Can someone please help me in this issue?

I've +5V low level digital output which need to connect to a alarm input board. The problem is the alarm input board only work with the dry contact. is there a way to convert the digital output to a dry contact output?

Please help.

 

Yes -- find an electromechanical relay with a 5V coil. or if you can ascertain the voltage used in the alarm system, you can use a solid state relay whose output is compatible with that voltage. Both of these will produce a voltage-free or dry contact controlled by your 5V output. BTW, why does your PLC have only 5V outputs? That's unusual.

 

Is there any other way instead of adding a relay? My system have 200++ output, can we add a transistor to convert to dry contact?

BTW, I'm using the camera motion detection to trigger output to the alarm input point. When there is a motion detect, relay will change to 0V to 5V.

 

Properly chosen, a FET might work, but the systems would need a common ground reference.

Regards
cww

 

Ans: No. The solution Mr. Myres recommends is exactly what the alarm input board spec calls for - dry contacts.

The alarm board design allows for a universal solution with practically no knowledge of electronics: dry contacts. Relay contacts are dry contacts as long as you don't connect any wires carrying (external) voltage to them.

A transistor has to have some voltage and some current to operate. Those voltage/current conditions are an unknown.

A relay is cheap, universally available solution.

 

> Properly chosen, a FET might work, but the systems would need a common ground reference.

What is FET? How is the connection?
Thanks.

 

That speaks to the proper selection. Probably better use the relays.

Regards
cww

 

Having 200 channels changes the entire issue (and for the better in some ways).

PLC outputs themselves are almost always "dry" (no voltage applied internally), albeit with one end of eight or sixteen outputs tied together. IOW, the current from one of the output terminals isn't provided by the PLC processor or the rack power supply, but via one or more terminals on the wiring connector of the module. The tying (or "commons") is probably not incompatible with the alarm system inputs as the power provided for use with the dry contact is probably all from one source anyway.

And if you have 200 I/O to interface, this is not something you're doing with a spare point here and there on a module whose common is already connected to an existing source. Find out what voltage the alarm system is feeding to the dry contacts and get PLC output cards whose output transistors are compatible with that voltage. Then just connect them directly.

 

Google is your friend, just search convert dry contact, came up a few results.

http://blog.solidremote.com/post/co...tact-dry-contact-to-power-output-contact.aspx