct operation. According to Table 1, the actual output from IC1 at -50°C is 228mV below the offset voltage. So using an offset of 1.25V leaves us with a comfortable 22mV margin above the critical 1V level. The 3.5-digit LCD panel meter used to display the temperature has a 200mV full scale reading (actually 199.9mV) for a reading of 1999. It’s basically connected to measure the voltage between IC1’s output (via a divider) and the offset voltage. This effectively removes the offset voltage from the reading. To prevent the meter from overranging and to get a reading in °C, we need to divide IC1’s output by 50. For example, if the temperature is 1200°C, the voltage between IC1’s output and the 1.25V offset will be 6V (ie, 1200 x 5mV). Dividing this by 50 gives 120.0mV and the panel meter is configured to show 1200 (ie, no decimal point). Note that, in the full circuit, either a fixed divide-by-50 attenuator or an adjustable divide-by-50 attenuator can be used. The desired attenuator is selected using jumper links and siliconchip.com.au the adjustable one allows for accurate calibration. The display can either show the measured temperature when switch S2 is in position 1 or the preset temperature (for the thermostat operation) when S2 is in position 2. VR1 sets the thermostat temperature. As shown, it’s connected to a 2.5V reference (REF1) and the voltage at its wiper drives op amp IC2d. As a result, IC2d’s output can range up to 7.5V, slightly more than the 7.25V at IC1’s output when the measured temperature is at the 1200°C maximum (ie, 1200 x 5mV plus the 1.25V offset). This allows VR1 to set the thermostat temperature anywhere from -50°C to 1200°C. IC2d’s output is fed to the inverting input of comparator IC2a where it is compared with IC1’s output. IC2a’s output thus switches low when the temperature is below the preset and high when the temperature is above the preset. This output then drives a relay via transistors Q1 and/or Q2. Links LK5 and LK6 can be selected so that the relay either switches on when IC2b’s output goes high or on when it goes low. Circuit details Refer now to Fig.2 for the full circuit diagram of the High-Temperature Thermometer/Thermostat. As well as the AD8495 (IC1) and the LCD panel meter, it includes an OP747 precision quad op amp (IC2), a 7805 3-terminal regulator, an LM285-2.5 precision voltage reference, transistors Q1 & Q2 and various minor components. IC1 is powered from a 12V DC plugpack supply via switch S1, diode D1 (for reverse polarity protection) and a 10Ω resistor. A 22V zener diode (ZD1) clamps any over-voltage transients while 100µF and 100nF capacitors are used to bypass the supply. In operation, IC1 draws just 180µA to minimise internal heating (note: internal heating would affect the measurement of the ambient temperature used for the thermocouple ice-point temperature compensation). The K-type thermocouple connects to its IN+ and IN- terminals (pins 8 & 1) via series 47kΩ resistors. These resistors and their associated 100nF ceramic capacitors prevent RF (radio frequency) signals from being detected by IC1’s sensitive input stages. The resistors acts as RF stoppers, while the siliconchip.com.au Table 1: AD8495 Output vs. Temperature Thermocouple Temperature (°C) Ideal Output <at> 5mV/°C (mV) AD8495 Output (mV) Display Reading (°C) ±1 Digit -50 -40 -20 0 20 25 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 420 440 460 480 500 520 540 560 580 600 620 640 660 680 700 720 740 760 780 800 820 840 860 880 900 920 940 960 980 1000 1020 1040 1060 1080 1100 1120 1140 1160 1180 1200 -0.25 -0.2 -0.1 0.0 0.1 0.125 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 6.0 -0.228 -0.184 -0.093 0.003 0.100 0.125 0.200 0.301 0.402 0.504 0.605 0.705 0.803 0.901 0.999 1.097 1.196 1.295 1.396 1.497 1.599 1.701 1.803 1.906 2.010 2.113 2.217 2.321 2.425 2.529 2.634 2.738 2.843 2.947 3.051 3.155 3.259 3.362 3.465 3.568 3.670 3.772 3.874 3.975 4.076 4.176 