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RE: How does capacitance change with voltage?

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John PA

In Reply to: RE: How does capacitance change with voltage? posted by rick_m on July 02, 2007 at 08:31:52

But is the effective capacitance changed with voltage as farads is a unit of energy so if the energy stored is increased with voltage is the effective capacitance increased? Is a 12,000uF cap a 12, --- at 5 volts and at 10 volts?

The Elaborate DAC=Johnny's Balloon Meshplate 27 Balanced Preamp=Balanced Six Pacs=Newform Research 645

Edits: 07/02/07

Follow Ups:

RE: How does capacitance change with voltage? - rick_m 11:57:17 07/02/07 (0)
A Fared is actually not a unit of energy, rather is the amount of charge the capacitor holds when charged to a volt. F=Q/V. In other words 1 Fared is a Coulomb per Volt. The total energy that the capacitor can store is .5CV^2, the integral of the charge curve.

The capacitance is essentially constant at all voltages, the capacitor's voltage rating is the upper limit that can be applied without risking punching through the dielectric. For practical uses, they are independent variables.

Typically, the maximum voltage in the circuit is known and relatively constant, so just choose a cap with a working voltage above that voltage and choose a value of capacitance to provide the desired energy storage at the voltage you are using.

Rick
No, let me explain - Russ57 11:55:02 07/02/07 (0)
The energy stored in a cap, in Joules, is 0.5 times farads times [(volts)squared]. So as the volts goes up the total energy is squared. But in no way is the amount of farads changed in accomplishing this feat.

Now as to current....it depends a lot on the cap in question. For example, IIRC you tried out motor run caps a bit ago. Since they are designed to directly power motors they will handle a lot of current. Typically they are rated for about 25 amps (basically coupling duty rating). Most other caps will have a current rating for "ripple voltage" only. This is the current the cap can handle passing when used for power supply filtering.

So the basic answer to your question is No:) Also keep in mind that with electrolytic caps the allowable tolerance is huge. Typically at least 20% and sometimes as much as (plus) 100%.

Russ

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