[TCML] Phase tuning gadgets and when to set the gap to discharge.
bartb at classictesla.com
Wed Jul 2 19:08:59 MDT 2008
Sounds like a fine way of identifying zero crossing. If it ends up
working well, you should do a "write up" so others can give it a go. I
don't use SRSG motors any longer, but when I did, the very best way I
found to adjust phase was to actually adjust phase while running the
coil and observing the spark output. I did something highly mechanical
to make this happen (the stationary electrodes were rotated via
extremely slow gear motor). Worked great!
Regarding the "best time to have the gap fire":
That depends if your running bps at 120, 240, etc. For 120 bps, just a
little after peak (say 112 degrees), if 240 bps, then about 135 degrees
past peak. You want it so that the cap has peaked in voltage and after
the 90 degree mark.
Regarding cap size:
Large caps are usually better for SRSG's, but a small cap can do well
also if designed in size to take advantage of resonant charging. I
typically run smaller caps then the norm and set it to about 1.4 x Cres
which is a decent value to take advantage of resonant charging without
going too far into resonance that could otherwise damage the NST.
Jeremy Scott wrote:
> Hi all,
> I designed and built a simple circuit to help adjust the phase of SRSG's.
> It's a timing light (white LED .. fancy) that pulses at 120 pps at each AC zero crossing. The pulse width is less than a millisecond or so with
> half of it before and the other half after the zero crossing. Kind of like
> an inverted rectified full wave if you can imagine that.
> It uses a 120VAC to 9VAC power adapter/transformer that I found in my junk box for the signal and power. It's a fairly simple two-transistor circuit.
> Turn the lights out, shine it at a rotary spark gap, and you can see if the electrodes line up like they should and if your motor is truly synchronous.
> So I used it on the propeller gap of my first coil which was tuned right but never worked very well. Hah. Way out of sync. Not at the zero crossing and not at a peak, but somewhere in between.
> I tried it with the SRSG phase adjust variac thing as discussed previously on this list and turns out the adjuster isn't doing anything. (Anyone know the proper size capacitor for one of those small square oriental motors?)
> So now I know why my first coil sucked and I can fix it!
> So I guess I got a few questions, now that I can visually see at which point in the AC cycle the gap is firing at.
> 1. So now that I can actually see and adjust the timing, when is the best time to have the gap fire, theoretically? (I know practically, it's 'whatever' ... adjust it while running until the streamers get bigger.)
> Is it at the zero crossing when the capacitor has had a full 8.3 ms to charge OR is it 90 degrees (4.16ms) sooner/later when AC cycle has peaked
> in voltage?
> For any given input current (say 60ma NST), would it be better to pick a larger capacitor size that takes a full 8.3 ms to charge then discharge it into the tank before the current reverses (at the zero crossing)
> OR would it be best to have a smaller capacitor and discharge it at the peak input voltage but potentially wasting the rest of the current available in the half cycle? (eg. step up the break rate to 240bps and fire the smaller capacitor twice.)
> 2. Is the timing light even accurate? The signal I'm using comes from a step down AC wall adapter. Is the secondary phase angle from this small transformer the same as the phase angle of the high voltage 15K secondary of the coil's transformer?
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