Tooth impedance

[grr]

Hi friends, I need your help! I am trying to measure tooth impedance, I thought to make a wien bridge oscillator and then to balance it and to connect it through a resistor to a tooth which the equivalent circuit is a R-C parallel. I found the equations but I don't know how I can calculate the phase shift, is any appropriate circuit for that?
Of course any other ideas how I can calculate the R-C of the tooth welcome.
Also I thought to use an apex locator but I can't find any circuit for that, Does anyone has any simple circuit for apex locator?
Thank you

 

[panic mode]

why not derive it? it is just a bit of complex number manipulation. for example check
https://en.wikipedia.org/wiki/Wien_bridge

let's write

Zx=Rx||Cx=Rx - j/(wC)

Z2=R2- j/(wC2)

Z3=R3

Z4=R4

in ballance, product of opposite branches is same so

Zx*Z4=Z2*Z3 (this is for circuit as shown on wiki page)

now plug it all in and you will get equation of two complex values.
complex numbers are same only when both real and imaginary component of their values are equal.
so that one equation is actually two (one real and one imaginary).
from real one you get Rx, from imaginary you get Cx.

 

[Boncuk]

Post removed for obvious misunderstanding.

Boncuk

 

[grr]

Thank you panic mode, I did that already and I got the solution that R3=2R4 and when is balanced w=1/RC
and then the output of the opamp is the input in the tooth circuit (R2 //C1) with an R1 in series and I got
Vo(s) = Vi(s)*R2/(R2+R1+C1*R1*R2*s). I got stuck there....any idea?

 

[Winterstone]

Thank you panic mode, I did that already and I got the solution that R3=2R4 and when is balanced w=1/RC
and then the output of the opamp is the input in the tooth circuit (R2 //C1) with an R1 in series and I got
Vo(s) = Vi(s)*R2/(R2+R1+C1*R1*R2*s). I got stuck there....any idea?

Hi grr,

I assume that you know how a WIEN oscillator looks like.
If the opamp gain is G=1+ R3/R4=3 with R3=2R4 the WIEN bridge is tuned if both resistors and both capacitors in the frequency dependent path are equal.
Thus, if the parallel combination R||C is replaced by a tooth, both elements (R and C) of the series connection have to be tuned separately until the circuit oscillates with a quasi-sinusoidal waveshape.
Is this your idea?
And what now is the problem?

Or do you intend to use the bridge without building an oscillator?

 

[panic mode]

there is no Vo or Vi. also there is no w=1/RC.... (what is R and what is C? where do they come from?)
you seem to borrow from wien oscillator... Huston, we have a problem...


WIEN BRIDGE is a type of bridge circuit used to measure impedance.
WIEN OSCILLATOR is a type of sine wave oscillator (used to produce AC signal of known frequency).

They are two DIFFERENT circuits with DIFFERENT equations - even if they share few things like name and some of components.
in fact there are not many circuits that don't use R and C.

IF you want to MEASURE impedance (of whatever sample), you would want to use the first circuit (bridge), not the oscillator.

actually one could use both: oscillator as source of AC, then bridge as test circuit. but what is it that you want?
you seem to want to MEASURE so...


equation for Wien bridge in balance was simply Zx*Z4=Z2*Z3, which need to be expressed in form

LHS=RHS

(left and right hand side) where LHS and RHS are complex numbers or more precisely
LHS=A+jB
RHS=C+jD

since circuit is in balance, the two complex numbers are equal:

A+jB = C+ jD

this is only true if A=C and jB=jD or just B=D

somewhere in that 'A' you will have Rx, so just rewrite that equation into Rx=....
somewhere in that 'C' you will have Cx, so just rewrite that equation into Cx=....

and this is what you seem to want.

 

[Winterstone]

Hi Grr,

here comes another circuit (bridge) which better fits to your needs.
I hope the following large link works:


https://www.google.de/imgres?imgurl...OoCc3ktQb626z4CA&sqi=2&ved=0CFQQ9QEwAA&dur=22


You can supply the bridge with any suitable frequency. The R1-C1 parallel connection will be replaced by the tooth.
Then, the bridge is tuned by varying succesively R3 and C3 until the bridge is tuned (zero volts in the middle path). Fixed values for R2 and C2.
Then, the unknown (tooth) values are:

R1=R2C3/C2 and C1=R3C2/R2

 

[Winterstone]

A+jB = C+ jD

this is only true if A=C and jB=jD or just B=D

somewhere in that 'A' you will have Rx, so just rewrite that equation into Rx=....
somewhere in that 'C' you will have Cx, so just rewrite that equation into Cx=....


Hi panicmode,

I am afraid - after rewriting the equations as proposed by you - it does not work. The WIEN bridge alone is not suited to perform impedance measurements.
Reason: The tuning condition (equilibrium) is frequency-dependent.

 

[panic mode]

The WIEN bridge alone is not suited to perform impedance measurements.
Reason: The tuning condition (equilibrium) is frequency-dependent.

so????

AC source is fixed frequency (60Hz in north america and 50Hz in rest of the world). It is pretty fixed and stable you know, in fact it is often used to generate time reference for other circuits such as clocks.

If you use function generator as an AC source of higher frequency, you read the frequency of the display of the unit. you are NOT going to take speaker wires from JukeBox playing 'smoke on the water' just to use them as AC source...

