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Chapter 10 When used with the LED display, a heatsink needs to be attached to the regulator. The thermometer is shown here with a stainless steel probe thermocouple. High Temperature Digital Thermometer It uses an LCD or LED readout, can measure to an incredible 1200°C and can switch devices on or off at a set temperature. Main Features •  Uses readily available K-type thermocouples •  Measures to +1200°C (range depends on probe) •  Adjustable trip point relay and high-intensity LED •  Dual double-pole changeover 5A relay contacts •  Switches on rising temperature •  Adjustable hysteresis •  High-intensity LED or low-current LCD readout 58 PERFORMANCE ELECTRONICS FOR CARS L OOK IN THE CABIN of any high-performance machine running a piston engine and you’ll find gauges for exhaust gas and cylinder head temperatures. For example, all piston engine aircraft use exhaust gas temperature and cylinder head temperature displays, while serious race cars also log or display these temperatures. An overly high cylinder head temperature can indicate cooling problems, while too high an exhaust gas temperature usually shows that the engine is running lean – or is working so hard that it’s on the edge of destruction! On a turbo car, exhaust gas temperature (usually abbreviated to EGT) is also a great indication of how hot the turbine is running. Why don’t more people use these gauges? There are two problems. First, exhaust gas runs at up to 900°C – hot enough to make the exhaust manifold glow bright red and hot enough to melt most temperature sensors into a pathetic congealed pool of plastic. What’s needed is a high-temperature thermocouple mounted in an inconel or stainless steel sheath. While these are commonly available (being widely used in industrial furnace applications), another problem then looms: the electrical output of a siliconchip.com.au Parts List Fig.1: this shows where each of the components is placed on the main PC board. Use this diagram, the photos of the completed board and the parts list to help you assemble it correctly. Link LK1 is normally in the “TEMP” position – see text. 1 PC board coded 05car041, 106 x 61mm 1 plastic case, 130 x 68 x 42mm – optional, not in kit 1 LCD panel meter or LED panel meter 1 K-type insulated thermometer probe 5 PC-mount 2-way screw terminals with 5mm pin spacing 1 12V PC-mount DPDT 5A relay (Relay1) 1 3-way header with 2.54mm spacing 1 jumper shunt with 2.54mm spacing 2 2-way pin header plugs 2 2-way pin header sockets 1 1MΩ multi-turn top adjust trimpot (VR5) 4 10kΩ multi-turn top adjust trimpots (VR1-VR4) 3 PC stakes 1 100mm length of 4-way rainbow cable 1 50mm length of 0.8mm tinned copper wire Semiconductors When constructed, your circuit board should look like this. When assembling the PC board, make sure that you insert the polarised components the correct way around (the diodes, ICs, LED, transistors, voltage regulator and electrolytic capacitors are the easiest to make mistakes with). 1 LT1025CN thermocouple cold junction compensator (IC1) 2 OP07CN op amps (IC2, IC3) 1 7805 3-terminal regulator (REG1) 1 BC337 NPN transistor (Q1) 2 LM336-2.5 reference (REF1,REF2) 1 5mm red LED (LED1) 1 16V 1W zener diode (ZD1) 1 10V 1W zener diode (ZD2) 2 1N4004 1A diodes (D1,D8) 6 1N4148 diodes (D2-D7) Capacitors thermocouple is tiny. Before you can read the output on a meter, the signal must be amplified and have other compensations applied. And the result of that complexity is cost; displays for thermocouples are normally expensive – $200 for a fairly cheap one! But as you may have gathered from this preamble, what we have here is a much more cost-effective way of displaying temperature. Depending on the type of thermocouple housing that you select, temperatures from -500°C to an incredible 1200°C can be shown on an LCD or LED display! And it gets even better than that – you can also set a temperature at which a relay siliconchip.com.au switches over and a high-intensity LED lights. So not only can you read off the temperature, you can also turn another device on or off when the temperature reaches a preset level. For example, in a high-performance turbo road or race car where you are measuring exhaust gas temperature, you can set the relay to click over at 850°C to sound a loud warning buzzer. Alternatively, at the other end of the temperature range, you can use a fast response (and tiny!) bead thermocouple to monitor the internal temperature of your sound system’s amplifier. Not only can you 2 100µF 16V PC electrolytic 7 10µF 16V PC electrolytic 2 100nF MKT polyester (code 104 or 100n) Resistors (0.25W, 1%) 1 120kΩ 2 82kΩ 1 68kΩ 1 22kΩ 1 15kΩ 4 10kΩ 1 9.1kΩ 1 2.2kΩ 2 1.8kΩ 1 220Ω 1 100Ω 0.5W 1 10Ω then read off the temperature on a fast reacting