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Re: Driving optocouplers

Cross-posted to 3 groups.

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On 8/7/2015 12:52 AM, DaveC wrote:
> http://pdf.datasheetcatalog.com/datasheet/vishay/83725.pdf
>
> The output transistor of the 4N25 optocoupler is spec’d at max 150 mA (no
> mention of on-resistance). I measure the resistance across the output
> terminals with the input driven with 17 mA and I see a little over 110 ohms.
>
> This is going to operate the enable input of a VFD: 10 vdc high-impedance
> digital input needs to be pulled to ground. I’m concerned whether or not
> the 4N25 will do the job. With an on-resistance of 110 ohms, will this pull
> the input low enough?
>
> Maybe I should be looking at another optocoupler? What spec is important look
> for?
>
> Thanks,
> Dave

A VFD (Variable Frequency Drive) is usually designed to be an industrial 
controller. It usually has isolated inputs. That is, the inputs for the 
enable function is usually passed through an optocoupler inside the VFD. 
Furthermore, those digital inputs usually accept dry switch contacts.

If the manual recommends otherwise then ignore my response.

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Thread

Driving optocouplers DaveC <not@home.cow> - 2015-08-06 22:52 -0700
  Re: Driving optocouplers Tim Wescott <tim@seemywebsite.com> - 2015-08-07 01:08 -0500
    Re: Driving optocouplers DaveC <not@home.cow> - 2015-08-06 23:16 -0700
      Re: Driving optocouplers John Robertson <spam@flippers.com> - 2015-08-06 23:53 -0700
        Re: Driving optocouplers upsidedown@downunder.com - 2015-08-07 13:03 +0300
      Re: Driving optocouplers Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> - 2015-08-07 08:21 -0400
  Re: Driving optocouplers John Larkin <jlarkin@highlandtechnology.com> - 2015-08-07 07:17 -0700
  Re: Driving optocouplers John S <Sophi.2@invalid.org> - 2015-08-07 09:29 -0500
    Re: Driving optocouplers "Ralph Mowery" <rmowery28146@earthlink.net> - 2015-08-07 11:59 -0400
      Re: Driving optocouplers John S <Sophi.2@invalid.org> - 2015-08-07 11:17 -0500
  Re: Driving optocouplers WangoTango <Asgard24@mindspring.com> - 2015-08-07 17:59 -0400
    Re: Driving optocouplers M Philbrook <jamie_ka1lpa@charter.net> - 2015-08-07 18:49 -0400
      Re: Driving optocouplers WangoTango <Asgard24@mindspring.com> - 2015-08-10 17:42 -0400

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