please help me with how this circuit works?

[pravvish]

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 ) .

thank you

 

[ericgibbs]

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

hi,
The R1 says +3500ppm/C so its a PTC.

The second stage is an inverting amp with a dc offset using the non inverting
input.

Whats the value of R9. EDIT: I see it 100K...

 

[tresca]

The topology of the second stage looks like a LPF filter as well. I could be wrong though.

But if thats the case, why ? Noise filter ?

 

[pravvish]

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??

 

[ericgibbs]

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??

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.

 

[pravvish]

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.

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?

 

[ericgibbs]

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?

Normally the integrator cap is added to reduce/eliminate any noise in the circuit.
As the required system is only slow acting, a large cap reduces the effective bandwidth of the system.

Does the application sheet give output voltage ranges for known Ph input values.?

 

[pravvish]

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??

 

[ericgibbs]

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??

hi,
This is a rough plot from LTSPice of your circuit.

Where does it say its a thermistor, I would use a 100K metal film resistor, choose one with a positive temperature coeff.

 

[pravvish]

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

 

[ericgibbs]

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

Can you post a link to the application sheet showing the circuit that you are using.?

 

[pravvish]

**broken link removed**

that is the link

 

[ericgibbs]

**broken link removed**

that is the link

Ill have a look, please look at this: **broken link removed**

 

[ericgibbs]

Looked at the datasheet:

As I suspected its not a thermistor, look at the last post for a suitable PTC resistor.

 

[pravvish]

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

 

[ericgibbs]

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

This is the link.?
**broken link removed**

 

[pravvish]

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...

 

[ericgibbs]

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...

I just Googled 100K 3500ppm/C resistor.............

100K 3500 ppm/C resistor - Google Search

 

[pravvish]

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??

 

[ericgibbs]

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??

You require a 'linear' change of resistance with respect to temperature of 3500 parts per million per degree Centigrade,
generally thermistors are not linear.

I have tried Farnell for you with no success.