Re: [TCML] Bleeding resistors for MMC

[Henry Hallam <henry@xxxxxxxxxxxxxxxx>]

In the general case you should use farads and ohms, then the result
will be in seconds.

But there's a handy shortcut, because:
1 Mohm = 1,000,000 ohms
1 uF = 0.000 001 farads
1 Mohm * 1 uF = 1,000,000 * 0.000 001 = 1 second

so you can actually multiply megaohms by microfarads and get the
answer in seconds, because the factors of 10^6 and 10^-6 cancel.


Henry


On Fri, Feb 26, 2010 at 6:48 PM, Brandon Hendershot
<brandonhendershot@xxxxxxxxx> wrote:
> For t=RC, what are the units for Capacitance? nf, uf, etc? Also, If I'm
> using a 10M ohm resistor, would I use 10 for the value of R, or 10,000,000?
> Thanks Tyler,
> Brandon
>
> On Feb 26, 2010, at 6:51 AM, Tyler Pauly <rpggod714@xxxxxxxxx> wrote:
>
>> 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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>>
>>
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>
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-- 
Henry Hallam

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