M AN773 Application Circuits of the TC620/TC621Solid-State Temperature Sensors Author: thermistor to sense a remote temperature. Internal heating will not affect the temperature sensing accuracy. Microchip Technology Inc. INTRODUCTION Figure 2 is a schematic of a heating and cooling controller using a single TC620 and a TC4469 Quad CMOS driver. In this example, the TC620 is programmed for maximum and minimum temperature set points with a hysteresis of 5°. The TC620/TC621are solid-state temperature sensors that are easy to program and interface with control equipment. The TC620 senses the temperature internally, while the TC621 uses an external thermistor. Figure 1 shows how the devices are connected. INPUT SECTION The TC620/TC621 data sheet (DS21439) describes how to calculate the correct resistance value for any desired temperature. It also gives a graphical depiction of the outputs for varying temperatures. Typically, a heating/cooling thermostat has a wide enough temperature range to allow heating and cooling from 45°F to 85°F (7°C to 29°C). The calculations that follow show how this range was incorporated into the design. The TC620 programming inputs have a resistance-to-temperature ratio of approximately 782Ω / °C. Since the TC620 senses temperatures internally, its outputs must be limited to 1 mA. The device can source or sink higher currents, but internal self-heating may cause errors in the temperature sensing. The TC621 can source or sink 10 mA, since it uses an external 8 1.2V REF High Set TC620/TC621 t° + Thermistor 1 (TC621 Only) Low Set AMP + 2 + AMP - 3 + AMP - + COMP - 7 + COMP - 6 S R Q FIGURE 1: Q Latch Gnd Low Limit High Limit “C” (Standard) Denotes Cooling Option (True High Control Output) 5 Regulate … 4 VCC “H” (Option) Denotes Heating Option (True Low Control Output) TC620/TC621 Block Diagram. 2003 Microchip Technology Inc. DS00773B-page 1 AN773 12 VDC + 10 µF + 0.1 µF Cool 14 2.2 kΩ 20 kΩ TC4469 8 7 6 93.1 kΩ, 1% 2 1 2 13 3 4 12 2.2 kΩ 2.2 kΩ Comfort Zone TC620-C 95.3 kΩ, 1% 3 Heat 5 4 5 6 11 8 9 10 7 Cooling Contactor 12 VDC Coil Heat/Cool FIGURE 2: Heating Contactor 12 VDC Coil Heating/Cooling Thermostat Controller. To get the desired range, we need a potentiometer that will provide a 22°C variation (29°C - 7°C = 22°C). Multiply this temperature range by the resistance versus temperature ratio to get the needed resistance for the potentiometer: EQUATION Plugging this value back into the resistance calculation formula, we will verify that the maximum trip temperature is greater than the desired high end of the window: EQUATION T= 782 x 22 = 17.2 kΩ A 20 kΩ potentiometer will meet this requirement. Now each programming resistor can be calculated. For the low end of the window, the minimum programming resistor value should be: EQUATION RTRIP = 0.783 x T + 91...RTRIP = 96.5 kΩ T = 7°C (45°C) By adding the 20 kΩ potentiometer value to this, we get: EQUATION 96.5 kΩ + 20 kΩ = 116.5 kΩ total resistance (RTRIP - 91) 0.783 T = 32°C (89.6°C) The previously calculated resistance values will span both ends of the desired heating and cooling window (45°F to 85°F). To program an acceptable hysteresis for the thermostat, the Low Set resistor must be lower in value than the High Set resistor. A resistance versus temperature ratio of 782Ω / °C for temperatures below 70°C will give a good guideline for calculating the hysteresis. For a hysteresis of 5°, the difference in resistance is: EQUATION RDIFF = 782 x 5...RDIFF = 391 kΩ Subtracting the 3.91 kΩ from the 96.5 kΩ will give the Low Set resistor value: EQUATION 96.5 kΩ - 3.91 kΩ = 92.6 kΩ DS00773B-page 2 2003 Microchip Technology Inc. AN773 two set points (our previously calculated 5° hysteresis), this indicator will be lit. The third driver controls the heating contactor. It is enabled when the Heat/Cool selector switch is open and the Regulate output is low. When the Heat/Cool selector switch is closed, the third driver is disabled and the fourth driver will be enabled to control the cooling contactor. This driver will turn on the cooling contactor when the Regulate output is high. The logic features of the TC4469 CMOS Driver are used to prevent the heating and cooling contactors from operating simultaneously. Choosing standard 1% resistance values closest to the calculated values gives: