Confirming PSU spec for steppers.

Next in the list (now that the motors are here) will be the PSU. I think I understand what I'm doing but I wanted to ask if someone could stop me before I make any (or at least too many) terrible mistakes :) I know this is a rehash of threads to probably occur once a fortnight but its for a reason. I'm setting out the details because it's not just about buying the right thing - I want to make sure my understanding of what I'm buying and why is correct.

I have these 3.1Nm steppers and I plan to wire them as parallel: 4.2A, 3.2mH and 2.73V. Doing some magic calculations (based on magic spells found elsewhere on this forum) leads me to an operating voltage of 57.2V. The PSU would therefore need to provide [email protected]. But wait...

I plan to buy PM752s which can provide, inter alia, 4.09A or 4.64A. In fact they support up to 5.2A but the motor has a maximum of 4.2A. This would mean the appropriate setting would be 4.09A therefore the PSU would now only need to provide [email protected]. But wait, again...

Now as far as I understand it, I don't need to make the full 56.2V available, as nice as the idea might seem to me. There would be a performance drop but as the motors are probably outperforming the rest of the machine anyway, would this necessarily present a problem? Common figures seem to be 42V and 48V. Is dropping from 56V to 48V a travesty of epic proportions?

So what about the current? If needed, I could run two PSUs wired in parallel to provide more current (ie 2 x 42V@7A would effectively be able to provide 42V@14A). However, much as the full 56.2V may not be needed, so too the full 12.3A may not be needed. For an unregulated supply, the PM752 docs suggest the PSU might only need to supply 50%-70% of the motor rating. Picking an arbitrary 75%, the PSU would now only need to provide [email protected]. While 50%-70% would actually be 6.2A-8.7A, I assume (no! not assumptions! nooooo!) that more safety margin is always better than less.

Zapp's SPS407 - with 42V@7A(continuous) and 9A(peak) - would work on its own using the 50%-70% figures but would need to be doubled up to make my fairly arbitrary 75% minimum - though if doubled then for the same price I could consider the SPS705, two of which would provide 68V@10A. I gather this could be magically fettled to provide the ideal 56V-57V at the slightly-less-than-ideal 10A, but would move the cost bracket from £60-£70 to £120-£140.

CNC4You's 48V@12A, by contrast, would provide 97.5% and a few extra volts all on it's lonesome, but would still be under the ideal voltage.

I notice that Zapp also bundle their PS806 68V@6A with 3xPM752s and 3Nm motors, but that 6A falls below the 50% so I am presuming it would be too far a stretch in this very-similar-but-a-tiny-bit-different scenario.

Then we move onto sourcing the bits and building one but for some reason that makes me a little jittery compared to getting something made by someone who is competent to build something I plan to plug into the mains!

All of this leads to a point, eventually. If I choose "money" then it's 48V@12A@£57. If I choose "voltage" then it's 56V@10A@£138.

Am I on the right tracks and am I considering the right things to balance? I've picked up this idea that I can run on less volts, but am I being daft (well, the anwer to that is always yes) to think I should be considering anything less than 56V in this scenario?

Looks like you're after an explanation of why, not what...

I would consider the voltage you get from the common formulas (20*V, 32*L^0.5 etc) to be a 'recommended minimum'. You will get better torque at high speed if you use 70-75V, plus seems a waste to get PM752 drivers if you're only going to run them on 42V.

A linear power supply handles current surges better than a switch mode power supply (smps), which means you can safely use a lower rated current than the smps. A 500VA transformer is fine for 3 motors, probably 4 and that's only 500/70=7.1A.

The current in a 2-phase stepper motor on a microstepping driver is, at intermediate speed, approximately sinosoidal and out of phase by 90°. This means that the maximum current is the maximum of I*(sin(thing)+cos(thing)), which is I*2^0.5 where I is the current in one phase - so 2.1*1.414=2.97A. This is 70.7% of 4.2A. Similarly if you work out the rms current for both phases (just integrate the previous formula squared etc), it is 2.1A - hence 50%. I'm guessing this is where your 50%-70% approximation comes from.

Ok the calcs you've done are close and correct in some ways but some of what you read I think you miss understood.? The 752s Manuel and 50-70% was referring to current not voltage. So you still need 57V but only 8-9A current.

That said has you know the voltage is what gives you the speed in a stepper so dropping to 48V means your only getting 85% of the speed you would at 57V. PLUS these motors will happly run at 70V thou I always run them at 65-68V to give a better margin for Back emf. So really at 48V your only getting 69% of the motors potential at 70V.?

Now in speed terms to the machine it can be dramatic in terms of the motors corner speed or point torque starts dropping away.
IE @48v then expect the corner speed less than 700rpm and @70v approx 1000rpm.
On a 5mm pitch screw then it makes the usable feed rate difference between @48=3500mm/min and @70v=5000mm/min.

Without knowing more about pitch and what you want to cut then hard to say best route to take.? .. BUT . If the voltage can provide the speed in combination with your screws to give the feed rates you need to cut what you want then it's not an issue really simple has that.!!

