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[SOLVED] How to insulate dry contact in a circuit (like PLC)?

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Ranbeer Singh

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How insolate dry contact in circuit ( like PLC )

Hello

Please anybody can tell me how to insulate a dry contact from my 5V circuit. Often in circuit optocoupler uses for this propose. But they required input voltage to drive photo sensor. But I have a dry contact and insulate it from my circuit. I d'not want to use relay for this.
 

Re: How insulate dry contact in circuit ( like PLC )

Either you are not describing the problem completely or you have a problem with understanding of "dry contacts".

By nature, a dry contact is isolated on the driver side so that it can be freely connected to the receiver, e.g. one side tied to local ground. A problem could arise if you have a contact closure sensor that is not safely isolated at the sender side, or should be not directly connected to the receiver circuit for specific reasons, e.g. achieving intrinsic safety.

If it's not possible to supply the circuit on the contact side, you would implement an isolated auxiliary supply.

PLC digital input section has either a common ground with the 24V supply or expecting external supply for opto isolated inputs.
 

Re: How insulate dry contact in circuit ( like PLC )

Hi fvm

I know correctly what is dry contact. I have made circuit like you but d'not have any other isolated supply with my circuit. I connected ground with dry contact. I mean when the contact is closed ... i will found ground at opto cathode pin and connected VCC with opto anode with a series resistance. I am not sure that it is fully safe for circuit.
What do you think...Is is safe or required some changing?

- - - Updated - - -

Small DC-DC isolators are used. Like a B0505S

It require another supply also.
 

Re: How insulate dry contact in circuit ( like PLC )

Don't fully understand what you are trying to achieve. If the contact is isolated on the sensor side (according to the definition of a dry contact) you don't need an auxiliary supply or opto coupler. Just a pull-up resistor to your digital supply and most likely a series resistor for the digital input to protect against possible overvoltage, ESD, whatsoever.
 

Re: How insulate dry contact in circuit ( like PLC )

Yes I am agree with your suggestion. If anyone put 230VAC on dry contact mistakenly...what will be at this time?
 

Re: How insulate dry contact in circuit ( like PLC )

I can imagine to use relative high resistor values respectively higher rated resistors for the receiver circuit so that it withstands 230 VAC without damage. A usual PLC digital input circuit however won't.

If a "dry contact" is coming from far away and you won't guarantee that it's always kept within safe voltage limits, an automation engineer will usually place an auxiliary relay to limit the possible damage in case of failure. Or accept a certain risk.
 

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Re: How insulate dry contact in circuit ( like PLC )

Thaks for your valuable advice.
You are right but we should be careful from our side if it is possible. I will try it with higher rasister and current limit circuit.
 

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