Analog signal isolator applications and selection

Ren Yunye Ankerui Electric Co.
, Ltd.
Jiangsu Jiangyin 202206 [Abstract] Starting from the application scenarios of signal isolators, combined with some practical application analysis of customers, the selection technology of industrial field signal isolators is introduced, and the application characteristics
of different types of signal isolators are analyzed.

【Keywords】Isolator; PLC； Signals; Grounding Introduction In industrial systems, signal isolators are mostly between peripherals and interface devices, ensuring the input and output isolation of the signal and the stable operation of the back-stage equipment, solving the potential difference between the reference points of different equipment and instrument signals, and at the same time can be interfered with filtering, and the signal can be completely transmitted
.

If the signals of all instruments and equipment have only one common reference point, good results can be achieved, but in the actual application environment, this is almost impossible
.

The reason why isolators can play this role is that the input and output are completely electrically isolated
.

There is no common "ground" between the input and output, such as the input signal is 0 ~ 5V or with +1V interference, after isolation is still 0 ~ 5V, that is, the newly established PLC "ground" after isolation has nothing to do with the external device, the instrument "ground": it is for this reason that the isolation between multiple external equipment instrument signals input to the PLC host is also realized, that is, there is no "ground" relationship
between them.

In the PLC to the external signal device outgoing signal also has a similar phenomenon problem, the use of isolators can also achieve the purpose of
solving the problem.

Figure 1
.

Figure 1 Most of the three-terminal isolated isolators must be applied to the working power, generally DC24V or AC220V
.

This power supply also ensures electrical isolation from
the input and output parts when supplying power to the input and output parts.

This isolator that is isolated from each other between input, output, and operating power supplies is called three-terminal
isolation.

Theoretically, this power supply method can be powered by one power supply regardless of the number of isolators, and will not introduce interference
.

1.
Support a variety of input modes three-terminal isolated input support a variety of input methods, namely current source, three-wire system, two-wire system
.

In the automatic control system, generally need the on-site pressure, water level, flow and other signals for measurement and monitoring, often use pressure sensors, water level sensors, flow sensors, etc.
, these non-power sensors are divided into 2-wire system, 3-wire system, 4-wire system and other types, all need to provide external working power, in order to ensure the safety of the power supply generally use DC24V, the common way is to use DC24V power supply
provided by the relevant control system equipment.

The number of sensors to be monitored in the automation control system is large, and the importance of the control unit is concerned and the operation safety of the field control unit is generally provided, and a separate DC24V power module
is generally provided.

As a bridge between the sensor and PLC, the signal isolator not only improves the measurement accuracy of the analog signal, the ability to resist electromagnetic interference, and the safety performance of electrical isolation, but also effectively solves the problem of the safety and relative independence of
the sensor power supply.

Three-terminal isolation type wiring diagram two, output power supply type in the instrument supporting the scene, due to various reasons may occur the following situations: 24 V two-wire interface may be connected to the signal access end of the power receiving device, on-site debugging will also appear instrument interface mismatch
.

For example, the field transmission equipment is a four-wire transmitter output 4~20mA, and the interface of the receiving end 4~20mA is a two-wire loop power supply mode, which is not convenient for external power supply, at this time, the output power supply type signal isolator is selected, and the power supply type isolator is connected to the output loop by the signal output line, which is powered by the receiving equipment, and the signal of the 4-wire transmitter at the site is isolated and output
.

Output power supply type wiring figure three, passive type If you handle the isolation of 4~20mA to 4~20mA current signal, here is recommended an isolator
that does not require additional power supply.

Obviously, eliminating external power supplies makes wiring simpler, with low power consumption, low self-heat, and high
reliability.

This feature is that it does not require an external power supply, it brings simple and reliable advantages, but also brings limitations in use, for the isolated transmission of 4~20mA signal, in another sense, the power loss inside the power transmission, the carrying capacity will be affected by the input voltage, therefore, the actual application process needs to consider whether the passive type is suitable
according to the rear load demand and the size of the input voltage.

Passive wiring diagram four, application selection for the field needs to isolate the 4~20mA signal and in order to simplify the wiring selection of passive type, you need to pay attention to the load demand of the later stage, if you can not meet the load demand, the passive type can not be used normally; In the case where the input loop of the back-stage product has a power supply in the output power supply type application scenario, the on-load demand of the output power supply type and the power supply size of the output loop are positively correlated; The disadvantage of the three-terminal isolation type in the application is that it requires independent power supply, but its accuracy, anti-interference ability, load carrying capacity, and ripple are better than the passive type and the output power supply type
.

V.
BM100 signal isolator 1.
Overview BM100 series signal isolator in industrial production in order to increase the load capacity of the instrument and ensure that the instruments connected to the same signal do not interfere with each other, improve electrical safety performance
.

The BM100 signal isolator collects, amplifies and calculates the input voltage, current, resistance or temperature signals and performs anti-interference processing, and then outputs the isolated current and voltage signals and safely transmits them to the secondary instrument or PLC/DCS
.

2.
Technical parameters six, the conclusion in the PLC / DCS control system to add signal isolator connection mode, effectively solve the interference problem in the control system, but the industrial field applications are diverse, so the selection of isolators is particularly important
.

Only by selecting the appropriate isolator according to the control needs can the accuracy and stability
required for industrial control be achieved.

Reference:[1] Enterprise Microgrid Design and Application Handbook.
2020.
6.
Chen Jie.
Application of isolator in industrial field parts[J].
Petrochemical Automation,2005[3]Wang Hongspoon.
Application and selection of industrial field isolation devices[J].
Sichuan Cement,2006,5(5):45-47.
[7] About the author:Ren Yunye, male, undergraduate, electronic information engineering, mainly engaged in the research and development
of electric power sensor.