Quote Originally Posted by Rogue View Post
Yeah, he likes to give me expensive advice. I'm still trying to work out how to safely situate this machine after he got me to check the rafters Keep an eye out for the upcoming threads on building waterproof, soundproof and giant-maneating-spider-proof low profile enclosures with built in dehumidifiers....

Not that I mind in the least of course. I'd rather have the advice to weigh up than not have it and blunder on blindly.

I'm intrigued though. What kind of thing would you be doing to benefit from the price difference?
The cheap capacitors are a bit marginal on ripple current for three but as Jazz/Jonathan says they'll do the job for the level of use you'll put them to esp if you use four of them. I did qualify my statement with the phrase... "or whatever you can get cheap on eBay :)" and they were examples. I have 2 x 7500uF 400V capacitors on my 68v supply, I paid £5 each on eBay - they list at £75 each, used in hi-quality 500W+ sound systems - the D-class output stage of a high-power switching amp looks remarkably similar to a stepper drive :). The only thing I don't like about the ones you've chosen are the solder terminations; its a personal preference but I like meaty screw terminals.


On other points...

Bleed resistors are there for safety. You might never power the supply on without the steppers connected but I can't know that and I would never advise building a hi-voltage (>48v) supply without them. Leaving them out isn't best practice (it ranks with wiring e-Stops with 230v and having plugs on the 'live' side in my book) and I've been caught out by that before. For the record, I've been in electronics manufacturing on and off for over 30y and have seen most levels of stupidity :)

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). If you look at the datasheet for the rectifier you used its rated at 35A @ 55degC case temp. At 35A its dissipating 35 x 1.1 x 2 = 77W so given the limiting junction temp is 125degC the internal thermal resistance must be (125-55)/77 = 0.9degC/W therefore case to ambient of 25degC must be 0.4degC/W to run with no heatsink (which is what the datasheet seems to suggest, though I find that very low, but it is a metal case so go figure). At 12A (which I accept is a worst case with all three motors at full chat, relatively rare) your case temp would be 35degC. On the face of it thats a better rectifier for this purpose. The one I suggested has an epoxy case so clearly needs the heatsink as detailed in its datasheet.