Construction of capacitor

  • #1
2
0

Main Question or Discussion Point

Hello,

I have made a capacitor with two aluminium foils separated with paper dielectric with,

dielectric constant (K) = 3.8 for paper material
paper thickness, D = 0.1mm = 1e-4 m
plate or foil area, A = (12.1 x 8.3)cm = 0.010043 m
air permitivity, Eo = 8.85e-12 F/m

Calculated capacitance = (K Eo A)/d = 3.3 nano Farad

When I measured the capacitance i got,

Measured capacitance = 125 micro Farad

Can any body explain why this huge error is existing? I have tried with different foil areas but could not find the reason of error.

I m looking forward, Cheers.
 

Answers and Replies

  • #2
1,168
210
Hello,

I have made a capacitor with two aluminium foils separated with paper dielectric with,

dielectric constant (K) = 3.8 for paper material
paper thickness, D = 0.1mm = 1e-4 m
plate or foil area, A = (12.1 x 8.3)cm = 0.010043 m
air permitivity, Eo = 8.85e-12 F/m

Calculated capacitance = (K Eo A)/d = 3.3 nano Farad

When I measured the capacitance i got,

Measured capacitance = 125 micro Farad

Can any body explain why this huge error is existing? I have tried with different foil areas but could not find the reason of error.

I m looking forward, Cheers.
!
It's not just huge error, It's a huge value of capacity for described capacitor. My guess is you made a serious error in measurement procedure, readings or misuse of the instruments .
 
  • #3
berkeman
Mentor
56,816
6,784
For one thing, this is incorrect: "plate or foil area, A = (12.1 x 8.3)cm = 0.010043 m" Convert the two lengths to meters before multiplying, and your final area units need to be m^2.

Also, how did you measure the capacitance? What type of meter are you using? What do you measure for the resistance between the plates? How are you holding the plate stack together? Can you post a picture?
 
  • #4
Svein
Science Advisor
Insights Author
2,025
649
Hello,

I have made a capacitor with two aluminium foils separated with paper dielectric with,

dielectric constant (K) = 3.8 for paper material
paper thickness, D = 0.1mm = 1e-4 m
plate or foil area, A = (12.1 x 8.3)cm = 0.010043 m
air permitivity, Eo = 8.85e-12 F/m

Calculated capacitance = (K Eo A)/d = 3.3 nano Farad

When I measured the capacitance i got,

Measured capacitance = 125 micro Farad

Can any body explain why this huge error is existing? I have tried with different foil areas but could not find the reason of error.

I m looking forward, Cheers.
Just by looking at your numbers, my feeling (based on 50 years of electronics experience) is that your calculated value is in the correct range. There is absolutely no chance at all that you can create a capacitor yourself with a value of 125μF.
 
  • #5
704
151
I'm actually not exactly sure how devices measure capacitance (RC circuit? Or LC?), but I wouldn't be surprised if they don't show the same accuracy across all ranges (kinda how you have to dial in the right resistance range to get an accurate reading of a resistor).
 
  • #6
berkeman
Mentor
56,816
6,784
I'm actually not exactly sure how devices measure capacitance (RC circuit? Or LC?), but I wouldn't be surprised if they don't show the same accuracy across all ranges (kinda how you have to dial in the right resistance range to get an accurate reading of a resistor).
My personal inexpensive B&K LCR meter definitely needs to be put on the capacitance range that is closest to (but not less than) the capacitor to be measured. It also has a D (= 1/Q} measurement setting to let you know if the capacitor you are measuring is very lossy. If it is too lossy, that messes up the capacitance measurement. That's why I asked the resistance question in my earlier post... :-)
 
  • #7
Svein
Science Advisor
Insights Author
2,025
649
I'm actually not exactly sure how devices measure capacitance (RC circuit? Or LC?), but I wouldn't be surprised if they don't show the same accuracy across all ranges (kinda how you have to dial in the right resistance range to get an accurate reading of a resistor).
I once built an LCR meter based on a Wien bridge. If anybody is interested, I could try to dig up the schematics.
 
  • #8
2
0
For one thing, this is incorrect: "plate or foil area, A = (12.1 x 8.3)cm = 0.010043 m" Convert the two lengths to meters before multiplying, and your final area units need to be m^2.

Also, how did you measure the capacitance? What type of meter are you using? What do you measure for the resistance between the plates? How are you holding the plate stack together? Can you post a picture?
hi,

0.010043 is in meter-sq not in meters. the unit was wrong sorry for that. I measuring capacitance with LCR meter.
 

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