Driving optocouplers

An SSR might be a better choice.

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.

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.

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.

er or not

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.

Excessive photons flying around the area?

Jamie

max 150 mA (no

hether or not

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.