"Thermostat using ic741"-circuit explaination please?

[ace.neerav]

**broken link removed**

can someone explain me this circuit?

 

[ericgibbs]

hi ace,
Which part of the circuit do you have a problem understanding.?

 

[ace.neerav]

here is what i understand...
there are 2 voltage divider ckts. the one with vr1 and vr3 supply the reference voltage to the inverting terminal. the thermistor provides the non-inverting trminal voltage. when the thermistor drop is less that that accross vr1 and vr3 the ompams o/p is 0 (and not -Vsat as 4 is connected to ground) when the thermistor detects enough temperature to develop a drop higher than that accross vr1 and vr3, the opamp o/p turns +Vsat.

now what i dont understand is this point onwards...
what does f/b vr2 actually do (haven't read the opamp chapter yet).
which mode is the transistor in? cb? or cc? and what does it do?
what is a relay and what does it do?

i guess the in4148 is the led which operates in reverse bias right?

so, any insight...

 

[Nigel Goodwin]

The opamp acts as a comparator, the transistor is just a switch (common emitter), and the diode is a protection device across the relay (without it the transistor will be destroyed the first time it switches OFF).

 

[ericgibbs]

hi,
Consider what would happen if the Vset and Vntc were at the same value, the relay would 'chatter' ON/OFF.
What VR2 does is to alter the Vntc voltage.

example: assume that the relay is de-energised, this means that pin6 would be high at +V [9-24],
so Vntc would be increased by the value set on VR2.

This means that the Thermistors resistance will have to fall [due to a rising tempr] futher in order to reduce the Vntc to less than the Vset, IF the VR2 voltage had not been present.

As the Vntc becomes lower than Vset due a rise in tempr, pin 6 goes low to 0V.Which energises the relay.
Also the +V voltage due to VR2 is removed from the Vntc voltage.[pin6 would be 0V]

This means the tempr must fall to a far lower level before the comparator can switch over the other way.

example: relay energises at 25Cdeg, Fan ON,,, the tempr would have to fall to 20Cdeg before the Fan switched OFF....
So there is 5Cdeg of hystereis.. when the Tempr reaches 20Cdeg,

Do you follow this.?

 

[ace.neerav]

hello friends, thanks for you efforts. the working is not exactly clear in my mind.

i dont quite understand what the transistor is doing there. is it in cut of or sat or active region initially? and what happenes when opamp o/p goes +Vsat to the Base collector junction?

i would also have to do some reading on relays. i have no clue what they are. can u provide me a link?

basically we got this project made from someone else and now its explaination time in college. in our project, based on the same circuit, an led glows when we provide heat to the thermistor. now i am not sure if the led is IN 4148.

this is the difficulty we are facing for not doing it ourselves.

 

[ericgibbs]

hello friends, thanks for you efforts. the working is not exactly clear in my mind.

i dont quite understand what the transistor is doing there. is it in cut of or sat or active region initially? and what happenes when opamp o/p goes +Vsat to the Base collector junction?

Its a pnp transistor, When pin 6 is High +V its OFF, when pin 6 is low 0V its ON

i would also have to do some reading on relays. i have no clue what they are. can u provide me a link?

basically we got this project made from someone else and now its explaination time in college. in our project, based on the same circuit, an led glows when we provide heat to the thermistor. now i am not sure if the led is IN 4148.

this is the difficulty we are facing for not doing it ourselves.

Hi
Try Google for Relay Basics

https://www.electro-tech-online.com/custompdfs/2008/04/relaydrv.pdf

https://www.electro-tech-online.com/custompdfs/2008/04/relay.pdf

https://www.the12volt.com/relays/relays.asp

 

[ace.neerav]

can some explain the ckt operation as text books would to a dumb student.

i know i am asking a lot, but i am in the process of clearing my concepts.

 

[crutschow]

i dont quite understand what the transistor is doing there.

The transistor operates as a switch to provide sufficient current to operate the relay. The op amp doesn't have enough output drive to power most relays.

is it in cut of or sat or active region initially? and what happenes when opamp o/p goes +Vsat to the Base collector junction?

