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Re: Relay vs. FET's

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On 3/31/2017 12:29 AM, mpm wrote:
> On Friday, March 31, 2017 at 12:04:02 AM UTC-4, rickman wrote:
>> How do you see using a relay making this easier?
>
>> Rick C
>
> Hi Rick,
> I'm not sure it would make it easier.  (other than in my head, maybe)
>
> In case I wasn't clear:  Our board is what provides the "contacts" that then get wired over to the alarm panel.  (I think you got that; just making sure.) Right now, those "contacts" are FET's, but I'd rather they be relays.
>
> Most alarm loops (provided by others) seem to run about 12 VDC with a 2.7k supervision resistor.  So, alarm loop current is generally low.  The wiring from our board over to the alarm panel is usually not well controlled.  Sometimes shielded, sometimes in conduit, sometimes just flapping in the breeze.
>
> The typical FET arrangement we're using will measure 0-Ohms closed, and several Meg's open (with the alarm loop disconnected).  But when you connect the alarm, sometimes the "open" condition can read as low as 1 Meg-ohm (which frankly, even that shouldn't be a big deal).
>
> But it seems everytime I get involved with this interfacing issue, I'd rather there not be a direct copper connection to any of our equipment other than via the business side of a Form-C dry contact relay.
>
> I occasionally get into situations where I can pull the alarm wires off our interface, and the interface works exactly as it should.  Then, open & short the alarm wire loop (now totally disconnected from our interface), and they work just fine too.  But hook them up as intended, and sometimes things get weird (granted, not very often -- maybe even borderline rare?)
>
> I don't think we have enough data and cases from the field to pinpoint what is going on, but my strong suspicion is that it has something to do with our FET output.
>
> Adding to this:  "back in the day"..., I think every residential or commercial burglar / fire panel sensor I ever saw used a relay.  Like a PIR motion sensor, or smoke detector.  (Maybe my sample size was too low!)  :)
>
> Anyway, it's late.
> I appreciate the answers and everyone's input.
> I know it's probably been beat to death... I just have "that nagging feeling" about this one...

Ok, so I get that you are emulating the relay contacts.  You still 
haven't explained what your problem is.  When you say things like 
"things get weird", I don't have any way to translate that into a 
description of the problem.

You seem to indicate you are concerned about a 1 Megohm resistance for a 
"relay open" state.  But then you say "even that shouldn't be a big 
deal".  This seems to be an emotional issue for you; "I'd rather there 
not be a direct copper connection to any of our equipment" is the best 
you can say as to what the problem is.

Why can't you explain what problems you have seen?

 From what you describe there should be no grounding problems... at 
least if the power supply to the isolated side of the optoisolators is 
done right that is.  You didn't describe that.  The transistor side of 
the opto has to have some power to control the FET.  This power source 
needs to be isolated from the rest of your box just as the opto isolates 
the signal path.  Do you know hwo this is done?

-- 

Rick C

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Thread

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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