RE: Terry's filter question

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Lau, Gary" <Gary.Lau@xxxxxx>

> Original poster: norman@xxxxxxxxxxxxxxxxxxxxxxxxx
>
> 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?

The trouble with safety gaps is that their breakdown voltage is
ill-defined and subject to misguided adjusting.  The threshold voltage
of a MOV string is fixed and assuming you use the correct, prescribed
number of MOV's, will result in a tamper-proof, effective clamp.  In
theory the safety gap alone should be adequate, but we're all human.

> 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?

No, the MOV's and resistor/capacitors form two unrelated protection
elements.  The MOV's form a last-ditch overvoltage clamp.  The R-C
network forms a low-pass filter to remove high frequency transients.
Each is independent of the other.

> 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.
> My coil uses a 15k 30mA NST

>From a voltage standpoint your plan is sound, but your resulting string
will only be rated for 28 Watts, so it will run a great deal hotter than
a 100W resistor.  Might be OK, just be prepared for hot!

> Norman Scheinberg

Regards, Gary Lau
MA, USA