Confirming PSU spec for steppers.

[Jonathan]

This bridge has a forward voltage drop of 1.1v [...] each diode will dissipate 1.1v * 12A = 13.2W so you'll have 26W of heat to dissipate.

The mean current will be significantly less than 12A, so a heatsink with a much higher thermal resistance could be used. On my stepper PSU, which also uses a 500VA 50V transformer with 3 motors, I used one of these (or this one) and no heatsink is required as I've never noticed the rectifier get 'very warm' let alone 150°C. Still, you can find good heatsinks for free in lots of things, or since this evidently doesn't need much you could just attach it to a reasonable size sheet of aluminium.

Agree with the calculation for the bleed resistor, but surely since the stepper drivers will be switched on until the voltage has dropped to about 20V, resistors aren't critical since the energy from the capacitors will dissipate into the motors? If you unplugged the stepper drivers before turning off the power supply then the resistor is needed, but that would be a strange thing to do since it risks breaking the drivers.

Thank you, that's very, very useful. I was looking at caps that cost about £3 each though, is there an important part of the spec that I didn't take into account?

The ones you have linked to will be fine, I'd get 4. The ones irving linked to are better quality (lifetime etc), but cheap capacitors are fine for this application.

[Rogue]

The ones irving linked to are better quality (lifetime etc), but cheap capacitors are fine for this application.

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?

[JAZZCNC]

I'm intrigued though. What kind of thing would you be doing to benefit from the price difference?

Well if this machine was working 14hrs days 6 days a week cutting £500 lumps of Ali then you'd want to know it's reliable and not going to break down just for the sake of £30.!!

Edit: Jons right thou for your use then they are fine and actually I have the same ones on my machine.!

[JAZZCNC]

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....

You forgot " That are electricly safe". . :)

[Rogue]

You forgot " That are electricly safe". . :)

Actually I'm counting on the sparks, arcing and random live surfaces to keep the giant-maneating-spiders away if I end up having to use the basement...

[irving2008]

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.

[JAZZCNC]

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 :)

Couldn't agree more and here's an example of why.!
I've just acquired an old cnc lathe out of a school and been ripping it's guts out has it runs a bespoke control/software and I want to run Mach3.
This thing has 3 boards Control boards, PSU board and spindle board. I'm keeping the PSU and Spindle so after pulling the control board out I was messing with the spindle board trying to work out what wire goes where when I got a pretty wicked kick I wasn't expecting seen has it wasn't plugged into mains.?? . . . .Yep it's got toroidal PSU with no Cap drain.

Not enough to hurt me but woke me up good and that was only small PSU running at 36V ish with relatively small caps.!!

SO I completely agree with Irving and always build them into my PSU's..!! . . Now the only thing I don't like about Irvings way is that the resistors are permanently draining while the machines running, this creates heat while running so warms the control box and therefore the drives etc.
I'm a bit Anal with my control box and build them with relays and full safety using 24V and control lots of things with relays, one of these is a NC relay which kicks in and out a resistor so it only drains when off.!! . . No such thing has being too safe IMO.

[Rogue]

SO I completely agree with Irving and always build them into my PSU's..!! . . Now the only thing I don't like about Irvings way is that the resistors are permanently draining while the machines running, this creates heat while running so warms the control box and therefore the drives etc.
I'm a bit Anal with my control box and build them with relays and full safety using 24V and control lots of things with relays, one of these is a NC relay which kicks in and out a resistor so it only drains when off.!! . . No such thing has being too safe IMO.

I've read elsewhere about people using lamps rather than resistors as a "visual" indicator, is this the same kind of thing? I'm intrigued if a lamp connected to a normally closed relay would work, though I have no intention of trying it out...

[Jonathan]

I've read elsewhere about people using lamps rather than resistors as a "visual" indicator, is this the same kind of thing? I'm intrigued if a lamp connected to a normally closed relay would work, though I have no intention of trying it out...

Don't see why not, but bear in mind a lamp can blow/fail - so even if it's not illuminated you don't know for sure that there's no voltage across it.

[JAZZCNC]

Don't see why not, but bear in mind a lamp can blow/fail - so even if it's not illuminate you don't know for sure that there's no voltage across it.

Took the words right out my finger tips.! . . Resistor is a safer bet, could always use a resistor and a bulb but why complicate things.