Basic Basic Question

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ele1200
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Basic Basic Question

Post by ele1200 » Sat May 10, 2003 8:13 am

I have a very basic capcitor question.<p>I know that a cap stores voltage and passes current. I'd like to know how a car audio cap, 1 Farad provides some 500 Amps to the system from a 14.4 V Battery/Alternator.<p>I'm probably missing somthing really simple here.....<p>thanks<p>bob

russlk
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Re: Basic Basic Question

Post by russlk » Sat May 10, 2003 8:43 am

The capacitor stores energy, not voltage. The energy is 1/2*C*V^2. A 1 farad cap with 12 volts on it stores 72 Joules. In order to discharge at 500 amps the resistance in the circuit has to be: R=E/I = .024 Ohms. The time to discharge to 63% will be: T=R*C = .024 seconds.<p>The only 1 farad caps that I am aware of, that would fit in a car, are the "gold" caps which have too much resistance to discharge at that rate.

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Edd
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Re: Basic Basic Question

Post by Edd » Sat May 10, 2003 9:05 am

Bob:
There are basically three factors involved in your situation. The capacitance value, the power applied and the TIME..the latter is what you were apparently overlooking.
A <500 amp figure would be more in line, also but this would be just an extremely short output burst if demanded until the stored charge is pulled down or depleted.
For a mechanical analogy , imagine a bucket getting filled with either a drip…slow charge…or a small stream…faster charge. You then have the capability to dump that buckets contents immediately and get ALL of its water (stored) in a large quantity in an INSTANT in comparison to the fill time….. but not in excess of the filled capacity, or stored capacity in the case of the capacitor. The stored power level can be impressive but the time element transition that occurs in, can be minute.<p>73's de Edd
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Edd
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Re: Basic Basic Question

Post by Edd » Sat May 10, 2003 9:07 am

Bob:
There are basically three factors involved in your situation. The capacitance value, the power applied and the TIME..the latter is what you were apparently overlooking.
A <500 amp figure would be more in line, also but this would be just an extremely short output burst if demanded until the stored charge is pulled down or depleted.
For a mechanical analogy , imagine a bucket getting filled with either a drip…slow charge…or a small stream…faster charge. You then have the capability to dump that buckets contents immediately and get ALL of its water (stored) in a large quantity in an INSTANT in comparison to the fill time….. but not in excess of the filled capacity, or stored capacity in the case of the capacitor. The stored power level can be impressive but the time element transition that occurs in, can be minute.<p>73's de Edd
[email protected] ............(Interstellar~~~~Warp~~~Speed)
[email protected]........(Firewalled-Spam*Cookies*Crumbs)<p>…….or in more appropo evaluation of my hi speeed internet service as of late from the new carrier replacing ATT……Commcast….and in light of their 50 % plus downtime you might more aptly call it (Unstellar,Warped and Speedless !!!!! )

ele1200
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Re: Basic Basic Question

Post by ele1200 » Mon May 12, 2003 11:33 am

Ok so the cap can provide the 500 Amps based on the voltage output through the resistor.<p>
So how high would the power rating on the resistor have to be? I calculated 6000 Watts.<p>Is that available??<p>bob

josmith
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Re: Basic Basic Question

Post by josmith » Mon May 12, 2003 12:18 pm

You would want as little resistance as possible between the cap and the amplifier.(heavy short wires) The point is to keep the supply voltage as steady as possible when the current drain peaks.<p>The maximum amps will depend on the internal resistance of the cap and the amplifier's ability to deliver power at a given supply voltage.<p>[ May 12, 2003: Message edited by: josmith ]</p>

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Chris Smith
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Re: Basic Basic Question

Post by Chris Smith » Mon May 12, 2003 2:14 pm

The best analogy for large caps in this instance is "Fly Wheel".<p>It smooths out all the starts and stops [ripple or power surges] of the lacking caused by long wiring from the battery, to the actual amp. <p>The car battery it self has many farads, but with long and relatively small wiring [even 10 gage] there is a delay in the amps that is needed to be more instantaneous when the bass kicks in, and needs a Sh** load of amps for one or two bass keys or drum beats. <p>With out such amps available, the base is clipped considerably making the music flat. <p>“Double Ought/odd” Wiring size, like “00" or welding cable helps between the battery and the amp, but still the cap is needed for “fidelity” at high base drawing.

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