For P=V^2/R, P is power dissipated in resistor, in watts. V is the
voltage across the capacitor (volts.) R is the resistance (in ohms)
of the resistor you are using.
For t=RC, the resistance of the resistor times the capacitance of
the capacitor it's wired across gives you a time, t, in seconds.
This time is the time it takes for the voltage to drop from Vo
(initial V) to Vo/e (as in e=2.718...) In other words, every time t
seconds passes for a given RC, the voltage drops to ~37 percent of
the voltage before t seconds had elapsed. So for 2 time constants,
it would be .37(.37Vo), or .135Vo.
The idea is to find a resistor that gives you a nice quick time
constant, without requiring a large power rating. It's usually not
too difficult to do though, MMC caps don't require fancy resistors.
Tyler
----- Original Message ----
From: Brandon Hendershot <brandonhendershot@xxxxxxxxx>
To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
Sent: Thu, February 25, 2010 10:33:36 PM
Subject: Re: [TCML] Bleeding resistors for MMC
Minor Bump,
Could you shed some light on what the variables in these formulas
represent?
Thanks,
Brandon
On Feb 22, 2010, at 2:48 PM, Henry Hallam <henry@xxxxxxxxxxxxxxxx>
wrote:
Design the bleeders on a per-capacitor basis. Each capacitor should
have its own bleeder.
If you are arranging your MMC as 3 parallel sets of 8 series
capacitors, each capacitor will see 1kV RMS (if you're not sizing for
mains resonance). Then a 10 Mohm resistor will dissipate 0.1 watt
(P=V^2/R), and will discharge the capacitor to a safe voltage in 3
seconds (1kV discharges to 50V in 3 time constants, t=3*R*C)
You could also use 2M resistors (0.5 watts, 0.6 seconds), or 20M
resistors (0.05 watts, 6 seconds), or ...
Be sure that the resistor will withstand the voltage, as well as
being
able to dissipate the heat. You should either
1. Find resistors that are rated for 1kV
2. Put two smaller resistors in series for each capacitor
or
3. Use them beyond their voltage rating and hope they don't arc over.
Henry
2010/2/22 John Byström <zrc@xxxxxxx>:
I know this question has been brought up before, but they
were a bit old posts and I don't know if there is anything new
in the "Tesla world" and opinions and rules maybe have
changed.
For my 8kV 50mA NST I'm gonna have a 37,5nF MMC bank,
with 0.1uF capacitors. If I've made the calculations right from
the formulas I found, I need a resistor with the value 20Mohm
and handle 3,2W. How important is this?
Because I probably gonna order both the capacitors and
resistors from Elfa, and they don't have any over 1/2W. Does
that mean I need around 6-7 resistors over each capacitor?
_________________________________________________________________
_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxx
http://www.pupman.com/mailman/listinfo/tesla
--
Henry Hallam
Sent from my Laptop
_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxx
http://www.pupman.com/mailman/listinfo/tesla
_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxx
http://www.pupman.com/mailman/listinfo/tesla
_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxx
http://www.pupman.com/mailman/listinfo/tesla