help please with toroidal transformers selection

[Neale]

A few months ago I said that I would have a look at the output of my power supply with an oscilloscope to get a feel for what was happening to the supply voltage with the router in operation. I was able to do this recently. I had some gcode running that was winding X and Y backwards and forwards at full rapid speed (gantry probably weighs in at around 50Kg, rapid speed on X and Y 4.5m/min, NEMA23/ballscrews, to give an idea of mechanical load), and I hung the oscilloscope off the supply to one of the drivers. Basically, there was the best part of 10V mains frequency ripple, with maybe a couple of volts of high-frequency noise on top of that. Mains ripple was presumably what the smoothing capacitors could not remove, and the high-frequency noise was the combined result of four unsynchronised switching drivers. That's all on top of a nominal 68V DC supply - the output voltage was swinging either side of that. The amount of ripple didn't change very much when the steppers were stationary although the stepper drivers are set to half-current mode. Clearly that doesn't make much difference to the PSU ripple.

Edit - have now seen that PSU has 2x6800uF smoothing capacitors, 100V/105C rated. Voltage limit is fine for this job and it's good to see the higher temp capacitors being used rather than the cheaper 85C - electrolytic capacitors are not the most reliable of electronic components but the higher-temp versions are better.

On the face of it, that doesn't sound like a great result. I was surprised by the amount of ripple - much more than expected. I can't read the capacitor values where they are, unfortunately, so I'm not sure how generous they are (the original PSU is the PS806 from Zapp although with a replacement transformer). On the plus side, the stepper drivers clearly don't give a damn and just keep on doing what they are supposed to be doing, and do it very well. Maybe, just maybe, it's possible to get a bit too hung up on the details sometimes and worry unnecessarily.