Confirming PSU spec for steppers.

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Originally Posted by Rogue

Ahh, wait a second. Is the 625VA shared between the two outputs or per output?

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Ok I see where your going wrong.? Nope forget the Va rating for now the only bit your interested in is the secondary outputs.

You have 2 and you can either have them separate so effectively making 2 completely separate PSU's or wire them together to make one large supply.
If wired in series then you add the voltages for each secondary together but keep the amps the same. Wire in Parallel then you do the opposite.

IE 625Va 2 x25V Can have this combination.?

1 x 25Vac x 12A = 1 x 35Vdc @12a PSU
1 x 25Vac x 12A = 1 x 35Vdc @12a PSU

OR
1 x 50Vac x 12A = 70Vdc @12A Psu = Wired series

Or
1x 25Vac x 24A = 35Vdc @ 24A Psu = Wired Parallel

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Originally Posted by Rogue

And you wonder why I get jittery with mains power :joker:

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This is the secondary winding, no mains there :wink:

The core of the transformer is primarily what determines the power rating, hence why changing the windings from series to parallel cannot change the power rating (VA) you get.

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Originally Posted by JAZZCNC

IE 625Va 2 x25V Can have this combination.?

1 x 25Vac x 12A = 1 x 35Vdc @12a PSU
1 x 25Vac x 12A = 1 x 35Vdc @12a PSU

OR
1 x 50Vac x 12A = 70Vdc @12A Psu = Wired series

Or
1x 25Vac x 24A = 35Vdc @ 24A Psu = Wired Parallel

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I have a few more questions I shall need to read into I think. You've already been more than patient with me! Thank you both.

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Originally Posted by Jonathan

This is the secondary winding, no mains there :wink:

The core of the transformer is primarily what determines the power rating, hence why changing the windings from series to parallel cannot change the power rating (VA) you get.

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But remember the old adage: Its volts that jolts but Amps that kills... so 70v at 12A still needs treating with respect

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Originally Posted by irving2008

But remember the old adage: Its volts that jolts but Amps that kills... so 70v at 12A still needs treating with respect

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I thought the old adage was "Of course it's not live, I checked it just a few miARRRggRGRgrGRGGRGRHHHHH"?

So, you have a 50v AC transformer giving 12A, what about the rest of the components?

Here are my suggestions and the calculations needed to ensure it all works reliably...

Bridge rectifier: Buy Bridge Rectifiers Bridge Rectifier Single 400V 35A GBPC4 Vishay GBPC3504-E4/51 online from RS for next day delivery. @ £3.60

This bridge has a forward voltage drop of 1.1v per diode so your output volts will be 50 X 1.4 - 2 x 1.1 = 68v! Also each diode will dissipate 1.1v * 12A = 13.2W so you'll have 26W of heat to dissipate. Assuming air temp of 25degC, and a rectifier temp of 125degC you need a thermal resistance of no more than (125-25)/26 = 3.8degC/W, since the rectifier has a thermal resistance of 1.4degC/W from the diode to the mounting base it'll need to be mounted on a heatsink rated at 2.4degC/W or better. Something like this: Buy Heat Sinks Heatsink 2.4K/W 97x50x25mm ABL Components 333AB0500B online from RS for next day delivery. @ £3.79 and don't forget the thermal compound between rectifier and heatsink and between heatsink and the case...

Smoothing Capacitors:

You need C = 0.1 * A/V for a 5% ripple = 0.1 * 12/70 = .017F = 17,000uF with a voltage rating of at least 100V and a ripple rating of 12A. (for derivation see here)

Doing this in one capacitor is expensive (18000uF, 400v, 12A ripple = £271!!!) So usually you make it up with a bank of capacitors in parallel. The exact value is not critical so 3 x 5000uF @ 100v/4A would do, or whatever you can get cheap on eBay :)

For example, 3 of these: Buy Aluminium Capacitors GP Al electrolytic capacitor,4700uF 100V Epcos B41456B9478M online from RS for next day delivery. would be £33, with the matching mounting clips @£1.70 for a pack of 5. These have screw terminals so you'll need the appropriate solder ring tags.

You'll also need a bleed resistor across each capacitor to discharge it when the power is turned off (for safety). The energy stored is 0.5CV^2 joules. For each capacitor spec'd above, the energy is 0.5 * .005 * 70^2 = 12joules. A watt is a joule per second, so for a 5 second discharge you need to dissipate 12/5 = 2.4W so we're going to need 3 or 5W rated resistors. Assuming the resistor dissipates the same 2.4W when the power supply is turned on the value will be R = v^2/W = 70^2/2.4 = 2000ohm. I'd use 2200ohm (2k2) @ 3W wire ended and solder them directly to the ring tags on the capacitor: Buy Through Hole Fixed Resistors ROX3S metal oxide film resistor,2K2 3W TE Connectivity ROX3SJ2K2 online from RS for next day delivery.. (pack of 10 @ 99p)

Don't forget fuses for primary and secondary sides, push-on tags for the rectifier, screw tags for the capacitors, suitably heavy wire (red & black, at least 1.5mm sq, e.g. 30 x 0.25mm)...

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Originally Posted by irving2008

Smoothing Capacitors:

You need C = 0.1 * A/V for a 5% ripple = 0.1 * 12/70 = .017F = 17,000uF with a voltage rating of at least 100V and a ripple rating of 12A. (for derivation see here)

Doing this in one capacitor is expensive (18000uF, 400v, 12A ripple = £271!!!) So usually you make it up with a bank of capacitors in parallel. The exact value is not critical so 3 x 5000uF @ 100v/4A would do, or whatever you can get cheap on eBay :)

For example, 3 of these: Buy Aluminium Capacitors GP Al electrolytic capacitor,4700uF 100V Epcos B41456B9478M online from RS for next day delivery. would be £33, with the matching mounting clips @£1.70 for a pack of 5. hese have screw terminals so you'll need the appropriate solder ring tags.

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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?

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Originally Posted by irving2008

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.

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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.

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Originally Posted by Rogue

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?

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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.

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Originally Posted by Jonathan

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

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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 :whistle: 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?

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Originally Posted by Rogue

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

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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.!