digital display, you can also automatically switch on fans when the temperature rises excessively. PERFORMANCE ELECTRONICS FOR CARS 59 How It Works K-type thermocouples comprise two dissimilar metal wires (Chromel and Alumel) which are alloys and are joined at the measuring end of the probe. The other end of the wire pair is normally connected to a 2-pin plug. The voltage at the plug provides a nominal 40.6µV (microvolts) per °C output, which is the difference between the probe end and the plug end of the wire thermocouple. If the plug end is at 0°C, we can directly read off the temperature measured by the probe since we know that the output will be 40.6µV per °C. In practice, it is impractical to keep the plug end at 0°C and so we simply compensate for the plug end temperature instead. In our circuit, we use a Linear Technology LT1025 thermocouple cold junction compensator (IC1), which provides a pre-calibrated 40.6µV per °C output to offset the thermocouple voltage. Op amp IC2 amplifies the thermocouple output by a factor of 2.4652, converting the 40.6µV per °C output to 0.1mV per °C. This provides the meter with the required voltage so that the display reads directly in °C. The OP07 op amp is a very low drift type with high gain and high input impedance, which ensures that the measurement remains stable with changes in ambient temperature. IC2 is powered from a 10V supply (pin 7) and its pin 4 is connected to 0V. However, the op amp’s output is not able to swing down to the 0V rail but only to about +2V. Consequently, we have biased the thermocouple to a +2.49V reference which means that the op amp output will be at around +2.5V, allowing it to operate correctly since its output is now well above the 0V rail. The problem with this is that the meter reading also needs to be compensated for the output voltage offset. This is simply achieved by connecting the INLO input of the meter to the same +2.49V reference. The meter then reads the difference between INLO and the output of IC2, connected to the INHI input. Trimpot VR3 provides offset adjustment for IC2, so that the meter can precisely read 0V at 0°C. Without this adjustment the meter may have an error of up to ±2°C. Note that the meter can read voltages below the +2.49V reference which means that the meter can theoretically show negative temperatures. The 68kΩ resistor connecting pin 3 of IC1 to ground effectively gives the output a means to go below +2.49V and provides the facility to continue compensation below 0°C. IC3 is another OP07 op amp, this time connected as a comparator to compare the output of IC2 with a reference voltage from VR4. Its output is low Here’s another view of the completed PC board. You can leave the relay out if you don’t need to switch other equipment. 60 PERFORMANCE ELECTRONICS FOR CARS when its pin 3 is below pin 2. When IC2’s output goes above the threshold set by VR4, the output (pin 6) of IC3 goes high. This drives transistor Q1 and the relay. LED1 also lights to indicate that the temperature threshold has been exceeded. The diode across the relay coil is there to quench the reverse voltage that is generated by the collapsing magnetic field of the relay coil when it is switched off. To prevent the relay from erratically opening and closing at or around the threshold temperature, IC3 has positive feedback from its output to the noninverting input, pin 3, via two 10kΩ resistors, trimpot VR5 and diode D6. When IC3’s output goes high, closing the relay, this hysteresis has the effect of pulling the pin 3 voltage higher than IC2’s output level. This means that the temperature must drop by a reasonable amount before pin 6 of IC3 goes low and the relay opens again. Diode D7 clamps the top of VR5 to +5.6V. This ensures that the hysteresis does not alter with changes in the 12V supply. Voltage Reference Two series-connected LM336-2.5 references (REF1 & REF2) are used to provide a 4.98V reference. Temperature compensation is included, comprising the series diodes and trimpots VR1 & VR2. Each sensor is stable with temperature changes when its trimpot is adjusted for 2.49V. The 4.98V reference is critical to the thermometer’s performance and it must remain stable over temperature so that the reading does not drift. The 4.98V reference provides IC3’s temperature trip point (via trimpot VR4). The 4.98V supply is derived from a 10V rail which is itself provided from 10V zener diode ZD2. Power for the circuit is obtained from the car’s battery and diode D1 gives reverse connection protection. The 10Ω resistor, 100µF capacitor and zener diode ZD1 provide transient protection at the input of 3-terminal regulator REG1 which provides a +5V rail to power the LCD or LED display module. The op amps, relay and LED1 are driven by the +11.4V (nominal) rail. siliconchip.com.au