[Prev][Next][Index][Thread]
RE: filter fundamentals
>>> Without a lot of thought, I fit 20 turns of 12ga solid
>>> THHN house wire on one of them before realizing that the
>>> wire is only rated at 600v.
>>I don't see how this would be a problem. There shouldn't be a large
>>voltage drop across the inductors (my brain isn't in gear so I don't
>>remember the formula for the impedence of an inductor, but you can
>>figure it out from V=IX, where X is the impedence of your inductor).
Let's say you want to keep the RF coming back at the neon sign transformer to
10% of the primary tank circuit. At 15Kv you need to attenuate the voltage
by 10 or 1500v by using an LC filter. Since the capacitor should not be
so large as to clamp the 60Hz high voltage, it might be choosen to be 10%
of the tank capacitance, say 1000pF. Picking some values to talk about...
200Khz resonance puts the Xc at 1/(2*PI*200000*1000*10^-12)=795 ohms.
The Xl needs to be about 8000 ohms to work as an RF filter. 8000=2*pi*200000*L
or L=0.00637H or 6mH. The voltage is 15000-1500=14500V If you had 100 turns
that would be 1450v turn to turn. I have seen a 1000 ohm 2W resistor with about
5 turns of 10ga wire wrapped around it fried to a crisp while the 'inductor' around
it remained intact. Impossible at DC, easily done with RF.
>How can the inductor do its job without dropping a lot of voltage across it?
Depends on what its job is. The drop 'across' will depend on the
frequency. If the job is to keep the hash (rf) out of the power
transformer, or out of the power feed, then the voltage AT RF may
well be small, but still need to be blocked. The voltage at power
if it's small why do we need to block it? I thought the RF was a neon xformer killer.
freq (ca 60 Hz) will be smaller yet, and thus irrelavant. Ditto the
cases on a line side filter.
I'm sure the RF coupling (capacitive) is large from primary to secondary to case and
transformer action is not an issue.
(in the special case of an inductor in a resonat circuit, or a
resonant configuration (lumped circuit or helical) the voltages
do go up.)
In any case of considering insulation, consider what is being insulated
from what. Ferrites are typically (always, i think) insulators
in and of themselves. Insulation from turn to turn, expecially
in primary or secondary filters would not see a LOT of voltage
normally, per turn, which is what the insulation rating on the
wire is about. (Some overrating on the insulation might save
destruction in a fault condition....)
Ferrite is a good insulator but if the starting end is close to the finish on a toroid
you better watch the insulation.
regards
dwp
just my view
Dave Huffman