help please with toroidal transformers selection

[Jonathan]

I don't want to get hung up on this transformer voltage business (except that I was surprised by the toroidal transformer I bought recently that measured about 5% above nominal voltage when off-load - the "regulation" factor that's mentioned above which I hadn't expected) but I am interested in the practical choice of smoothing capacitors.

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.

Stepper drivers can cope with fairly wide voltage ranges. From memory, the EM806 which I am using is rated at 24-80V absolute min/max, for example, although the Leadshine recommendation is to stay at least 10% away from either extreme.

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.

We are not building audio amplifiers here, so we're not interested in hum levels on the output.

Yes!

The capacitors are being charged at 100Hz (assuming the usual bridge rectifier) but the steppers are pulsed at a few KHz, so any given pulse will see only the variation in voltage during that pulse - small compared with the variation across a half-mains cycle?

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.

On top of all that, one of the stepper driver's main jobs is to control and limit current through the stepper coils, so one thing it can do very well is cope with a range of input voltages while trying to maintain the target current through the stepper coils. So do I really care if the DC output voltage sags by a few volts, across a half mains cycle?

Probably not, for a few volts...

In practice, then, just what size capacitor will do an adequate job without going OTT? What is the effect of a too-small capacitor?

That is the question everyone should be asking - the answer is not straightforward.

Don't know the answer myself, but once my machine is back running I'll stick an oscilloscope on the PSU output and see what's happening, just in the interests of science.

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...

The thing is, that 1000uF per A is an arbitrary figure provided as a guideline.
I just had a quick scan through the Gecko Drive guide, and they list the following formula for minimum capacitance in uF-
(80'000 * I)/V

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.

I'd imagine they've done some calculations and research to reach that figure, and it'll involve the maximum voltage ripple their drive internal capacitors can handle safely under various conditions.

But do they apply to every driver - maybe close enough, maybe not?

Anyone wanting to make one Jonathan is selling these http://www.mycncuk.com/threads/10475...al-transformer. A bargain I think.

Thanks Clive - I still have two left and more capacitors on the way.