4.275 4.374 4.473 4.571 4.669 4.766 4.863 4.959 5.055 5.150 5.245 5.339 5.432 5.525 5.617 5.709 5.800 5.891 5.980 -46 -37 -19 0 20 25 40 60 80 101 121 141 161 180 199 219 239 259 279 299 320 340 361 381 402 423 443 464 485 506 527 548 569 589 610 631 651 672 693 713 734 754 774 795 815 835 855 875 895 914 934 953 973 992 1011 1030 1049 1068 1086 1105 1123 1141 1160 1178 1196 May 2012  43 44  Silicon Chip siliconchip.com.au + – CON2 10k 10k 10k 1 8 K 10 9 6 5 SENSE 5 100nF* 100 F IC2c IC2b –Vs 3 2 8 7 * CERAMIC REF 6 IC1 OUT AD8495 100nF –IN +IN 7 +Vs 100 F 10 UNCAL 10 LK1 LK2 CAL OUT GND V+ IN REG1 7805 +2.5V K A 15k VR1 100k 100 F +5V 10k VR4 100 LK3 CAL UNCAL LK4 100nF THERMOSTAT PRESET  LED1 POWER 470 HIGH TEMPERATURE THERMOMETER/THERMOSTAT A +2.5V A K D1 100nF* VR3 100 REF OFFSET 10k 100 47k A 100nF* ZD1 22V 100nF* 47k S1 POWER 100nF 20k IC2d 4 1.8 39 1 THERMOMETER 13 12 100nF B 10k A K D1, D2: 1N4004 1k 51k 2 THERMOSTAT 470 IC2: OP747 S2 14 10 F NP 9 7 5 COM 6 INLO 8 RFL 1  K 1 +5V B A IN E CON3 A ZD1 K 2 NO NC COM OUT K A NC C BC337 7805 B A K LEDS GND LM285-2.5Z/LP Q2 BC337 GND E C RELAY1 . .8:8.8 10k 2.2k D2 K 3.5-DIGIT LCD PANEL METER VR2 1M + ROH RFH INHI 100k LK6 L/H 11 IC2a 10k 10 3 2 E Q1 BC337 C H/L LK5 LED2 A Fig.2: the complete circuit diagram for the High-Temperature Thermometer/Thermostat. An accurate 1.25V reference is derived from REF1 via IC2b or IC2c and this is applied to the REF input of IC1 to enable measurements down to -50°C. IC1’s output drives the LCD panel meter via a 50:1 divider and also drives the noninverting input of comparator IC2a. IC2a compares IC1’s output with the thermostat preset temperature, as set by VR1 & IC2d, and drives relay 1 via transistors Q1 and/or Q2 when the preset limit is reached. Links LK5 & LK6 allow the relay to be driven on either a rising or falling temperature. SC 2012 +5V K 4.7k REF1 LM285 -2.5Z/LP – + CON1 K-TYPE THERMOCOUPLE 12V DC INPUT +12V +11.4V 100nF capacitors effectively shunt any remaining RF signal to ground. In addition, the negative terminal of the K-type thermocouple is tied to ground via a 100Ω resistor. This prevents the probe from picking up noise and mains hum, which would cause erratic operation. Note that the 100Ω resistor is included so that the circuit can be used with both earthed and insulatedsheath thermocouples. Basically, the thermocouple probe wires are housed in a cylindrical metal sheath or rod. Some units connect the negative thermocouple wire directly to this metal sheath (an earthed probe), while others fully insulate the metal sheath from the thermocouple wires (an insulated probe). For an insulated probe, it doesn’t matter whether the negative terminal is connected directly to ground or connected to ground via a 100Ω resistor. That’s because an insulated probe can connect to a point that’s not at 0V without affecting the operation of the probe. By contrast, an earthed probe does require the 100Ω resistor. That’s because the probe could make an external connection to the 0V supply rail and this might not be at exactly the same voltage as the 0V rail inside the unit. This type of situation could easily arise, for example, when measuring engine heat or brake disc heat in a car and the unit is being powered by the vehicle’s battery. In this situation, the probe point and the internal 0V rail will be at slightly different voltages due to current flowing in the vehicle’s chassis. The difference in voltage may only be small but the thermocouple’s output only varies by about 40µV/°C, so only small variations can mean a huge error in temperature readings. The 100Ω resistor eliminates this problem by preventing significant current flow between the thermocouple’s negative terminal and the 0V rail within the thermometer. Deriving the offset The 1.25V offset for IC1 is derived from REF1, a precision 2.5V voltage reference, via a resistive divider. This divider comprises four 10kΩ resistors and a 