siliconchip.com.au PERFORMANCE ELECTRONICS FOR CARS 61 Fig.2: the circuit uses op amp IC2 to amplify the thermocouple output to provide a 0.1mV per °C output at pin 6. This output if then fed to either an LCD module or to a LED module to display the result. IC3 is wired as a comparator; its output goes high when IC2’s output exceeds a threshold voltage set by VR4 and this then turns on transistor Q1 and Relay1 to switch another piece of equipment (ie, at a preset temperature). IC1 is the thermocouple cold junction compensator (see text). Use It To . . . •  Display any car temperatures: oil, exhaust gas, coolant, intake air, brakes, etc. •  Trigger alarms, warning lights, fans, etc at a selectable temperature. If you wish, even real-time brake temperatures can be monitored with this display! Construction Fig.3: here is a typical connection set-up, with the Digital Thermometer shown measuring exhaust gas temperatures (EGT) via a high-temperature thermocouple. This temperature is displayed in real-time on the LED display. In addition, a warning alarm is wired to the relay so that if the EGT exceeds 800°C, the driver is alerted by the buzzer (as well as by LED1). To power the buzzer, one of the relay’s normally open (NO) connections is made to an ignition-switched +12V rail, while the adjacent Common terminal is connected to the buzzer itself. The other side of the buzzer is earthed. This engine dyno test shows just how hot the exhaust system gets on an engine working under sustained full load. Measuring the exhaust gas temperature requires a top quality thermocouple and a dedicated thermocouple display, like the one described here. 62 PERFORMANCE ELECTRONICS FOR CARS The design is easy to build and fits on a compact PC board – see Fig.1. Either an LCD or LED display can be used – each has advantages and disadvantages, depending on the situation in which you are going to use the meter. The advantage of the LED display is that it can be readily seen at night – in fact, it’s a very bright display that will also be visible in nearly all daytime conditions except direct sunlight. However, when the LED display is used, the 5V regulator will need to be fitted with a heatsink and this can be provided in a number of ways. You can use a sheet of aluminium at least 100 x 60 x 2mm in size or a heatsink similar to the one shown in the photos (this heatsink was salvaged from an old car radio). Alternatively, you can use the car’s body as a heatsink and bolt the regulator directly to the metalwork. The alternative LCD (liquid crystal display) module can be seen even in direct sunlight but will need to be externally lit at night (eg, by white LEDs). It draws less current than the LED display and so a heatsink is not required for the regulator if you use this option. The decision about which type of display to use can be made after the design is built; apart from the presence or absence of the heatsink, it is identical in either configuration. When assembling the PC board, make sure that you insert the polarised components the correct way around. These parts include the diodes, ICs, LED, transistors, voltage regulator and electrolytic capacitors. The three PC stakes are installed at TP1, TP2 and TP GND. Most thermocouples are provided siliconchip.com.au The High Temperature Digital Thermometer can use any K-type thermocouple. Here it is shown with a low-temperature bead type thermocouple and LCD readout. with a plug already installed on the lead. This will need to be removed so that the wires can be inserted into the screw terminal strip on the PC board. (Note: if the reading goes down when it should go up and up when it should go down, reverse the thermocouple lead connections). Calibration The Digital Thermometer needs to be calibrated when construction is complete. This is easy to do, needing only a multimeter and, as an option, a glass of water mixed with ice: (1). Connect a wire between TP2 and the thermocouple + input. With the meter display module installed, RESISTOR COLOUR CODES Value 4-Band Code (1%) 5-Band Code (1%) 120kΩ 82kΩ 68kΩ 22kΩ 15kΩ 10kΩ 9.1kΩ 2.2kΩ 1.8kΩ 220Ω 100Ω 10Ω brown red yellow brown grey red orange brown blue grey orange brown red red orange brown brown green orange brown brown black orange brown white brown red brown red red red brown brown grey red brown red red brown brown brown black brown brown brown black black brown brown red black orange brown grey red black red brown blue grey black red brown red red black red brown brown green black red brown brown black black red brown white brown black brown brown red red black brown brown brown grey black brown brown red red black black brown brown black black black brown brown black black gold brown Fig.4: these two diagrams show the wiring to the LCD module (left) and