Wiring one signal to an input on 2 plcs. 2

[Larf03]

I have a common sytem using dry contacts for level monitoring and the dry contacts send signals to two machines. I have wired the input on the same contact i.e: Input 13 of machine 1 is wired to the same contact as Input 13 of machine 2. When both machines are powered up, everything works fine but when I shut down power feed to one machine, the inputs are activated even if the contacts are not closed.

There is obviously a leak from input card of machine 1 to input card of machine 2 up to the power supply of machine two (wich acts as a resistive circuit when power is down). I work with 24 volts supply and when the problem occure, I read 13 volts at the contact wich is supposed to feed the input)

I have tried putting a diode on the input card's commom pin but it doesn't work. I really need a solution because the whole goal of this common system was to have one unit running while the other one was serviced and still be able to monitor the level application

 

[itsmoked]

Okay OperaHouse, nbucska, I was under the understanding that the sensor/switch sourced 24V not a sink.

Sure, diodes are the way to go in this case!

Ashereng; Answer: Yes!
Unless you cause the diode to avalanche by increasing the reverse voltage to the point where the diode can no longer block the voltage. This is usually destructive unless the current is limited.


Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Ashereng]

Thanks.

I'll have to remember this one.

Sometimes, the sensor has additional contacts that can be used. I wonder if this problem will still exist if a second set of contacts was used?

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[itsmoked]

Ashereng; Very often "switches" as in pressure, flow, position, etc. are based on some mechanical device that eventually operates a Microswitch. This allows the company to ride on the switch makers coat-tails with respect to the UL, CSA, and TUV, (ad nauseum) electrical certifications.

The standard Microswitch is a "form C" with a common terminal, a normally open and a normally closed terminal. So.. There normally would NOT be two separate normally open contacts available or that would be the best solution!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Ashereng]

Keith, many of my instruments now come with additional 24V dry contact switches (SPDT). I guess more and more people would like the same signal sent to different PLC/DCS/whatever.

One example that readily comes to mind, since I am working on it, is my limit switch on a valve.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[ScottI2R]

Ashereng,
See wikipedia.com for a pretty good definition of the various types of diodes. Just do a search on the word diode and look at the links. All diodes do NOT function the same way.

Hope this helps,

Scott

In a hundred years, it isn't going to matter anyway.