100Ω trimpot (VR3). As shown, the 1.25V midpoint of the 10kΩ fixed resistive divider is fed to pin 5 of IC2b, while the voltage on siliconchip.com.au VR3’s wiper is fed to IC2c. VR3 allows the offset voltage to be varied over a small range either side of 1.25V. IC2b and IC2c are both connected as unity gain buffer stages. When LK1 is installed, IC2b provides a fixed 1.25V offset for IC1 at its REF (pin 2) input. At the same time, IC2c provides the variable offset output to the panel meter at its IN LO input. Alternatively, if LK2 is installed, IC2c drives both the reference input of IC1 and the INLO input of the LCD panel meter. In this case, the voltage applied to both IC1’s REF input and the panel meter’s INLO input are exactly the same and this is the linking option to use if you do not want to accurately adjust the temperature calibration. LCD panel meter As stated previously, the LCD panel meter measures the difference between its INHI (pin 7) and INLO (pin 6) inputs. In this circuit, IC1 drives the INHI input via one of two 50:1 voltage dividers (one fixed, the other variable) when S2 is in position 1. IC1 is capable of delivering in excess of ±5mA to a load but the fixed 50:1 divider draws just 115µA maximum when IC1’s output is producing 7.25V for a 1200°C measurement. This low current minimises any internal heating of the IC. The fixed divider is selected using link LK4. It’s made up using a 51kΩ resistor in the top section and 39Ω, 1.8Ω and 1kΩ resistors at the bottom. Assuming the values are exact, the division ratio is very close to 50:1. However, resistor tolerances can shift this to within a range of around 50.05:1 to 49.95:1. The variable divider shares the 51kΩ and 1kΩ resistors but uses a 100Ω trimpot in place of the 39Ω and 1.8Ω resistors in the fixed divider. This allows the divider to be adjusted. It’s selected by installing link LK3 instead of LK4. The LCD panel meter itself is based on an Intersil ICL7106 3.5-digit LCD analog-to-digital converter (ADC). Its INLO, COM (common) and RFL (reference low) pins are all connected together, ie, they are all fed with the reference offset voltage at IC2c’s output. In addition, the ROH output is connected to the RFH (reference high) input and this sets the panel meter to 200mV full scale. A 5V supply rail for the LCD is derived from regulator REG1 (7805). The OP747ARZ Quad Precision Op Amp The OP747ARZ quad precision op amp specified here has features that are not found in general-purpose op amps. First, it features a low offset voltage of 100µV maximum and the input bias and offset currents are in the very low nA range. Second, it can handle input voltages ranging from the ground supply rail up to within 1V of the positive supply. And third, the output can reach close to each supply rail. Taken together, these characteristics make the op amps ideal for this circuit. REG1’s input and output rails are both filtered using 100µF electrolytic capacitors, while LED1 in series with a 470Ω current-limiting resistor provides power indication. This regulated 5V supply also drives the 2.5V reference (REF1), this time via a 4.7kΩ resistor. As well as providing a source for the offset voltage, the resulting 2.5V rail is also fed to the top of VR1 which sets the thermostat preset. VR1 is connected in series with a 15kΩ resistor across this supply and its wiper provides an output which ranges from 326mV up to 2.5V. IC2d amplifies this by three, as set by the 20kΩ and 10kΩ resistors in the feedback path. The resulting voltage at the output of IC2d can range anywhere from 978mV up to 7.5V and that more than covers the possible voltage range from IC1, for temperatures ranging from -50°C to 1200°C. As described previously, op amp IC2a is wired as a comparator. It monitors IC2d’s output and compares this with IC1’s output. IC2a thus switches its output high when the measured temperature is above the preset temperature or low when the measured