Definitions, use cases, and conditioning methods of dry contacts (dry nodes) and wet contacts (Wet nodes)

The dry contact seems to be commonly known, but in fact, it is already a standard term in the field of industrial control.

 

Definition of dry contacts:

Passive switch; two States are closed and disconnected; two contacts have no polarity and can be exchanged;

Common dry contact signals include:

1. Various switches, such as limit switch, travel switch, Foot switch, rotation switch, temperature switch, and liquid level switch;

2. Various buttons;

3. Output of various sensors, such as water immersion sensors, fire alarm sensors, glass crushing, vibration, smoke and condensation sensors in Environmental power monitoring;

4. Output of relays and cines;

 

If there is a dry contact, there is a wet contact, which is for the switch quantity.

 

The wet contacts are defined as follows:

Active switch; two states with and without power; two contacts with polarity, cannot be reversed;

Common Wet Contact signals include:

1. If the above dry contact signal is connected to the power supply, and the other pole of the power supply is used as the output, that is, the Wet Contact signal; industrial control, the voltage range of common wet contacts is dc0 ~ 30 V, the standard is dc24v; ac110 ~ The output of V can also be a wet contact, although this is relatively small;

2. Use the TTL level output as the wet contact, which is not acceptable. In general, the TTL level needs to be buffered, such as 7407, 245, and 244, VCC and other components constitute a loop; 244, 245 can also constitute a loop with Gnd; in order to drive the remote coupling.

3. the collector output and the VCC of the 2nd transistor;

4. dington tube collector output and VCC;

5. Output of the infrared reflection sensor and the radiation sensor;

In the industrial control field, the use of dry contacts is much more than the wet contacts, because the dry contacts do not have polarity to bring about the advantages:

1. Easy access to reduce engineering costs and requirements of engineering personnel, and increase engineering speed

2. handling a large number of dry contacts

3. the wires connected to the dry contacts do not damage local control devices or remote devices even if they are short-circuited for a long time.

4. Easy access and easy interface unification

 

Conditioning methods for dry and wet contacts:

Photoelectric isolation using optical coupling.

The optical coupling of the dry contacts in the following sequence:

VCC-traffic limiting resistor-photocoupler led-dry contacts 1-dry contacts 2-Gnd

You can also:

VCC-"dry connection 1-" dry connection 2-"throttling resistance-" photocoupler led-"Gnd

Wet contacts are connected to the photocoupler in the following sequence:

Wet Contact 1-traffic limiting resistor-photocoupler led-Wet Contact 2

If the wet contact is a DC power supply, if the Wet Contact 1 and 2 are reversed, it is obvious that the optical coupling cannot be opened;

If the wet contact is an AC power supply, it can be opposite;

 

Take an example of the 8-way relay output and 8-way isolated Digital Input card PCI-1760 of inghua to show that the dry and wet contacts are already a standard term.

PCI-1760 is a PCI bus relay output and an isolated digital output card. It has 8 Optical isolated Digital Input and provides 2500vdc isolation protection for digital input in a noise environment. It has 8 relays and can be used as a switch-controlled circuit breaker for small power supply. Two user-defined isolated pulse width modulation PWM outputs. For easy monitoring, each relay is equipped with a red led to display the on/off status. Each isolated input supports both dry and wet contact signals, so that the signal can be collected when the external circuit has no power supply. "

See:

Note:

1. There is a DC/DC module on the PCI-1760, the module output to the dry contacts to provide a power supply isolated from the PC;

2. Use a jumper to select the dry or wet contact input;

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