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Re: Cap Safety Question

Hi Adam,

	If you ask around, most shocks coilers get are from partially charged caps
zapping them at one time or another.  So bleeder resistors are very useful
especially since they do not affect performance in any way.

	However, most Tesla coil setups will bleed down automatically since the
transformer is wired across the capacitor.  You can check this by removing
the cap and using an ohmmeter check for resistance across the circuit where
the cap was (with the power off, of course).  The transformer windings will
probably measure around 5K ohm.  Of course, you may want to be double safe
and use resistors too incase something goes wrong...

	There are all kinds of theories about how to design them but this is the
method I use.


1.  Decide how long you would like it to take for the cap to discharge.  I
like 5 seconds.  Too long and you may get shocked, too short and the power
dissipation and such becomes hard to deal with.


2.  Find the resistance needed.  Using the equation:

Resistance = Time / (5 x Capacitance)

In my case (15kV neon and 27nF) that works to:

5 / (5 x 27 x 10^-9) = 37 Meg ohm.


3.  To find the power dissipation:

Power = Voltage^2 / Resistance

or 15000^2 / 37000000 = 6.1 watts

Not much power at all.


4.  How many resistors you will need to string together?  This is
determined by how much voltage a given resistor will stand.  Digi-Key sells
1/2 watt Carbon film resistors that are rated for 350 volts each for a
price of $3.96 per 200.  So if you want 20kV max voltage you simply divide
to see how many resistors you need.  Note that a 15kV RMS transformer will
reach about 21kV peak.  You "can" get "real" high voltage resistors but the
price is stunning...

Number of resistors = Total voltage / Voltage per resistor

21000 / 350 = 60 resistors


5.  To find the resistance of each resistor, divide the total resistance by
the number of resistors:

Resistance per resistor = Total resistance / Number of resistors

37000000 / 60 = 616000 ohm.  The closest match is 620000.


So, You need 60  620K  1/2 watt carbon film resistors for a 15kV system
that will dissipated 6 watts (1/10 watt per resistor) and will bleed of a
27nF cap in 5 seconds and it will cost about $1.20.  The hardest part is
stringing 60 resistors together in a reasonable way.  "Perf Board" is
probably the best way I have seen.  Use Radio Shack 276-1396 ($3.49)  Don't
buy perf board from digi-key unless you want the "finest" and want to pay
out the nose for it.  If you want to keep the size small, you can pack them
very close together and put them in a container of oil for insulation but
that is usually not worth the effort... 

Hopefully, this has demonstrated the equations and methods you need.

Cheers,

	Terry




At 04:02 AM 4/25/99 -0400, you wrote:
>I've heard that it's a good idea to put a high value resistor across the
>capacitor, so that the capacitor will discharge safely and calmly very
>quickly after the coil is turned off, and also to prevent against
>"ghost" charges.
>
>My question is:
>
>How do I determine the value of the resistor (both Ohm and Watt), so
>that maximum safety and maximum performance are both achieved?
>
>And, of course, where can I get these resistors and how much should I
>expect to pay for them?
>
>Thanks,
>Adam