temperature goes below the preset (ignoring hysteresis). Trimpot VR2 (1MΩ) and the 100kΩ and 470Ω resistors provide hysteresis. With VR2 set at 1MΩ, the hysteresis is at its minimum and there is less than 1°C hysteresis. At the other extreme, with VR2 set for 0Ω, the hysteresis is more than 9°C. Relay driver circuit IC2a drives transistor Q1, which in turn drives Q2, when link LK5 is inserted. Alternatively, if LK6 is selected, Q1 is bypassed and IC2a drives Q2 direct. May 2012  45 1M 100 47k 100 100nF 100nF 22V ZD1 100 10 LK1 LK2 LK3 LK4 1.8 51k 39 1k 15k 0V +5V 100nF A S2 LED2 LED1 VR4 10k REF1 2.2k A VR3 TEMPERATURE THERMOMETER /THERMOSTAT 100nF LM285 -2.5Z/LP 470 HI 47k BC337 10k 100k 10k 12150112 EPYT K RHIGH ETE M O MRE HT 100nF 100nF 20k 10k IC2 (UNDER) VR2 100nF 10 470 VR1 4.7k 100 F 4004 100 F S1 100 F 100k 100nF IC1 (UNDER) ROH RFH RFL InHi InLo COM COIL 10k REG1 7805 D1 Q1 RELAY1 4004 D2 LK5 LK6 10k Q2 10k 10k CON3 – Thermocouple K type + CON1 – LOW © 2012 BC337 10 F NP + TO THERMOCOUPLE SOCKET NO COM NC CON2 12VDC IN RELAY CONTACTS 13 12 11 10 9 8 7 6 5 4 2 1 3.5-DIGIT LCD PANEL METER (REAR) Fig.3: follow this diagram to build the unit but note that the first job is to install surface-mount devices IC1 & IC2 on the underside of the PCB (see below). You can omit the relay, CON3, S2 and transistors Q1 & Q2 if you intend using the unit as a thermometer only and don’t need the thermostat function. (UNDER SIDE OF PCB) 1 IC1 04105121 K TYPE THERMOMETER 1 IC2 46  Silicon Chip Fig.4 (left): this diagram and the above photo show how surface-mount devices IC1 & IC2 are mounted on the underside of the PCB. Make sure that both devices are correctly orientated (pin 1 is identified by a small dot on the device body) and follow the step-by-step procedure described in the text to solder them into position. These two links select whether the relay turns on for a low-to-high temperature transition (LK6 in place) or a high-to-low transition (LK5 in place). When LK6 is in circuit, Q2 turns on when IC2a’s output goes high (ie, when the temperature rises above the preset) and this turns on relay 1. The relay subsequently turns off again when IC2a’s output switches low (ie, when the temperature falls below the preset). Conversely, when LK5 is in circuit, Q1 inverts the logic. In this case, Q2 and the relay are normally on since Q2’s base is pulled high. However, when IC2a’s output switches high (as the temperature rises above the preset), Q1 turns on and pulls Q2’s base to ground. As a result, Q2 and the relay turn off and remain off until the temperature falls below the preset again. LED2 lights whenever the relay switches on to indicate that the set temperature threshold has been reached. The associated 2.2kΩ resistor limits the current through LED2, while diode D2 protects Q2 from damage by quenching the back-EMF voltage spikes generated when the relay turns off. The relay provides both the usual common (COM), normally open (NO) and normally closed (NC) contacts, so it can also drive a load on or off depending on the selection of the NO or NC contacts. So it may seem that links LK5 and LK6 are not really necessary to reverse the switching sense. However, there are reasons why you may wish to select whether the relay is normally powered or not, especially when the relay contacts are required to switch a heating or cooling operation. One reason is that less current is drawn by the circuit when the relay is off and you might want to choose the link and contact configuration that draws the least power. Another reason is that you might want to ensure fail-safe operation if power is cut to the circuit. By using the COM & NO contacts to do the switching, you can ensure that power is not provided for heating or cooling if the power to the Thermometer/ Thermostat fails. Construction The assembly is straightforward with all parts except the probe socket and the LCD panel meter mounted on a PCB