Thank you Steve.? That's the information I was looking for.?
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In fact I'm surprised that they are stating 10V for the input supply voltage since even rail to rail op-amps don't quite get to the rail unless they have some very expensive parts.? But I can see how it's harder to specify 12V and then have someone use a 12V wall wart that floats unloaded at 16V.
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I've attached my reverse engineered schematic of the cheapie far east BoB that has PWM from the DB-25 pin 1.? The LM358 won't go down to zero either but then most VFDs tend not to turn the AC motors much below about 10% anyway so I doubt it matters.?? They also limit the output voltage to no more than 10V by using a 10V supply on the op-amp.
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Partly why I went step/dir for my spindle AC/Servo and 1 RPM is trivial with a S1 M3 G-Code.
The circuitry works better at 10 or 12 volts, but it will function at 5 volts.
The issue is that the output range is limited to between approximately Gnd plus about 0.3 volts and Supply Voltage minus 0.6 volts. This does not change as you lower the supply voltage so the limitation gets worse percentage wise as the supply voltage is reduced.
If you need better results you could provide an isolated 12 volt supply and then use a resistor divider on the output to scale the range to 0-5 volts.
The circuit also uses an active filter to help smooth out the PWM frequency more than a capacitor alone would smooth it.
