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# Circuit Theory of Operation

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#### kurt.brinker

##### New Member
New guy here and i could use some help on this. I can't get a clear answer from anyone i work with so here we go.

We are using a digital output as the input to a vfd. The only way i know to do this is with pwm so i assume that is how its being done. This is the exact circuit. Part of what confuses me is the fact that the opto-coupler is listed as having a 5V input.... We can use a 5V output so why use 24V?

If someone could throw out some theory of operation, that would be great.

I have rarely ever had to go down to component level on things so bear with me.

24V-------680Ω----------A1 on opto coupler
Digital Out----------------A2 on opto coupler

This is very vague. Some more background info and drawings, schematics would be very helpful.

What is the issue, how is this variable frequency drive being used? The application?

It is being used to control a 1hp motor. the opto-coupler is a DEKO-OE-5DC/24DC/100KHZ

There is no issue with it, i just want to know why we use 24V on the input side of the opto-coupler. I assume the 680ohm resistor is just used to limit circuit current.

You said this is the exact circuit and posted no circuit to view.. Apparently your control circuitry runs on 12 volts, it's just what it uses, the resistor limits current. What exactly is confusing you?

the circuit is written at the bottom of the post. I can take a screenshot of the actual drawing but it doesn't tell you more than the one in the first post.

My question is why did they choose to use 24V for the power when the optocoupler input is rated at 5V and we have a 5V source available. Why does it work instead of letting out the magic smoke? Is it possible they adjust the duty cycle and hz to not let it go above 5V? I am going to slap a meter on it today but equipment has been flying out the door so no test parts were available.

And the resistor limits the circuit current to 30mA?

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