Re: [CAD_CAM_EDM_DRO] Re: Control Box Isolation

Thank you Marco, for the forwarded troubleshooting proceedure.

Tom Eldredge

----- Original Message -----
From: Marco C. Mason <marco@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Thursday, April 05, 2001 9:31 AM
Subject: [CAD_CAM_EDM_DRO] Re: Control Box Isolation


> Hello, gang--
>
> (I read from a digest, and am often a bit behind. So if this problem is
> already resolved, just ignore this...)
>
> I once worked on a plasma cutting system and we had some similar
> problems.
> We found several sources to the problem, perhaps one (or more) of these
> are what you're running into:
>
> 1) Make sure you minimize loop area in your circuits. When you get a
> large
> loop in an input circuit, you can couple to a *lot* of magnetic flux
> generated
> by high-current loops. (Such as welders, plasma cutters, stepper
> controllers,
> etc.) A loop with enough flux coupling to it will definitely get you.
>
> For example, if you have a few series switches, wiring like this would
> be
> BAD:
>
>
> +------o->o--------o->o---------o->o--------o->o---+
> | |
> | LOOP AREA --- BAD! |
> | |
> +-------------------+ +--------------------------+
> | |
> input
>
> Using twisted pair is good because while you must still have some loop
> area,
> some if it is in one direction, and some is in the other direction,
> causing
> magnetic interference to cancel itself out:
>
> A B A B A B A B A
> +-\/-\/-\/-o->o-\/----\/-o->o-\/-o->o-\/-\/-o->o-+
> +-/\-/\-/\------/\ /\------/\------/\-/\------+
> | |
> input
>
> So the voltage induced is amount induced in all the A loops minus all
> the B
> loops.
>
> 2) Make sure that you have only a single, large, LOW IMPEDANCE ground.
> If you
> have multiple grounding points, then when one circuit activates or
> deactivates,
> the current flowing through the grounding system can cause voltage
> shifts at
> multiple other grounding points. Multiple paths in your ground circuit
> can
> often cause "voltage dividers", so as currents through one part of your
> ground
> change, the voltage at your ground point in other parts of the circuit
> can
> change. (For example, if you switch a 10 amp current and your ground
> impedance is even a half an ohm, you can easily cause a 5V difference in
> your
> ground potentials.)
>
> Making things worse is that wire has a natural inductance in it, so it
> resists
> instantaneous current changes across it, causing brief voltage
> variations. So
> don't use wires any longer than you need to. (Especially don't coil
> them up
> in a corder if they carry variable current loads. This causes
> fluctuating
> magnetic fields which can couple to other circuits, as in (1)).
>
> 3) Parallel conductors are effectively capacitors, and capacitors can be
> very
> effective conductors of high-frequencies. So a stepper-motor control
> line in
> parallel with any logic signals is a great way to cause yourself
> headaches.
> When possible, run your logic signals at right angles to any lines
> containing
> high-frequency transients. Failing that, maintain as much separation as
> possible.
> It's also a good idea to shield any logic lines. (See rule 2, though.
> Don't
> ground both ends of your shield, because providing multiple ground paths
> can
> be BAD!)
>
> Sorry for the verbose message. (You'll be happy to know that I *did*
> edit for
> length. Normally, I run at the keyboard for much longer stretches...)
>
> --marco
>
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