RE: Filters & Chokes

["Lau, Gary" <Gary.Lau-at-xxxxxxxxxx> (by way of Terry Fritz <twftesla-at-xxxxxxxxxx>)]

I use a standard unmodified 15/60 NST and the filter uses a pair of 1.6K
113W resistors.  For a very long time I was puzzled at just how hot these
got.  By standard logic, the power is I^2 R, or .06^2 * 1600 = 5.76Watts.
But after building a fiber-optic VI probe, I was able to scope the NST
secondary current with a digital scope, which also performs true-RMS
calculations.  Guess what?  My 60mA NST is pushing 184 mA, RMS.  It appears
that some unexpected things happen when charging an LTR cap, speculation is
that the current shunts saturate and allow far more current to pass.  So the
184 mA causes a dissipation of 54 Watts per resistor.

Gary Lau
Waltham, MA USA

	Wait a minute.  The resistor dissipation is i^2 R.  For example, if
you're running 60 ma and using a 5000 ohm resistor, the dissipation is
(0.06)^2 x 5000, or 18 watts.  You can certainly run three or four times
that power for a few seconds or even minutes, without melting anything. 
For the cited case of 45 watts and 400 ohms, the current would have to
be about 335 ma to just equal the 45 watt power.

	Or was this whole discussion about protecting pole pigs, which
probably
don't need any?

	By the way, power resistors are pretty cheap in the surplus market.
As
an example, look at C & H Sales site:

	www.aaaim-dot-com/CandH/

Ed

> I think that the 100 watt rating of the filter circuit resistors is much
> larger than is really required.  Unless you intend to run your coil for
> long continuous runs, about half that value should be more than enough.
> 
> In my own case, I have a two section filter circuit on my coil with two
> 400 ohm, 45 watt, resistors in each leg.  The common point of the two
> resistors is bypassed to ground with a 500 pF. doorknob cap, and another
> cap is connected from each HV bushing of the transformer to ground.
> After runs of a minute or so, the resistors are just barely warm with a
> measured input power of 1000 watts.
> 
> You can even parallel a number of smaller wattage resistors to give you
> an equivalent resistance and wattage to one larger unit.  For instance,
> three 1200 ohm, 15 watt, resistors equals one 400 ohm, 45 watt resistor.
> In fact, due to more surface area, the group of three might radiate any
> heat produced better than a single one would.
> 
> That's just my $.02 worth.
> 
> 73, Weazle, VE3EAR/VE3WZL