Re: resistors

To: "'Tesla List'" <tesla-at-poodle.pupman-dot-com>
Subject: Re: resistors
From: Tesla List <tesla-at-stic-dot-net>
Date: Thu, 13 Nov 1997 21:35:53 -0600
Approved: tesla-at-stic-dot-net

From:	Esondrmn-at-aol-dot-com [SMTP:Esondrmn-at-aol-dot-com]
Sent:	Thursday, November 13, 1997 1:25 PM
To:	tesla-at-pupman-dot-com
Subject:	Re: resistors

In a message dated 97-11-12 22:51:12 EST, you write:

<< 
 >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
  >>
Gary,

Interesting.  I am using a 12 kv 60 ma neon transformer for power.  The
resitors get hot but not red hot.  I have no inductance in series with them.

Ed Sonderman