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From: zimmer@calvin.stanford.edu (Andrew Zimmerman)
Newsgroups: sci.electronics
Subject: Re: Good Clamping Circuit
Message-ID: <1991Mar10.093752.18289@neon.Stanford.EDU>
Date: 10 Mar 91 09:37:52 GMT
References: <1991Mar06.181426.10500@cdsmn.mn.org> <1041@eplunix.UUCP>
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In article <1041@eplunix.UUCP> raoul@eplunix.UUCP (Nico Garcia) writes:
>In article <1991Mar06.181426.10500@cdsmn.mn.org>, plate@cdsmn.mn.org (Doug Plate) writes:
>> *----------------------------------------------------------------*
>> 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.  
>
>Ummm, like this?
>                    -----------------
>                    |  |\            |  
>                    ---|- \          |
>                       |    \        |
>V1--------R1----|      |      \______|  Vout
>                |------|      /
>V2--------R2----|      |    /
>                       |  /
>                       |/
>
>Why not just use a good 5 Volt supply op-amp, with rail-to-rail capability?
>
>-- 
>			Nico Garcia
>			Designs by Geniuses for use by Idiots
>			eplunix!cirl!raoul@eddie.mit.edu

I don't see how the above circuit solves the original posters question.
I believe that Vout = (V1+V2)/2.  (No unity gain)
I don't see how it clamps, unless it makes use of the upper rail of the
op-amp supply.  

How about the following circuit instead?

     --------R1----------R1--------
     |            |  |\           |       a
    ---           ---|  \         |     --|>|-- +5
     -               |    \       |     |
                     |      \_____|_____|_____  Vout
V1------R1----|      |      /           |
              |------|    /             |  /  z
V2------R1----|      |  /               |--|<|---
                     |/                    /    |
                                               ---
                                                -


This actually is two different methods.  (a) The first just uses a diode.
(The +5 reference should be closer to 4.3 to take into account the voltage
drop on the diode.)

The second method (z) uses a 5 volt zener diode to clamp the circuit.

(I haven't done op-amps in a while, but I believe the above is correct.)


Andrew
zimmer@calvin.stanford.edu