Photocoupler and its application circuit diagram

Science and technology are inseparable from measurement. The purpose of the measurement is to obtain information about the physical or chemical properties of the object under test, in order to evaluate or control the object to be tested based on this information. The device that performs this function is called a sensor. Sensors are cutting-edge products of information technology. They are widely used in industrial and agricultural production, scientific research and biotechnology, especially temperature sensors. They are widely used and have a large number of sensors. The development of temperature sensors has gone through the following three stages; (1) traditional discrete temperature sensors (including sensitive components); mainly capable of switching between non-electricity and power. (2) Analog integrated temperature sensor/controller; (3) Intelligent temperature sensor. At present, the new international temperature sensors are developing from analog to digital, from integrated to intelligent and networked.

2 There are many classification methods for sensor classification sensors. There are two kinds of commonly used sensors: one is divided according to the parameters to be measured, and the other is divided according to the principle of transformation. Usually classified according to the measured parameters, can be divided into thermal parameters: temperature, specific heat, pressure, flow, liquid level, etc.; mechanical parameters: displacement, force, acceleration, weight, etc.; physical parameters: specific gravity, concentration, calculation Monitoring, etc.; state quantity parameters: color, crack, wear, etc. The temperature sensor is a thermal parameter. The temperature sensor can be divided into two categories according to the contact mode of the sensor on the measured medium: one is a contact temperature sensor, the other is a non-contact temperature sensor, and the temperature measuring component of the contact temperature sensor and the object to be tested are good. Thermal contact, through the heat conduction and convection principle to achieve thermal equilibrium, the value at this time is the temperature of the object under test. This method of measuring temperature is relatively accurate, and can also measure the temperature distribution inside the object to a certain extent, but for objects that are moving, have a small heat capacity, or have corrosive effects on the temperature sensing element, this method will produce Great error. The temperature measuring element of the non-contact temperature measurement does not contact the object to be measured. The most commonly used principle is the radiant heat exchange principle. The main features of this type of temperature measurement method are small targets that can measure motion state and objects with small heat capacity or rapid changes, and temperature distributions of temperature fields, but are greatly affected by the environment.

3 Principle and Development of Sensors 3.1 Traditional Discrete Temperature Sensors & Thermocouples Thermocouple Sensors Thermocouple sensors are the most widely used temperature sensors in industrial measurement. They are in direct contact with the object under test. Influenced by intermediate medium, it has high precision; wide measuring range, continuous measurement from -50 °C to 1600 °C, special thermocouple such as gold-nickel-chromium, minimum detectable -269 °C, tungsten-铼 Up to 2800 ° C. Thermocouple sensors work primarily with thermoelectric effects. Two different conductors A and B are connected to form a closed loop, which constitutes a temperature sensing element, as shown in Figure 1. When there is a temperature difference between the two contacts 1 and 2 of the conductors A and B, an electromotive force is generated between the two, and a certain amount of current is formed in the loop. This phenomenon is called a thermoelectric effect, also called a thermoelectric effect. . Thermocouples use this effect to work. One end of the thermocouple is a pair of A and B conductors welded together, called the working end, placed in the measured medium of temperature t. The other end is called the reference or free end and is placed at a constant temperature of t0. When the temperature of the measured medium at the working end changes, the thermoelectric potential changes accordingly, and the thermoelectric potential is sent to the computer for processing, and the temperature value is obtained.

Photocoupler and its application circuit diagram (transfer) - zmurder - The thermoelectric potential difference between the two ends of the blog thermocouple can be expressed by the following formula: Et=E(t)-E(t0) where: Et— The thermoelectric potential of the thermocouple E(t)— the thermoelectric potential at temperature t(t0)— the thermoelectric potential at temperature t0 When the temperature t0 of the reference end is constant, the thermoelectric potential is only related to the temperature at the working end, ie Et=f(t). When the material of the hot electrode constituting the thermocouple is uniform, the magnitude of the thermoelectric potential is independent of the length and diameter of the hot electrode itself, and is only related to the composition of the hot electrode and the temperature at both ends.

3.2 Integrated (IC) Temperature Sensor (1) Analog Integrated Temperature Sensor The integrated sensor is fabricated using a silicon semiconductor integrated process and is therefore also known as a silicon sensor or a monolithic integrated temperature sensor. The analog integrated temperature sensor was introduced in the 1980s. It is a dedicated IC that integrates a temperature sensor on a single chip to perform temperature measurement and analog signal output. The main features of the analog integrated temperature sensor are single function (measuring temperature only), small temperature measurement error, low price, fast response speed, long transmission distance, small size, micro power consumption, etc. It is suitable for long-distance temperature measurement and control measurement. Non-linear calibration is required and the peripheral circuitry is simple. An integrated sensor that is still widely used at home and abroad, the following describes an IC temperature sensor with high sensitivity and high precision — AN6701. The schematic diagram of AN6701 is shown in Figure 2. It consists of a temperature detection circuit, a temperature compensation circuit, and a buffer amplifier.

Optocoupler and its application circuit diagram (transfer) - zmurder - the starting point of the blog IC temperature sensor detection circuit is to use the transistor to the difference between the current density of the two emitters to generate the base-emitter voltage Working with the principle of difference (VbC). Figure 3 shows the temperature sensing and temperature compensation circuit diagram. In Fig. 2, T1-T5 are detection circuits, and the circuit composed of T8-T11 and RC generates a current proportional to its absolute temperature. This current is injected into T7 through T12 and T13 to obtain a compensation temperature corresponding to the injection current. RC is an external resistor, which makes the calibration of the sensor more convenient.

Photocoupler and its application circuit diagram (transfer) - zmurder - the starting point of the blog (2) Intelligent temperature sensor The sensor (also known as digital temperature sensor) was introduced in the mid-1990s. It is the result of microelectronics, computer technology and automated test technology (ATE). At present, a variety of intelligent temperature sensor products have been developed internationally. The smart temperature sensor internally contains a temperature sensor, an A/D converter, a signal processor, a memory (or register), and an interface circuit. Some products also come with a multiplexer, central controller (CPU), random access memory (RAM), and read only memory (ROM). The intelligent temperature sensor is characterized by the ability to output temperature data and associated temperature control, adapting to various microcontrollers (MCU); and it is based on hardware to implement test functions through software, and its intelligent harmony also depends on At the level of software development.

4 The new trend of intelligent temperature sensor development After the 21st century, intelligent temperature sensors are developing high-precision, multi-functional, bus standardization, high reliability and safety, developing virtual sensors and network sensors, and developing monolithic chips. High-tech directions such as temperature measurement systems are developing rapidly. 4.1 Improving Temperature Measurement Accuracy and Resolution The earliest intelligent temperature sensor introduced in the mid-1990s uses an 8-bit A/D converter with low temperature measurement accuracy and resolution of only 1 °C. At present, a variety of high-speed, high-resolution intelligent temperature sensors have been introduced in foreign countries. The 9- to 12-bit A/D converters are used, and the resolution is generally 0.5 to 0.0625 °C. The DS1624 high-resolution intelligent temperature sensor newly developed by DALLAS Semiconductor Company of the United States can output 13-bit binary data with a resolution of up to 0.03125 °C and a temperature measurement accuracy of ± 0.2 °C. In order to increase the slew rate of multi-channel intelligent temperature sensors, some chips use high-speed successive approximation A/D converters. Taking the AD7817 5-channel intelligent temperature sensor as an example, the conversion time for the local sensor and each remote sensor is only 27 & mu; s, 9 & mu; s.