The circuit below works fine when I connect 2n2222 or 2n3904 NPN transistor. But I don't know the reasons why anytime I add 2n3906 PNP transistor or any other pnp transistor the relay keep turning ON and OFF and making noise. And that is not how I want it because I want the comparator to feed the 2n3906 pnp transistor with small volt for the 2n3906 to able to activate the coil and turn on the normal open and then if the non inverting input is greater than the inverting input, it should be able to turn off relay once but it's not working like that and also it keep turning it on and off the relay. Please any help from you I will be highly appreciated thank you

 

That picture tells us little about your circuit, and your description is confusing.
Without a complete schematic diagram we cannot help you.

 

That picture tells us little about your circuit, and your description is confusing.
Without a complete schematic diagram we cannot help you.

Below is the circuit diagram

 

Why do you want to use the 2N3906 PNP? Your schematic shows an NPN transistor, which should work.
If you want to invert the signal, just interchange the plus and minus op amp inputs.

To eliminate relay chatter you could add some hysteresis (small amount of positive feedback) to provide a small difference between when the relay actuates and when it releases .
This can be done with a small resistor in series with the op amp plus input and a large resistor from the plus input to the op amp output. For example 100Ω in series with 100kΩ in the feedback will give about 10mV of hysteresis.

Using the 1N4148 diode as a voltage reference will give a voltage variation of about 0.3%/°C for any change in ambient temperature.
Is that acceptable for your application?
Usually better to use a stable reference voltage, such as the TL431.

 

Why do you want to use the 2N3906 PNP? Your schematic shows an NPN transistor, which should work.
If you want to invert the signal, just interchange the plus and minus op amp inputs.

To eliminate relay chatter you could add some hysteresis (small amount of positive feedback) To provide a small difference between when the relay actuates and when it releases.
This can be done with a small resistor in series with the op amp plus input and a large resistor from the plus input to the op amp output. For example 100Ω in series with 100kΩ in the feedback will give about 10mV of hysteresis.

Using the 1N4148 diode as a voltage reference will give a voltage variation of about 0.3%/°C for any change in ambient temperature.
Is that acceptable for your application?
Usually better to use a stable reference voltage, such as the TL431.

Wow, its well explained, I think I need more of this from you. Anyway, thank you

 

Added note:
To use a PNP in your circuit, the transistor emitter would need to be connected to the +12V and the collector to the relay coil with the other end of the coil grounded.
But that requires the op amp base be near +12V to turn it off.
Since the shown op amp output only goes to about +10V max, you would need additional circuitry to have the transistor base be near +12V.

What is the thermocouple output voltage?

 

I think some decoupling on the voltage reference would be a good idea, especially if you want to continue to use the diode.
As @crutschow said, a bandgap reference would be a better idea.
Depending on the quality of the 12V supply, the supply voltage may fall slightly when the relay switches on, this would make the reference voltage fall slightly, which may be one cause of the relay chattering.
A better reference would remain stable if the supply voltage changed.
Hysteresis would be a good idea too. You could put a small resistor between the transistor emitter and ground, and connect the thermocouple to there instead of ground.

 

Added note:
To use a PNP in your circuit, the transistor emitter would need to be connected to the +12V and the collector to the relay coil with the other end of the coil grounded.
But that requires the op amp base be near +12V to turn it off.
Since the shown op amp output only goes to about +10V max, you would need additional circuitry to have the transistor base be near +12V.

What is the thermocouple output voltage?

Turning the multimeter to 200k ohm it starts it counting from 0 point 0 upwards

 

Added note:
To use a PNP in your circuit, the transistor emitter would need to be connected to the +12V and the collector to the relay coil with the other end of the coil grounded.
But that requires the op amp base be near +12V to turn it off.
Since the shown op amp output only goes to about +10V max, you would need additional circuitry to have the transistor base be near +12V.

What is the thermocouple output voltage?

Do you have any diagram that demonstrates it .

 

I think 2N3906 which is a PNP type should be used as feed (emitter to 12 V and collector to relay where cathode of rectifier is connected).The other terminal of relay should be connect to GND.
NPN type should be used as open collector while PNP as feed.

 

Turning the multimeter to 200k ohm it starts it counting from 0 point 0 upwards

That measures resistance not voltage, which I asked for.

 

I think 2N3906 which is a PNP type should be used as feed (emitter to 12 V and collector to relay where the cathode of the rectifier is connected). The other terminal of the relay should be connected to GND.
NPN type should be used as open collector while PNP as feed.

It's working fine. I have connected 1k resistor from the output of the opamp to the base of the pnp transistor to the emitter connected to the ground and the collector to the minus of the relay and plus of the relay is connected to the ground But didn't use 2n3906 I rather used bc559