opto-isolator and PWM speed control.

[reg.miller]

Hi,

I have a chinese variable frequency driver for my spindle. It provides a 10V supply and takes a 0-10V input voltage for spindle speed. I want to integrate this with my GCODE interpreter so that I can control machine speed from GCODE. This means a level change from the digital voltage level and I want to use opto isolation to protect the controller chip.

Output will be a PWM signal which needs a low pass filter after the FOD817B opto isolator .

I have a circuit but just realised that it has different charge and discharge resistance which means the output voltage will not correctly reflect the PWM ratio.

Can someone suggest a better way of doing this?

Thanks.



[EDIT] Ah, I've just realised that if I increase the filter resistor to 100k, the difference becomes unimportant. Does this look like a valid way to drive VFD?

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[skrubol]

I think R3 should be connected to the + of C5, and its value changed to match R4.

[reg.miller]

I think R3 should be connected to the + of C5, and its value changed to match R4.

Thanks, could you explain the reason that should be done?

[skrubol]

Hm, I think my logic was off. I think you would be best off putting a logic inverter after the opto and then just have a single resistor after that.
I guess using a much larger value resistor for R4 as you mentioned will work as well, but keep in mind with no signal to the opto, you will get full speed commanded. You might be better off connecting the opto to vcc rather than ground (basically flip the whole circuit.)

[reg.miller]

The opto is on the same cct board as the MPU and the input if configured with a pull-up. That does leave the question of what happens if the spindle controller is power on but the Gcode interpretter board is down. Good point. It's like the limit sw situation where limit switch active is the better failure mode as you previously suggested.

I was intending to have a spindle enable so I will try to ensure that that goes into disable state if there is not power on the MPU.

Thanks for you input.

[ericks]

I made a pwm to 0-10V converter a while back. It worked well but i did not use it cause i still needed to make it "isolated". This is something i will look into sometime when i have a bit of spare time. To be honest i am thinking of designing a bob with all the stuff that might be needed for the proper control of the cnc router etc. All the best with your design....

[reg.miller]

Yes, using an opto like FOB817B provides both level shift and galvanic isolation in simple cheap and compact way. A four pin dil with minimal additional components.

[ericks]

Thanks for the info.....much appreciated

[reg.miller]

For anyone interested, here is what I settled on. Works well, following skrubol's suggestion of doing it the other way up.