Trying to make a parallel plate capacitor

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  • #51
vk6kro
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I did a calculation as per post 21 above, but with a spacing of 2.54 cm (1 inch) and an air dielectric to an outer cylinder that was grounded. I got about 12 pF.

This is imperfect because the outer cylinder has a greater surface area than the inner one, but it indicated that a 1 inch spacing would not introduce much extra capacitance. Compared with the 500 pF maximum capacitance, 12 pF doesn't matter much and may be an advantage. It would limit the frequency range of the oscillator.

Possibly a sleeve of polystyrene foam would be OK too.

Or, you could just have a 1 inch gap between the pipe with adhesive copper on it and any grounded objects, including the ground.
 
  • #52
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Is polystyrene a good insulator for electric field? I gues its just a thermal insulator...Any way i gonna try it.
Is there a way to detect a change in peak to peak voltage and obtain this change as high or low output?? For example, to detect such a change as in attachment ( 80mV change)
 

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  • #53
vk6kro
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Could you draw a picture of the waveform you have and what you want to get?

I find MS Paint good for this.
If you are using Windows XP, it is available under Start/Programs/Accessories.

Show the relative DC voltages as well.

I think polystyrene foam is an excellent insulator, although it can not cope with heat or some solvents. In this case, it does not have to deal with high voltages, just provide a non-conducting space filler.
 
  • #54
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I am using MS paint all the time. :p

When i use a astable oscillator that produces 4Khz output and connect it to a T bridge and an Op AMplifier. I tried to use an online circuit simulator http://www.falstad.com/circuit/" [Broken]
[Click Circuits on menu bar>555 timer chip > squarewave generator, chnage voltage to 9 V , add C and op Amp by right clicking the circuit"passive components and active components"]

I tried to use PSpice for simulation , but I am new to it and the simulation did not work for me.( although i created a schematic)

When i give the parameters as in figure 1 ( R1 = 10K Ohm, R2 = 160 KOHm, C = 1 nF) F is supposed to be = 4.36KHz. The waveform that is obtained at the output is shown in figure 1. For both 10pF and 11pF values. Basically they are sqaure waves.since the frequency is too high they are too crowded. If we redce the frequency one can see it.
When the C ( red marked one)= changes, the output voltage also changes accordingly.

In figure1, upper one is the circuit and the lower 2 diagrams shows the output for 10pF and 11pF. The values shown in them are the upper and lower peak values. The difference between them is the peak to peak volatge and that is waht changes when C varies.
 

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  • #55
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Hey all..

i tried another method for designing the circuit. Used an astable oscilator with the C as my test capacitor. ( producing around 6 KHz to 130 Hz )( from C = 10 pF to 500pF)
then used a frequency to voltage converter and get voltage values from 5.6 V to 0.1 V.
I am now in designing and testing the circuit.
Hope it will work.

I have another question.. I saw this in another site http://www.electronicspoint.com/capacity-voltage-converter-two-555s-t33728.html" [Broken]
a method by using 2 555 timers ( atable and monostable) . Can anyone explain the working of that one? I mean my C range is from 10pF to 500pF. So what frequecy output should have the astable and R& C for monostable??


Also Vk6kro: I need a bit theoretical explanation for this type of capacitance measurement. means this is not actually a parallel plate capacitor. But depends on the conductivity for change in C. am I right?? So the basic principle is "Capacitance due to conductance" or what??
 
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  • #56
vk6kro
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If you have a string of very short pulses and send them to a low pass filter, the output of the filter will depend on the width of the pulses or on the number of the pulses.

In this case, the number of pulses is kept constant, but the width of the pulses is varied with the capacitance.

So, this circuit works like that. The circuit should be fairly linear and it will give a greater reading for a greater capacitance.
This would be a good time to graduate to CMOS 555's though. They have many advantages and don't cost much more than ordinary 555s.

If we assume the water is a conductor, then the surface of the water touching the inside of the pipe is one plate of a capacitor and it connects through the water to the stainless steel or copper electrode at the bottom of the pipe. The other plate of the capacitor is the copper stuck to the outside of the pipe.
 
  • #57
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Heyy all.... I connected the test capacitor in place of C1 in an astable oscillator. There was very speculative and perfect chnages in frequency. Now I am trying to connect the output of the astable oscilator to monostable and then use a level detector (somethin like a schmitt trigger)

and vk6kro, i experimented both with tap water and rain water. there is no much difference, the results werre same....:D. So it only the dielectric that makes the change, i guess...

thanks guys...
 
  • #58
vk6kro
Science Advisor
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Heyy all.... I connected the test capacitor in place of C1 in an astable oscillator. There was very speculative and perfect chnages in frequency. Now I am trying to connect the output of the astable oscilator to monostable and then use a level detector (somethin like a schmitt trigger)

and vk6kro, i experimented both with tap water and rain water. there is no much difference, the results werre same....:D. So it only the dielectric that makes the change, i guess...

thanks guys...
That sounds promising.

If you have a moving coil meter, you could set it up as a 0 to 5 v voltmeter (assuming a 5 volt supply for your 555s). Then if you put this on the output of the monostable, the reading on the meter will be something between 0 and 5 v depending on the pulse width of the pulses from the monostable.

You probably want the readout to increase with depth of water. This would correspond with increased capacitance.
So, the pipe capacitance should be in the monostable, not in the astable.
In the astable, increased capacitance would result in a lower frequency and hence a lower reading.
 
  • #59
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I connected an astable oscillator ( F = 5.6KHz i.e t = 0.18ms) . The output was connected to a monistable oscilator, whose F = 10KHZ i.e t = 0.1ms). It was thenconnected to an RC low pass filter.

I need a lilttel help with next part. WHen the test Capacitor changes from 10F to 500 pF the peak voltage changes from 5.48 V to 800 mV. So how can I implement a schmitt trigger to set a HIGH when voltage drops down below 5.48 V and LOW when voltage = or greater than 5.48V.??? See attahcment
 

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  • #60
vk6kro
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Do you just want to know if there is water in the pipe at any level?

That can be achieved a lot easier than with this elaborate capacitor arrangement. That was to give you a continuous readout of how deep the water was in the pipe.

All you would need is a pair of contacts in the water to detect conductance through the water.
 

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