Re: Hysteresis needed in 741 Op-Amp circuit

From: Terry Pinnell (terrypinDELETE_at_THESEdial.pipex.com)
Date: 06/10/04 

Date: Thu, 10 Jun 2004 13:01:31 +0100

"Robert C Monsen" <rcsurname@comcast.net> wrote:

<Snipped excellent explanation>

>Now, for your heater circuit, you are varying the + input using the
>thermistor, rather than the -. Thus, by putting a resistor from 6 to
>3, and another one from 3 to where the old 3 was connected, at the
>node connecting the 10k and the thermistor, you'll add some
>hysteresis.
>
>Adding it to 2 won't be good, because that means that when the output
>goes low, the negative input will go lower, meaning the + will have to
>go down even further, thus turning it back on, etc... Not good. Also,
>you really want your opamp acting like a comparator, not an opamp, so
>negative feedback is not a good thing.
>
>I'd use a 330k potentiometer between pin 6 and the junction of the 10k
>and the thermistor in your diagram (the node that used to be attached
>to pin 3). Then, attach pin 3 to the wiper of the pot, and set it for
>the proper hysteresis.
>
> 12V
> +------------------------------+
> | |
> | |
> .-. |
> 10k | | |
> | | |
> '-' 330k |
> | ___ |
> +----|___|--------+ |
> | A | |
> | | | |
> | | |\ | 3.3k |
> | +----|+\ | ___ |<
> | | >--+--|___|---|
> .--------+--. Vref-|-/ |\
> | | |/ |
> | Sensor | .--------.
> | 10k | | |
> | | | Relay |
> '--------+--' | Etc |
> | | |
> === | |
> GND '--------'
>created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

I think you need to reverse the +ve and -ve input connections for that
to work in CF's single polarity supply, as shown here:

http://www.terrypin.dial.pipex.com/Images/741Hysteresis.gif 

-- 
Terry Pinnell
Hobbyist, West Sussex, UK
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