The difference between signal isolation safety barriers and signal isolators
Signal isolation module
Signal isolation safety barrier
I. Definition
1. Signal isolator: Generally refers to a signal isolator in a low-voltage system, which protects the lower-level signal system from the influence and interference of the upper-level system.
2. Safety barrier: A device installed between intrinsically safe and non-intrinsically safe circuits to limit the voltage or current supplied to the intrinsically safe circuit within a safe range. Safety barrier is a general term, divided into Zener barriers and isolation barriers; isolation barriers are simply called isolation barriers.
II. Working Principle
1. Signal isolator working principle: First, the signal from the transmitter or instrument is modulated and transformed by a semiconductor device. Then, it is isolated and converted by a photoelectric or magnetic sensor. Finally, it is demodulated and transformed back to the original signal before isolation. At the same time, the power supply of the isolated signal is isolated. This ensures absolute independence between the transformed signal, power supply, and ground.
2. Working principle of Zener barrier
The main function of a safety barrier is to limit the entry of hazardous energy from a safe location into a hazardous location, and to limit the voltage and current supplied to a hazardous location.
A Zener diode is used to limit voltage. When the circuit voltage approaches the safe voltage limit, the Zener diode conducts, keeping the voltage across it below the safe voltage limit.
The resistor R is used to limit the current. When the voltage is limited, by selecting an appropriate resistor value, the circuit current can be limited to below the safe current limit value.
The function of fuse F is to prevent the Zener diode from burning out due to a large current flowing through it for an extended period, thus preventing the circuit's voltage limiting from failing. When a voltage exceeding the safe voltage limit is applied to the circuit, the Zener diode conducts. Without a fuse, the current flowing through the Zener diode will increase indefinitely, eventually burning it out and causing the circuit to lose its voltage limiting capability. To ensure the circuit's voltage limiting safety, the fuse must melt at a rate ten times faster than the Zener diode could burn out.
The basic energy-limiting circuit structure of the safety barrier with three redundant Zener diodes, as shown in Figure 1, can ensure that the output energy of the safety barrier is limited to the range specified by the safety parameters under normal operation, one fault point, and two fault points, thereby meeting the requirements of intrinsically safe circuit of level ia.
3. Working principle of isolated signal isolation safety barrier
Compared to Zener barriers, isolation barriers, in addition to voltage and current limiting, also provide current isolation. An isolation barrier typically consists of three parts: a loop limiting unit, a current isolation unit, and a signal processing unit. The basic functional circuit is shown in Figure 2. The loop limiting unit is the core of the safety barrier. Furthermore, it includes a loop power supply circuit for driving field instruments and a detection circuit for acquiring instrument signals. The signal processing unit performs signal processing according to the functional requirements of the safety barrier.
III. Functions and Advantages
1. Signal isolator function:
a. Protects the control loop of the lower level;
b: Reduce the impact of environmental noise on the test circuit;
c: Suppresses interference from common ground, frequency converters, solenoid valves, and unknown pulses to the equipment; at the same time, it has the functions of limiting voltage and current for downstream equipment, providing faithful protection for transmitters, instruments, frequency converters, solenoid valves, PLC/DCS input/output and communication interfaces.
2. Isolated Safety Barrier: This type employs a circuit structure that electrically isolates the input, output, and power supply, while also meeting intrinsically safe energy limiting requirements. Although more expensive than Zener barriers, its superior performance brings greater benefits to users.
1. Due to the adoption of a three-way isolation method, there is no need for a system grounding line, which greatly facilitates the design and on-site construction.
2. The instrument requirements for hazardous areas are greatly reduced, and isolated instruments are no longer required on site.
3. Since the signal lines do not require a common ground, the stability and anti-interference capability of the detection and control loop signals are greatly enhanced, thereby improving the reliability of the entire system.
4. Isolated safety barriers have stronger input signal processing capabilities, and can accept and process signals such as thermocouples, RTDs, and frequencies, which is something that Zener safety barriers cannot do.
5. The isolated safety barrier can output two mutually isolated signals to be used by two devices using the same signal source, ensuring that the signals of the two devices do not interfere with each other, while improving the electrical safety insulation performance between the connected devices.
Therefore, a comparison of the characteristics and performance of Zener and isolated safety barriers reveals that isolated safety barriers have significant advantages and wider applications. Although their price is slightly higher than that of Zener barriers, considering the costs of design, construction, installation, commissioning, and maintenance, their overall cost may actually be lower. In demanding engineering projects, isolated safety barriers are almost universally used as the primary intrinsically safe explosion-proof instruments. Isolated safety barriers have gradually replaced Zener barriers and are increasingly widely used in the field of explosion-proof safety.
Application examples
Application principle diagram
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