Re: High voltage protection for a breakout board.

> A tech finally measured voltages, and found a 30v difference in
> ground reference between the two buildings.

I have had a 20V difference between a computer and a piece of
equipment. Both having 3 wire cords plugged into the same outlet
box. I knew this was impossible and at maximum I should be seeing
millivolts so I opened up the aux piece of equipment and found it was
grounded by connecting the ground wire to a capacitor and connecting
the other end of the cap to ground. I remember looking at the
equipment label and wondering how this thing could have a UL rating,
but it did.

I was hoping the current flow thru the capacitor was so low it would
not cause any damage. WRONG. The jolt of plugging it in and out a
few times fried a chip. Ever try trouble shooting a piece of
equipment you have no schematic for and know very little about? It
took a full day to find the bad chip.

In all fairness I will not say you have to have optical isolation in
all CNC applications. As many people are getting away without it.
But you almost have to have a crystal ball to know if you will be one
of the fortunate ones that do not. With so many people doing there
own wiring there is no guaranty how it will turn out.


Wally






--- In CAD_CAM_EDM_DRO@yahoogroups.com, Tom Harrison <TomH-CNC@T...>
wrote:

> I'll add my two cents to this, if it's of value:
>
> Optical isolation provides the system with immunity from grounding
> differences, which can be quite catastrophic. I recall a computer
> installation where a terminal was in a separate building from the

computer,

> and both the communications board in the computer and the terminal

kept

> failing. A tech finally measured voltages, and found a 30v

difference in

> ground reference between the two buildings. Bizarre, yes. Bad, yes.

But

> everything in both buildings worked just fine when isolated from

each

> other. An opto-isolation interface between the two machines worked
> perfectly: no more problems.
>
> In CNC applications, the motor start/stop changes and loading

changes when

> a tool moves through a cut induce large swings in current, which

always

> means fluctuations in observed voltage, since no conductors are

perfect.

> Opto's keep those fluctuations from becoming destructive to

expensive

> computers and the like.
>
> Tom Harrison (been there, done that, and likes opto-bulletproofing)
>
> At 5/4/2005 10:16 PM, you wrote:
> >Hi Arturo,
> >
> >I don't know if you realize that optoisolators are not simply used
> >to "protect the computer from high voltages that stepper drivers
> >handle." Your original post also seemed to show a bit of a
> >misunderstanding in this area as you were thinking a simple 74xx245
> >buffer might be a replacement for an optoisolator because of its

2kV

> >ESD protection. As I said, ESD is a minor consideration and has
> >really has nothing to do with protecting the computer from anything
> >going on with the motor. It has more to do with static discharge

as

> >what happens under low humidity conditions when someone walks

across a

> >carpet and decides to touch an exposed electronic connection.
> >
> >At the risk of repeating myself (please take this as constructive
> >criticism), optoisolators are very useful for separating circuits
> >entirely--include grounds. Sans an optoisolator, you risk making

for

> >an unreliable system if you join a digital signal ground with a

noisey

> >analog ground that potentially could also create ground loops.

Even

> >if you manage to have a system work on your workbench, you are only
> >asking for trouble if you expect any arbitrary wiring or driver a
> >potential user could use would work without intermittent/hard-to-

find

> >problems.
> >
> >Cheers,
> >Adrian
> >
> >--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Arturo E. Duncan"
> ><aduncan@c...> wrote:
> > > Alan,
> > >
> > > Opto-Isolation is to protect the computer from the extreme high
> > > voltages stepper drivers handle(yes, I know geckos have opto-
> > > isolators, there are other drivers that don´t).
> > >
> > > I understand your opinion with regards to breakout boards. I

myself

> > > have setup machines without them. But it makes things easier

and

> > > safer to setup. This breakout board I hope to price it the 20

to 30

> > > range, making it affordable to any budget.
> > >
> > > Thanks,
> > >
> > > Arturo Duncan
> >
> >
> >
> >
> >
> >Addresses:
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wanliker@a...,

> >timg@k...
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> >
> >OFF Topic POSTS: General Machining
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> >sister site to the CCED group, as many of the same members are

there, for

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> >
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EXCEPTIONS........

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