Re: Lost steps

Thanks for the help Ballendo,
Today I tested the three boards, two phases on each board as in
Camtronics instructions. Each phase swung from nearly 37 volts to
negative same amount.

I connected one of the motors to the controller they came with, a
black box labeled "The Motion Group", with direction and on/lock
switches and a speed knob. The motor ran smoothly and quietly from a
very slow speed, all the way up to max. I was able to hear 2 or 3 of
the resonance speeds that were mentioned here,a slight noise but no
perceptible change in shaft speed or smoothness. I conclude the
motors
are okay as well as the lead wires.

I pulled the X axis board from the cabinet because that was the only
way to reach all the spots you suggested testing.
I could get no continuity between any of the mosfet bodies and the
heatsinks.
Testing the resistence between each of the legs and the ground trace
proved confusing at first until I realized the same three numbers
kept
coming up in different sequences. The board uses two different
mosfets
arranged alternately down each side of the board. When I tested the
same leg of the same mosfet, all readings were the same. I presume
that means I can conclude they are good???

Next I wrapped the motor wires from that axis in foil and grounded
it,
and wrapped the fan motor wires and grounded it. I reasoned that
these
might be generating electrical noise, and though I didn't know if the
wrap would help, it couldn't hurt. (I'd already wrapped the signal
wires, you might remember.)I tested all the wraps for ground.

I put it all together and the motor ran much better but not without a
few stutters, and the bare shaft does not get back to the same radial
position after an increment that should have brought it to the same
position, e.g. an inch.

It may be my imagination but it seemed the longer I tested the worse
the condition became, but maybe I just became more observant. In any
case it never became as bad as yesterday.

I also tested for continuity between computer case and driver case,
and that was good.

What next, any ideas.
I suppose there are other wires I could wrap, but if that were needed
wouldn't Dan at Camtronics have mentioned it. I've seen pictures of
his drivers and the wires are not shielded??
The insulators between the mosfets and the heatsinks are the ones
supplied by Dan, as is every part in the case, and the case.

Should I try replacing all the mosfets in one board, I did that once
before I got a board to test right.

Jerry

--- In CAD_CAM_EDM_DRO@egroups.com, ballendo@y... wrote:
> Jerry,
>
> I have been watching your posts for some time. Sorry to see you
> having such problems. You have been getting some good advice but I
> think something has been overlooked. Mariss mentioned it in
> passing,i.e. the phase outputs of the driver itself!
>
> You are using the camtronics 5A drivers which use MOSFETS for
output.
> I hope the next part adds your knowledge, but if you already know
> this, stay with me:
>
> These transistors are VERY sensitive to static electricity. They
are
> constructed of many hundreds (or thousands) of little electronic
> switches all wired in parallel (so the resistance drop is as little
> as possible, this is what makes them good for our uses!). Each
little
> switch carries part of the workload (the current being switched is
> shared among all the workers, as it were).
>
> So heres the rub, I think you have some lazy workers in your output
> transistors!
> There are a couple of reasons these little guys get lazy. The first
> has already been mentioned, static electricity. You may be
surprised
> to know that the "scoot across the floor and shock someone" trick
we
> have all played involves some hundreds of volts. Yet, these
> transistors are rated only about 80V.
>
> What happens is SOME of the switches(inside) get blown up; now
there
> are less workers available to do the work, so the others have to
pick
> up the slack. This means THEY are now working harder than they
> should... So some of them eventually quit also. The process
> continues, and at some point becomes noticeable to us Big guys, in
> almost exactly the way you have been describing.
>
> A few other causes with the same results are mentioned in the
> camtronics assembly instructions. First, poorly flowed solder to
the
> legs of the MOSFETs. This increases the resistance on the path
> through the transistor, with the result that those overworked,
> underpaid electron carriers say ,ENOUGH already, I'm outta here!
(as
> above)
>
> These guys also don't like too much heat. (they can be destroyed
> during soldering into place) As I mentioned, they may not be hurt
> enough to notice at first (during circuit checks) but they are
still
> injured.
>
> I've saved the hardest to find(and deal with) for last: You CAN
have
> electrical isolation of the heatsink to the transistor body (when
> measured with an ohmmeter) and STILL have all the problems above!
> When you check the heatsink to MOSFET continuity,you are only using
a
> few volts at a tiny current, and this CAN fool you. BEEN THERE,DONE
> THAT. There is a world of difference between this and 70V at 5amps!
>
> I had some driver boards where the solder mask(the green film on
> PCboards) was the only thing between a heatsink at GND, and 36V at
> 2A. Checked out ok with the meter, but under power, the electrons
> were arcing through the solder mask to the heatsink with EXACTLY
the
> results you have been describing. (poor board design, but thats
> another matter)
>
> Theres some whys, now to the whats:
>
> 1.Check ALL of the transistor mount tabs continuity to ground.(the
> heatsink) Be SURE they are OPEN circuits. And compare the readings
> you get, you're looking for one(s) that are different.
> 2.Measure EACH leg of the MOSFET to its trace on the board. Again
> looking for variations. You may need to re-flow the solder to each
> lead (as mentioned in the instructions).
> 3.Be aware of the parallel construction described above and realize
> that these parts do not always fail catastrophically.
> 4.Mariss is right, its ALMOST NEVER the motors.
> 5.Jan is right too, still could be a shielding problem.
>
> But my money is on the output transistors being partially blown for
> the reasons described in this (long) post.
>
> Thanks for listening. Hope this helps
>
>
> Ballendo