please help me with how this circuit works?

pravvish;703459]hi i am a newbie .. i would like to know how this circuit works ?? please help me .

i want to know wheter we have to use a PTC thermistor or NTC thermistor as the variable R1 resistor .and also i cant understand how the second section of the ph meter works?? can any one help me?

as far as i understood the second stage is a differential amplifier with non inverting input of .35 volts. but shouldnt it be .700v ??? so as to display PH in the range of 0-1400mv?? (0-14 PH ). please do help me and forgive me if this has been asked earlier .

thank you

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pravvish said:

thank you for replying .

i dont get how the second stage really works?? the ist stage works as a non inverting amplifier with gain of 1.7 .

but the second stage is a bit confusing . as mentioned in the post above ... what will be the offset at the non inverting input of the opamp??

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ericgibbs said:

hi,
The gain of the 2nd stage is -G = 100k/100K = 1
and the NInv gain +G = 1+ 100K/100K =2

So the +0.35V on the NI input gives a fixed voltage on the output of +0.7V

The Ph signal from the first stage changes the level of the +0.7V

Have you details of the output from the Ph sensor, so that we can complete the calculations.?

The 2.2uF cap across the 2nd stage acts a signal integrator LPF.

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pravvish said:

ok .. the ph probe is a standard Ag-AgCl probe with 59.16mv/ph . and the ist stage gives a gain of around 1.7 (since this varies with temperature i cant be exact).

and why do we need an integrator ?? and only thing that comes out is the ph value in DC . so what is its use?

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pravvish said:

yeah ... the out put voltage for 0 -14 ph is 0 -1400mv .


another problem that i see is that i cant get a 100k PTC thermistor.

i got a 100k NTC thermistor. what can i do about that??

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pravvish said:

and what about the 3500ppm/c thing??? dosnt that mean its a thermistor??

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pravvish said:

**broken link removed**

that is the link

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pravvish said:

can you give me a link ???? like farnell.com ???

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pravvish said:

can you spot the correct one ??? i am asking for a help cause i cant find the correct one ... i first thought it would be a thermistor...

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pravvish said:

i googled it too ... i got random sites ....

that why i asked to spot the correct resistor from a site like farnell.com ... only if you have the time for it..


as far as i have learnt thermistors are temperature varying (compensating) resistors .. so why cant we use them??

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