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Re First Coil Up and Sparkin'
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From: Gary Lau 27-Jun-1998 1803 [SMTP:lau-at-hdecad.ENET.dec-dot-com]
Sent: Saturday, June 27, 1998 5:43 PM
To: tesla-at-pupman-dot-com
Subject: Re First Coil Up and Sparkin'
>Q's: What type of wire should I use from cap to primary. I'm currently
>using 10g copper but if the wires get close to each I get corona (?) between
>wires. I have plenty of large 2g and 4g stranded wire that is more heavily
>insulted but thought I had read that this stranded wire presented losses
>also.
If you're getting corona, the problem isn't the gauge, they're just too
close. #4AWG stranded is a fine choice for a coil your size. I use
uninsulated copper straps. The tank components should be laid out such
that wiring is as direct and short as possible.
>NST Protection: Currently, I've bypassed my protection. Chokes wound with
>22awg on 3" x 4" cores scavenged from ancient flyback transformers. I had
>some arcing problems and I'm now sealing with 3M IVI spray sealer. Has
>anyone tried this product? I originally had a 500ohm 65watt on each leg of
>transformer and 1000pf of ceramic caps across NS, I received a tip I'm lucky
>I didn't blow NST and should connect caps from NST to ground?? Both legs?
>What size caps best?
I suspect the arcing was from the wire to the core, I doubt a spray
sealer will help here.
I think the tip you refered to was mine, where I pointed out that your bypass
caps, rather than being in parallel with the resistors, should be from the NST
terminals to (RF) ground. Your parallel configuration is no more likely to blow
the NST as with no protection network at all, just that it was doing no good in
that configuration.
The protection network should have a cap from each HV terminal to RF ground.
Then a series resistor (what you had are fine) from each HV terminal to the
main spark gap. And also, a safety spark gap, from each NST terminal to RF
ground (in parallel with the bypass caps). Some coilers additionally use a
choke in series with each resistor (either side). The jury is still out on
whether this is a good thing.
The value of the bypass cap is generally a few hundred pF on each side of
the NST. The more pF, the more effective is the low pass filter it
forms. However, the larger it is, the more power is wasted. Most don't
realize it, but with each 60 Hz half-cycle, you charge two caps to your
NST's voltage: The tank cap, and the bypass caps. When the main gap
fires, the tank cap's energy goes into the tank and into sparks, but the
bypass cap's energy goes into the series resistors, doing nothing useful.
If the bypass caps (two in series) had the same value as the tank cap,
fully half of the transformer's power would be wasted as resistor heat.
In your case, the two 1000pF caps in series are 500 pF and represent 10%
of your tank cap's value, so about 10% of your NST's power is being wasted
here. You might try series-connecting some for 500 pF per side.
Regards,
Gary Lau
Waltham, MA USA