Re: Terry's filter question

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Vardan <vardan01@xxxxxxxxxxxxxxxxxxxxxxx>

Hi,

At 07:48 PM 3/26/2006, you wrote:
> I have several questions about the components used in Terry's filter.
>
> 1. Since the safty spark gap acts somewhat like an MOV (a nonlinear resistor
> why
> use an MOV?

MOVs have a relatively precise voltage that cannot be "fiddled 
with".  Sprk gaps are always being set wrong, getting dirty, failing 
and causing NSTs to blow up.  MOVs are far more reliable.

MOVs cannot take a lot of power however.  So the spark gap is still 
needed to take most of the typical over voltage events.  In a way, 
the MOVs are sort of the last resort "fuse" that will save the NST 
even if they are destroyed themselves (they fail shorted).



> 2. Whenever I see MOVs in protection circuits they are always shunted by small
> caps as in Terry's filter.  Is this because the MOVs are slow?

The caps are part of the filter circuit.  The MOVs also have high 
capacitance but they are not "perfect".  I suppose it could be argued 
that JUST the MOV capacitance would supply the needed capacitance for 
the filter without the regular filter caps.


> 3. I plan on using twenty 56 ohm 1/2 Watt resistors in series in place of the
> 100 Watt 1k resistor.  If the safty gap fires occasionally will the resistors
> fry?  The instantaneous power is huge but the average power is low.

That is only 10W of resistors...  I suspect they will get 
"hot".  Might just be able to get by with it, but it is real close.

Cheers,

        Terry



> My coil uses a 15k 30mA NST
>
> Norman Scheinberg