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Anyone wanting to make one Jonathan is selling these http://www.mycncuk.com/threads/10475...al-transformer. A bargain I think.
Last edited by Clive S; 06-01-2017 at 11:29 AM.
..Clive
The more you know, The better you know, How little you know
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06-01-2017 #22
Another thing to play with is adding or subtracting turns from the transformer to tweak the voltage. But I wouldn't recommend that by design, only if you the insulation materials and enameled wire lying around... It's a neat way to make an auxiliary supply though - just add on an extra winding.
Having scoped the turn-off overshoot voltage of the MOSFETs in a DQ860MA, I have no qualms about running them at 80V. They use 100V mosfets and the overshoot was only a few volts, so still some headroom. They do skimp on the high frequency decoupling, so I added an extra ceramic capacitor on the empty footprint. I can't comment on other drives though.
Yes!
Correct. The stepper driver is a current controller with a switching frequency of 10-20kHz, so the current control bandwidth will be plenty sufficient to deal with some input voltage ripple.
Probably not, for a few volts...
That is the question everyone should be asking - the answer is not straightforward.
Maybe I'll try it this evening, if I can find that spare stepper driver...
Meanwhile, I just dug out these graphs I captured whilst repairing a stepper driver, showing the voltage across one mosfet Vds (blue) and the line current through the same (yellow).
The first graph shows a normal situation - we see the current rising when the voltage is applied, and falling otherwise with the inductance of the motor smoothing this current. Observe how little overshoot there is on the voltage.
For this one I removed the dc-bus capacitor (can't remember if it was the small or large one) and supplied from a lab PSU. You can see the voltage and current now oscillating horribly.
Same again, but with a lower supply voltage - notice the current is the same (so the regulation is working) and the duty cycle has increased.
This doesn't tell us anything about sizing the supply capacitor, still it's mildly interesting...
This is what I like to call a "Forum Formula": formula's commonly distributed with little concern for how they were derived. Often empirical, commonly including a seemingly arbitrary constant designed to make the user think they are learning something.
Just like the formula's in the first post to estimate the supply current - these bare little relevance to reality.
Where does the 80,000 come from? Which is the I, motor or supply current? etc...
The 1000uF per amp is derived from .. The gecko formula will be something similar.
There is some better information in this thread. Post #2 is pertinent.
This one too.
But do they apply to every driver - maybe close enough, maybe not?
Thanks Clive - I still have two left and more capacitors on the way.Last edited by Jonathan; 06-01-2017 at 03:47 PM.
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27-04-2017 #23
A few months ago I said that I would have a look at the output of my power supply with an oscilloscope to get a feel for what was happening to the supply voltage with the router in operation. I was able to do this recently. I had some gcode running that was winding X and Y backwards and forwards at full rapid speed (gantry probably weighs in at around 50Kg, rapid speed on X and Y 4.5m/min, NEMA23/ballscrews, to give an idea of mechanical load), and I hung the oscilloscope off the supply to one of the drivers. Basically, there was the best part of 10V mains frequency ripple, with maybe a couple of volts of high-frequency noise on top of that. Mains ripple was presumably what the smoothing capacitors could not remove, and the high-frequency noise was the combined result of four unsynchronised switching drivers. That's all on top of a nominal 68V DC supply - the output voltage was swinging either side of that. The amount of ripple didn't change very much when the steppers were stationary although the stepper drivers are set to half-current mode. Clearly that doesn't make much difference to the PSU ripple.
Edit - have now seen that PSU has 2x6800uF smoothing capacitors, 100V/105C rated. Voltage limit is fine for this job and it's good to see the higher temp capacitors being used rather than the cheaper 85C - electrolytic capacitors are not the most reliable of electronic components but the higher-temp versions are better.
On the face of it, that doesn't sound like a great result. I was surprised by the amount of ripple - much more than expected. I can't read the capacitor values where they are, unfortunately, so I'm not sure how generous they are (the original PSU is the PS806 from Zapp although with a replacement transformer). On the plus side, the stepper drivers clearly don't give a damn and just keep on doing what they are supposed to be doing, and do it very well. Maybe, just maybe, it's possible to get a bit too hung up on the details sometimes and worry unnecessarily.Last edited by Neale; 20-06-2017 at 09:38 AM.
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