Re: Bleed Resistor for Homemade/Large Caps - THE FULL DESIGN NOTE S

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

At 05:54 PM 10/29/2002 -0700, you wrote:
>Original poster: "Jeremy Scott by way of Terry Fritz <twftesla-at-qwest-dot-net>" 
><supertux1-at-yahoo-dot-com>
>
>Okay,
>
>I've had one person say that stringing
>10Mohm 1/2 Watt resistors would work...
>and another saying that they all have
>to be 5 Watts ...


The resistors will be dissipating less than 1/2 watt, but, you just might 
want to use bigger resistors:
1) higher breakdown voltage
2) more margin between power dissipation and rated max dissipation

Since resistors are cheap, why not overdesign/overbuild a bit...

Other ideas.. use the 1/2 watt resistors, but use them in 4's, series 
parallel.  Basically a 2W resistor of the same value, but, more redundancy 
and failure tolerance (except that if one fails open (the usual failure 
mode), the one that was parallel will now be dissipating 4 times the power 
that it was before (but, since your design only disspates 1/3 watt anyway, 
you're still under the limit)...

The advantage is that your bleeder can have a failure, and still perform 
it's assigned safety function, which is really what it's all about.


>So which is it?
>
>As I understand it a resistor dissipates
>energy as heat. The larger the resistor,
>the more heat it can dissipate due to
>increased contact with the air. Thus, a
>resistor's length and cross section has
>a direct effect on how much power it can
>handle.
>
>So stringing together 15 10Mohm 1/2 Watt
>resistors *should* give 15 times more
>surface area than a single 1/2 Watt resistor.
>This *should* then be able to handle 7.5 watts,
>providing all of the resistors are the same value.
>
>Calculations based on 21000V, 15 1/2 Watt
>10Mohm resistors.
>
>I = V / R
>I = 21000V / 150 000 000 Ohms
>I = .00014A
>
>W = I * R
>W = 21000V * .00014A
>W = 2.94
>
>Voltage across each resistor in this string
>of 15 is going to be about 1400V
>
>Ir = Vr * R
>Ir = 1400 / 10 000 000
>Ir = .00014
>
>Wr = 1400 * .00014
>Wr = .196 W
>
>W * 15 = 2.94