Re: [TCML] Bleeding resistors for MMC

[Tyler Pauly <rpggod714@xxxxxxxxx>]

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?
>>
>> _________________________________________________________________
>>
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>>
>
>
>
> -- 
> Henry Hallam
>
> Sent from my Laptop
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