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Re: site update + filter question
Hi Jan,
At 10:39 PM 04/23/2000 +0300, you wrote:
snip...
>
>Other thing, I'd like to ask about RC filter design. I've a 3kV
>50mA NST for that tiny twin TC. The tank cap is 8.5nF.
>
>The current filter looks like this
>
>NST side tank side
>---/\/\/\---+---/\/\/\----
> |
> === 470pF 6kV max (ceramic)
> |
>---/\/\/\---+---/\/\/\----
> 15kOhm power resistors
>
>It's symmetric because the NST has no center tap and is not grounded.
>
>Those 60 kOhm total at 3kVAC (=>50mA) shouldn't be a problem, as it is
>50mA max from the NST anyway? Or?
>
>The filter should now be a low-pass below 10kHz. Is that ok, or should
>the freq be even lower?
>
>The problem now is that once the filter is connected, there's a dramatic
>performance drop (about half of streamer length), so there's probably
>something wrong with that filter design?
>
>How do you calculate the resistor values? And what wattage should they be?
>
The resistors are too high of value. With a 3kV/50mA NST you have about
150 watts available. If you put 50mA through 60k ohms the power lost to
the resistors is 0.05^2 * 60000 = 150 watts!! You are probably dropping
half you coil's power right now!
I try to drop about 5 to 10 percent of the system power in my filters. I
no longer use the resistors between the NST and filter cap but beware of
resonant rise. Your systems resonant capacitance is 48nF so your fine
there. So put the cap directly across the NST output and only use two big
resistors to the rest of the system. They should drop say 10 watts. I
would put a safety gap at the primary side of the resistors to stop
anything bad from getting through. I also use MOVs right at the NST
terminals as a last resort to be sure to stop anything bad from getting the
NST.
The formula for resistor power is P = I^2 R
We want 10 watts at 50 mA so R = P / I^2
or
10 / 0.0025 = 4000 ohms total which is 2000 ohms per leg.
If you still want to use four resistors, make them 1000 ohms each. I would
get at least 25 watt resistors to hold off the 3000 volts of your NST (Neon
Sign Transformer).
Cheers,
Terry
>many thanks,
>and happy easter coiling to all :),
>
> Jan
>
>--
>*************************************************
> Jan Florian Wagner
> http://www.hut.fi/~jwagner
>
>
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