coded 21105121 (117 x 102mm). This is housed in a plastic instrument siliconchip.com.au The thermocouple socket is connected to an adjacent screw terminal block via two short leads. Alternatively, the screw terminal block could be omitted and a couple of flying leads soldered direct to the PCB. case measuring 140 x 110 x 35mm. Begin by carefully checking the PCB for any defects. Check also that the hole sizes are correct for each component to fit neatly. The corner mounting holes and the regulator mounting hole should all be 3mm in diameter. Our prototype used a double-sided PCB and Fig.3 shows the parts layout. The first step is to install IC1 and IC2. These are both surface-mount devices (SMDs) and mount on the underside of the PCB – see Fig.4. To install these, you will need a fine-tipped soldering iron, some fine solder and some quality solder wick. A magnifying lamp or at least a magnifying lens will also be handy. It’s best to install IC2 first. This is the 14-pin device with the wider pin spacings. First, place the PCB copper side up and apply a small amount of solder to the top-right pad, then pick the IC up with tweezers and position it near the pads. Check that it is orientated correctly (ie, with its pin 1 dot positioned as shown on Fig.4), then heat the tinned pad, slide the IC into place and remove the heat. Now check the IC’s alignment carefully using a magnifying glass. It siliconchip.com.au should be straight, with all the pins centred on their respective pads and a equal amount of exposed pad on either side. If not, reheat the soldered pin and nudge the chip in the right direction. Once its position is correct, solder the diagonally opposite pin, then recheck its position before soldering the remaining pins. Don’t worry too much about solder bridges between pins at this stage; they are virtually inevitable and can easily be fixed. The most important job right now is to ensure that solder flows onto all the pins and pads. Once you’ve finished, apply a thin smear of no-clean flux paste along all the solder joints and remove the excess solder using solder wick. You should then make a final inspection to ensure that there are no remaining solder bridges and that the solder has not “balled out” onto a pin without flowing onto the pad. If there are still bridges, clean them up with more flux and solder wick. Once IC2 is in place, you can install IC1 in exactly the same manner. Through-hole parts The larger through-hole parts can now be installed on the top of the PCB. Start with the resistors and diodes, then install zener diode ZD1, the MKT and ceramic capacitors and the electrolytics. It’s a good idea to check the value of each resistor using a multimeter before installing it. Take care with the polarity of the electrolytics, the diodes and the zener diode. They must be orientated as shown on Fig.3. Transistors Q1 & Q2 and the LM3852.5 precision voltage reference (REF1) can go in next. REG1 can then be installed. This mounts horizontally with its tab against the PCB, so you will have to bend its leads down at right angles to match its mounting holes. Secure its tab to the PCB using an M3 x 6mm screw and nut before soldering its leads. Don’t solder the leads before securing the tab; you could crack the copper tracks at the mounting screw is tightened if you do. Trimpots VR1-VR4 are next on the list. These must all be mounted with the adjustment screw to the right. Follow with the three 3-way pin headers for links LK1-LK6, then install the 6-way and 2-way polarised headers for the LCD panel meter connections. May 2012  47 The cable gland on the rear panel allows an external lead to be fed into the case and connected to the relay contacts at CON3. The LCD is secured to the front panel by running a couple of beads of silicone adhesive or hot-melt glue down the vertical inside edges. Be sure to orientate these headers as shown, ie, with their vertical tabs towards the panel