can someone explain me this circuit?

 

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

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Eric
"Good enough is Perfect"

PIC tutorials:
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Bill's: www.blueroomelectronics.com/

 

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...

 

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).

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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.?

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

 

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.

 

Hi
Try Google for Relay Basics

http://www.jaycar.com.au/images_uploaded/relaydrv.pdf

http://www.physics.unlv.edu/~bill/PHYS483/relay.pdf

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

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

 

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.

 

 

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.

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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?

 

Look at the attached drawing.

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

 

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

 

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.

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