EQUATION RHIGH Set = 95.3 kΩ RLOW Set = 93.1 kΩ With the 20 kΩ potentiometer connected to both programming resistors, the Low Set resistor's 5° hysteresis will track the High Set resistor, as the potentiometer is manually adjusted by the user for different temperatures. The TC620/TC621 will operate with any supply between 4.5 VDC and 18 VDC. The TC4469 Quad CMOS Driver can source 300 mA continuously. The coils on the Heating and Cooling contactors must be of the appropriate type and voltage rating for the circuit. OUTPUT SECTION The Low Limit and High Limit outputs will go high when the device (or thermistor, TC621) reaches the programmed temperature for each corresponding input. The Regulate output is a latch that goes high when both programmed temperatures have been reached, and goes low when the device temperature decreases to below both set points. Figure 3 shows the outputs with respect to input set points and temperature changes. 24 VAC EQUIPMENT Most heating and cooling equipment is designed to operate with a 24 VAC secondary voltage. The schematic in Figure 4 is an example of a 24 VAC system that drives 24 VAC relays and operates on an internally self-generated 15 VDC. Because the TC620 and the TC4469 are CMOS devices, their current requirements are extremely low. Using triac switches to energize the relays keeps the component costs to a minimum, while reliability stays high. The application in Figure 2 uses a TC4469 Quad CMOS Driver. This device has four independent drivers, each with a logic AND gate as an input. The AND gate has one non-inverting input and one inverting input. The first driver is used to drive an LED indicator. Depending on the position of the Heat/Cool selector switch, either the Heat or Cool LED indicator will be lit. The second driver is used to drive the “Comfort Zone” LED indicator. When the temperature is between the Regulate ON “H” Option This design requires only four wires from the thermostat to the main control for a heating/cooling system. An additional fifth wire for a manual fan switch would make it compatible with standard 5-wire residential and commercial heating/cooling systems. Regulate ON “C” Option Regulate ON “H” Option High Limit ON High Set Point Low Limit ON Low Set Point Temperature FIGURE 3: TC620/TC621 Input and Output Logic. 2003 Microchip Technology Inc. DS00773B-page 3 AN773 1N4001 750, 1/2W + 0.1 µF Cool 15V 14 20 kΩ Heating Contactor Cooling Contactor 120 VAC + 220 µF 24 VAC 93.1 kΩ, 1% 2 95.3 kΩ, 1% 3 2.2 kΩ 13 7 6 3 4 12 5 5 6 11 8 9 10 8 TC620-C TC4469 1 2 4 2.2 kΩ Heat 2.2 kΩ Comfort Zone 2N6071B 7 2N6071B 3 kΩ 3 kΩ FIGURE 4: Heat/Cool 24 VAC Heating/Cooling Thermostat Controller. SOLAR HEAT CONTROLLER Figure 5 shows an external temperature sensor for a pool solar heating panel pump control. The TC620 should be a part with the “H” option bonding. This inverts the Regulate output logic. This is necessary when using “NTC”-type thermistors because the internal logic is designed to function with a “PTC”-type thermistor. The external thermistor used in this design is an NTC (negative temperature coefficient) thermistor. One manufacturer is Keystone™ Carbon. Their part is RL1006-53.4K-140-D1, which has a resistance of 100 kΩ at 25°C. Another vendor is Thermometrics®. Their part is D200B104L. This thermistor assembly is attached to the solar panel in a manner that will allow it to sense heat generated by direct exposure to the sun. EQUATION RTRIP = 0.783 x T + 91 RTRIP Low = 111.9 kΩ ≈ 113 kΩ 1% RTRIP High = 120.6 kΩ ≈ 121 kΩ 1% As the sun heats the thermistor assembly, the pump will turn on at 100°F and stay on until the thermistor assembly temperature decreases to 80°F. This ensures that the solar panel has time to heat up before the pump is energized, and that the pump will turn off before the solar panel has cooled below the pool temperature. The complete controller consists of nine low-cost components. This, then, energizes the pump when the sun is heating the panels, and turns off the pump when the sky becomes cloudy or the sun goes down. To prevent rapid cycling of the controller during partly-cloudy skies, the hysteresis is set for a wide (20°F) span. The thermal time constant of the solar panel will also aid in the prevention of rapid pump cycling, if the thermal resistance between the