hi everyone,

I am building a PI temperature controller and I am having some problems tunning it. In my design both the proportional term and integral term are negative before they are fed into an inverting summer, which gives me a positive output. The problem I am having is that the integral terms time constant is perfect on the way towards the desired temperature. But once it gets to that temperature, it swings to the positive rail and when the temperature drops , it takes to long to swing back negative, and the temperature drops to much. I have been playing with the time constant with the intergrator but I can only get so close. I am using a 741 opamp (spec sheethttp://www.national.com/mpf/LM/LM741.html#Overview ) for the integrator and tried grounding the positive rail, but I get a weird offset of 0.2 volts that is to big to be an expectable 0. I also have been messing around with a 324 opamp (spec sheet: http://www.national.com/mpf/LM/LM324.html#Overview ) but have been unsuccessful.

So my question is, how can I get the integrators output to be within the negative rail and 0 volts using either the 741 or the 324 opamp?

 

Without a schematic, much is guesswork. However, pitch the 741. That is currently the lowest-performing op- amp available (unless some 709's turn up). It's operation is only characterized for +/- 15 volt supplies.

Your 324 might work, but we need that schematic.

Meanwhile, why not use a modern op amp that is built for single supply operation? One example - http://cds.linear.com/docs/Datasheet/1006fa.pdf