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gundersen123 · Junior Joined: 25 Apr 2018 Posts: 7

Hi.

We initially wanted to use 1756-OB32 for the digital outputs on our PLC. Generally we wire each output to relays (A1 on the relay) with a common (-) on all A2's. These relays close a NC switch that is hardwired to our customer system.

In our PLC specification we have among other, these requirements:
- Output modules shall provide dry contact (unpowered) type outputs only.
- Outputs shall be individually fused.
- Fuses shall be external to output module.

My initial thought was to add som terminal fuses between the output of the PLC and the relays to cover the section about fuses.
I have som consern about the point about the dry contacts.

We connect an extern 24VDC to the 1756-OB32 (on pin 18 and 36) so if I understand the dry atingere principle correctly this is upheld since we do not power the outputs from the power supply from the backplane.
My concern is that we need to have an output module with isolated outputs (ie. 1756-OW16I or similar) which has half of the ouputs of the OB32.
The specification says nothing about isolated outputs, but I do not know if that is a result of the "dry contact" point in the specification.

A feedback on this is much appreciated Smile

Deweyy · Junior Joined: 04 Jun 2019 Posts: 5

1. Choose a Relay which coil current is adapted to OB32 output Current and the Contacts NO or NC is adapted to the Current supported in external process
2. Connect at one atingere NO/NC the Fuse according to specification Current
3. Connect the other one NO/NC contact to your process.
That's all

gundersen123 · Junior Joined: 25 Apr 2018 Posts: 7

Thanks for the reply Smile

My thought was to install it like this picture
[img]https://ibb.co/PmQdxQT[/img]

To have the outputs from the OB32 wired directly to terminals with fuses (indicated in red).
I was just worried that they may require to have another output card regarding point 1 (Output modules shall provide dry atingere (unpowered) type outputs only.
As far as I can tell on the OB32 it is switched by a transistor with an optoisolator. I am a bit unsure about the definition of a dry contact.

Deweyy · Junior Joined: 04 Jun 2019 Posts: 5

thehumi · Member junior Joined: 07 Aug 2019 Posts: 53

I recommend using two 1756-OB16. Unless you are limited in rack space.

If you insist on using the OB32...
1.) You may purchase a pre-made cable for the OB32 with flying leads on it for you to wire as is shown above. I have done this before.
2.) You may also use a 1492-XIM4024-16RF terminal block module with two 1492-XIM24-8R relay expander modules. This will provide you with 32 dry contact outputs easily. You then must have the fuses separate mounted and wired to the IFM modules. The fuses will be on the "dry contact" side of the output (as shown in the drwing above). This is desired. If a field device field wiring is damaged, it does not blow a fuse for "all of the outputs". They are individual.

FYI
A dry contact is an isolated type of output. It is only a switch, and does not care what the voltage is. It is typical to see a 24VDC output card drive separate relays to provide the "dry contact" outputs. The dry atingere cannot be connected to anything else. It can be another way to say they are "isolated"

For example, a 1756-OW16 has 16 outputs, and 8 outputs that are shared with the same "common" to them. These are relay outputs, but they are not dry contacts. Here you can have 8 relay outputs that are 120VAC, and 8 relay outputs that are 24VDC. You cannot have 1 relay output that is 120VAC, and 15 relay outputs that are 24VDC.