Re: Ignition Coil Power Supply QUestions (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: List moderator <mod1@xxxxxxxxxx>



---------- Forwarded message ----------
Date: Sun, 20 May 2007 16:01:56 -0500
From: resonance <resonance@xxxxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: Ignition Coil Power Supply QUestions (fwd)




Yes, an OBIT --- oil burner ignition transformer.  Most local 
heating/cooling shops would donate a used one free for a high school science 
project.  They provide much more current than an ignition xmfr can (usually 
only 1-3 mA --- very slow charging of your capacitor).  Obits are generally 
rated 10 kV at 23 mA and will charge an MMC cap bank without any problems.

Jim Heagy in SFC CA has dozens of these OBITs in stock.

If you go the route of an ignition xmfr use an alternistor not a triac for 
switching (Digi-Key).


Dr. Resonance



Subject: Ignition Coil Power Supply QUestions

Hello,

My ARSG project has been temporarily (for a few days) interrupted so I can 
make a small coil for someone as a gift, because they have supplied many of 
the parts necessary for my larger coil (old microwaves, piping, fans, etc.). 
I've wound a small secondary and filter chokes, but am trying to figure out 
a good power supply for it that would be small and low power.  I think the 
cheapest way of doing this would probably be to use a few ignition coils. 
(If there's a better way for about the same price and power level, 
suggestions are welcome.)  Anyway, I had a few questions about ignition coil 
power supplies relating to Tesla coils.
The first type of ignition coil power supply that I thought of would be to 
use one or two ignition coils as normal autotransformers.  With 
approximately a 1:100 turn ratio, 120V on the primary could yield a very 
reasonable 12kV on the secondary in a nice sine wave.  They would have to be 
ballasted, of course, probably capacitatively on the primary, like the 
common mains ignition coil driver.  The problem I see is that I think most 
of the voltage would be dropped at the ballast.  From searching the 
internet, it seems to me that the unloaded inductance of the primary of a 
typical ignition coil is around 3mH, yielding around 1.1 Ohms of reactance 
at 60Hz.  A reasonable capacitative ballast would be a 10uF motor run 
capacitor (which I happen to have on hand), which would yield about 165 Ohms 
of reactance.  This means that the majority of the voltage would be dropped 
by the ballast representing a corresponding decrease in the high voltage 
produced, right?  And is there any good way to get around this?
The second type I considered was the typical ignition coil mains power 
supply, consisting of a capacitor, triac lamp dimmer, and ignition coil 
primary in series.  I know that some people have used these to power Tesla 
coils, but wouldn't the high voltage waveform be far from ideal for Tesla 
coil use?  Examining the waveform, the first part of a cycle on the primary 
(before the triac kicks in) would be a normal sine wave rise, but during 
this portion, wouldn't the high voltage produced be severely limited by the 
series capacitor?  Then, when the triac cuts in and gives the inductive 
kick, a much higher voltage is produced, but since the gap fires at that 
point, it doesn't go into charging the capacitor.  Is this correct?  And am 
I correct in thinking that it would be a problem?  Also, the second 
inductive kick on the first half cycle of the 60Hz primary waveform would 
pretty much go to waste, as well as the part of the cycle after that before 
the waveform crosses 0, right?  And if these are really problems, is there a 
good way to fix them?
Is there any better way to drive ignition coils than these two for Tesla 
coil use?

Thanks a lot,
Chris