Quote Originally Posted by m_c View Post
I'm sure they only supply that resistor to minimise people asking how they set 3.5A if they've not got a resistor.
Probably true! Just slightly puzzled why it's not a round 3.5K, but there we are.

Quote Originally Posted by m_c View Post
...by plucking figures out of no where. Which is essentially what you've done by saying your 65V supply hasn't given you any problems, despite you not understanding the theory behind why it probably really isn't a good idea.
My problem with those kinds of rule-of-thumb formulae is that they take on a kind of mystical significance, where really they are just a "do this and you won't be far wrong" guideline. In fact, my problem with this one is that I don't see any theoretical basis for it (and I don't think that Ohm's Law and the di/dt stuff has changed that much since I studied it 40-odd years ago). However, I do see a reason why higher voltages can give better performance when used with appropriate drives and settings. I won't use a higher voltage still because that means much more expensive drives and power supplies, and why risk the insulation breakdown in the motor? I won't use a lower voltage because I do not believe that I am unreasonably stressing the motor at 65V as the driver is controlling the motor current for me and any excess power is being dissipated in a fan-cooled heatsink - but I'm going to get faster di/dt hence higher peak initial torque this way, so better performance. Can I quantify this? No, but it costs me very little to do it. I'm still tweaking my new machine but I have every intention of sticking an oscilloscope across various bits of the power supply to see what's happening. I'm interested both in the main PSU ripple given that it's a conventional linear PSU, and what's happening across the motor windings under load, just to see what voltages do appear there in practice.

Good engineering is a compromise. Understanding the trade-offs from both a theoretical and practical perspective is useful if you are going to go outside the "just do it my way and it'll be OK" approach. A lot of forum members here do lack an electronics background and look for advice and help, which they often find. Unfortunately, blindly following a published rule of thumb is a bit like buying a kit of electronic bits - it will probably work but won't be optimum in all but a few cases.

In the OP's case, it sounds as if the PSU is at the source of the problem as all the other bits seem to work. Whether it's a paranoid PSU overload circuit which doesn't like the pulse load of four chopper stepper drivers, or is underspecified, or is faulty, is a bit moot but replacing it, even on a temporary basis, with something that is known good is one way to find out. I would probably try reducing motor current in all four motors as well, if appropriate current-set resistors are to hand, to see if that helps. That might also point to whether it's the PSU.