Help with op amp buffer information.

[cbiblis]

Can someone please tell me why it is necessary to use a 5v+ supply to the pin 10 of the IC1C if the idea is to get 12V+ out of the pin 8? I understand that it is termed as a buffer, but what is it actually buffing? Seems to me that i could just ground the IC1C just like the IC1B & IC1A and feed the pin 8 junction with a 12V+? The V+ is 12v+

 

[MRCecil]

Is ZD1 a 5V zener? The buffering refers to the isolation of load to the divider of ZD1 and R1. Is the design you show really supposed to give an output of IC1c of +12V? Your schematic and request have many unknowns. Further information is required for evaluation of the issues, or I have misread/misunderstood something.

 

[cbiblis]

yes the zd1 is a 5v zener. The x1 are 12v for input. When probed the pin 8 is 12v. That's why i don't understand the 5v thru the op amp to amp out 12v+. Is there other information that your require to make the analysis?

 

[MRCecil]

If the output is on the rail with the non-inverting output @ the ~5V of the zener, then something must be setup wrong on your breadboard or whatever construction you have to test the output. If what you say about X1-1 and X1-2 is true why would one put Vcc (12V) to the output of an op amp? It would appear that certain assumptions made are faulty, else why place any device expected to produce a signal across zero potential as shown between 1X-1 and 1X-2? Ensure you have the correct connections made to the correct pins. It would be more likely that the potential between 1X-1 & 1X-2 is the difference between the two of ~7V across the unknown sensor rather than 0V.

After many, many years of breadboarding, solderballing, etc, I still have brain cramps from lack of attention to detail. Good luck!

 

[MikeMl]

Can someone please tell me why it is necessary to use a 5v+ supply to the pin 10 of the IC1C if the idea is to get 12V+ out of the pin 8? I understand that it is termed as a buffer, but what is it actually buffing? Seems to me that i could just ground the IC1C just like the IC1B & IC1A and feed the pin 8 junction with a 12V+? The V+ is 12v+

The pin numbers in your posted schematic are unreadable (at least to me).

The Zener makes 5V out of 12V. The 5V is "buffered" by the voltage follower stage, so it's output is 5V +- a tiny offset. Since the voltage follower stage can source ~25mA, it supplies the bridge consisting of the thermistor and associated resistors.

Is the buffer opamp stage really needed? No.

Does it hurt anything? No.

Is the Zener the best way of producing a stable voltage to drive the bridge? No. There are much better ways.

Does the bridge voltage have to be 5V? No. It could be almost anything.

 

[cbiblis]

Thanks for the replys.

 

[cbiblis]

Here is the really application. Will this work? the op amp use to replace the 12+. I show the resistance value of the ntc sensor at the set temperature that i want to be close to.

 

[cbiblis]

Here is the supply portion.