Issues with my investigation in capacitance

[Filippo Ficarra]

This is a simpler issue but I need some help nonetheless. I have been carrying out an investigation on how the dielectric size of a capacitor affects its time constant. In theory the larger the dielectric the smaller the capacitance and therefore the smaller the time constant. However although I am changing my dielectric by 5 mm at a time every single time I have measured the time constant with an oscilloscope I get the same value for the time constant of 302 ms which is also off of the theoretical value by a factor of ten. I think it may be due to the fact that my capacitor is in the pico farad range therefore it is hard to measure acureately.

 

[berkeman]

This is a simpler issue but I need some help nonetheless. I have been carrying out an investigation on how the dielectric size of a capacitor affects its time constant. In theory the larger the dielectric the smaller the capacitance and therefore the smaller the time constant. However although I am changing my dielectric by 5 mm at a time every single time I have measured the time constant with an oscilloscope I get the same value for the time constant of 302 ms which is also off of the theoretical value by a factor of ten. I think it may be due to the fact that my capacitor is in the pico farad range therefore it is hard to measure acureately.

Yes, it sounds like the parasitic capacitance of your setup is too large. Can you post a schematic and picture of your setup?

 

[Filippo Ficarra]

Yes, it sounds like the parasitic capacitance of your setup is too large. Can you post a schematic and picture of your setup?

http://imgur.com/a/0Sm5z here is the link to my setup image. The resistor is 100 k ohm btw.

 

[berkeman]

http://imgur.com/a/0Sm5z here is the link to my setup image. The resistor is 100 k ohm btw.

Yeah, looks like many tens of pF in just the setup and wiring alone. What are you using for an oscilloscope? Can you compensate the probe capacitance, or are the probes too simple?

You can look into how oscilloscope probe capacitance can be nulled out, to see if you can use a similar technique on your setup (after making all of the connections as small and tight and low-capacitance as possible)...

 

[Filippo Ficarra]

Yeah, looks like many tens of pF in just the setup and wiring alone. What are you using for an oscilloscope? Can you compensate the probe capacitance, or are the probes too simple?

You can look into how oscilloscope probe capacitance can be nulled out, to see if you can use a similar technique on your setup (after making all of the connections as small and tight and low-capacitance as possible)...

Ok my school did not have an accurate oscilloscope so I am using logger pro to collect my data. I set it to collect 6 seconds and it takes samples every 100 milliseconds while the capacitor discharges. Is it possible that since many wires are involved their is some stray capacitance that is affecting my data?

 

[berkeman]

Ok my school did not have an accurate oscilloscope so I am using logger pro to collect my data. I set it to collect 6 seconds and it takes samples every 100 milliseconds while the capacitor discharges. Is it possible that since many wires are involved their is some stray capacitance that is affecting my data?

Yes, that setup will not work for measuring the effects of small capacitors. You could probably measure caps in the several uF range, but not down in the pF range.

Are you familiar with opamp circuits? You could build a circuit with a few opamps that would isolate the capacitance and buffer the signals to minimize the effects of stray capacitance. You could even "bootstrap" the stray capacitance with opamps to effectively null it out...

 

[Filippo Ficarra]

Yes, that setup will not work for measuring the effects of small capacitors. You could probably measure caps in the several uF range, but not down in the pF range.

Are you familiar with opamp circuits? You could build a circuit with a few opamps that would isolate the capacitance and buffer the signals to minimize the effects of stray capacitance. You could even "bootstrap" the stray capacitance with opamps to effectively null it out...

I will look into that but thanks for your help

 

[berkeman]

You're welcome. You might try Googling something like Measuring Small Capacitance Using Opamps...

EDIT -- use Google Images...