Abstract: This paper analyzes the original control systems of commonly used drying and heat treatment equipment in factories. It focuses on the principle and performance of solid-state relays (SSRs) and compares them with AC contactors. A method using SSRs to replace AC contactors for furnace temperature control is proposed, and a schematic diagram is designed and applied in practice. This effectively improves temperature control accuracy and the working environment. Keywords: Heat treatment furnace; Furnace temperature control; Solid-state relay Many devices used in factory production for workpiece drying and heat treatment include box-type electric furnaces, infrared heating furnaces, melting furnaces, drying ovens, pit-type tempering furnaces, quenching furnaces, high-temperature salt furnaces, medium-temperature salt furnaces, and oil furnaces, with operating temperatures between 0 and 1200℃. Some of these devices are purchased externally, while others are designed and manufactured in-house. Most of these control cabinets are severely aged, having been in use for nearly 20 years. These devices typically use recorders to directly control the contactor's on/off state for heating, resulting in low control accuracy (generally ±5%), high noise, and high energy consumption. With the development of the company's manufacturing capabilities and technology, the requirements for workpiece surface treatment and temperature control are becoming increasingly stringent. During the "Eleventh Five-Year Plan" period, the company invested some funds in system transformation. During the transformation, foreign technology was referenced. The new system adopts a three-meter separation design, with multiple temperature measurement points, uniform distribution, and high temperature control accuracy, generally within -4% to 2%. In particular, the use of solid-state relays reduced both noise and no-load losses. [b]1 Basic Situation and Working Process of the Original Furnace Temperature Control System[/b] The principle of the original furnace temperature control system is shown in Figure 1. The system uses a recorder to directly control the AC contactor for switching heating on and off. The AC contactor operates frequently, generates a lot of noise, and suffers severe contact erosion, requiring replacement every so often. The limited number of temperature sampling points resulted in inaccurate reflection of the actual furnace temperature; the temperature control accuracy was low, with large temperature fluctuations and poor recording performance. [b]2 Furnace Temperature System Controlled by Solid-State Relays 2.1 Classification of Solid-State Relays[/b] A solid-state relay (also known as a solid-state relay, or SSR) is a contactless electronic switch with relay characteristics developed using discrete electronic components, integrated circuits (or chips), and hybrid microcircuit technology. Solid-state relays are characterized by long lifespan, high reliability, fast switching speed, low electromagnetic interference, no noise, and no sparks. Based on the output load power supply, they are classified into AC solid-state relays, DC solid-state relays, and AC/DC solid-state relays. AC solid-state relays are further classified by switching method into zero-crossing conduction type (abbreviated as zero-crossing type) and random conduction type (abbreviated as random type); by output switching element into bidirectional thyristor output type (ordinary type) and unidirectional thyristor anti-parallel type (enhanced type); by installation method into pin-type (natural cooling, no heat sink required) for use on printed circuit boards and device type fixed to a metal base plate (cooled by a heat sink); additionally, there are wide-range input (DC 3～32V) constant current source type and series resistor current limiting type, etc. DC solid-state relays are classified by input terminal into opto-isolated type, high-frequency magnetically isolated type, and transformer-coupled type; by output terminal into high-power transistor type and power MOSFET type. AC/DC solid-state relays are classified into photovoltaic coupler type and magnetically isolated type. 2.2 Principle Analysis of Solid State Relays As shown in Figure 2, a solid state relay (SSR) is a contactless switching device with relay characteristics that uses semiconductor devices instead of traditional electrical contacts as a switching mechanism. It is a four-terminal active device with two input control terminals and two output terminals. The inputs and outputs are optically isolated. When a DC or pulse signal is applied to the input terminals to a certain current value, output terminals 1 and 2 can switch from the off state to the on state, thus connecting the AC or DC circuit. The SSR is essentially a contactless switch, as shown in Figure 3. By providing a signal to the input control terminals, the on/off state of the power supply (AC or DC) is controlled. In the case of inductive loads, when the SSR is turned off from the on state, due to the phase inconsistency between the current and voltage, a large voltage rise rate du/dt (commutation du/dt) will be generated across the bidirectional thyristor. If this value exceeds the commutation du/dt specification of the bidirectional thyristor (typically 10V/gs), it will lead to delayed turn-off or even failure. Unidirectional thyristors operate in a unipolar state, only affected by the static voltage rise rate du/dt (typically 100V/gs). An enhanced SSR composed of two unidirectional thyristors connected in anti-parallel has a significantly improved commutation du/dt compared to a conventional SSR composed of a single bidirectional thyristor. Therefore, an enhanced SSR is preferable for inductive or capacitive load applications. 2.3 Advantages and Disadvantages of Solid State Relays Compared to conventionally used AC contactors, solid state relays have advantages, but also disadvantages such as a larger on-state voltage drop and leakage current during off-state operation. See Table 1 for details . 2.4 Temperature Control System Based on Solid State Relays Based on the characteristics of zero-crossing AC solid state relays, the Et Yokogawa UT350 series or Dahua Ichino KP1000 series temperature controllers are selected, using an SSR-driven output type. This type of temperature controller can store 19 program segments, each with 19 steps displayed on the LCD. The currently executing step and the steps before and after it can be freely set with PID constants corresponding to various objects using a self-tuning function chip. The program can run in master-slave mode. The alarm meter uses the XMT series, and the recorder uses the Sichuan Instrument ER186 series instrument. Voltage and current monitoring are added to the main circuit, and alarm output is added to the control circuit. Since solid-state relays are heat-generating components, they are prone to burnout if not properly cooled during long-term operation. In this system, the solid-state relay is installed on an aluminum heat sink with a cooling fan, and a temperature relay is installed on the surface of the heat sink. Its contacts are connected in series with the main circuit control coil. Once the temperature is too high, the solid-state relay can automatically stop working, effectively protecting the component. The design principle is shown in Figure 4. The entire system draws on foreign technology, using three separate meters (temperature controller, recorder, and alarm meter) and three separate thermocouples. The temperature measuring points are evenly distributed in the furnace and mutually constrain each other, effectively detecting the furnace temperature. The overall temperature control accuracy is high, and the system operates with virtually no noise. The system workflow is shown in Figure 5. [b]3 Conclusion[/b] The modified system, due to the unification of instruments and control components, is interchangeable, highly versatile, and easy to maintain. Moreover, it boasts high temperature control accuracy, low noise, and energy savings. It has already been applied in the modification of several pieces of equipment in the company, achieving good results. [b]References:[/b] [1] Wang Zhaoan, Huang Jun. Power Electronics Technology [M]. Beijing: Machinery Industry Press, 2000. Click to download: Application of Solid State Relays in Furnace Temperature Control System Modification Editor: Chen Dong