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Re: Terry's NST protection filter.



Original poster: "Terry Fritz" <twftesla-at-uswest-dot-net>

Hi Alan,

At 12:16 PM 1/6/2001 -0800, you wrote:
>
>Hi -
>
>Firstly thanks to all who replied about a source of Al duct in the UK...I
>tracked some down at the weekend and now have a gleaming new toroid :-)
>
>I'm planning to use Terry's NST filter as it appears in his NSTFilt.jpg with
>my 10kV 100mA 50Hz NST. Thought I'd better check, first of all, if I need to
>make any changes to the component values ?

http://hot-streamer-dot-com/TeslaCoils/Misc/NSTFilt.jpg

The filter is designed for 15kV so you may want to reduce the number of
1800 volt MOVs.

10kV x SQRT(2) =14.142 kV peak.  The MOVs minimum voltage is 1800 * 0.9 = 1620

14142 / 1620 = 9  so maybe 5 MOVs per leg would be better than 7.  You
could also probably use 6 since when NSTs blow from over voltage, the
voltage levels are usually pretty dramatic.  It is not terribly critical.
The MOVs normally only act if all else fails.

>
>One thing that did occur to me.....matching the transformer impedence to
>that of the capacitor at 5O Hz gives a value of about 0.032microF. With the
>filter in place does it change the situation at all.....that is, does the
>filter drastically affect the impedence across the terminals that the
>primary capacitor "sees"......or, the other way around, the impedence of the
>filter and tank circuit that the transformer "sees" ....and so is this
>impedence matching process to find the right capacitor altered significantly
>?

Your primary cap is 32 nF and the filter caps are (series) 0.275nF.  They
will have nill effect.  The filter will add some loss but this will not
affect anything significantly.

>
>One last thing....if there's a spike formed in the tank circuit then
>presumably the RC filter works by the impedence of the capacitor being very
>very small compared to the resistor so that all the p.d. is across the
>resistor and it dissipates all the energy associated with the spike
>harmlessly as heat ? Or, put another way, that the capacitor looks like a
>direct connection to earth which a current spike then follows rather than
>going through the transformer windings.

Yes.  You want to let the 60 Hz (50Hz) power through while knocking down
the high frequency stuff and also provide overvoltage protection.

>I was trying to sort out, then, what R & C values would have to be used and
>got puzzled because I couldn't get the kind of values that Terry uses. Is
>there a webpage somewhere that explains Terry's choices ?

The "choices" I used were based on many things like parts availability,
robustness out in the real world coiler's hands ( :-)) ), and what would
handle "most" coils which was just as important and pure circuit theory.

General electrical details are at:
http://hot-streamer-dot-com/TeslaCoils/Misc/Filter.jpg
http://hot-streamer-dot-com/TeslaCoils/MyPapers/rcfilter/rcfilter.html
http://hot-streamer-dot-com/TeslaCoils/MyPapers/rlcfilter/rlcfilter.html
http://hot-streamer-dot-com/TeslaCoils/MyPapers/MyPapers.htm
http://hot-streamer-dot-com/TeslaCoils/TeslaCoils.htm

Pictures of mine are at:
http://hot-streamer-dot-com/TeslaCoils/MyCoils/MyCoils.htm
http://hot-streamer-dot-com/TeslaCoils/MyCoils/BigCoil/protection.jpg
http://hot-streamer-dot-com/TeslaCoils/MyCoils/SmallCoil/small_protection.jpg

Cheers,

	Terry


>
>With thanks,
>
>Alan.
>
>
>
>
>
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