Difference between relay PLC and transistor PLC

Relay output:

The advantage is that different common points can carry different AC and DC loads, and the voltage can also be different. The load current can reach 2A / point; but the relay output mode is not suitable for high-frequency operating loads. This is due to the life of the relay. decided. Its life decreases with the increase of the load current, generally between hundreds of thousands to Jl million times, and some companies’ products can reach more than 10 million times, and the response time is 10ms.

(2) Thyristor output:

With load capacity of 0.2A / point, it can only carry AC load, can adapt to high-frequency action, and the response time is 1ms.

(3) Transistor output:

The biggest advantage is that it is suitable for high-frequency operation and has a short response time, generally about 0.2ms, but it can only carry a DC 5-30V load, and the maximum output load current is 0.5A / point, but every 4 points must not be greater than 0.8A .

When your system output frequency is less than 6 times per minute, the relay output should be preferred because of its simple circuit design, strong anti-interference and load capacity. When the frequency is less than 10 times / min, the relay output mode can be used; the Darlington triode (5-10A) can also be driven by the PLC output, and then the load can be greatly reduced.

1. Different load voltage and current types

Load type: The transistor can only carry DC load, while the relay can carry AC or DC load.

Current: transistor current 0.2A-0.3A, relay 2A.

Voltage: The transistor can be connected to DC 24V (generally the maximum is about DC 30V, and the relay can be connected to DC 24V or AC 220V.

2. Different load capacity

The ability of a transistor to carry a load is less than the ability of a relay to carry a load. When using a transistor, other things are sometimes added to drive a large load (such as a relay, solid state relay, etc.).

3.Transistor overload capacity is less than relay overload capacity

In general, when there is a large inrush current (such as a light bulb, an inductive load, etc.), the transistor’s overload capacity is small and it needs to be derated more.

4.Transistors respond faster than relays

The principle of the relay output type is that the CPU drives the relay coil to make the contacts close, and the external power source drives the external load through the closed contacts. The open-circuit leakage current is zero and the response time is slow (about 10ms).

The transistor output type principle is that the CPU turns the transistor on and off through optical coupling to control the external DC load, and the response time is fast (about 0.2ms or less).

The transistor is mainly used for positioning control, and the output of the crystal is used to send pulses. However, the relay cannot be used to send pulses, so it cannot be positioned and controlled.

If a relay is used to control the positioning servo or stepping, a positioning module is also added, which is not economical. The servo output can be controlled by a transistor output.

5. Under the rated working condition, the relay has the life of the number of operations, the transistor only has aging and there is no limit on the number of uses.

Relays are mechanical components and have an operating life. Transistors are electronic components. They are only aged and have no limit on the number of uses. Relays have a limited number of switches per minute, while transistors do not.

6. The relay provides dry contacts, which can be used for AC and DC, and has a wide range of voltages, but in some cases, the signal is not “clean” (burr);

Transistor (including thyristor) output is an active switch. It is required that the load must be matched or adapted to the corresponding power supply. For inductive loads, sometimes it is necessary to consider a diode.

If you use pulse output, it is better to choose a PLC with a transistor output. The relay should sometimes be unstable. The optocoupler used for the pulse output port is a high-speed optocoupler, which is very expensive.

Relay advantages: both AC and DC loads can be driven; the load rated current is large;

Disadvantages: the operating frequency cannot be too high, and the relay has a life span, generally 1 million times;

Transistor advantages: the operating frequency can reach several hundred KHZ, no contact, so there is no statement of mechanical life;

Disadvantages: can only be connected to DC loads (generally below DC30V), the current is relatively small;

Bidirectional thyristor: can only be connected to AC load, the operating frequency is relatively high, and the life is long, but the rated current of the load is also relatively small;