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From: plate@cdsmn.mn.org (Doug Plate)
Newsgroups: sci.electronics
Subject: Good Clamping Circuit
Message-ID: <1991Mar06.181426.10500@cdsmn.mn.org>
Date: 6 Mar 91 18:14:26 GMT
Organization: Crosfield Design Systems, Burnsvill, MN.
Lines: 26

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I'm looking for a simple circuit that will clamp the output of a summing
amp to 5 volts.  The circuit uses an OP amp in a non-inverting
summing configuration (unity gain) with 2 inputs, either of which 
can go from 0 - 5 volts, however the  output must come up to 5 volts 
and stop (plus or minus about .009v) even if the sum of the two 
input > 5 volts.  

I've tried several different clamping techniques, but keep running
into the same two limitations.  One condition I run into is that I
can clamp the output so that if the sum of the outputs > 5v, the
output = 5v, however the problem is that a sum of exactly five volts
in gives me an output a few hundered millivolts short of the 5v output 
I need.

The second condition occurs when I clamp  so that a 5v input sum
produces a 5v ouput.  Unfortunately in this case, the clamp doesn't
kick in until my output is about 5.22v.

So basically what I'm looking for is a clamp that allows the output to 
follow the input all the way up to 5v then clamp to 5v for anything
above that.  Also, I'm trying to keep things as simple as possible.

		Doug Plate
		plate@cdsmn.mn.org