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Re: Driving optocouplers

Cross-posted to 2 groups.

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On Thu, 06 Aug 2015 22:52:21 -0700, 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?

It's a bipolar junction transistor, not a FET.  "On resistance" isn't 
really a concept that applies -- figure that it'll pull down to about 
0.2V if you're pumping enough current in.

You don't specify what your "high impedance" is, but it'll probably pull 
the enable input down low enough.

-- 
www.wescottdesign.com

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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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