meter. Once they’re in, you can install the two LEDs but first you have to bend their leads down through 90° some 9mm from their bodies. The best way to do this is to first cut a cardboard spacer 9mm wide. This is then be used as a template when bending the LED leads. Make sure that each LED is correctly orientated before bending its leads – the (longer) anode lead must be on the right when looking at the lens. Having bent their leads through 90°, the two LEDs must be installed with their leads 5mm above the PCB. This is best done by pushing them down onto a 5mm spacer, then soldering the leads to the PCB pads. Switches S1 & S2 are right-angle types and are mounted directly on the PCB. Push them down onto the board as far as they will go before soldering their leads. The PCB assembly can then be completed by installing the relay, 48  Silicon Chip the DC socket (CON2) and the 2-way and 3-way screw terminals. Connecting the panel meter The panel meter is wired to the 6-way header plug and to the 2-way header plug using short lengths of ribbon cable. These leads can be obtained by separating an 8-way ribbon into 6-way and 2-way strips. Cut these strips to 50mm in length, then strip about 2mm of insulation from the individual wires at one end and crimp them to the header pins. The pins can are then inserted into the headers. The other ends of these leads can then be stripped and soldered to the LCD panel meter pins. Check carefully to ensure that each wire goes to the correct pin on the panel meter and that there are no shorts between them. In fact, it’s a good idea to slip a short length of heatshrink over each wire before soldering it and then pushing over the soldered joint to insulate it from its neighbours. Jumper links LK2 & LK4 should now be installed and either LK5 or LK6. Install LK5 if you want the relay to switch on when the temperature drops below the preset. Alternatively, install LK6 if the relay is to switch on when the temperature rises above the preset. Final assembly Fig.5 shows the front and rear panel artworks. You can purchase finished panels from SILICON CHIP or you can download the artworks in PDF format from our website. Mounting the panel meter The LCD panel meter is mounted by sliding into its front-panel slot (which is open at the top). Check that the top of the meter sits flush with the top of the panel. If it protrudes slightly, it will be necessary to make the slot slightly deeper until it does sit flush. The meter is secured in place by running a bead of silicone sealant or hot-melt glue along the two vertical inside edges, adjacent to the front panel. siliconchip.com.au + its mounting slot from the rear (terminal screws facing up) and fitted with the supplied clip to hold it in place. Once that’s done, the rear panel can be slipped into the case and two short wires run between the thermocouple socket and the screw terminal block on the PCB. The lid can now be test fitted to make sure everything is correct. Note that it will be necessary to file the RELAY CONTACTS 10A MAX & 30V MAX K-Type Thermocouple Relay Temperature Power SILICON CHIP THERMOMETER/ THERMOSTAT Thermostat RELAY OUTPUT 12V DC . Once the meter is in place, the front panel and the PCB assembly can be slid into the case. The PCB is then secured to the base using four self-tapping screws that go into integral mounting bushes. That done, the leads from the panel meter can be plugged into the headers on the PCB. The rear panel carries a cable gland (for the relay outputs) and the thermocouple socket. The latter is fed through Fig.5: these front and rear panel artworks can be copied and used as drilling templates. Finished panels are also available from SILICON CHIP. two ridges at the front of the lid down where they meet the panel meter. Testing To test the unit, first apply power Table 1: Resistor Colour Codes o o o o o o o o o o o o o o o siliconchip.com.au No.   1   1   2   1   1   8   1   1   