Feedback potentiometer circuit QUESTION

[fta_25]

Hi I just have a quick question for a feedback potentiometer circuit what is the purpose of the resistor R25? Please see circuit diagram below (attach)

Thx a lot
Fta_25

 

[unplebeian]

Since V+ = V- that terminal has voltage = Vout. Then you have a voltage divider type of thing with R24 and R25. Also given that the input at V+ has two inputs (two wires) I would say that R24 and R25 are providing some kind of offset voltage.

 

[fta_25]

thx a lot
fta_25

Since V+ = V- that terminal has voltage = Vout. Then you have a voltage divider type of thing with R24 and R25. Also given that the input at V+ has two inputs (two wires) I would say that R24 and R25 are providing some kind of offset voltage.

 

[berkeman]

See posts #43-45 in this tutorial thread here in the EE forum:

https://www.physicsforums.com/showthread.php?t=178516

BTW, I think R25 should have a smaller value, closer to 100kOhms. But that would depend on the potentiometer datasheet. Quiz question -- why?

 

[fta_25]

thx a lot...=)

 

[berkeman]

You're welcome, but what about the quiz question?

 

[omnikey]

hi,

i know it is an old topic but i need a little bit help. I try to made that circuit diagram, my question is where do i connect 0V for supply, and where measurements are taken. I connected yust like circuit above, and measure on pin 1 (V out), and on - 12V pin?,and with 20k pot i get measurements from 18V to min. 7V, i believe that measurements must be taken on pin 1 and ground (0V), to get full scale (i need scale from 0V-10V) but there is no ground in circuit?


need help

regards

 

[vk6kro]

Ground isn't shown, but there is one.

It is where the positive supply's negative terminal and the negative supply's positive terminal are connected.

Incidentally, R25 is present because the potentiometer can be unplugged and this would leave the non inverting input open circuited. This would produce random output, depending on the static charge on this input.

Putting R25 in makes certain that the output goes fully positive if the potentiometer is unplugged.

 

[omnikey]

thanks for reply but i still can't determine where that should be "It is where the positive supply's negative terminal and the negative supply's positive terminal are connected", could u please mark it on diagram.

thanks

 

[vk6kro]

I have shown the power supply connections and added some changes to the resistor network to give you the 0 to 10 volts. This is approximate and you will need to adjust it later.

[PLAIN]http://dl.dropbox.com/u/4222062/Opamp%20buffer.PNG

 

[omnikey]

thanks

regards,

omnikey