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Re: Relay vs. FET's

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On 3/30/2017 11:51 PM, mpm wrote:
> So, is my argument weak?
>
> One of our products needs to interface to various, unknown, ever-changing, commercial building alarm panels.  Our designer is using opto-isolators to drive a pair of N-Channel FET's and this arrangement supposedly is identical to the "old-fashioned" way of just using a general purpose small signal relay to construct a Form-C output.  (Similar to: Digikey Z773-ND, which is an Omron G5V-1-DC5.)  Maybe I can draw the circuit in ASCII ART if needed.  Although our board and the alarm panel are usually in the same room, there is no guarantee they share anything else in common.  Might be the same "ground", might not be.
>
> Anyway, I'm the guy who usually gets stuck dealing with this interface board when problems creep up in field installations.  Sure, we can figure them out, but that takes time, labor, and typically.. aggravation.
>
> I say that because we simply don't know (and therefore cannot control) whatever voltages and currents are present in the alarm panel loops (or god knows what else!? - third parties connect some pretty weird stuff sometimes!), that we should just use relays and skip the FET approach entirely.  To me, the added benefit of using a relay is it at least establishes a clear demarcation between where our obligation ends, and the third-party world begins..  (But maybe I am just old and stupid?  I do like FET's for many things... just not this.)
>
> BTW:  These alarms are for things like AC-Fail, charger fail, high temp, etc...
> It's not like we need the speed or lifetime cycles that a FET arrangement provides.  Also, build quantities are low enough that a few general purpose relays per build isn't a deal breaker.
>
> Thoughts?  Relays or FET's.  (or does it matter?)
> Horror stories appreciated, especially if they can be used as ammunition to advocate for a relay solution.  Thanks!!

I don't have lots of horror stories to share, but I'm not clear on what 
problems you have with the solid state versions of relays.  Which 
direction are the signals going?  If they are driving your optos, I 
don't see how that is significantly different from driving your relay 
coils.  You would have an input voltage min and max and a spec on the 
current that must be supplied.

For an output that is driving the remote panel, then you have a driver 
(contacts in the relay) that have a max voltage and current rating.

How is this different between the relay and the solid state version?

How do you see using a relay making this easier?

-- 

Rick C

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Relay vs. FET's mpm <mpmillard@aol.com> - 2017-03-30 20:51 -0700
  Re: Relay vs. FET's rickman <gnuarm@gmail.com> - 2017-03-31 00:03 -0400
    Re: Relay vs. FET's mpm <mpmillard@aol.com> - 2017-03-30 21:29 -0700
      Re: Relay vs. FET's rickman <gnuarm@gmail.com> - 2017-03-31 01:22 -0400
        Re: Relay vs. FET's mpm <mpmillard@aol.com> - 2017-03-31 04:07 -0700
          Re: Relay vs. FET's rickman <gnuarm@gmail.com> - 2017-03-31 13:41 -0400
          Re: Relay vs. FET's Jon Elson <jmelson@wustl.edu> - 2017-03-31 14:04 -0500
            Re: Relay vs. FET's rickman <gnuarm@gmail.com> - 2017-03-31 16:07 -0400
          Re: Relay vs. FET's mpm <mpmillard@aol.com> - 2017-04-02 10:41 -0700
            Re: Relay vs. FET's rickman <gnuarm@gmail.com> - 2017-04-02 14:59 -0400
              Re: Relay vs. FET's tabbypurr@gmail.com - 2017-04-02 17:44 -0700
                Re: Relay vs. FET's mpm <mpmillard@aol.com> - 2017-04-02 18:41 -0700
                Re: Relay vs. FET's rickman <gnuarm@gmail.com> - 2017-04-02 23:51 -0400
      Re: Relay vs. FET's tabbypurr@gmail.com - 2017-03-31 00:52 -0700
  Re: Relay vs. FET's John Larkin <jjlarkin@highlandtechnology.com> - 2017-03-30 22:11 -0700
    Re: Relay vs. FET's BM2335 <BM265@DMM.com.au> - 2017-03-31 14:27 +0800
      Re: Relay vs. FET's Phil Allison <pallison49@gmail.com> - 2017-03-30 23:50 -0700
        Re: Relay vs. FET's BM2335 <BM265@DMM.com.au> - 2017-03-31 15:07 +0800
  Re: Relay vs. FET's John Devereux <john@devereux.me.uk> - 2017-03-31 08:22 +0100
  Re: Relay vs. FET's "Tim Williams" <tiwill@seventransistorlabs.com> - 2017-03-31 17:58 -0500

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