Confirming PSU spec for steppers.

[Jonathan]

You might never power the supply on without the steppers connected

To be fair it's a good idea to power the supply on without the steppers connected when you've first built it to check the voltage is correct, so yes have the bleed resistor.

As regards the heatsink Jonathan, of course the case doesnt get to 150degC, its what the junction mustn't go above (actually 125degC for your device).

I should have clarified I've used both on different power supplies. It's 150°C for the first one I linked to and 125°C for the other.

I just put an ammeter on my 500VA PSU and ran a few programs on the router. With 4 motors maximum current was 2.66A and at standstill 0.72A. Just going by eye (not great I know, should use my oscilloscope) the mean current looked to be about 1.9A. The drivers are set to 50% current at standstill, so since the power supply is outputting 68V we expect the current current when stationary for 4 motors to be 4*0.5*2.73*4.2/68*2=0.674A, so the measurement is fairly close.

So lets be very safe and calculate it with the peak current since we know if the temperature is fine at that current (2.66A) it must be fine for the lower current when running. The 8A rectifier I used is 2.2°C/W junction to case and forward voltage is 1V, so 2.66*1=2.66W to dissipate. (150-25)/2.2=68°C/W which is a large value, hence why it works with no heatsink. Also the datasheet specifies up to 3.2A with no heatsink, so clearly we're fine especially as in reality the mean current for 3 motors is less than 2.66A.

Edit: Just tried measuring using the oscilloscope with a shunt resistor and running one motor at the speed it draws the highest current I get 1.58A mean, so 4.75A for 3 motors which implies if I get all 4 motors running at their peak, it will draw 430W from the 500VA transformer.