Driving optocouplers

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
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An SSR might be a better choice.
--

John Larkin Highland Technology, Inc
lunatic fringe electronics
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On 8/7/2015 12:52 AM, DaveC wrote:

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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That has been my limiated experance with the VFDs. Some of them have adapter boards that use optical isolators.
Ran into that problem when the company I worked for was in a big rebuilding project. The engineer had just used the solid state output of a PLC to turn on a VFD. The VFD wanted a hard contact, so I used an interposing relay as a quick fix as it was the weekend and needed to get it up and running.
It is really difficult to say the resistance of a solid state device is so many ohms. As pointed out is is the ammount of currrent it can sink and how low the voltage will go.
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On 8/7/2015 10:59 AM, Ralph Mowery wrote:

You are correct, AFAIK. The important thing is to know the input circuit of the VFD. Without that, you are forced to do tests. Which is what I would do.
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In article < snipped-for-privacy@news.eternal-

150 mA (no

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I've used them for isolation of digital signals in the past. I did run into a problem where, even though it is a bipolar transistor, in a noisy environment there could be enough noise conducted in by the bonded out base of the transistor to cause the thing to turn on sporadically. For the speed of the signals I was interested in I moved to good old PC817 parts and haven't had any issues since.
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snipped-for-privacy@mindspring.com says...



Excessive photons flying around the area?
Jamie
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jamie snipped-for-privacy@charter.net says...

max 150 mA (no

0 ohms.

ance hether or not

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ant look

Not that I know of, I think just good old near-field coupling or capacitive coupling to nearby traces. The output was acting like the opto was always in the linear region, where they needed off or saturated. Cutting the Base pin off corrected the issue, presumably because of a smaller antenna length or exposure to capacitance between it and other traces on the laminate. I didn't spend anymore time on it beyond changing the opto family and relaying the PCB. Mind you, this was 20 some years ago, and I was working for a different company, so all the details are a bit dragged through the filter of time.
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