the LED display module (right). Your choice of module will depend on the conditions under which the Digital Thermometer is to be used (see text). siliconchip.com.au PERFORMANCE ELECTRONICS FOR CARS 63 The panel meter is connected to the PC board by flying leads. These can be made long enough so that the display can be remote mounted; eg, on the dashboard, with the PC board in a box tucked away elsewhere. adjust trimpot VR3 for reading of 0 on the meter. (2). Use a multimeter to measure between TP1 and TP2. Adjust trimpot VR1 for 2.49V (3). Use a multimeter to measure between TP2 and TP GND. Adjust VR2 for 2.49V (4). Further refinement: adjust VR3 for a reading of 0 when the thermocouple is placed in and stirred in an ice and fresh water solution. Alternatively, if you live near sea level, place the thermocouple in boiling water and adjust VR3 for a reading of 100. Note that when power is first applied, it will take about 10 seconds for the display to settle. Trip Point & Hysteresis Thermocouples are available with different lead lengths and in different housings. Industrial suppliers are the best bet for really high temperature designs. This thermocouple (complete with extension lead) is suitable for measuring up to about 250°C. For higher temperatures, you’ll need a thermocouple with a stainless steel braided lead. Note: the thermocouple probe must be insulated to prevents shorts to the vehicle’s chassis. 64 PERFORMANCE ELECTRONICS FOR CARS When link LK1 is placed in the “SET” position, the temperature at which the relay trips can be read off the display. To set this trip point, move the link to this position and adjust trimpot VR4 until the desired trip temperature is shown. Turning this pot clockwise will increase the temperature at which the relay trips. The hysteresis (the difference between the switch-on and switch-off siliconchip.com.au The heatsink attached to the regulator was salvaged from an old car radio. Any heatsink of around this size is suitable for when the LED meter is being used. temperatures) is set by trimpot VR5. Start off with this trimpot fully clockwise; this gives minimum hysteresis. If you find that the relay chatters, or you’d like the relay to stay on longer after the temperature starts to fall, turn this trimpot anti-clockwise. Fitting In most applications, the LED or LCD will be mounted on the dashboard, connected to the PC board by flying leads. This allows the PC board to be mounted where there is plenty of space and access is easy for when the trip point or hysteresis needs to be changed. Note that the high-intensity LED indicator can also be mounted on the dash – make sure that you get the polarity of the LED wiring correct when extending the leads. Thermocouples are available in a wide variety of configurations and any insulated K-type thermocouple (ie, the probe is insulated from the outer sheath) will work with this unit. Bare bead-type thermocouples react to temperature changes very quickly but are relatively fragile and their cable insulation is not usually rated for very high temperatures. They can be used for monitoring ambient, heatsink and intercooler core temperatures, provided the probe doesn’t touch the chassis (or connect to the chassis via other parts). Thermocouples mounted in stainless steel probes are also available. Real time brake temperatures can be monitored using a thermocouple. On this brake pad research vehicle, the yellow lead running to the thermocouple mounted behind the brake pad can be clearly seen. These are suitable for higher temperatures (eg, engine and transmission oil) but again are often let down by their insulation. For really high temperatures (eg, exhaust gas and brake temperatures), you need a specific high-temperature thermocouple. These are normally sold in a stainless steel or inconel sheath, complete with special hightemperature braided cable. Note that you cannot extend the length of a thermocouple lead without using the correct metals in the cable. If a long reach is required, you will need to buy a thermocouple complete with a long lead. Industrial controls often use K-type thermocouples and companies specialising in this area are the best sources for good quality, high-temperature thermocouples.  Ambient Temperature This Toyota 1G-GTE turbocharged engine is fitted with several thermocouples – one on each exhaust and another mounted after the turbo. Usually, a single thermocouple mounted either before or after the turbo is sufficient to indicate what’s going on. siliconchip.com.au Want to measure just the ambient temperature? Its easy to do. If the thermocouple is replaced with a copper wire link, the meter will show the ambient temperature reading as measured by IC1, the thermocouple ice-point compensation chip. PERFORMANCE ELECTRONICS FOR CARS 65