Working principle and status of dry and wet contacts of the circuit

Dry contact is a passive switch, it has two states, respectively closed and open, there is no polarity between the two contacts, in some cases can also be exchanged. In our daily life around, more likely to encounter the following dry contact signals.

The first is a variety of switches like limit switches, travel switches, foot switches, etc.

The second type is the various push buttons.

The third type is the output of sensors like flood sensors, fire alarm sensors, etc.

The fourth type is the output of things like relays, reed switches, etc.

There are two main types of dry contact, one is passive and normally open and the other is passive and normally closed.

It is characterized by the fact that it does not carry a power supply, but has a certain capacity. Dry contacts can often withstand DC30V/1A in a fire protection system, all usually connected to a fire protection module.

As long as there is a dry contact in the circuit, then there must be a wet contact. A wet contact is an active switch, which has both electrical and non-electrical states. It is the opposite of a dry contact because it has polarity between the two contacts and cannot be reversed.

The wet contact signal has the following types.

1.Under normal circumstances, the voltage range allowed for wet contact is often in the range of DC0~30V, the more standard is DC24V. AC110~220V output can also be wet contact.

2.It is also possible to treat a TTL level output as a wet contact. Usually, TTL levels need to be buffered outputs, such as 7407, 245, 244, etc., to form a circuit with VCC, etc. 244, 245 can also form a circuit with GND, in order to drive the remote optocoupler.

3.The collector output and VCC of NPN triodes.

4.Collector output and VCC of a Darlington tube.

5.The output of the IR reflection sensor and the opposite sensor.

The advantages of a dry contact are as follows.

1.The requirement for access is lower, which will result in much lower project costs and engineering staff requirements, which will enable the speed of the project to be increased.

2.The number of switches that can be handled with dry contacts is relatively high.

3.Even if the wires connected to the dry contact are short-circuited for a long time, this will not only not damage the local control equipment, but will also not lead to damage to the remote equipment.

4.It is relatively easy to intervene and to make the interface uniform.

The following two examples give a better understanding of dry and wet contacts.

1.The signal sent by the fire linkage controller to the fire pump control cabinet is a wet contact, because both signals are active contacts, regardless of whether the device is automatically activated or manually operated. 2.The signal received by the fire linkage controller from the fire pump control cabinet is a dry contact, because the pump operation and the fault signal issued is sent out in the form of a passive contact, which is then returned via the bus module.

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