Re: Some capacitor questions

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jason by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jasonp-at-btinternet-dot-com>

Jon

> What is the difference between pF uF, and the others? If there was just a
> site that tells conversions, that is all I would need, since I know what
> pico, micro and the lot mean.
1F = 1000mF
1mF = 1000uF
1uF = 1000nF
1nF = 1000pF

or in more useful terms:
a standard cap for a 9/30 is about 8nF, or 0.008uF
a standard cap for a 10/50 is about 12nF, or 0.012
a standard cap for a 10/150 is about 40nF, or 0.04uF

It is very rare to see caps over 500nF (0.5uF) in a TC tank circuit... I
daresay that some of the masive coils like TTR's ones or possibly some of
the kVA effects coils use huge caps such as these, but the most i have ever
really seen for a 10KVA pig is about 200nF

> I see how capacitors do their job, but I don't understand how they are
> charged by weaker power sources and how they get discharged in a circuit.
Imagine this: you have a 10KV power source, and it runs at 10mA. A certain
capacitor may take 1 second to charge.
Imagine this: you have a 10KV power source and it runs at 100mA. The same
cap will take 0.1 seconds to charge.
However this is an inverse square curve, and a cap can never be completely
charged, i.e. the current flowing through is never zero, just really close
to it :)

The speed at which a cap charges is determined by the amount of input
current. The higher the input current then the faster the cap will charge.
In a capacitor as it approaches its peak charge, the backward motive force
from the cap limits the current. So initially the inrush current to charge a
large cap will be huge, but then after a certain amount of time it will fall
to almost nothing. It is mainly for this reason that TCs represent a
complete short to a transformer. The capacitance of the cap relates to the
amount and type of dielectric used in the cap.

The voltage at which a cap can charge to is only limited by the supply
voltage. If you charge a suitible cap at 500V then its pd will be 500v. If
you charge the same cap to 5000V then the pd across the cap will be 5000v.
The limiting factor is the strength of the dielectric, i.e. what voltage
will punch through the guts of the cap.

The output current of a cap depends on hw fast it is disharged. If a cap is
discharged over a long period of time then its output will be in the
mlliamps. However if ti is discharged as in a TC circuit then it will dump
all its energy instantly, resulting in a very high peak current.

> Also, what makes some capacitors discharge all their energy at once, or
> slowly over time?
The load. A load with a high impedance will cause the cap to discharge over
a long time. A good example of this is the bleeder resisistors used in an
MMC.If you discharge it say through your safety gap then the cap will be
discharged almost instantly.

Hope this helps,
Jason