Transistor switch
Transistor switch
(OP)
Can anyone explain the difference between dry contacts and a solid state switch. I know that a voltage needs to be provided from the DCS for dry contact to operate but I'm just not 100% sure how a solid state input works with the DCS.
Thanks very much
Thanks very much





RE: Transistor switch
"Dry contacts" is a specific term meaning "relay contacts". A relay's contacts are exactly a mechanical switch operated by electromechanical means instead of by a finger. The "dry contacts" mean additional things. Things like the contacts are completely electrically isolated from everything else. "Dry contacts" also infers that when the contacts are open there can be NO leakage of current across the 'switch'.
While an SSR can be "isolated" this isn't guaranteed until it's investigated. They also typically have leakage across their terminals, sometimes a great deal of it.
As for INPUTS. Virtually all PLCs and DCSs have "solid-state" inputs. These are nothing more than light emitting diodes that are driven by the field connected devices. The light from the LED is seen across a transparent electrical barrier by a photo-transistor that can signal the controller's processor that the input is now active. There is no "dry" analog with INPUTs in DCSs or PLCs these days. However a reed switch is a dry contact, a type of relay, that could be driven by typical field connected devices and would provide the same mechanical aspect into the processor.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Transistor switch
Keith Cress
kcress - http://www.flaminsystems.com
RE: Transistor switch
Thanks very much
RE: Transistor switch
http://www.ia.omron.com/support/guide/18/overview....
Keith Cress
kcress - http://www.flaminsystems.com
RE: Transistor switch
http://www.tf.uni-kiel.de/matwis/amat/elmat_en/kap...
RE: Transistor switch
Thanks very much
RE: Transistor switch
http://en.wikipedia.org/wiki/Transistor
TTFN

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