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Registered Member #2470
Joined: Tue Nov 17 2009, 05:47AM
Location: Simi Valley, CA
Posts: 43
I had an idea today at work. What if i kept the safety spark gap, and the MOV's but removed the rest. Won't this provide the same amount of protection (for the NST) as the original design? Or does the RF kill them too? If RF isn't what kills these, i plan on buying a cheap RF filter for the mains (which will cost 5 dollars and i wont have to build it). The real reason that i want to go this route is because the resistors are proving hard to find. I am running 2 Franceformer 15kV 60mA transformers, so if this is a bad idea i would rather try harder to find the parts (and spend the money) then be out two expensive transformers. Thanks for reading.
P.S. Would a plexiglass plate over the primary eliminate the risk of any transformer death?
Registered Member #2298
Joined: Sat Aug 15 2009, 08:16PM
Location: ex UK, now Santa Cruz, CA
Posts: 35
Until you really, really know what you are doing it is best to build the filter complete, as designed. The resistors and capacitors are there to protect the transformer, not just to keep RF out of the mains. If you are in the US, Digikey has power resistors at $7 for 100W, $10 for 225W.
The plexiglass plate is a good idea (make sure it's removable or there's some other way to access the primary for tuning adjustments). It reduces, but does not eliminate, the risk of primary strikes. It does nothing for other causes of transformer death. Your complete Terry filter with the correct number of MOVs and a properly adjusted safety gap makes a big difference to NST lifetime, though even that does not completely guarantee survival.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The resistors are a vital part of the filter. Terry Fritz spent a long time designing it so you don't have to reinvent the wheel: just build it the way he says.
If you can't find a single resistor of the right power rating, you can use a bunch of those white cement block resistors in series. Series is better than parallel because we're running the resistors at the limits of their voltage ratings.
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Terry Filter Idea
