Isolator principle

Isolator design and working principle

Schematic diagram of system circuit with multiple isolators connected at the same time

In the above figure, I4=I1+I2+I3, I5=0, I6=0, I7=0, I8=0, I9=0, I10=0, I11=0, why is the current of I5I11 zero?

Loop current theorem: In an independent power supply loop, the current flowing out of the positive pole of the power supply is equal to the current flowing back to the negative pole of the power supply. This conclusion is based on the principle of conservation of energy, and the inflow and outflow currents are completely equal, as shown in the figure.

According to this principle, in the system, as long as the input signal, output signal and power supply have independent power supply loops, the loop current will not flow into the earth or the reference point of the PLC, generating a common mode voltage error, but floating at the ground level.Even if the power supply and the ground are connected by a very thin and long wire, the ground error potential will not be accumulated because the current flowing into the reference point is zero, so the ground level can be truly zero potential.

In the isolator, this independent power source is generated by the power transformer side winding, and each functional circuit in the isolator, such as the input loop output circuit, needs to be configured with such an independent power supply, as shown in the figure.

If the input loop and output loop of the meter do not use isolated power, how much ground error potential will be generated? The following figure illustrates.

Suppose there are 100 4~20mA signals on the site. If the signals are 20mA, 100 times 20mA is 2A. If the ground resistance of 100 meters is 0.5Ω, the ground potential will be up 1V, which affects the 250Ω sampling PLC card. It is 25% of the range. For the 25Ω sampling PLC card, the impact has reached 250% of the range.

In actual use, the ground connection resistance of 100 instruments is far greater than 0.5Ω, and the output of each meter is constantly changing within 4~20mA, so no isolation is used, and copper bus is used to reduce the ground resistance. It is possible to stabilize the system.

The above isolation mode is clear and simple, but the elements must be complete.

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Note on the use of isolated two-wire transmitter

In the application of the isolated two-wire transmitter, its output is not isolated from the power supply, but the input and output are isolated. Only in the special case when one power supply is only supplied to one transmitter can the three be realized. Isolation mode. When a power supply supplies power to multiple isolated two-wire transmitters, the output current is returned to one power supply, and the ground potential error occurs, as shown in Figure 1.

The way to overcome the error is still to increase the distributor according to the conventional scheme, to achieve the isolation of the output signal, that is, the isolator must ensure the input/output/power supply three isolation principle, in order to eliminate the loop mutual generated when the output signals of multiple isolators are connected together. Disturbance, but then use the isolated two-wire scheme, the input signal is isolated twice, resulting in higher costs and slower speed, as shown in Figure 2.

Therefore, the two-wire instrument in the system application should still be based on the classic scheme, using a non-isolated two-wire transmitter, plus the isolated power distribution, to achieve isolation of power / input / output, without adding additional cost, as shown in Figure 3.