When the op amp output is high, the transistor is off. When the op amp output is low the transistor is on

now i am not sure if the led is IN 4148.

The 1N4148 is a regular diode used to suppress the relay inductive spike. There is no LED shown in the schematic. You could add one in parallel with the relay coil, if desired, along with the 1N4148. (Place cathode to ground and add the appropriate resistor in series to limit the LED current)

 

[aljamri]

I have seen alot of simillar circuit on web, all of them used an NPN to control the relay, for that circuit it will be easy to understand the operation.

 

[ace.neerav]

alright! i did some reading on relays and have understood majority of the circuit. just a few questions...

1.when the opamp o/p is 0 what is the state of the be jn and the bc jn and why?
2. when the opamp o/p is high what is the state of the be jn and bc jn and why?

 

[ericgibbs]

alright! i did some reading on relays and have understood majority of the circuit. just a few questions...

1.when the opamp o/p is 0 what is the state of the be jn and the bc jn and why?
Vbe=0.7v, due to the current flowinhg thru R3 and R4 and the transistor base/emitter junction.

2. when the opamp o/p is high what is the state of the be jn and bc jn and why?
Vbe=0, there is no current in R3/4 so NO Vbe

Look at the attached drawing.

 

[ace.neerav]

so u mean the transistor is operated in active region right?

 

[ace.neerav]

cause i read in relay articles that the transistors is to be operated in saturation region. and that means ic would be in opposite dirction. and that would also mean when the opamp o/p is 0 and transitor is on, the relay should be energised but would not cause the forward biased diode would nearly short the relay. things are getting confusing.

 

[Nigel Goodwin]

cause i read in relay articles that the transistors is to be operated in saturation region. and that means ic would be in opposite dirction. and that would also mean when the opamp o/p is 0 and transitor is on, the relay should be energised but would not cause the forward biased diode would nearly short the relay. things are getting confusing.

Read my first post, the transistor acts as a switch, not as a linear device.

The diode is also reverse biased, to protect the transistor from back EMF when the relay field collapses.

 

[ace.neerav]

i still dont understand the feedback concept?

 

[ace.neerav]

OMG! there is a huge confusion. when does the opamp turn on? when the temperature rises or when the temperature falls?

 

[ericgibbs]

OMG! there is a huge confusion. when does the opamp turn on? when the temperature rises or when the temperature falls?

hi ace,
I have already previously explained in detail and included a drawing which answers these questions.

May I ask are you studying electronics or is this just to explain the circuit diagram you posted.?

Are you planning to build this circuit.?

 

[ace.neerav]

hi. i am in the nd of second year of electronics and telecommunication engineering. i asked the above question because in our project (which we have already got made from some one and which now only requires to be explained to our professors) when we touch the thermistor the LED would glow. and that would happend only if the opamp o/p tunred high when the the voltage drop accross the thermistor increases and that would also mean that the thermistor has +temp coefficent.

if ur talking about the feed back then as i have not yet come up to the topic of opamp in my self study i m not quite sure what hysterisis is and how the f/b works

 

[ericgibbs]

(which we have already got made from some one and which now only requires to be explained to our professors)
when we touch the thermistor the LED would glow.
Your circuit does not show an LED???

and that would happend only if the opamp o/p tunred high when the the

voltage drop accross the thermistor increases and that would also mean that the thermistor has +temp coefficent.
Your circuit is showing a NTC thermistor, reducing resistance with increase inTempr

if ur talking about the feed back then as i have not yet come up to the topic of opamp in my self study
There are basically two types of feedback:
negative, this is where a portion of the output signal is fed back to the input in antiphase to the input.

positive this is where a portion of the output signal is fed back to the input in phase to the input.

i m not quite sure what hysterisis is
I would suggest as a starting point that you look [Google] for hysteresis.
Look at magnetic hysteresis also schmitt inputs.
and how the f/b works

hi,
Google for the topics and then come back and ask specific questions.