In PLC applications, it is not recommended to directly connect the alarm to the output terminal, especially in safety alarm and emergency handling situations. 

PLC output modules are divided into relay output type and transistor output type. The former is suitable for driving high-power loads, while the latter is not suitable. 

The working current and voltage of the alarm are relatively high, and direct connection may damage the PLC or cause safety issues. When using a relay output PLC, 

the alarm can be connected to the output terminal, but attention should be paid to independent power supply, reasonable control signal, reliable reset operation, 

and alarm safety performance. When designing an alarm system, appropriate solutions should be selected based on the PLC output type and load characteristics to 

ensure system reliability.

In the application process of PLC, there is a common misconception that many people will directly connect the alarm to the output terminal of the PLC. In fact, this 

practice is not recommended, especially in situations involving safety alarms and emergency handling.

Firstly, we need to clarify the output types of PLC. The output modules of PLC are mainly divided into two types: relay output type and transistor output type.

The output module of the relay output type PLC contains relays, whose output signals are switch signals, suitable for driving various loads such as motors, solenoid 

valves, indicator lights, etc.

The output module of transistor output PLC uses transistors (such as MOSFET or IGBT) as switching elements, and the output signal is also a switch signal. However, 

due to the characteristics of transistors, their output current and voltage range are relatively small, and they cannot directly drive high-power loads.

As a common safety device, the working principle of the alarm is to receive signals from the PLC or other controllers, trigger the internal sound and light alarm device, 

and remind the staff to pay attention to abnormal situations.

Due to the high working current and voltage of the alarm, if it is directly connected to the output terminal of the PLC, especially the transistor output type PLC, it may 

cause damage to the output module of the PLC and even lead to serious consequences such as fire.

So, why can the alarm be connected to the output terminal when using a relay output PLC?

This is because the output module of the relay output type PLC contains relays, which have certain isolation and protection functions and can withstand large current 

and voltage surges.

However, this does not mean that any load can be freely connected to the output terminals of the PLC. When connecting the alarm, we also need to pay attention to 

the following points:

1. Do not use the 24V power supply of the PLC to directly drive the alarm

Although PLCs typically provide a 24V DC power supply for external devices, using the PLC's power supply directly may cause power overload or damage to the PLC 

due to the potential high power of the alarm. Therefore, it is recommended to use an independent power supply to power the alarm.

2. The design of control signals should be reasonable

Due to the working characteristics of the alarm, we need to ensure that the control signal output by the PLC can accurately trigger the action of the alarm. 

This usually requires setting reasonable pulse width and frequency in PLC programming to ensure that the alarm can work properly.

3. The reset operation of the alarm should be reliable

Once the alarm is triggered, we need a reliable way to turn it off. This can usually be achieved by setting a reset signal in the programming of the PLC, or by setting a 

manual reset button on the alarm.

4. Consider the safety performance of the alarm system

When choosing an alarm, we need to ensure that it complies with relevant safety standards and regulations to ensure reliable alarm signals can be issued in emergency 

situations.

When designing an alarm system, it is necessary to choose a suitable design scheme based on the output type, load characteristics, and reliability requirements of the 

PLC. For critical safety related alarm systems, a more reliable design should be adopted, such as using relay output PLC and ensuring that the alarm has an 

independent power supply.