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fluctuating temperature control?

hello.. pls manage with me as i am weak in english.. the controller i am discussing below is theoretical and the values i am using is completely ideal and not getting into complete circuit detail but focusing on just the function of them..
suppose i build a simple temperature controller which would just work as a simple on/off switch using any temperature sensor attached to a buffer circuit... a relay is connected to the buffer and the relay performs the switching action at a temperature set by a potentiometer in the sensor circuit..

suppose a fan is switched on when temperature reaches 60degree celcius and the place starts cooling down to 59.9 (+/- some percent) d celcius...the sensor sees the change and switches off the fan even at .1 degree change...and when the temp rise by .1d it would switch on again...(just my theory..not sure)

question: how to make the fan be switched on until the temp drops till 55d celcius(or any other value)? (or some time gap like 15mins)?

thanks for bearing my long question but i wanted to clear myself... as i am a little new in electronics and want to start by making such simple things

thanks

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Re-design7 years ago
A thermostatic switch usually has two switches. One controls when the fan goes on and one controls when the fan goes off.

That would be easy to do with two temp switches and a relay.  The on switch trips the relay and it locks on.  The off switch is in series with the relay coil.  When the temp drops enough the coil is turned off and the relay opens.

I could maybe draw you a diagram later in the week if you don't fully understand how to implement it.
hussainb (author)  Re-design7 years ago
humm right...i got the concept now...as i have said i am new in elect so such get around ideas dont come to me easily but thanks to you i got the point...it wouldnt be difficult to develop the circuit now as i have got the point....but if unsuccessful i may ask you...dont worry most probably i wont be disturbing you with it...but just in case..and thanks again
rickharris7 years ago
Your best bet would be to turn to a microcontroller. One of the problems with heating systems is hysteresis - when the temperature is rising - as it gets to the required temperature you turn off the heat.

BUT there is already heat in the system so the temperature over shoots,

The same happens when cooling - you turn on the heat but it takes time for it to have an effect. This is generally solved by the use of a PID controller

As the temperature approaches the target the heat input is turned down so that the heating effect is slower close to the target temperature. In this way very close control may be acheived.
You can virtually eliminate hysteresis effects by careful placement of the sensor relative to the heater.
hussainb (author)  steveastrouk7 years ago
thakns for you have given me a point to consider for proper control....will manage the placement
hussainb (author)  rickharris7 years ago
thanks sir for your detailed answer but, for me, using a microcontroller would be somewhat difficult as i am not so experienced in electronics so i would like to go with analog circuits at the moment... but i also got the digital side view of the controller...thanks for it and the concept will surely be helpful for me furthur.... wished they gave 2 best answer select options...