Re: Question on bleed off resistor

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

Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>

Hi Michael,

In the past, people who made rolled poly caps often used a "3X" voltage 
rating for poly thickness.  The reason was that homemade caps were 
"homemade" and they tended to need extra "help" anyway.

If something goes wrong in a resonant cap/transformer system, the voltage 
could go far higher too.  LTR designs have largely eliminated that problem 
as have good filters now days.

Since commercial caps will be destroyed "instantly" if they go over voltage 
and arc through, many people prefer to have plenty of margin there.  Also, 
high voltage caps can take more abuse and may last longer in general in the 
tough Tesla coil environment.

All that said, more modern LTR designs, MMC caps, effective filters, etc. 
have all but eliminated the need to over-design for voltage.  We still have 
to remember that the peak voltage is 1.414 times the RMS voltage (so a 
7500Vrms transformer will have a peak voltage of 10600 volts) and keep in 
mind those variacs that go to 117% of the line voltage.  But the days of 
blindly rating caps to 3X the voltage "just because", are long gone.  In 
this case, the resistors can take 3000 volts if they had to.  MMC caps can 
also take substantial over voltage but are not destroyed if there is some 
arcing through.  But the systems of today should have primary voltages 
right where we expect them to be and we are pretty good at keeping them there.

Cheers,

         Terry


At 03:55 PM 7/22/2003 +0200, you wrote:

>>Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>
>>[...]
>>So you could use 20  10Meg 1/2 watt MMC resistors in a 2 x 10 array which 
>>would give 50Meg ohms at 10kV and 20 watts.
>>[...]
>
>
>Hi!
>
>I'm new to coiling and to this list, so I don't know if i'm going to ask a 
>silly question or not. It is always said that the capacitor sees the 
>multiple voltage of the transformer's output and thus needs to be able to 
>withstand these high voltages. So why are 10 resistors in a row which 
>results in 10KV enough? The voltage that lies across the resistors should 
>be the same, than the one that lies across the cap, shouldn't it? Could 
>you please explain this to me?
>
>Greetings,
>Michael Zwerger
>