Remember high rapids mean pretty much nothing in real world cutting unless you intend to do lots of positional moves like drilling or have a massive long machine.? . . Mostly just bragging rights.!!

That said I prefer building my own supplies so obviously can build it to my exact needs so nearly always 65-68V using 75V drives simply because I can.! The amps just depends on how motors and number of them.
It's easy building a toroidal PSU and often cheaper plus safer for the drives has they handle back Emf much better and gives a nice consistent supply of power.

For me it would be "Build own to my exact needs" other wise if speeds/feeds it allows are OK use the 48V@12A and save the money.! Certainly wouldn't buy 2x68V@5a has could easily build for less than price one.!!!

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Originally Posted by Jonathan

Looks like you're after an explanation of why, not what...

I would consider the voltage you get from the common formulas (20*V, 32*L^0.5 etc) to be a 'recommended minimum'. You will get better torque at high speed if you use 70-75V, plus seems a waste to get PM752 drivers if you're only going to run them on 42V.

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I think that's it. It's easy to ask for the answer (and I often do) but it's sometimes better to ask for correction as a learning exercise.

I agree about the drivers. It was my understanding that the system could be run with lower voltage, I was trying to establish if doing this would lead to an earthshattering change or just knock a few percentage off the numbers. I would prefer to go for the higher-spec drivers and utilise them as best I can.

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Originally Posted by JAZZCNC

Ok the calcs you've done are close and correct in some ways but some of what you read I think you miss understood.? The 752s Manuel and 50-70% was referring to current not voltage. So you still need 57V but only 8-9A current.

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Yep I got that bit, though I guess I didn't really make it clear. The reduction to 42V or 48V wasn't because I was applying a percentage reduction, I was just observing that several of the CNC places were offering that size of PSU and if I wanted to buy off the shelf then I was limiting myself to what was on the shelf in the first place!

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Originally Posted by JAZZCNC

Without knowing more about pitch and what you want to cut then hard to say best route to take.? .. BUT . If the voltage can provide the speed in combination with your screws to give the feed rates you need to cut what you want then it's not an issue really simple has that.!!

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Ahah. Of course, the benchmark against which the performance needs to be measured. I was asking the wrong questions again! Not "will it work" (because the answer can only be "it depends") but "what do I need it to achieve?"

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Originally Posted by JAZZCNC

For me it would be "Build own to my exact needs" other wise if speeds/feeds it allows are OK use the 48V@12A and save the money.! Certainly wouldn't buy 2x68V@5a has could easily build for less than price one.!!!

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I keep looking at it and thinking "that looks quite straightforward", but then I think about plugging it into the mains and I have a vision of mushroom clouds rising over Derby. Some might say it would be an improvement, of course.

I'm going to end up trying to build this, aren't I? Bugger. Well, most of the components are cheap enough,

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Originally Posted by Rogue

I'm going to end up trying to build this, aren't I? Bugger. Well, most of the components are cheap enough,

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Yep probably.?? BUT it is Honestly so easy you'll wonder why you fretted afterwards and if you need a hand holding then just shout.!

So...

...this 500VA 35V-35V transformer would let me connect the two outputs in series to get [email protected]. By the time it has lost a few volts through the additional components (rectifier etc) I would have 68.6V (I think it's 1.6V drop for suitable rectifiers?). This 30V-30V would give me 58.4V - with 57.2V being the minimum rather than the optimal that I had thought it represented - so it would be the bottom line option.

Daft question, is it feasible to knock the output down a bit from a transformer, short of fiddling with the number of turns?

No the 35V is AC so needs to be rectified which makes it times 1.4 so that would be 35 +35=70 x 1.4= 98Vdc. . .Boom.!!

You need 25V to give 70Vdc.

Yes can fiddle with turns but why would you.? Just buy the appropriate transformer.??

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Originally Posted by JAZZCNC

Yep probably.?? BUT it is Honestly so easy you'll wonder why you fretted afterwards and if you need a hand holding then just shout.!

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I fret because I'm used to being an expert in my chosen field and here I'm dealing with things that I have absolutely no experience (or confidence) in. I am very good at imagining all the horrible ways things could go tragically wrong, though. Each action taken or order placed commits scarce resources, and every mistake is a painful waste. Otherwise I'd quite happily glue everything to everything else and pump lightning through it to see what would happen :whistle:

Hand holding? I would rather throw my ideas out and be corrected so that my understanding improves, but I'm sure some handholding creeps in unwittingly. My apologies if it does because that's not what I want to do.

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Originally Posted by JAZZCNC

No the 35V is AC so needs to be rectified which makes it times 1.4 so that would be 35 +35=70 x 1.4= 98Vdc. . .Boom.!!

You need 25V to give 70Vdc.

Yes can fiddle with turns but why would you.? Just buy the appropriate transformer.??

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Ahh, yeah that might not be what I'd want then. I was asking about the fiddling because I was wondering, I certainly don't want to fiddle...