thermistor assembly and the solar panel itself is low. The Low Set temperature is set for 26.7°C (80°F) and the High Set temperature is set for 37.8°C (100°F). The resistor values are calculated: DS00773B-page 4 2003 Microchip Technology Inc. AN773 Pump Relay 240 VAC 100k @ 25°C + T + 220 µF Time Clock NTC Pump Motor 8 1 120 VAC 9 VAC - 113 kΩ, 1% 121 kΩ, 1% 2 7 TC621-H 3 6 5 MTP3055E 240 VAC 4 FIGURE 5: Pool Solar Heat Control. SUMMARY The TC620 and TC621 are programmable logic output temperature sensors. These sensors feature dual thermal interrupt outputs (high limit and low limit) which can be programmed with a single external resistor. The TC620 and TC621 can be used to provide simple on/off control for a wide range of applications, such as a cooling fan or heater. 2003 Microchip Technology Inc. DS00773B-page 5 AN773 NOTES: DS00773B-page 6 2003 Microchip Technology Inc. Note the following details of the code protection feature on Microchip devices: • Microchip products meet the specification contained in their particular Microchip Data Sheet. • Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the intended manner and under normal conditions. • There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip's Data Sheets. Most likely, the person doing so is engaged in theft of intellectual property. • Microchip is willing to work with the customer who is concerned about the integrity of their code. • Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not mean that we are guaranteeing the product as “unbreakable.” Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our products. Attempts to break microchip’s code protection feature may be a violation of the Digital Millennium Copyright Act. If such acts allow unauthorized access to your software or other copyrighted work, you may have a right to sue for relief under that Act. Information contained in this publication regarding device applications and the like is intended through suggestion only and may be superseded by updates. It is your responsibility to ensure that your application meets with your specifications. No representation or warranty is given and no liability is assumed by Microchip Technology Incorporated with respect to the accuracy or use of such information, or infringement of patents or other intellectual property rights arising from such use or otherwise. Use of Microchip’s products as critical components in life support systems is not authorized except with express written approval by Microchip. No licenses are conveyed, implicitly or otherwise, under any intellectual property rights. Trademarks The Microchip name and logo, the Microchip logo, KEELOQ, MPLAB, PIC, PICmicro, PICSTART, PRO MATE and PowerSmart are registered trademarks of Microchip Technology Incorporated in the U.S.A. and other countries. FilterLab, microID, MXDEV, MXLAB, PICMASTER, SEEVAL and The Embedded Control Solutions Company are registered trademarks of Microchip Technology Incorporated in the U.S.A. Accuron, dsPIC, dsPICDEM.net, ECONOMONITOR, FanSense, FlexROM, fuzzyLAB, In-Circuit Serial Programming, ICSP, ICEPIC, microPort, Migratable Memory, MPASM, MPLIB, MPLINK, MPSIM, PICC, PICkit, PICDEM, PICDEM.net, PowerCal, PowerInfo, PowerTool, rfPIC, Select Mode, SmartSensor, SmartShunt, SmartTel and Total Endurance are trademarks of Microchip Technology Incorporated in the U.S.A. and other countries. Serialized Quick Turn Programming (SQTP) is a service mark of Microchip Technology Incorporated in the U.S.A. All other trademarks mentioned herein are property of their respective companies. © 2003, Microchip Technology Incorporated, Printed in the U.S.A., All Rights Reserved. Printed on recycled paper. Microchip received QS-9000 quality system certification for its worldwide headquarters, design and wafer fabrication facilities in Chandler and Tempe, Arizona in July 1999 and Mountain View, California in March 2002. The Company’s quality system processes and procedures are QS-9000 compliant for its PICmicro ® 8-bit MCUs, KEELOQ® code hopping devices, Serial EEPROMs, microperipherals, non-volatile memory and analog products. In addition, Microchip’s quality system for the design and manufacture of development systems is ISO 9001 certified. 2003 Microchip Technology Inc. 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