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In Reply to: RE: How does capacitance change with voltage? posted by tvi on July 01, 2007 at 17:47:01
I guess I should say the amount of current in relation to the voltage rating of the cap and what voltage the cap is actually being used at.The Elaborate DAC=Johnny's Balloon Meshplate 27 Balanced Preamp=Balanced Six Pacs=Newform Research 645
Follow Ups:
Hi,
I'm not quite sure what your concern is. However as already posted the capacitance is basically constant with voltage.
The amount of power stored goes up with V^2 so if you run it at twice the voltage, it's got four times the energy available. Naturally you must stay under the rated voltage.
As far as peak current goes, it's mostly limited by the ESR (effective series resistance) of the Cap. Nowadays that specification is getting a lot more attention than it used to and many capacitors are being designed to optimize it. You can thank switching power supplies for this as the ESR of the Caps. is a major loss factor.
So if you want to store more energy, increase the capacitance or voltage.
If you want more peak current, use capacitors with low ESR and consider running several in parallel.
Good luck, Rick
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
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
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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