Need appropriate decoupler

Greetings everyone,
I'd like to hear your opinions about the following issue...
I need DCS digital output module's signal (+24 V) to use to energize a relay. Relay contacts are wired with longer cable to other relay logic that is located near AC power cables. I fear that induction can cause my module output card to fail.

24 V relay is in the same cabinet with digital output modules, and from that cabinet cable goes to the field near AC power cables. Naturaly, I need some kind of decoupler, perhaps, most appropriate would be an optocoupler.

Can you recommend me one?
In any case I need to think about the way to protect module since it is very expensive and there are other important signals wired on it. I need to have galvanic isolation between the field and the equipment in the control cabinet?

What would you do?

Thanks

If you use "shielded" wire for the DC circuits (it has an overall metal outer jacket), according to Gauss's Law and the Faraday Cage*, that will help to reduce the AC induced voltage on the DC cables.

The shield should extend as close as possible to each device. Leaving 10 centimeters of exposed unshielded cable is a common mistake. These unshielded ends serve as good antennas for AC induction.

It is almost impossible to adequately shield all the DC cables where relays are used, but you have to do the best you can.

*Faraday used a metal cage to prove that a conductor completely surrounded by another metal conductor could not have a current induced upon it.

Thank you Lancie1,

I'm afraid shield cable is not an option, since cable is already placed there before. Now, I'm using free wires. Will optocoupler or some other form of galvanic isolation will do the job? If so, can you recommend any?

Mikas,

I do not understand how any type of optical isolator could help you to transmit 24 volt DC power to a relay.

I need DCS digital output module's signal (+24 V) to use to energize a relay.

Click to expand...

Sure, you could install one, but the relay STILL must have power from somewhere. So you use the opto-isolator to transfer the signal across a gap, but on the other side of the optical gap, when the opto switch sees the signal, it must switch 24 VDC power to the relay to energize it. Where will that power come from?

I am sure that some company makes a 24 VDC power conditioner or power filter that will filter out most of the induced AC voltage. That would be an effective solution.

A much better, permanent, low-maintenance, no-headache solution is to remove the 24 VDC power leads and replace them with a 24 VDC shielded cable.

If you still want an optical isolator, one of these might work:

http://www.beiied.com/PDFs2/oim.pdf

http://www.nteinc.com/relay_web/RIM_output.html

http://www.opto22.com/site/pr_details.aspx?item=ODC24&qs=100310071001,,,23,32&

mikas_m,

I might be miss understanding your problem, so this reply might not be what you want.

I would think you have more a problem from the induced back voltage that comes from the collapsing field of the relay coil. One way to stop the dammage to your outputs is to use a "reverse bias diode" in parrallel with the relay coil.

If the relay is placed near the where the "existing dirty wires" are but still inbetween the DCS unit the relay will give you some isolation. When used in this fashion these relays are often called "interposing relays" At this point I have been talking about what we call "ice cube relays". If you had given us some more information about the type of load the relay would drive then we might be able to suggest a "solid state relay". The mechanical "ice cube relays" have a operation cycle life some where between 100,000 cycles and 10,000,000 cycles. The solid state relays can be much more. The solid state relays will also not harm your outputs like an unmodified relay coil.

Thank you for your replies,

maybe, I wasn't clear enough what I actually want.

Please look at the attached picture. Basically I'm seeking a way to protect my output card, I know that channels in output card are not galvanically isolated and every disturbance that can get to my output1 channel can ruin other channels on the card. I want to prevent this. Maybe induced voltage will not the big deal at all, I don't know since we didn't wired everything yet, but want to prevent any bad scenario. So I want to use some kind of isolation between +24V (card's output) and relay. That optoisolator would be placed in the same cabinet as dig. output card.


Life cycle of relay is not very important since it will not be energized often.

All I'm trying to achive is to protect digital output card as best as I can.

http://www.beiied.com/PDFs2/oim.pdf

http://www.nteinc.com/relay_web/RIM_output.html

http://www.opto22.com/site/pr_detai...0071001,,,23,32&

mikas,

From your drawing the PLC output is only subjected to the transients from your relay coil it is by its design electrically isolated from your load. A reverse bias diode should be added to reduce the wear and tear on the output.

