Re: 9.5 questions on capacitor, LED, resistor

<leon.park@xxxxxxxxx> wrote in message
news:1129881685.570623.286590@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> 1. Fast charging capacitors-Possible? Is the charging speed of a
> capacitor determined by the input voltage and current?

Uh...yeah...
I = C * dV/dt.

> 2. I don't understand the differences between these myriad of capacitor
> chemistries. But I'm looking at aerogel caps. Does output voltage stay
> constant while current varies?

Huh!? By definition, a capacitor's voltage will always vary in proportion
to the current!

> 3. Also, do capacitors accept charge from an a/c source or will it need
> a diode?

If you connect a capacitor, say 0.1uF, to the 120V line, the voltage on it
will be 120V AC, which I think even you can comprehend. If you disconnect
it at any random moment, the voltage left on its terminals will be equal to
the voltage at the instant you disconnected it. You have a random chance of
getting peak, i.e., 160V left on the capacitor, but you also have the same
chance of getting zero or a negative value.

A diode, however, will ensure that current is always in the same direction,
not random.

> 4. LEDs require a resistor, but how do you calculate the required
> resistance if you connect the LEDs in series?
>
> 4.5. Then the voltage has dropped across each LED, yes?

No. First you assume each LED has the same terminal voltage, independent of
current. This is a "bad" assumption, but it works remarkably well for LEDs
(and other diodes). Since voltage is independent of current, you merely
subtract the total LED voltage drop from the supply voltage and divide the
remainder by the desired current to find your resistor.

> 5. What determines the capacity of a resistors resistance, i.e., what
> is the max voltage/amperage input of a resistor with impedance x.

Not impedance, resistance.

There are three limits on all components: voltage, current and power. Too
much voltage can cause electrons to go where they shouldn't, for example
high voltage loves to jump across air between wires that you might want
insulated. Too much power (P = I*R) causes things to burn. Too much
current causes too much power to be spent somewhere it doesn't belong, wires
for instance.

The voltage limit of a resistor is V = sqrt(P*R), and the respective maximum
current is I = V/R = sqrt(P*R)/R = sqrt(P/R), which you'd know if you looked
at a damn Ohms Law table.

> Or do I have it all wrong and is impedance rated at the specified
> input power?

I don't know what that was supposed to mean.

> 7. Will a resistor pass proportionally more power through if less than
> the power the resistor is rated at is experienced?
<and>
> 6. Is the power sent through a resistor merely attenuated or burnt off
> as heat?

Conservation of energy, it "wastes" it as heat. It also can waste ONLY as
much power as it is sent, i.e., volts times amps.

> 8. Can caps handle overvoltage/overcharge (unlike a battery)?

Almost all capacitors are rated for some sort of overvoltage; electrolytics
might be rated for a mere 10% while film capacitors are often rated for 50%
extra, or more. Self-healing caps can withstand even more, if you don't
mind weeding them out first!

> 9. Are cap chemistries what determine a caps discharge rate?

If chemistry determines ESR (equivalent series resistance) and ESL
(inductance), then yes...

But it doesn't.

> Are carbon aerogel caps "fast?"

I don't know, but I doubt it. Faster than a battery but slower than say, an
electrolytic.

> What do they use for railguns?

High voltage film capacitors. They build them specially to handle the
extreme currents required in a railgun. Likewise, such capacitors can
discharge very quickly, under a microsecond.

Oh, and learn to count too- that was two questions there.

> And because I'm too lazy to look it up, is there a simple way to figure
> how many watts/hour a cap of capacitance xF will put out before
> draining?

I don't know what the hell watts per hour are. Did you mean watt-hours?

The equation for energy stored in a cap is E = 1/2*C*V^2, where E is in
joules, C is in farads and V is in volts.

There are 3,600 joules in a "watt-hour" (i.e., one watt consumption for a
duration of one hour).

Tim

--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms


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