Compared with Zener safety barrier, isolation safety barrier not only has the function of limiting voltage and current, but also has the function of current isolation. Isolation grid usually consists of three parts: loop energy limiting unit, current isolation unit and signal processing unit. The basic functional circuit is shown in Figure 2. The loop energy limiting unit is the core part of the safety barrier. In addition, the loop power supply circuit of auxiliary driving field instrument and the detection circuit of instrument signal acquisition. The signal processing unit performs signal processing according to the functional requirements of the safety barrier.
Generally, a two-wire power distributor is needed in industrial field, which not only provides 24V power for primary instruments such as pressure transmitter, but also collects, amplifies, calculates and anti-jamming the input current signal, and then outputs isolated current and voltage signals for subsequent secondary instruments or other instruments. However, some special industrial sites need not only two-wire transmission, but also power distribution and signal isolation functions. At the same time, they also need a kind of safety spark explosion-proof performance, which can reliably prevent the mixed contact between high-voltage power supply and signals, and limit the energy entering dangerous places below the safety quota by using the dual limiting circuit of current and voltage.
There are basically two kinds of isolation safety barriers: detection end safety barrier and operation end safety barrier. The safety barrier at the detection end is matched with the two-wire transmitter; The safety barrier at the operating end is used in conjunction with the electric converter or electric valve. There are also isolation safety barriers such as signal input.
Because the isolation safety barrier adopts measures such as voltage limiting, current limiting and isolation, it can not only prevent dangerous energy from entering dangerous places from intrinsically safe terminals, but also improve the intrinsically safe explosion-proof performance of the system, and also increase the anti-interference ability of the system and greatly improve the reliability of the system operation. After DC- AC -DC conversion, the 24VDC power supply outputs various voltages required by the module circuit.
The principle of the isolation safety barrier at the detection end is that the module circuit converts the current or voltage signal input through the intrinsically safe energy limiting circuit into 0.2- 1VDC, and then sends it to the module for acquisition, amplification, operation and anti-interference processing, and then modulates it into isolated current and voltage signals through the transformer for use by subsequent secondary instruments or other instruments. The module also needs to output an isolation voltage of 18.5∽28.5VDC, which is used as the working voltage of the two-wire transmitter through the intrinsically safe energy limiting circuit. Intrinsically safe energy limiting circuit can limit dangerous signals with high current or high voltage from entering dangerous places.
The principle of the isolation safety barrier at the operating end is to isolate the 4-20mDC signal output by the regulator or operator, and then output the 4-20mDC signal, which is provided to the on-site electric converter or electric valve positioner through the intrinsically safe energy limiting circuit.