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Re: resistors
rom: Gary Lau 11-Nov-1997 1428 [SMTP:lau-at-hdecad.ENET.dec-dot-com]
Sent: Tuesday, November 11, 1997 12:35 PM
To: tesla-at-pupman-dot-com
Subject: Re: resistors
>> I am working on protection of my transformer. I have bypass caps off
>> each HV bushing to ground and I also have two chokes on each HV bushing.
>> I have read stuff about using resistors. Could someone give me some
>> info on what they are used for, where they go, and some values to look
>> for?
>>
>> thanks
>> Kevin Wahila
>
>Kevin,
>
>On my smaller coil (3.5" dia) powered by neon sign transformers, I use two
>3000 ohm 50 watt resistors. One in each high voltage leg between the
>transformer and the tank circuit. That is the only protection I use on this
>system other than a safety gap. Have not had any problems.
>
>Ed Sonderman
I had been using a 3K/50W resistor (actually two 1.5K/25W in series) on
each side of my 15kv/30mA transformer (with .006 uF cap), and the
resistors got pretty damn hot. I recently upgraded to a 15kv/60mA
transformer (with .01 uF cap), and using these same resistors, smoked
them. I've since replaced the smoked ones and rather than series wiring
them, used them in parallel pairs, 750 Ohms/50W on each side, and even
these get hot after a minute of operation. I do know that for both
transformers, my caps were well sized for my transformer and were
definately seeing 60 Hz resonance. With the 15kv/60mA unit, the lights
in the house literally dim when I crank things up! I have yet to measure
the wall plug power.
My understanding of using resistors is to dampen the self-resonant
ringing of chokes, so I'm not sure of what value R's are without chokes.
Simulating the circuit on PSpice clearly shows the consequence of using
various value R's vs. none at all, the difference being the amplitude
and decay time of the choke's self-resonant ringing following each gap
quench. Unless the R is above a certain value, the amplitude rings
higher and higher on each gap firing. I know, GI-GO, but it does no
doubt have some bearing on the real world. It would be interesting to
actually scope choke self-resonance in actual Tesla coil operation to
verify the simulations, but have no idea how to attempt this or how
significant the problem actually is.
Did I hear someone suggest that damping R's were mainly needed on air
core chokes? Could this be because they have higher Q's? Do they? I'm
using ferrite torroids and would love to dump this 100W power sink if
they're not needed, but don't want to risk my beloved tranny!
Gary Lau
Waltham, MA