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

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

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!