Re: [TCML] Bleeder resistors: [Please read in Courier-New font, 10 point. ]

[Quarkster <quarkster@xxxxxxx>]

Dave -

(Are you familiar with the TCML Archives at www.pupman.com? The last 16 years of TCML postings are contained there, with a search function that lets you directly search the archives. Just about any question related to Tesla coils has probably been asked and answered multiple times previously, and searching the archives may provide exactly the answers you are looking for.)

The primary circuit with the spark gap connected directly across the HV transformer's secondary is the preferred layout. During operation, there are lots of HV transients, RF harmonics, etc in the primary circuit. When the spark gap is "firing", it is essentially a closed, low-impedance switch. This effectively "shorts" the transformer's secondary, and prevents most of this HV "noise" from finding its way back into the transformer windings, where iot can damage the insulation.

As far as Tesla coil arc length or efficiency go, both layouts yield identical performance.

The required value of a bleeder resistor is defined by the R-C time constant, which is the procuct of R X C that yields the time (in seconds) required to discharge the capacitor to ~37% of its original voltage.

The resistor must be capable of withstanding the maximum voltage that will appear across it during coil operation. If your coil design uses a single primary capacitor, then either a single special HV resistor, or a series string of  resistors individually having a lower voltage rating can be used. In an "MMC" capacitor design, each series-connected capacitor will usually have a single bleeder resistor connected across it.

The power handling capability of the resistors must not be exceeded, but this is almost never an issue in Tesla coil applications because the total energy in the capacitor bank is relatively small, and the current through the resistor(s) is very low. For use in a typical MMC design, 1/2 watt bleeder resistors are fine.

Regards,
Herr Zapp





--- On Thu, 6/4/09, Davetracer@xxxxxxx <Davetracer@xxxxxxx> wrote:

From: Davetracer@xxxxxxx <Davetracer@xxxxxxx>
Subject: Re: [TCML] Bleeder resistors: [Please read in Courier-New font, 10 point. ]
To: tesla@xxxxxxxxxx
Date: Thursday, June 4, 2009, 11:39 AM


Looks like it's time for me to go on another  journey up the learning curve 
...
 
I've seen Tesla Coil designs where the capacitor  is connected directly 
across the high voltage output "legs" of the neon sign  transformer. One leg 
then goes through a spark gap, through the primary coil,  and then connects to 
the bottom leg. 
 
Note: (If this drawing comes out as  a bunch of wildly disconnected lines, 
try viewing it in Courier New font at  10 point, like I did. I had to use a 
mono-spaced font to make the  drawings.)
 
I'll call this Design #1.  [Because of the  nature of "ASCII Art" for 
drawing, I'm not putting in the inductors on the NST's  legs, nor a Terry 
Protective Device ...]
 
This is kinda the "Classic NST Tesla Coil" design  that looks something 
like this... (below, Design #1)
 
Design #1
 
                  TTTT
                         )
NST   CAP     SG    PRI  )
) -----+----->  <---)    {
(       |            {    {
}    -----           )   {
)    -----           {   {
(       |            }    }
{------+------------}   {
                        GND
But I've also seen this design (#2). 
 
Design #2
 
 TTTT
                         )
NST     SG     CAP  PRI  )
) -----+------| |---)    {
(       |            {    {
}       \/           )    {
}         )    {
)       /\           {    {
(       |            }    }
{------+------------}   {
                        GND
 

Here's my question. Is either design more  efficient, or for other reasons, 
a better design, than the other?
 
   Then I have a couple questions on  bleed-down resistors.
 
 
I understand (well, I thought I did) about  bleed-down resistors. Is there 
a "rule-of-thumb" for how much resistance for how  much voltage? 
 
Or is it "Calculate how long it will take to bring  the cap down to zero 
volts in (for example) 10 minutes, and use that value?"  type of rule? Seems 
like with really big capacitors you'd have to figure in the  heat generated 
during the bleed-down, e.g., watts = I^2R. 
 
 It seems like the resistor would also have  to be physically designed so 
that at, say, 12kv, an arc doesn't develop around  the resistor as dust and 
so forth collect on it.
 
Anyway, thanks for the help, I do appreciate it.
 
 
  Thanks,
 
Dave Small
 


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