1   2   1   1   2   1 Value 100kΩ 51kΩ 47kΩ 20kΩ 15kΩ 10kΩ 4.7kΩ 2.2kΩ 1kΩ 470Ω 100Ω 39Ω 10Ω 1.8Ω (5%) 4-Band Code (1%) brown black yellow brown green brown orange brown yellow violet orange brown red black orange brown brown green orange brown brown black orange brown yellow violet red brown red red red brown brown black red brown yellow violet brown brown brown black brown brown orange white black brown brown black black brown brown grey gold gold 5-Band Code (1%) brown black black orange brown green brown black red brown yellow violet black red brown red black black red brown brown green black red brown brown black black red brown yellow violet black brown brown red red black brown brown brown black black brown brown yellow violet black black brown brown black black black brown orange white black gold brown brown black black gold brown not applicable May 2012  49 Parts List 1 PCB, code 21105121, 117 x 102mm 1 plastic instrument case, 140 x 110 x 35mm 1 12V DC 500mA plugpack 1 3.5-digit LCD panel meter (Jaycar QP-5570 or similar) 1 front-panel label or 1 front-panel PCB, code 21105122 1 rear-panel label or 1 rear-panel PCB, code 21105123 1 K-type thermocouple probe (Jaycar QM-1292 -50°C to 250°C, QM-1283 -40°C to 1200°C) 1 K-type thermocouple probe socket (Element14 Cat. 708-6386) 1 SPDT 10A 12V relay, Jaycar SY4050 or equivalent (RELAY1) 2 SPDT PCB-mount toggle switches (S1,S2) (Altronics S1421 or equivalent) 1 PCB-mount 2.5mm DC socket (CON1) 1 2-way PCB-mount screw terminal block, 5.08mm spacing (CON2) 1 3-way PCB-mount screw terminal block, 5.08mm spacing (CON3) 1 cable gland for 3-6.5mm diameter cable 1 2-way polarised pin header, 2.54mm spacing 1 6-way polarised pin header, 2.54mm spacing 1 2-way header sockets to match above header 1 6-way header sockets to match above header 2 3mm LED bezels (optional) 3 3-way pin headers, 2.54mm spacing (LK1-LK6) 3 jumper shunts 4 No.4 x 6mm self-tapping screws 1 M3 x 6mm pan-head machine screw 1 M3 nut 1 100mm length of 0.8mm tinned copper wire 1 50mm length of 8-way ribbon cable Semiconductors 1 AD8495ARMZ precision thermocouple amplifier with cold junction compensation (IC1) (Element14 Cat. 186-4707) 1 OP747ARZ quad precision single supply op amp (IC2) (Element14 Cat. 960-4405) (IC2) 1 LM285Z/LP-2.5 micropower voltage reference diode (REF1) (Element14 Cat. 966-5447; Jaycar ZV1626) 1 7805 5V 3-terminal regulator (REG1) 2 BC337 NPN transistors (Q1,Q2) 1 22V 1W zener diode (ZD1) 2 1N4004 1A diodes (D1,D2) 1 green 3mm LED (LED1) 1 red 3mm LED (LED2) Capacitors 3 100µF 16V PC electrolytic 1 10µF 50V non-polarised electrolytic 4 100nF ceramic 4 100nF MKT polyester Trimpots 1 1MΩ top-adjust multi-turn trimpot (code 105) (VR2) 1 100kΩ top-adjust multi-turn trimpot (code 104) (VR1) 2 100Ω top-adjust multi-turn trimpots (code 100) (VR3,VR4) Resistors (0.25W, 1%) 1 100kΩ 1 2.2kΩ 1 51kΩ 1 1kΩ 2 47kΩ 2 470Ω 1 20kΩ 1 100Ω 1 15kΩ 1 39Ω 8 10kΩ 2 10Ω 1 4.7kΩ 1 1.8Ω 5% Note: PCBs for this project are available from SILICON CHIP. The unit can be used with any K-type thermocouple, eg, the Jaycar QM1292 or QM1283. and off when the preset goes just over or under the measured temperature. VR2 can now be adjusted to give the required amount of hysteresis (clockwise for more hysteresis and anticlockwise for less). Calibration If you wish, the unit can be left uncalibrated in which case its accuracy will be as shown in the specifications panel. Alternatively, if you wish to calibrate the unit for improved accuracy, the procedure is as follows: (1) Remove jumper links LK2 & LK4 and install links LK1 & LK3 instead. (2) Place the thermocouple probe in a cup of distilled water brimming with ice (note: the ice also needs to be made from distilled water to ensure accuracy and the ice-water mixture has to be constantly stirred to maintain a 0°C temperature). (3) Adjust VR3 so that the thermometer reads 0°C. (4) Place the thermocouple probe in boiling distilled water