[sdiy] Taming things that run to the rails...

[Harry Bissell]

some opamps phase reverse when you saturate them (or exceed the common mode range.

This can be very bad in some circumstances, you are expecting positive and you get
negative. It can latch up many circuits or cause damage (for example, in a servo
system the motor woulod run backwards, the furnace would get HOTTER).

If your opamp runs to the rail politely and you don't need the speed of recovery, its safe to ignore.

Zeners in the feedback are not a panacea for everything. (too much parasitics, etc)

H^) harry


----- Original Message -----
From: David G Dixon <dixon at mail.ubc.ca>
To: 'Justin Owen' <juzowen at gmail.com>, 'SDIY List' <synth-diy at dropmix.xs4all.nl>
Sent: Tue, 26 Jul 2011 17:40:50 -0400 (EDT)
Subject: Re: [sdiy] Taming things that run to the rails...

> Is it considered good practice to limit signals (where 
> possible) that would otherwise try and head for the rails 
> under normal operating conditions? i.e. Comparator outputs, 
> some integrator outputs perhaps?
> 
> I guess I'm referring mainly to trying to keep op amps out of 
> saturation by using zeners across the feedback path.
> 
> Good? Bad? Vital? or Don't worry about it unless it's causing 
> problems?

Generally, I let opamp comparators run to the rails, and have never had
problems.  Otherwise it never really comes up as an issue.

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-- 
Harry Bissell & Nora Abdullah 4eva