 

[Winterstone]

so????
AC source is fixed frequency (60Hz in north america and 50Hz in rest of the world). It is pretty fixed and stable you know, in fact it is often used to generate time reference for other circuits such as clocks.
If you use function generator as an AC source of higher frequency, you read the frequency of the display of the unit. you are NOT going to take speaker wires from JukeBox playing 'smoke on the water' just to use them as AC source...

Sorry panic mode - did I say something concerning frequency stability?
With respect to your post#6 it would be interesting to know about the tuning procedure - in case the WIEN bridge is used for impedance measurement.
As mentioned - for my opinion it does not work, unless you show me how...
In post#6 you did not come to an end ... instead you left us with the advice "so just rewrite that equation into Rx=...."

Explanation: The bridge is tuned when the resistive (real) as well as the capacitive (imaginary) parts are matched - as mentioned by you in post#6. No doubt about it.
However - this applies to one specific frequency only, which depends on the matching conditions. That means: you have to vary THREE parametrs in sequence: R and C and f.
Or tell us your equations something else?
Regards
W.

 

[panic mode]

hello Winterstone,

before this goes any further, id like to apologize for being being harsh, wasn't my intention.

i don't promote or defend any particular bridge not i get royalties from their use.
to me they are more of a curiosity or academic subject. can't say i've ever seen one outside school labs.

that is why i did not derive equations, because it sounds suspiciously like homework in disguise.
and that is why i stayed with choice of bridge OP mentioned instead of suggesting alternatives.

Sorry panic mode - did I say something concerning frequency stability?

well... here are your words:

The WIEN bridge alone is not suited to perform impedance measurements.
Reason: The tuning condition (equilibrium) is frequency-dependent.

does not work because it is frequency dependant. this only makes sense if frequency varies or am i missing something?

That means: you have to vary THREE parametrs in sequence: R and C and f.

why would you have to vary frequency? frequency can be fixed. you just need to know it's value for calculations.
you don't keep on changing it while using bridge. the only components that need to be adjusted to bring wien bridge into equilibrium are two potentiometers.

granted, even that process is not user friendly by today's standards but neither they had many capacitors kicking around in those days.

 

[Winterstone]

Hello panic mode,

before this goes any further, id like to apologize for being being harsh, wasn't my intention.

No problem - don't mention it.

i don't promote or defend any particular bridge not i get royalties from their use.
to me they are more of a curiosity or academic subject. can't say i've ever seen one outside school labs.

Nevertheless, as it was proposed by the questioner, I think we should comment it.

that is why i did not derive equations, because it sounds suspiciously like homework in disguise.
and that is why i stayed with choice of bridge OP mentioned instead of suggesting alternatives.


OK, instead deriving equations by ourselves we can use wikipedia. Unfortunately, a detailed treatment of the bridge under discussion in wiki is only in german. However, the formulas for equilibrium are the same. Look on pages 2 and three of the attachement.

As far as the frequency is concerned there was certainly a misunderstanding. By speaking on frequency-dependence, of course, I mean dependence of the measurement result from the selected frequency - not the problem of frequency uncertainties. The formulas in the attached document show what I mean.

My summary in short:
* To determine an Rx-Cx parallel combination a capacitive ac bridge (similar to the one under discussion) can be used if the frequency-dependent part of the bridge is also an R-C parallel combination. In case of a series R-C combination (as in the circuit under discussion) the formulas for equilibrium show that Rx depends on Cx and vice versa.
* Only if the valus of Rx is very large (small loss angle) there are approximative formulas which can be used. But this is an approximation only.
* However, the ac bridge containing a series R-C circuit can be used to determine the values of the device under test (tooth) in form of series R-C equivalent values. In this case, the series elements have to be converted into an equivalent parallel combination (as desired by the questioner).
However, this is not recommended.
* Why not use a bridge (as I have proposed earlier), which is well suited for the purpose?
W.

 

[panic mode]

Deutch ist kein Problem...

deriving expressions for Rx and Cx is no problem either - only algebra is needed and steps are very straight forward.

about practical side of the bridge - it is not simple plug it in and read the value on display.
i didn't dwell on it but one possible argument against it could be that variable capacitor may be a limitation. potentiometers can be obtained easily in various ranges but variable capacitors have low values (pF range). based on what i've come across so far, the biggest ones are in some 500pF range. of course one can use different values for other components to compensate but that means even more knobs to treak (selector switches). just a thought...

 

[Winterstone]

What surprises me is the following:
We are arrived now at post#13 - and the latest response from the questioner was with post#4.
Perhaps he considers his problem as solved? Or he isn't interested anymore?
W.

 

[grr]

Thank you to everyone for the help! Obviously from what I understood it is not a very good idea to use this bridge for that. Could you please tell me any other circuit which I can use to measure the impedance of a tooth? I was reading in the net about the apex locators, maybe this is an idea, does anyone has any circuit diagram for that also?
All ideas are welcome!
Thank you all again.

 

[Winterstone]

Thank you to everyone for the help! Obviously from what I understood it is not a very good idea to use this bridge for that. Could you please tell me any other circuit which I can use to measure the impedance of a tooth? I was reading in the net about the apex locators, maybe this is an idea, does anyone has any circuit diagram for that also?
All ideas are welcome!
Thank you all again.

In post#7 I have proposed another bridge circuit that was developed for the purpose of impedance measurements.
By the way: What are the approximate values you are expecting for the equivalent RC circuit?

 

[grr]

Hi Winterstone, thank you for your replies! to be honest I don't know what values I must except. I will try your circuit and probably I will use an oscillator too.
Do you know if apex locator work this way?