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# Calculating leakage resistance of a capacitor?

#1
01-12-08, 05:53 AM
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Calculating leakage resistance of a capacitor?

Can anyone help me answer this question? I need to know how to work out the following for an assignment....

100pF capacitor. Mica dielectric relative permittivity Er=12 and resistivity p=10(power14) ohmM. How to calculate leakage resistance?

Cheers for any help!

#2
01-12-08, 06:17 AM
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Check this out in PDF format. May have the forumulae you need. Googled leakage resistance of capacitor.http://www.keithley.com/data?asset=6076.

#3
01-12-08, 06:32 AM
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Thanks Chandler, read through it but it's not in there :-(

#4
01-12-08, 07:50 AM
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I think you are looking for a figure in MegOhms per Microfarad. Check this link: http://www.avxcorp.com/docs/techinfo/eqtant.pdf

If that doesn't help, check the text and reference books for the class you are taking, and let google be your friend!

#5
01-12-08, 08:08 AM
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Thanks Tough, I think the answer may be in there 'somewhere!'. I've spent hours upon hours with Google and today decided to join a forum to try and find the way to solve this...

My course books just don't explain how to get there at all

I'll read through that link some more and see if it takes me there

#6
01-12-08, 09:52 AM
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#7
01-12-08, 10:23 AM
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you show already a resistivity number

although your units don't make sense. What is ohmM? Do you mean Megaohms (Mohm)? In any case, if your given the parallel resistivity in ohms, that already is your "leakage" resistance. Trick question...

#8
01-12-08, 11:24 AM
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Yes I thought that too, the leakage is already specified....

Ohms/metre is the OhmM by the way.

There is definately something I'm missing here! Thanks for the help so far though

And ray2047, what can I say..................

#9
01-12-08, 12:18 PM
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OK, new plan

The problem wants IR in ohms. You are given C, Er, and the specific IR of mica R(ohms/Meter).

1) You first need to calculate both the area of the the plate in Meter*2, AND, the thickness of the insulator in Meters. for this, use C, Er and a constant. You gave us C and Er; the constant you need to know or look up.

2) So, with area A(meters*2) and thickness T(meters), use the given mica IR to get the actual capacitor resistance. That's your answer.

you won't get actual thickness and area. You will get a ratio to get the given capacitance. Since voltage rating is not part of the problem, you could use thin dielectric and smaller area, or thicker and more area. Its only the ratio you need to solve the given problem.

Last edited by telecom guy; 01-12-08 at 12:57 PM. Reason: forgot something
#10
01-12-08, 12:41 PM
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Thanks Telecom guy! I'll try and apply that tomorrow. it's getting late here and I've spent all day working out electrical equations, so my brains fried for now

Thanks for all the help!!!

#11
01-13-08, 02:36 AM
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Thanks again for your help. I'm not positive I'm right yet but I get to a conclusion at the end of the equations my leakage resistance is 1 Gig Ohm....

#12
01-13-08, 04:54 AM
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Originally Posted by Puppydog
Thanks again for your help. I'm not positive I'm right yet but I get to a conclusion at the end of the equations my leakage resistance is 1 Gig Ohm....
At what frequency?

#13
01-13-08, 05:25 AM
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I didn't include a frequency

The question is multi part and the following part I calculated time to dissipate at open circuit as 100 seconds.

#14
01-13-08, 06:44 AM
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not frequency dependant

Originally Posted by Rick Johnston
At what frequency?
Although some capacitor parameters will be freq. dependant, leakage is typically thought of as a DC parameter.
Mica is a very good insulator, and 100pF is a small cap. 1Gohm could be correct, but I haven't run the numbers.

#15
01-13-08, 08:14 AM
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Thanks Telecom Guy, a bit of reassurance I'm on the right path

#16
01-17-08, 02:07 PM
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leakage resistance

Puppy dog, I think I'm stuck on the same question as you.
So now looking for your help. Did you get to the answer? If so can you give me some pointers. I've seen the other help about using the constants for area and thickness but dont get it?:

#17
01-18-08, 09:01 AM
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Originally Posted by joe mc
Puppy dog, I think I'm stuck on the same question as you.
So now looking for your help. Did you get to the answer? If so can you give me some pointers. I've seen the other help about using the constants for area and thickness but dont get it?:
Hi Joe, if wer'e doing the same course we may be able to help each other... I do have the solution now to this question Send me an email to [email protected]

Cheers!

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