and adjust VR4 for a reading of 100°C at sea level or deduct 1°C for every 300m above sea level. That completes the calibration. The lid can now be attached to the case and the unit is ready for use. Ambient temperature display and check that the power LED lights. The display should also show a temperature reading with S2 (Thermostat/ Thermometer) in position 1 (Temperature). If it does, check the power supply voltages on the board. REG1’s output should be close to +5V, while pin 7 of IC1 should be about 11.4V as should 50  Silicon Chip pin 4 of IC2. REF1 should have close to 2.5V across terminals 1 and 2. Now check that the display shows a temperature that’s close to the ambient when the connected probe is exposed to room air. Assuming it does, switch S2 to position 2 (Thermostat) and check that you can adjust the preset using VR1. The relay should click on There are a couple options available if you just want the unit to measure the ambient temperature. First, you can use the thermocouple as the sensor and simply sit it in free air. Alternatively, you can disconnect the thermocouple and short its inputs on the PCB using a short length of wire. The unit will then display the ambient siliconchip.com.au Controlling Mains Voltages temperature (in °C) as measured by the AD8495 itself. Note that this will really be the temperature inside the case rather than the room temperature. However, this will be close to room temperature, since there is little warming inside the case. If you intend using this project simply as an ambient temperature thermometer or to measure temperatures up to 199°C only, then the divider resistors can be changed so that they divide by five instead of 50. That way, siliconchip.com.au As presented in the diagram and photos, the Digital Thermometer/Thermostat is capable of controlling external loads running at 30V DC and up to 10A. However, it can control 230VAC loads, provided the relay and the wiring itself is rated for 250VAC mains operation. This will mean that a larger case must be used to accommodate the extra wiring and mains input and output sockets (note: the plastic case used here is not suitable; it’s too small and the back is too flimsy to safely anchor mains cables). The mains input wiring will need to include a mains fuse and we suggest an IEC chassis-mount male socket that includes a switch and fuse (eg, Jaycar PP-4003). For the output mains wiring, use a chassis or panel-mount female IEC socket (eg, Jaycar PS-4176) or 3-pin mains panel-mount socket (eg, Jaycar PS-4094). All mains wiring should be run in 250VAC 10A-rated cabling. Cable tie and clamp the internal mains wires so they cannot possibly come adrift and contact any low-voltage section of the circuit. It’s a good idea to secure the terminal block wires to the PCB; eg, by using silicone sealant or a cable tie that loops through a couple of holes drilled through the PCB adjacent to the terminal block. A metal enclosure will need to be securely earthed. For a plastic case, any exposed metal screws used to secure the IEC connector or other parts near to the mains wiring will also need to be earthed. Nylon screws can be used as an alternative to earthing the screws. The relay should be an Altronics S-4197 or exact equivalent, with contacts rated for 250VAC operation. Finally, for the 3-way terminal block, CON3, we recommend using a Weidmuller type (Jaycar HM-3132) so that it has sufficient voltage rating. the display can show the temperature with a 0.1°C resolution. To do this, change the 1kΩ resistor to 12kΩ, the 39Ω resistor to 750Ω and the 1.8Ω resistor to a 0Ω resistor (or wire link). The 100Ω trimpot (VR4) on the adjustable side of the divider should be changed to 1kΩ. Finally, the decimal point in front of the righthand digit can be displayed by connecting the LCD panel meter’s DP3 pin to the +5V supply. The details are shown on the instruction sheet SC supplied with the meter. May 2012  51