how to design a thermostat without Microcontroller?

1. Re: how to design a thermostat without Microcontroller?

Originally Posted by c_mitra
You still need to work on the span control (the hysteresis range) and the temp setting.

In the present case, they are strongly interacting. You should be able to set the actual temp and the hysteresis control independently.

That will involve only a couple of resistors and one more potentiometer.

You are using film capacitors but for this application, ceramic capacitors will be somewhat better.

See the application note for LM35 for more ideas.
How to do span control?

2. Re: how to design a thermostat without Microcontroller?

How to do span control?
Great to know it's working! I'm sure all contributers will be happy to read this.

My advice - check the operations / play with this circuit until you know exactly how it operates / behaves for some days now.

Only AFTER that let's talk about supposed enhancements to the functionality.

3. Re: how to design a thermostat without Microcontroller?

How to do span control?
This should control the amount of hysteresis; in other words turn on and turn off points.

The difference between the turn on and turn off points is called the hysteresis.

It should turn on at temp t1 and turn off at temp t2. The difference between t2 and t1 is the one we call span or hysteresis.

The difference should be about 1C or about 0.5C- in terms of voltage it turns about 10-5 mv.

This voltage is derived from the output of the opamp. This has no feedback and is simply working like a comparator.

This particular one cannot give 5V output if the supply voltage is 5v; we take 3.5v as the max output voltage when the turn on is in effect.

using a voltage divider, and also a small value potentiometer, you need to scale this voltage to a 0-5mv (or 0-10mv; as the case may be). The potentiometer shall provide the span control. A trimpot is ideal for this purpose because the user does need to change this regularly.

You already have this but the resistor values are wrong. A trimpot is also useful.

R1 and RV1 work as a voltage divider and the voltage at the wiper should be 370mv (the set point or turn on point). Assume the input of the opamp does not take any current the voltage at the other end of R3 is also 370mv.

If the output of the temp sensor falls below 370mv, the output (of the opamp) goes high. Now some current flows through R4, R3 and RV1. This should now increase by 5mV (or 10mV; as the case may be).

If you can figure out yourself, good. Else come back as often as you like.

4. Re: how to design a thermostat without Microcontroller?

Originally Posted by c_mitra
This should control the amount of hysteresis; in other words turn on and turn off points.

The difference between the turn on and turn off points is called the hysteresis.

It should turn on at temp t1 and turn off at temp t2. The difference between t2 and t1 is the one we call span or hysteresis.

The difference should be about 1C or about 0.5C- in terms of voltage it turns about 10-5 mv.

This voltage is derived from the output of the opamp. This has no feedback and is simply working like a comparator.

This particular one cannot give 5V output if the supply voltage is 5v; we take 3.5v as the max output voltage when the turn on is in effect.

using a voltage divider, and also a small value potentiometer, you need to scale this voltage to a 0-5mv (or 0-10mv; as the case may be). The potentiometer shall provide the span control. A trimpot is ideal for this purpose because the user does need to change this regularly.

You already have this but the resistor values are wrong. A trimpot is also useful.

R1 and RV1 work as a voltage divider and the voltage at the wiper should be 370mv (the set point or turn on point). Assume the input of the opamp does not take any current the voltage at the other end of R3 is also 370mv.

If the output of the temp sensor falls below 370mv, the output (of the opamp) goes high. Now some current flows through R4, R3 and RV1. This should now increase by 5mV (or 10mV; as the case may be).

If you can figure out yourself, good. Else come back as often as you like.
LOL
the discussions are focused on hystersis for 10 days and now you find it out?

the probelm has been fixed and the feedback network provides about 1C HYSTERSIS. (330K/1K). the thermostat is now ON and keeps the temperature bteween 37-38C accurately.moreover the relay chattering has been removed.