# How do I calculate the breakdown voltage of a capacitor's dielectric?

#### electronice123

Joined Oct 10, 2008
323
How do I calculate the breakdown voltage of a capacitor's dielectric?

Capacitance=.05uF
Dielectric constant of dielectric=45

At what voltage would the dielectric breakdown?

If the capacitance was changed, would it change the breakdown voltage of the dielectric?

#### SgtWookie

Joined Jul 17, 2007
22,227
A dielectric constant of a material is unrelated to the breakdown voltage of the dielectric material.

Dielectric materials have separate ratings for their dielectric constants and the breakdown voltage.

The breakdown voltage generally decreases as the dielectric material becomes thinner.

If you used the same dielectric material, but decreased the thickness to increase the capacitance, the breakdown voltage would decrease.

If you used the same kind of dielectric material, but only increased the area of the plates, the capacitance would increase but the breakdown voltage would remain the same.

#### Pavlo138

Joined Feb 26, 2009
28
So, reverse breakdown is proportional to the distance between the plates then? Assuming the dielectric becomes thinner while the distance between the plates decrease. However, since the capacitance is directly proportional to the dielectric constant, given; C=kE*A/d, so the capacitance could only change if either the dielectric constant and plate area, or distance changed. But, strictly speaking of changing capacitance in relation to break down voltage, yes the dielectric constant appears to have nothing to do with the break down voltage, but obviously the distance does.

But I agree, it does seem to me that different dielectric material would have different levels of leakage current and give rise to either more or less resistance to the source current, air, mylar, paper, ceramic; ceramic appears to have more resistance to current then paper, and might account for a ceramic capacitor's high dielectric constant and small size as compared to other caps of the same value. It's a good question though as to what role does the selection of dielectric material have to do with the breakdown current or voltage of a capacitor. Please let me know what you discover

#### SgtWookie

Joined Jul 17, 2007
22,227
So, reverse breakdown is proportional to the distance between the plates then?
This is an entirely different question from that asked by the original poster.

Polarized capacitors (such as aluminum electrolytic, tantalum) are rated for voltage only in one direction.

They will not withstand charging in reverse polarity for long, if at all.

Assuming the dielectric becomes thinner while the distance between the plates decrease. However, since the capacitance is directly proportional to the dielectric constant, given; C=kE*A/d, so the capacitance could only change if either the dielectric constant and plate area, or distance changed. But, strictly speaking of changing capacitance in relation to break down voltage, yes the dielectric constant appears to have nothing to do with the break down voltage, but obviously the distance does.
I think I said that already.

But I agree, it does seem to me that different dielectric material would have different levels of leakage current and give rise to either more or less resistance to the source current, air, mylar, paper, ceramic; ceramic appears to have more resistance to current then paper, and might account for a ceramic capacitor's high dielectric constant and small size as compared to other caps of the same value.
I don't know where you're going with this.

Leakage current was not a parameter of interest to our original poster, although it is certainly an important characteristic of capacitors.

Might interest you to know that pure water has a very high dielectric constant; about 78 at room temperature.
It's a good question though as to what role does the selection of dielectric material have to do with the breakdown current or voltage of a capacitor. Please let me know what you discover
Here is a table of dielectric constants of various materials: http://www.asiinstr.com/technical/Dielectric Constants.htm

#### Pavlo138

Joined Feb 26, 2009
28
Thanks for the resource

My reasoning behind the breakdown voltage being affected by the dielectric material, is along the lines of the the different amount of breakdown voltages for different types of insulators as well. Some materials make better insulators then others for the same thickness, as some materials are better suited as dielectrics in capacitors then others

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