design of temperature regulator

Thread Starter

assuc

Joined Mar 23, 2012
77
Greetings,
Could anyone help me explaining the following circuit
lcs project error.jpg

Actually i have been assigned to build a circuit with pi controller to maintain the temperature set point of 40C of an incubator. the above is proposed to be used as the first stage before pi controller.
Explanation with mathematical relations would do great.
 
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Thread Starter

assuc

Joined Mar 23, 2012
77
I have made the following pi controller design but in this the relay does not switches when temp goes above 40
Untitled (1).png
What could be the problem?? Please figure it out asap.
REFERENCEhttp://www.ecircuitcenter.com/Circui...pid/op_pid.htm
 

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bertus

Joined Apr 5, 2008
22,270
Hello,

How are the opamps powered?
It can NOT be a single supply, as some of the inputs of the opamps are grounded.

Bertus
 

Brownout

Joined Jan 10, 2012
2,390
You don't need all those amplification stages. And what is the purpose of the U2 stage? You need one stage configured as a wheatstone bridge ( as the first stage in your textbook is configured ) and a transistor switch. Maybe one stage of amplification on the front end, but probably not.
 

Thread Starter

assuc

Joined Mar 23, 2012
77
Hello,

How are the opamps powered?
It can NOT be a single supply, as some of the inputs of the opamps are grounded.

Bertus
All opamps are poered via a single 12volt variable power supply and all the grounds of opamps are common
 

Thread Starter

assuc

Joined Mar 23, 2012
77
You don't need all those amplification stages. And what is the purpose of the U2 stage? You need one stage configured as a wheatstone bridge ( as the first stage in your textbook is configured ) and a transistor switch. Maybe one stage of amplification on the front end, but probably not.
Basically its a PI controller to control the temperature(as in this case 40 C) of the incubator box. U2 is the integrator whose output is summed up with the proportional controller output
 

Brownout

Joined Jan 10, 2012
2,390
I answered that in post #7.

You need one stage configured as a wheatstone bridge ( as the first stage in your textbook is configured ) and a transistor switch. Maybe one stage of amplification on the front end, but probably not.

There are many schematics that implement this simple control method. Quickly searching "opamp wheatstone bridge" and "temperature controller" should produce a torrent of information.
 

bertus

Joined Apr 5, 2008
22,270
Hello,

As I told you before: the inputs of the opamps can NOT be put to ground on a single supply.
The opamps can NOT have any input voltages below the ground value.

You will need to compare the output of the LM35 with a reference voltage.
At 40 °C the output voltage of the LM35 will be 0.4 Volts.

Adding a hysteresis will prevent the relays from scattering.
On this page of the eBook you will find some info on this:
http://www.allaboutcircuits.com/vol_3/chpt_8/12.html

Bertus
 
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Thread Starter

assuc

Joined Mar 23, 2012
77
There are many schematics that implement this simple control method. Quickly searching "opamp wheatstone bridge" and "temperature controller" should produce a torrent of information.
if that was the only case i wouldn't have posted here, its difficult for me to choose out of tons of schematics so this was the only source to seek infromation!
 
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