Re: Filters & Chokes

["Ed Phillips" <evp-at-xxxxxxxxxxx> (by way of Terry Fritz <twftesla-at-xxxxxxxxxx>)]

> >
> > I'm just costing out the filter as per Terry's circuit diagram
> > (http://hot-streamer-dot-com/TeslaCoils/Misc/NSTFilt.jpg) and at the UK prices
> > I've found, it's gonna set me back over £110! £80 of that being the 100W
> > wire-wound resisters.
> >
> > Any cheaper options?!
> >
> > Cheers
> >
> > neil
> >
> Hello Neil,
> 
> 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

	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