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

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

Original poster: "Jeremy Scott by way of Terry Fritz <twftesla-at-qwest-dot-net>" <supertux1-at-yahoo-dot-com>

Okay


143Mohm -at- 5 watts...

Now how does one make such a resistor, since
I'm sure that one of that specification does
not exist in a single package...

Would it be safe to series fifteen 10M resistors
rated for 1/2 watt each? Or would each of the
resistors have to be rated -at- 5 watts?





--- Tesla list <tesla-at-pupman-dot-com> wrote:
 > Original poster: "Mccauley, Daniel H by way of Terry
 > Fritz <twftesla-at-qwest-dot-net>"
 > <daniel.h.mccauley-at-lmco-dot-com>
 >
 >
 >
 >  >>>>>>>>>>>>>How about instead of just giving
 > arbitrary numbers, as is done
 > almost 99.99% of the time regarding bleeder
 > resistors on this group, we
 > actually do a bit of design regarding bleeder
 > resistors.
 >
 > First, lets ask why we need them period.  Safety of
 > course.  When capacitors
 > are charged up, they have the ability to retain a
 > lethal charge for long
 > periods of time after system power is disabled.
 > Therefore, we need to put a
 > way to safely discharge the capacitors once system
 > power is disabled.
 >
 > How fast do we want the system to discharge?  Well,
 > this is usually provided
 > in some design specification.  For tesla coil use,
 > an arbitrary figure is
 > that you want your capacitors to discharge to less
 > than 30 volts in 10
 > seconds.  From this specification, you can determine
 > your bleeder resistor
 > size.
 >
 > For a simple system running a 15kV, 0.0106uF
 > capacitor, we'll assume worst
 > case that power is turned off right before the
 > primary circuit discharges
 > leaving a maximum voltage of 21.2kV on the
 > capacitor.  Neglecting any effect
 > of impedance from the primary coil itself, we want
 > out 0.0106uF capacitor
 > which is worst case charged to 21.2kV to discharge
 > to less than 30 V in 10
 > seconds.
 >
 > Using the following equation:
 >
 > Vfinal = Vinitial * exp(-t/(C*R))
 >
 > Vfinal = 30 V
 > Vinitial = 21200 V
 > t = 10 seconds
 > C = .0106 uF
 > R = unknown
 >
 > Solving this equation (if my calculations are
 > correct - and i'm doubtful i
 > am) yields a bleeder resistor value of
 >
 > R = 143.6 Megs
 >
 > So a 0.0106uF capacitor charged to 21.2kV (peak
 > value of 15kVAC) will
 > discharge to a safe voltage of 30 V in about 10
 > seconds with a bleeder
 > resistor of 143.6 Megs.
 >
 > Next we need to determine the proper size of this
 > resistor.  So during
 > operation, the RMS voltage seen at this resistor
 > will be 15kV.  Solving for
 > power:
 >
 > P = (V^2) / R = 1.56 Watts
 >
 > Properly derating this value we should get a
 > resistor of at least 3 Watts -
 > I'd make it 5 Watts.
 >
 >
 > So there you are, a sample design for a bleeder
 > resistor in a capacitvely
 > charged system.
 >
 >
 > Now another problem is the case where we need to
 > discharge a system in a
 > very fast amount of time say the same example of
 > above, but in less than 2
 > seconds.  Because we do not want to waste a large
 > amount of power during
 > normal operation to a very large bleeder resistor,
 > we design what is called
 > a "Shorting Switch."
 >
 > Shorting switches are usually never used alone, but
 > along with a standard
 > bleeder resistor in a system.  Basically the
 > shorting switch is another
 > bleed resistor path (which discharge very quickly)
 > and are wired in series
 > with a High Voltage relay which is normally closed
 > with no power applied.
 > So when power is disconnected, the system will
 > discharge very quickly.
 > These shorting switches are also used as part of
 > interlock controls, e-stop
 > controls, and other fault modes of high voltage
 > systems.
 >
 > And finally of couse, if you want to discharge your
 > system extremely fast
 > (mainly to protect expensive components), you can
 > always use a triggered
 > spark gap as part of a crowbar system.  However, a
 > crowbar can be EXTREMELY
 > hard on a system and you is typically only used for
 > emergency fault
 > conditions.
 >
 > Dan
 > Captain Corona
 >
 >
 >
 >
 > Hi there,
 >
 > I was wondering what you use for bleeder
 > resistors on non-MMC (that is, large
 > commercial caps, rolled poly, or
 > "bucket caps")
 >
 > eg, how many ohms, how many watts.
 >
 > Thanks!
 >
 >
 >