Re: solution for optoisolator 3.3 or 5 volt

--- In CAD_CAM_EDM_DRO@yahoogroups.com, denis <denisc1@...> wrote:

>
> Hi Tom,
> thanks for the info. I knew there was a bandwidth limitation but

didn't know

> it was that low. The stepper drives are not opto isolated. I'll try

buffering

> the PP output lines to drive the 6N136 cathodes. The separate

supplies are

> there already - totally isolated supply rails but common grounds.

Those aren't isolated supplies if they share the same ground. It's
the common ground that is the conductor for the common noise. Motor
DC has motor noise and PWM stuff on it. With the same ground on both
sides the only use of the Opto's is to do some level translation and
since the response of the opto is so poor it will kill high freq noise
ON THE SIGNAL LINE ONLY. If noise is on the signal ground (since it's
tied to the "dirty" outside ground) it just skipd right on through and
you can still have noise problems.

If you have a scope do this: Ground your scope probe over on the PC
logic side then go out to the Motor ground at the driver card and see
wha the signal looks like. Should be a straight line.....You will be
amazed at the spikes you see.





I need

> isolation on the limit switches for noise immunity, and was thinking

of using

> TIL117 - speed is probably not an issue for this application, do you

think

> that's a good choice for the limit switches?
> cheers,
> Denis

Virtually any opto will work for switch isolation. No speed required
there. Once again though if you run the same ground most of the
advantages of isolation are defeated.

In a lot of driver cards you have no choice because they use a common
ground with the logic. Most of those cards are for smaller motors and
the noise issue is not as critical. I do know that the Xylotex card
(which uses a common ground) had noise problems early on and they had
to make some circuit mods to get rid of it.

Designing a motor drive is easy (stepper or servo). Making it work
reliably and survive driving big inductive loads is not!

I admire anyone that wants to build and design their own stuff if the
objective is to learn. If you are doing it to try and save money it's
marginal over buying a ready to run BOB. Even more so for a motor
drive (unless you you have a free source of parts and your time has no
value).

>
> On Monday 07 January 2008 22:39:33 caudlet wrote:
>
> > Depending on the opto you choose and parallel port drive capability it
> > may or may not work.
> >
> > First problem is that some ports can't turn on an opto that takes 15
> /snip
> > TOM CAUDLE
> > www.CandCNC.com
>