* Original msg to: Sroys-at-radiology.ab.umd.ed
SR> Greetings Richard,
SR> First, this is a re-post.
If at first you don't succeed... My connection appears to be rather
intermittant at times.
SR> I don't have a pole pig yet, but I have wired up some 4kV
SR> microwave oven transformers in series, and I'm not really
SR> sure what precautions I need to take. In this light, I
SR> have a few questions that I'd like to ask you before I do
SR> anything ignorant to damage myself or my surroundings...
Ho boy! Jury rigged HV supplies are really FUN! Keep a fire exting-
guisher handy at all times...
Yes you can connect more than two transformers (we are talking about
microwave transformers specifically, since we have sticklers here)
in series to get higher voltages, but as someone else mentioned here,
it can be tricky, and the "jury-rig" can be prone to failure.
The classic transformer reference text: ELECTRICTY AT HIGH PRESSURES
AND FREQUENCIES, by Henry L. Transtrom, shows the specific modifi-
cations to producing high voltages using "staged" secondaries; see the
core modifications and secondary coil arrangement pictured on page 145
where over 20 kV is produced from eight secondaries placed in series.
SR> 1. A few of the xformers have the secondary connected to the
SR> core - is this a problem, and if so, should I simply disconnect
SR> the wire from the core?
I don't really have enough info here on the placement of the center
tap ground wire coming out of the winding to say for sure. You may
not have a choice of disconnecting the center tap ground if the
voltage is going to arc to the core anyway, possibly destroying some
of the winding in the process. You also mention that only some of the
xfmrs are wired this way.
I would leave the center taps alone. Perhaps use one center tapped
xfmr in the middle, placing one untapped xfmr on either side to get
12kV across (6 kV from each side of the center tap). A trick that
helps is to magnetically link up the core by butressing the ends
and binding the array up with electrical tape.
SR> 2. Is it better to let the secondaries float or should I set
SR> it up so that one side of the middle transformer secondary is
SR> grounded? In general, are center tap grounded transformers
SR> better, safer, etc... than ones without the center tap grounded?
Tesla would almost always ground the center tap of the step up xfmr if
there was a tap available. There is no reason not too from a perform-
ance standpoint. Safety? Perhaps. If the center tap ground is connected
to the base of the secondary coil, it is possible for deadly 60 cycle to
appear in the clean RF discharge from the top of the secondary, but it
is a remote possiblity. Always, always, always, use a low impedance
dedicated ground for this type work; both for performance and safety.
I always say that the ground strap is the bottom line in high-voltage
SR> 3. How many transformers can I hook up in series before getting
SR> into trouble? I currently have 3 of them wired to (nominally)
SR> give me 12kV and I was thinking of wiring in the fourth to give
SR> me 16kV.
Assuming that the windings are not out of phase, and the voltage
produced is not arcing and burning up the coils, you can add as many
as you need, but 12kV would about as high as I would practically want
to take it.
SR> 4. I have a 35kV, 3kVA potential transformer that I bought
SR> surplus. I was going to use a variac to keep the voltage
SR> down to a more reasonable level, but will this really be
SR> suitable for coiling or will it just end up being a killer
SR> Jacob's ladder?
That transformer is a little extreme, even for me, but could be made to
work by placing a heavy stepdown transformer in the control circuit
after the variac. I have seen large step up/downs with a 2:1 winding
ratio that would be ideal for this. The problem here is that every
coiler pushes the equipment in search of a bigger spark, and if you
depend on just the variac alone to limit the voltage, you will inevit-
ably push it a little too far and something will blow. By putting in a
2:1 stepdown you will still have plenty of output voltage to play with,
and you will greatly reduce the chance of accidental component failure.
SR> I have a couple of large .04uF, 80kV DC rated caps that came out
SR> of an XRay generator and I was wondering if these would be good
SR> for Tesla coil work? They were used to filter the high voltage
SR> DC output to the XRay tube, so I'm not really sure if they would
SR> be good for pulse work?
Yeah I have one of those: .05uF, 50kVDC Sprague filter cap. Mine
is a black cylinder about 18 inches long, 6 inches in diameter.
Yes you can use them, and yes they are very poor for Tesla work.
One way you can tell if a cap is going to be suitable for Tesla work
is to look at the connectors (or terminals) provided by the manu-
facturer. My .05uF Sprague cap has recessed threaded mounts (doorknob
type) for a #10 screw. This is way too small to effectively pulse
discharge a .05uF capacitor, but is just fine for filtering out a
high-voltage DC power supply.
When I hook the cap up in the Tesla Tank circuit, the spark it pro-
duces is thin and violet, that is to say spindly and weak. Then the
cap starts to heat up (1-2 min). Next the case starts to sweat resin
(3-5 min). I usually shut down about here, though I don't know why,
I only paid $20.00 for the cap, and it came with no guarantee...
In other words it is definately not a Tesla rated component.
SR> Thanks in advance for you time. I've really enjoyed your posts;
SR> they've been very informative and have given me something to shoot
SR> for. I hope to have the time over the Christmas holiday to work on a
SR> 6" coil built as per your posts. Keep up the good work!
... If all else fails... Throw another megavolt across it!
___ Blue Wave/QWK v2.12