Centroid Community CNC Support Forum

I went out to my machine today to see if a cap would help but my vfd was rock solid from 5000 all the way to 24000. I tried about 8 different speed settings from 5000 rpm up to 24000 rpm and I was actually shocked the vfd never fluctuated on any of the speeds. Needless to say I didn’t add the cap. Just lucky I guess.

There is no problem with the analog output under normal circumstances. The output is an opamp. The most basic form of the circuit has worked well on hundreds of Centroid products over the years. It looks like this:
The problems occur from external non-resistive loads or electrical disturbances. This could be from wire routing, cable type, poor quality VFDs, or other sources of noise. Sometimes the noise sources are extreme enough to damage the ACORN's output. To solve this, ACORN revisions 180625 and newer have a zener diode added for protection.
Now, the external noise might be enough to cause some instability, but not damage. In this case it may be useful to add a capacitor to absorb external disturbances. Opamps are not designed to drive large capacitive loads, so the capacitor should be as small as possible to solve the original problem. If too much capacitance is added, the output may oscillate (like a sine wave).
A larger capacitor can be used if the opamp load is reduced with a resistor before the capacitor. The resistor improves stability, but also creates a voltage divider with the input impedance of the VFD. A higher value resistor will be more stable but create more error in the voltage level at the VFD.
Another approach is a snubber circuit. The resistor and capacitor added in this version prevent oscillation from the extra capacitance. There is no resistor in series, so the average voltage to the VFD is not reduced. This circuit requires 3 parts, so it is not as convenient to hand wire.