Re: Filters & Chokes

[Terry Fritz <twftesla-at-xxxxxxxxxx>]

Hi Ed,

At 08:06 AM 11/14/2000 -0800, you wrote:
snip...
>
>	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.
>

Usually, NSTs supply a bit more than their rated current especially in LTR
systems, so I put the power dissipation at 90mA for a 60mA NST.

0.09^2 x 5000 = 40.5 watts

Then, you actually discharge the filter caps into the resistors too at each
bang so add:

1/2 x 500pF x 10kV^2 x 120 = 3 watts

Then remember that at full power, these resistors run at a toasty 300
degrees C!!  So I use double the size to get the temperature down to only 150C:

(40.5 + 3) x 2 = about 100 watts.

I have 1000 ohms in my diagram for up to 240mA as sort of a universal
filter.  A resistance between 1 and 5 Kohms is fine.  Don't underestimate
how hot those things can get!  I got a number of people were concerned
about the high temperatures originally, so I added more power dissipation
to keep the temperatures down.  Power resistors are nothing more than
heating elements and the high temperatures they "can" work at will melt
down nearby things.  Best to run them a bit cooler...

cheers,

	Terry