Transistor switch 1

[Lc85]

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

 

[itsmoked]

Your question is confusing because you ask about the difference between dry contacts and an SSR, both OUTPUTS, then start talking about "solid-state INPUTS".

"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

 

[itsmoked]

I hope you're not just copying this into a school report...

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Lc85]

Sorry for the confusion, my question is how does a solid state transistor switch actually "switch" if it has no physical contacts?

Thanks very much

 

[itsmoked]

Liamc85:

http://www.ia.omron.com/support/guide/18/overview.html

Keith Cress
kcress -

http://www.flaminsystems.com

 

[VEBill]

I believe that our OP is asking, 'By what magic does a transistor work?'

http://www.tf.uni-kiel.de/matwis/amat/elmat_en/kap_5/backbone/r5_0_1.html

 

[Lc85]

Thanks for all the feed back guys. I have attached a data sheet that shows a flow meter we are going with the version shown on page 6 of 52 of the data sheet. Our customers DCS can handle the output without going through an isolated switch amplifier. my question is how can their DCS know that a low or lowlow has been initiated if there is no physical "Contact" as such.

Thanks very much

 

[IRstuff]

You already answered the question in your title. A solid state "switch" uses one or more transistors or other multilayer semiconductor devices to control the flow of current. The base of the transistor for a normal bipolar junction transistor controls whether current flows between the emitter and the collector. The gain of the transistor determines how little the base current needs to be to do the controlling.

http://en.wikipedia.org/wiki/Transistor

TTFN
faq731-376

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