[sdiy] The annual I/O impedance/protection thread! (revised)

[Harry Bissell]

imho, if you want to protect just within your +/-12V system, you don't need zeners.  If you consider patching to a +/-15V system, you
might need more protection.

The big problem is the non-inverting input, Plugging a 15V input there will cause trouble, maybe blow the opamp. 

I'd suggest this... use only inverting inputs (which co-incidently can function as summing amplifiers) Use back to back shottky
diodes between the inverting and non-inverting inputs. As long as the input signal does not exceed what the feedback resistor
can supply (to keep the inverting input at 0V, everything is OK.  If you DO overdrive the input, the shottky diodes clamp to .3V
(or so) away from ground. You have a 100K input resistor, so your voltage could go WAY up.

For an non-inverting stage, use two inverting stages in series.  Make those stages both SUMMING amplifiers and you can get both
inverting and non-inverting and summing all at the same time... You can even use some (summing) input resistors with series caps
to have AC coupling as well (you still need the 1M pull-down to ground at the input jacks...

Now I'll think about you output case...

H^) harry


----- Original Message -----
From: Justin Owen <juzowen at gmail.com>
To: Harry Bissell <harrybissell at wowway.com>
Cc: SDIY List <synth-diy at dropmix.xs4all.nl>
Sent: Wed, 13 Jul 2011 14:32:46 -0400 (EDT)
Subject: Re: [sdiy] The annual I/O impedance/protection thread! (revised)

Thanks Harry - that's awesome.

You mentioned in one of your emails yesterday that you didn't think the Zeners were necessary. Is that for all of these inputs, outputs and summers - or have I misinterpreted that?

It does seem like by the time you protect them and sort out any impedance adjustments that causes then you're already on your way to just wearing the cost of a repair.

Anyone else got any thoughts on that?

Can't say I see many other people using them - but then I don't see many people using PTCs on their power supply - and I'm completely converted on that one.

Latest changes made and uploaded if anyone else is interested...

http://www.sdiy.org/juz/euro_input_03.pdf
http://www.sdiy.org/juz/euro_output_03.pdf
http://www.sdiy.org/juz/euro_summing_03.pdf

J


-- 
Harry Bissell & Nora Abdullah 4eva