Re: 10 amps to drive steppers

> From: Dan Falck <dfalck@...>
>
> As I increased the current above 6 amps (minus the AND gates), the signal
> was still getting flakey. The circuit was missing steps all over the
> place. My input signals are shielded using single conductor braided audio
> cable, grounded on one end only and tied together at a common ground point.
> I physically moved the cables around and found that by turning cables 90
> degrees to other cables things improved somewhat. So, I tucked cables in
> and wiretied them down the best that I could. Are the mosfets acting as
> little antennas, emitting lots of noise from any sharp corner on them?

No, the FETs themselves are too small to radiate much RF. The traces
connected to them, however, are large enough to radiate. You have to
minimize the area inside the loop of the current paths. I did a lot of work
minimizing the current loops on my 100 KHz PWM servo amps, and
apparently I must have done it right, as I have no RFI, and can even run
a radio within a couple feet of the CNC system. (Umm, that is FM,
of course. AM radios don't work so well anywhere in my house, due
to all the electronic gear. But, it doesn't bother the TV, either.)

> Things that I could have done better:
> Shielding- use better quality shielded cables and then shield them some
> more. I think I used Radio Shack brand on the previous controller.

Twisted pair cables are very good at shielding. Maybe this is why
a lot of people use opto-couplers on their stepper drivers. Not only
does it provide isolation, but can be quite effective in reducing
EMI glitches.

> I will use an etched circuit board next time. I suspect the milled board
> probably contributed to noise due to ragged edges etc...

Raggd edges have no effect (below the GHz range, anyway). But
layout is REALLY important in any power switching circuitry.

>
> Layout- I will make sure that signals are miles away from motor leads and
> power supplies.
> Cooling- how about three fans on the mosfets? Maybe an air conditioner in
> the shop right next to it!

Yeah, well I just don't understand why these fets are running so hot.
They may not be driven into the proper gate charge to cut conduction
losses. My servo amps are switching continuously at 100 KHz, and often
idle at about 1 Amp output (plus the 1.5 Amp or so triangle wave
going into the filter). The transistors run cool, even with NO fan at
all. Just to be sure, I have a whisper fan on them.

Jon