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Registered Member #3353
Joined: Sat Oct 23 2010, 11:21PM
Location: Greece
Posts: 90
Most people recommend a NST filter (most common, a “Terry Filterâ€). This is a RC filter , or more accurately a low-pass filter , meaning it allows for DC-x and frequencies to pass, and cuts out x+ frequencies ( x is of course the cut-off frequency). Terry has well-documented the use of this filter ( ) so there can be no doubt that his filter works. BUT , some people argue that this is “unnecessary loss†(and not just for “Terry Filters†but also for simple RF chokes).
So , finally my question :
Could someone just use an over-voltage (opposed to and RF ) protection circuit?
Could only MOVs set to a higher (than NST) voltage protect the NST (without the cost of “lots of Wattsâ€-resistors and “High-Voltageâ€-Caps) do the job? In real-life, could a safety-gap with some MOVs protect the NST? This really comes down to this:
If RF voltage is just a voltage, then you don't not need a RF filter, just a voltage filter, right?
Like this :
(I am not an expert, i am just asking to see if i am right, or else why am i wrong)
In the case that my suggested circuit works, but RF noise ( lower voltage than NSTs output) will get to mains, would my suggested circuit with an EMI – RF filter connected to the NST primary work ?
Registered Member #3093
Joined: Mon Aug 09 2010, 11:40PM
Location:
Posts: 68
The reason the caps and resistors are there, is to catch high voltage pulses that are too fast too actually break down the air and jump over the spark gap, I think. I may not be correct, but If you really want the kind of protection the terry filter supposedly provides, I'd not risk it.
Registered Member #3353
Joined: Sat Oct 23 2010, 11:21PM
Location: Greece
Posts: 90
What about the two 1K resistor watt rating? I *think* that Terry Fritz stated that he first used 50W resistors but they got a bit hot, like 300 degrees ( ) so then he recommended higher watt ratings (250W). I may be wrong about all that, please correct me if that is the case.
Here Terry measured 100mA of RF energy (i think), using just 4KV for testing. He states that in real life this can be up to 500mA (using 20KV power supply).
How can you calculate how much power will be "spend" on the resistors (according to the power of the NST used)? If i just use Ohms Law i get something like 6amps for a 12KV NST (assuming the NSTs output current is much less, this is like a short-circuit right?) but this is just if you put the two resistors directly across the NST... I know this calculation is wrong and useless, but it is the best i can think of
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NST over-voltage protection?
) so then he recommended higher watt ratings (250W). I may be wrong about all that, please correct me if that is the case.