Hi Albert,
You're looking at the resistance of the NST secondary incorrectly.The 10K
resistance is the DC-component of the SOURCE (not load) impedance. If you
had an imaginary perfect NST wound with superconducting wire, it would have
zero resistance, and this would be a GOOD thing, not something that
attempts to draw infinite current. The available current would still be
limited by other factors but that loss-component would be eliminated.
You're correct that the NST secondary will *normally* discharge the caps
after the NST is switched off, but let me highlight that sometimes "stuff"
happens...
I once had this brilliant idea to determine which primary tap gave the best
sparks. Normally one would try a tap, power off, move to the next tap, and
hope that you remember what the previous tap's sparks looked like in
comparison. I decided to attach the tap wire to the end of a PVC pipe, and
drag the tap wire across the primary turns so I'd immediately see how the
different tap points compared. It sounded reasonable, but when the tap
wire approached the primary, the sparks between the wire and the primary
were REALLY scary. I immediately abandoned that idea and powered off the
rig. I picked up the tap wire to fasten it to the primary, touched the
primary, ZAP!!!! Fortunately there was considerably less than a full
charge left in the cap, but it could just as easily have been worse. If
you think hard, you can probably come up with other uncommon but plausible
scenarios where the normal discharge path is rendered inoperative. An
internal NST failure, a bad crimp connection, a broken wire, etc. Long
storey short, it's prudent and inexpensive to have a bleeder soundly and
permanently across your caps.
Regarding the reply suggesting that the resistors spread the charging
voltage evenly across the series capacitors, this is untrue. The impedance
of the bleeder resistors is huge compared with the impedance of the
individual capacitors at charging frequency. If we were working with DC,
the resistors would work, but that's not the case. Unless the resistance
of equalizing resistors is so low that more current flows through them than
to charge the caps (i.e. more than half of available power is burned up in
the resistors), the voltage division across multiple series capacitors will
be governed strictly by the ratios of their capacitance. If all caps have
the same capacitance, the voltage will divide equally. But caps should
still be used across individual series-connected caps for other reasons.
Regards, Gary Lau
MA, USA
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On Sun, Feb 4, 2018 at 8:28 AM, Albert <Animation@xxxxxxxxxxxxxx> wrote:
>
> Hello all, my first posting ....
>
> I am building a standard tesla coil (NST, 15kv at 30 m/a etc)
>
> When working, if I switch off when the caps are charged but the spark gap
> hasn't yet fired,
>
> (say they are charged to 10kv), than the caps can now discharge back
> through my NST secondary
>
> via the tesla coil primary coil as all are in series.
>
> My NST secondary resistance measures 20k ohms. So 10kv is put
>
> across this equating to 1/2 AMP !!. Can the NST fine secondary windings
> take this continually?
>
>
> Also as the caps discharge quickly via the NST anyway,do I need bleed
> resistors across the caps?
>
> (4 doorknobs in parallel)
>
>
> Thanks.
>
>
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