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Originally Posted by Rogue

Ahh, yeah that might not be what I'd want then. I was asking about the fiddling because I was wondering, I certainly don't want to fiddle...

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You can wind a few extra turns on, or vice versa, then add the appropriate insulation. It's a crude but successful way of tweaking the voltage of a toroidal transformer that's not quite what you want...it shouldn't be needed here though as you can just buy the right one.

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Originally Posted by Rogue

Hand holding? I would rather throw my ideas out and be corrected so that my understanding improves, but I'm sure some handholding creeps in unwittingly. My apologies if it does because that's not what I want to do.

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Yep I understand but Wasn't offering to do it for you just over look and backup.!! An experienced eye watching over never hurts only affirms you've learnt correct.

Ahh hold on, wait a second. It would be 25V if the outputs were connected in series (thereby doubling the V while maintaining the A), so 25+25*[email protected]. If a transformer with 50V-50V 625A had the outputs wired in parallel, would this give 50V*1.4=70 @ 625+625 / 70= 17.8A?

Yes 50v wired parallel is double the amps but they are half the rating of the 25V so workout the same.

Could you just clarify what you mean by rating in that context? VA? I ask because the only way I understood that the 25V wired in series would equal the amps from the 50v wired in parallel would be for the 25V to start with double the VA, ie

50V-50V 500VA parallel would be: 50*1.4 = 70V@500+500 / 70 = 14.2A
25V-25V 500VA series would be: 25+25*1.4 = 70V@500 / 70 = 7.14A
so you would actually need 25V-25V 1000VA to get: 25+25*1.4 = 70V@1000/70 = 14.2A

There's clearly something fundamental I'm missing here.

I really appreciate both of your inputs, time and patience, just in case I didn't mention it already!

When you put the windings in series add the secondary voltages, but at the same current. In parallel you get the same voltage as for one winding but twice the current. Either way the VA rating has to remain constant for the same transformer.

Just what Jonathan says but to make it clearer look at the transformer output current ratings for these two transformers.

Both are 625Va but first is 2x25V second is 2x50v.

Standard Range Toroidal Transformers: CM0625225: 625VA 230v to 2x25v

Standard Range Toroidal Transformers: CM0625250: 625VA 230v to 2x50v

If you wired the 2x25V in parallel not series you'd still have 25V but @ 24A So obviously 2x50V in paralle is 50V @12A (6A x 2)

Now, are you agreeing with me or correcting me? Because what you said is what I thought I said, if you follow me.

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

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Originally Posted by Rogue

Now, are you agreeing with me or correcting me? Because what you said is what I thought I said, if you follow me.

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Who you asking Jon or Me.? Either way we are both saying series you 2 x volts 1xamps or parallel 1 x volts 2 x Amps. . . BUT . . the amp output rating for 25v transformer are double the 50V.!

So we were saying mostly the same thing. That's a good start :)

I think I've worked out where my mental block is for this element at least. I was treating parallel as increasing A rather than decreasing the division of A. In other words, I was thinking of two outputs, each with 50V 625VA. Parallel would make this one output of 50V 1250VA. Rather, I should be looking at it as two outputs of 50V sharing 625VA between them - effectively making it 100V / 625VA = 6.2A. Wiring it in parallel would then have the same VA but divided by less, which has the same end effect as doubling it: 50V / 625VA = 12.5A. When I look at it like this, your previous comments make more sense so I guess I'm on the right track.

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

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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.!

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Originally Posted by Rogue

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....

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

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Originally Posted by JAZZCNC

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

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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...

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Originally Posted by Rogue

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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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.

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Originally Posted by irving2008

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 :)

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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.

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Originally Posted by irving2008

For the record, I've been in electronics manufacturing on and off for over 30y and have seen most levels of stupidity :)

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And I'm grateful for the chance to be able to draw on that experience, along with the experience and knowledge that Jazz and Jonathan have brought to the discussion.

Fuses - now that never occured to me but sounds like a rather good idea...

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Originally Posted by Rogue

...

Fuses - now that never occured to me but sounds like a rather good idea...

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Err.. yes. Fuse the primary side with a slow-blo fuse rated approx 50% - 100% more than the likely operating current... for a 625VA transformer thats a 4 or 5 amp fuse. You can fuse the secondary with a 15A slo-blo as well if you want. Put the fuse between the transformer and the rectifier.

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Originally Posted by irving2008

You might never power the supply on without the steppers connected

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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.

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Originally Posted by irving2008

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).

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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.

Don't all these figures make your head hurt.?? . . . . For sake of £1 Just stick it on a heat sink to be sure then it's done and forgot about.!!

Quote:

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Originally Posted by JAZZCNC

Don't all these figures make your head hurt.??

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No, they make life more interesting.

Here's a graph to brighten things up:

Attachment 6849

(0.47 ohm shunt resistor and one motor going at speed which obtains the highest current)

Quote:

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Originally Posted by Jonathan

No, they make life more interesting.

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Love pretty pictures but told you before you need to get out more pal. .:couple_inlove: