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RE: Chokes/Transformers


>Original Poster: "Tabraze Malik" <tab-at-malik13.freeserve.co.uk> 
>Hi it's Tab,
>I dont know if you remember but I asked a couple of questions about chokes
>before. Thanks for the replies guys. The replies said that chokes are
inductors
>that are more resistive to high frequencies and voltage. Bearing in mind
that
>resonant frequency is calculated using inductance and capacitance - 
>What I want to know is, (since inductors are coils and coils have
capacitance
>and inductance) will the extra inductance and capacitance affect the
>calculations of the resonant frequency? will I have to take this into
account
>when making my calculations?

Chokes and bypass caps will have no effect on the tank circuit's resonant
frequency.

>I have seen many coils schematics showing that the chokes must be grounded.
The
>only way I can think of doing this without all the transformer output going
to
>ground is to ground the choke by way of em induction. If there are any
other
>ways to do this please tell me.

I've never seen a schematic suggesting that a choke (in a Tesla Coil
application) should in any way be grounded.  Both ends of a choke are "hot",
neither is grounded.  Bypass caps which you didn't mention _are_ grounded at
one end.

>I.e wrap the choke wire around a PVC cylinder and place the ground inside
the
>cylinder without it touching the choke wire. This would be good for when
high
>voltage/ frequency comes back to the transformer, but wouldn't the coke
waste
>input power even at low frequencies?

At low frequencies (like 50/60Hz), the choke has a very low impedance and
wastes no power.  However, using chokes to protect the NST in a protection
circuit has largely fallen out of favor with many on this list, including
myself.  It is now widely believed (though many will still disagree) that
the use of chokes contributes as much high voltage oscillations due to
resonance with bypass caps as the tank circuit that it's trying to protect
against.  Better to use an R-C filter.

If you do want to use chokes, you'll also have to use resistors in series
with the chokes to reduce the Q of the LC circuit formed by the chokes and
bypass caps.  This will _somewhat_ reduce the duration of the oscillations
of the choke/bypasscap circuit, but not the magnitude.  The resistors will
waste power, the same as if an R-C filter was used.  There's no perfect
solution.

>I've been able to find a NST rated at 10Kv -at- 60Ma. Trouble with this is
that it
>is actually made up of 2  -  5Kv Transformers connected in series. Since I
do
>not have the transformer yet I'm guessing that each separate one has an
output
>of 120Ma. I'm basing this on the capacitors connected in series affect of
>doubling voltage but decreasing capacitance by the same ratio, the same
affect
>must apply to transformers connected in series. Is this right?

Nope.  A 10KV 60mA NST is made out of two 5KV 60mA windings.  It's just like
having two batteries in series.  The voltage is simply added and the current
through each one is the same.

>The transformer is old (don't know how old) so will it have any PCB's, I
intend
>to ask the dealer, but are there any laws in the UK against PCB's? If the
>transformer does have them then I'm not buying it (asking price £35).

Neon transformers always use a solid insulation inside them, usually
asphalt, not a liquid oil.  No problem.
		
Regards, Gary Lau
Waltham, MA USA