Soft Start Inrush Current Limiter- PCB design

[Neale]

It's interesting that the original design talks about using this kind of inrush limiter with toroidal transformers "up to 500VA." This surprised me as I have been cheerfully using toroidal transformers in CNC machines in the 500-650VA range for more than 10 years and never had any issues. Not so much as a single MCB trip. But then I see that the original design was for audio applications and I wonder if it is quite so relevant for stepper driver PSUs. At least, at the "home user" scale. Big commercial machines are a different kettle of fish.

There was a discussion on this forum a while back of capacitor sizing for stepper PSU use and learned discourses of ripple against capacitance, none of which took into account what level of ripple was acceptable, and this is where stepper and audio applications are very different. I put an oscilloscope on my router supply and ran some gcode to exercise all three axes. I saw around 10V ripple. And, frankly, while that sounds a lot on a nominal 68V supply, it doesn't make any practical difference to performance when the drivers are happy to accept 24V to 70V and are switching at rates way over the 100Hz ripple frequency. So, capacitors do not have to be that big. But for audio, I suspect that that would be entirely unacceptable leading to much larger smoothing caps and hence much higher inrush currents. Supply ripple may have audible consequences!

In short, I reckon that typical "domestic" stepper supplies do not need this level of sophistication. It won't hurt, but I wouldn't worry about installing such devices. Above, say, 1000VA, maybe it's a different story but I have no experience at that level. All I'm saying, I think, is that you shouldn't read across from one situation and assume that it applies in another very different one!