Hollysys PLC - three forms of PLC output circuit

1. Relay output circuit

This type of output circuit form can be either DC or AC. The output circuit form of PLC relay allows the load to be generally below AC250V/50V, the load current can reach 2A, and the capacity can reach 80-100VA (voltage × current), therefore, the output of PLC is generally not suitable to directly drive large current load (generally through a small load to drive large load, such as the output of PLC can be connected to an intermediate relay with relatively small current, and then the intermediate relay contact drives a large load, such as contactor coil, etc.).

The service life of the relay contact in the form of the output circuit of the PLC relay is also limited (generally about hundreds of thousands of times, depending on the load, such as the life of the inductive load when connected is less than the resistive load). In addition, the response time of the relay output is relatively slow (about 10 ms), so it is not suitable to use this type of circuit output in situations where fast response is required (the two output forms described below can be used depending on the situation).

When connecting an inductive load, in order to prolong the service life of the relay contacts, for the case of an external DC power supply, overvoltage suppression diodes should usually be added at both ends of the load; For AC loads, RC suppressors should be added at both ends of the load.

2. Transistor output circuit

The transistor output circuit form has a faster response than the relay output (generally less than 0.2 ms), making it suitable for applications that require a fast response, and because the transistor has no mechanical contacts, it has a longer service life than the relay output circuit form. The external power supply of transistor output circuits can only be direct power supply, which is one aspect of its application limitations. In addition, the transistor output drive capacity is smaller than the relay output, the allowable load voltage is generally DC5V-30V, and the allowable load current is 0.2A-0.5A. Pay attention to these two points when using the transistor output circuit form.

3. Bidirectional thyristor output circuit

The bidirectional thyristor output circuit can only drive AC loads, and the response speed is also faster than that of relay output circuit form, and the life is longer. The driving capacity of the bidirectional thyristor output is smaller than that of the relay output, and the allowable load voltage is generally AC85-242V; The output current of a single point is 0.2A-0.5A, and the output current of each point should be reduced when multiple points share the common end (for example, the output of a bidirectional thyristor with a single point drive capability of 0.3A is 0.8A/4 points when the common end is shared at 4 points). Note: In order to protect the thyristor, usually the two ends of the thyristor in the internal circuit of the PLC are connected to the RC resistor-absorbing element (generally about 0.015uF/22Ω) and the varistor, so when the thyristor is turned off, the output of the PLC still has an open-circuit leakage current of 1-2mA, which may lead to the situation that some small relays cannot be turned off when the PLC output is OFF.

Hollysys PLC - three forms of PLC output circuit

Hollysys PLC - three forms of PLC output circuit

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