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

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

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <dhmccauley-at-spacecatlighting-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 ...
 >
 > So which is it?

 >>>>>>>>>>>>>>You comments below are correct.  Although for a bleeder
resistor calculation, you should use the
RMS value of your transformer output which in the example is 15kV.  21kV is
the peak value which was used as a maximum
worst case charge the capacitor may hold if power was disconnected from the
system.


 > 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
 >
 >
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 >
 > --- Tesla list <tesla-at-pupman-dot-com> wrote:
 >  > Original poster: "by way of Terry Fritz
 >  > <twftesla-at-qwest-dot-net>" <Mddeming-at-aol-dot-com>
 >  >
 >  > In a message dated 10/29/02 2:50:00 PM Eastern
 >  > Standard Time,
 >  > tesla-at-pupman-dot-com writes:
 >  >
 >  >
 >  >
 >  > >Original poster: "Jonathan Peakall by way of Terry
 >  > Fritz
 >  > ><twftesla-at-qwest-dot-net>" <jpeakall-at-madlabs.info>
 >  > >
 >  > >In series, the resistors current rating will be the
 >  > value of the lowest
 >  > >resistor in the series and the resistance values
 >  > add up. In parallel, the
 >  > >current rating is added, and the resistance value
 >  > added. So in this case,
 >  > >each resistor must be rated for 5 watts.
 >  > >
 >  > >Regards,
 >  > >
 >  > >Jonathan Peakall
 >  >
 >  >
 >  > Hi Jonathan,
 >  > Let's go back to Ohm's Law 101
 >  > I = E / R and W=ExI
 >  > If there are 10 equal resistors in series, each one
 >  > passes current of I and
 >  > has a voltage drop across it of E/10. Therefore the
 >  > power dissipated by
 >  > each resistor will be I x E/10= W/10, not W. Ten
 >  > equal 1/2 watt resistors
 >  > can dissipate 5 watts total. Current passing through
 >  > each resistor is I,
 >  > total voltage drop across the string is E, total
 >  > power is E x I.
 >  >         I made this same mistake last month while
 >  > trying to think at 3AM. ;-)
 >  > Matt D.
 >  >
 >  >
 >