Quote Originally Posted by diycnc View Post
As I slowly edge closer to the electronics step, I was wondering why we have to over spec a switch mode power supply.

The stepper drivers we use limit the current to the motors, so why do we need to make sure we choose a power supply that can supply say +20% more current? Surely a driver set to say 2A will only draw 2A from the PSU?

I'm sure there is a very logic explanation.
Well, there's a very technical explanation! Actually, the stepper driver does not quite limit current to the nominal value. In fact, it's often unclear what that nominal value means anyway - peak current? Average current? So the value that you set the stepper driver to will be somewhere near the maximum current drawn from the PSU but not necessarily exactly the value you expect. The other problem is that the stepper driver doesn't actually supply a steady current over the whole pulse, but chops it alternately on/off so that the average is around the desired value. So peak current draw might be higher than expected during the pulses, even if the average is around what you would expect. The SMPS must be able to cope with these short-duration pulses without going into "overload" when it might shut down altogether for a brief period, until the "overload" has gone away. This can be pretty catastrophic for the machine overall and losing steps might be the least of your problems. Hence the need to over-spec a SMPS just to make sure that it never goes into current-limiting or overload mode. I would be worrying if the SMPS is only capable of supplying 20% over nominal current - doesn't seem like much headroom to me. That's why the usual advice (even from people like Leadshine who should understand these things) is to use a "linear" power supply which is very tolerant of pulse current overloads.

This is a bit of an over-simplification, but the fact is that the current draw is very "peaky" and not steady, you are adding together the loads of multiple drivers without any idea of whether their pulse loads will occur simultaneously or not, and you have a power supply that is not capable of handling overloads gracefully. A linear PSU will be heavier, probably larger, more expensive - but more reliable. And they're easy to build - there are several "how-to" threads on the forum. If you go SMPS, over-spec the rating to make sure!