I see no reason why you would need another level of isolation... perhaps I am missing something

Andrew

Corkers,

Mikas said it is a DCS output module, not a PLC. It may not be optically isolated already.

In which case one of the 3 stand-alone 24 volts DC optical isolators that I linked should do the trick.

The first one is designed for encoders, and the 2nd and 3rd ones are made for PLC outputs, but there is no reason any of them will not work on a DCS system.

PS: For we US guys, "galvanically" means it works through resistance.

maybe, I wasn't clear enough what I actually want.

Click to expand...

Exactly. I always ponder why almost every poster with a question only describes part of the problem, leaves out critical details, brand names, and part numbers, then cannot figure out why no one can understand his problem.

I suppose it is due to at first thinking that the question must be so simple that it doesn't even deserve a good description.

Perhaps some know that there may be a simple solution, so leave out the relevant details to make the conversation longer?

I don't know. I just cannot figure it out.

Lancie1, you are right. I leave details because of lack of experience, I suppose. Since I don't have much experience (and knowledge) I thought that many of you delt with this already and therefore, there is no need to be more specific.

It is function module for Siemens SPPA T2000 (older TXP). That is function module for binary conditioning. In manual I read that all inputs and outputs are not isolated, and since there are other channels which are important (both inputs and outputs) I'm trying to be as careful as I can. This output which I plan to use is going out from cabinet.

Since I didn't work with optocoupler earlier, I'll need your help in choosing the right one. I think that ODC24 5-60 V will be OK.
I'll just connect leads from output module to terminals 3 and 4, and relay to terminals 1 and 2, and this should be it.

I also found this:
http://www.weidmuller.com/portal/page?_pageid=33,1686326&_dad=portal&_schema=PORTAL
On this page you can see RSS and RCL relay couplers that provide "galvanically isolating signals between the field and the controls". Since you mentioned that galvanically doesn't mean that there is no physical connection, I wonder if RSS could be suitable?

I think that opto isolators are best solution anyway...

mikas_m,

.

Hello,
that line is representing a boundary between field and control cabinet. I want to ensure that nothing (as low as possible number of disturbances) can cause problems and destroy output card. So right from that boudary line is place when I expect some disturbance to appear.

mikas_m said:

Hello,
that line is representing a boundary between field and control cabinet. I want to ensure that nothing (as low as possible number of disturbances) can cause problems and destroy output card. So right from that boudary line is place when I expect some disturbance to appear.

Click to expand...

Ahhhh! This should keep disturbances out of your gear

.

Since I didn't work with optocoupler earlier, I'll need your help in choosing the right one. I think that ODC24 5-60 V will be OK.
I'll just connect leads from output module to terminals 3 and 4, and relay to terminals 1 and 2, and this should be it.

Click to expand...

Yes, the RIM-ODC24 should work as an isolator between your signal voltage and a relay. However you will still need to supply the relay with 24 VDC power, with the RIM-ODC24 serving as a switch to energize the relay using the 24 volt external power. The RIM isolator module does not pass on the 24 volt power. If it did, then it would not be an optical isolator. You probably will also want to purchase the DIN rail mounting socket RLY9141, so you will have screw terminals.

The Weidmuller RSS and RCL models are simply two different types of relays. A relay can serve as a galvanic isolator between the relays energizing source and the loads on the relay contacts. If that is all you need, then practically any brand of 24 VDC relay will work. I suspect that this actually will be adequate for your case, even though it provides no noise filtering or no induced-voltage isolation. If you use shielded cable for the relay coil leads and keep them short, then you should be okay.

Here is a relay that takes no more space than a terminal block and in fact can be used in place of the terminal block.

http://eshop.phoenixcontact.com/phoenix/treeViewClick.do?UID=2966676&parentUID=null&reloadFrame=true

<disclaimer - I work for a Phoenix Contact distributor>