# Re: [Fwd: Spark gap not firing]

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

Hi Luc,

The source and load impedances play into this but I think the R and C is
dominant for the case we have here.  Probably close enough...

Fc = 1 / RC

I always let the computer figure it out ;-))

Cheers,

Terry

At 11:24 PM 2/9/2001 -0500, you wrote:
>Hi Terry, list,
>
>I'm in the process of designing a power supply and I try to find a formula to
>create a RC low pass filter (Terry NST filter protection ) I look at your
>paper,
>on the web, I remember I read about it in the past but were ? I know if you
>change the value of R and C you change at what frequency the filter start to
>cut
>and how fast the db cut rise with the frequency. I could apply your value
but I
>like to understand what I do . Can you help me to find the formula or if my
>memory is good the graph to calculated it ?
>
>Luc Benard
>
>
>
>> snip
>>
>> The voltages levels by themselves are within the NST's rated maximum.
>> However, what worries me is the frequency.  If and NST gets hit with 15kV
>> at 60 Hz it is happy.  But think of what happens when the output terminals
>> are hit with 15kV at 400kHz!  The secondary coils of the NST are not going
>> to look like nice inductors anymore but rather complex L's and C's.  I
>> suspect that instead of the high voltage being evenly distributed across
>> the output windings, as they are at low frequency, much of the voltage is
>> spread across just a few windings at high frequency.  At 400kHz, the high
>> frequency voltage is just not going to get very far in the output windings
>> and that high voltage my hit only a few layers and BLAMMO!  Since trying to
>> figure out how the high frequency voltage distributes in the output winding
>> of an NST is rather messy.  I figure it is best to simply stop it from ever
>> going there...
>>
>> Reports like yours of NSTs taking this high frequency voltage are
>> encouraging.  However, I suspect that is an exceptional case.  Since the
>> strong emphasis on using protection filters, and putting the gap across the
>> NST began, the number of NST failures has dropped dramatically...
>>
>> Cheers,
>>
>>         Terry
>>
>
>snip
>

```