Re: Primary Coil Power Supply...
Subject: Re: Primary Coil Power Supply...
Date: 12 Jun 97 17:17:57 EDT
From: Alan Sharp <100624.504-at-CompuServe.COM>
To: "INTERNET:tesla-at-pupman-dot-com" <tesla-at-pupman-dot-com>
Neil, Tom and all
> I've had the same problems in the UK trying to find a source of
>transformers for my first coil. The only idea I've had is to buy
>transformer kits from Maplin, wire them up for 12.5 KVac, then put them
>parrallel??? to raise the Current. The only other possibility is going
>neon sign shop, and buying one from there. I guess if you are really
>adventurous you could try to get hold of one of those transformers that
>supply electricity to houses away from sub-stations, directly from the x
My first solution was to build a solid state coil!
Winding your own is going to be very difficult - 1000's of turns of very
wire. Plus the need for lots of insulation on small bobbins -
a life times work and the expense eek!
I remember someone in the UK getting a polepig for L50 from the
electricity board. But these can be very heavy.
The other possibilty is oil ignition transformers - 10kV 23mA -
fine for starting out - try heating engineers.
Dead micro waves are another possibility, I put a note in my parish
netted 4. Father Tom reports good results using two in series,you can
centre earth connections if you wire them in
anti phase. 2 gives you about 4.5kV at 1500W. Micro waves also contain
a 0,2uf 2000V AC capacitor. A number of these can be wired in series
to give an AC rating at least double the transformer output. They are
lossy but useable.
For current limiting I used a switchable resistor bank design to
deal with 750W, or 1250w or 2000W at 240V - yes an old convector
heater in the mains path (earth it). I was using it to current limit
a Jacobs ladder, I hope to fire up a conventional coil in the next
month - if I get time off! Resistors loose power of course but initially
I'll be happy with any length of spark. I've got a 5KV 330mA transformer
and two 0.01uf 30kV DC caps (one off finds).
I attach some previous postings on using uwave transformers and the
UK source guide - which is very incomplete.
(If all else fails get a long thin wire, a piece of string,
a kite and an over head cable......... :)
(or there is a completely alternative - very early lightning
machine design using 12 stones, bullocks, and 100 gallons of
water in 1 Kings chapter 18 verses 30 - 39)
Previous postings follow:
I once connected 4 microwave oven transformers together (secondaries in
series, primaries in series parallel) , two of the cores were grounded,
two I kept floating
at high voltage, (these floating cores have to be kept insulated from
everything else). These transformers all had shunts in them, BTW. The
primary arrangement allowed me to use 240 volts input for greater
But these transformers are inefficient in general, anyhow the system
60 " spark and drew 6 kW. You could see a corona effect between the
and the windings, on the floating core transformers because they' re not
supposed to be used in this manner. In time, they'd probably break
I didn't have a problem. For longer life, these floating cores could be
emersed in oil. I'd recommend this technique only if you absolutely
get better transformers, or just want to build a bizarre system!
From: Richard Quick <richard.quick-at-SLUG.ORG>
Organization: St. Louis Users Group
Subject: Transformers in the UK
To: Multiple recipients of list USA-TESLA <USA-TESLA-at-USA.NET>
* Original msg to: Tesla-at-grendel.objinc-dot-com
Quoting <i_hopley-at-wintermute.co.uk> (Ian Hopley):
> My two transformers give me about 1.1KVA after unpotting and
> removal of a couple of shunt plates, and are probably more than
> ample for my first attempts, although i'll probably soon
> outgrow and/or destroy them in my first efforts.
> This has led me to look at alternatives and have considered
> microwave units as a bit of a safty liability, lower power
In Volume 14, #3 (July-Sept 1995) of the Tesla Coil Builder
Association publication _NEWS_ there is an excellent article
on placing four microwave transformers with the primaries in
parallel and the secondaries in series (hot coring the end two
units) to produce a workable power supply.
Last night my good coiling buddy Dave Hartwick calls me and asks
if his 11.5 kV 300 mA microwave power supply would be of interest
to anybody... Anybody?
Dave has been making some really simple modifications to these
transformers and is coming up with solid HV power supplies for
Tesla coils. He indicated by phone that on 9 out of 10 microwave
transformers he has worked with, the grounded lead wire from the
secondary can be freed from the core with minimal effort. The
freed up wire is then soldered to an extension lead, the
connection is insulated with a section of heat shrink tubing.
Epoxy, hot glue, or polyester resin is used to beef up the
insulation between the core and the two windings.
He then places six units with the secondaries in series to obtain
the required voltage rating without hot coring.
He admits that there is a possibility of breakdown with the
possibility of the HV getting back to the LV mains; so he uses
inductive current limiting and an isolation transformer with
grounded core for safety.
Dave is giving up this line of experimentation soon because he
has a new pole pig on it's way, but I have implored him to write
a paper on this for publication (he has only a 286 with no modem
and a 5-1/4 floppy drive, I have also begged him to upgrade
systems and join us...)
I hope this sparks as much interest as I feel it is worth,
especially to you guys in the UK.
... If all else fails... Throw another megavolt across it!
___ Blue Wave/QWK v2.12
From: Thomas McGahee <tom_mcgahee-at-sigmais-dot-com>
Subject: Microwave Transformers / Capacitors
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Greetings! I am new to the 'net, but have been involved with many
scientific projects over the years, and would like to contribute to the
general pool of information that we all draw out of.
Concerning microwave oven transformers: They are usually about 2,500V
the current varying from model to model. 250ma is what a small one is
at. They go up to about an amp in some models. If you try to directly
measure the voltage with a regular voltmeter, you stand a good chance of
frying the meter. Even though your meter may have a high voltage scale,
read the manual carefully, because many of them have an absolute upper
limit of about 2KV AC RMS. At this voltage the internal insulation and
voltage divider just can't handle it any more.
A sneaky way to measure the voltage is to apply the full 120VAC from
power line to the SECONDARY, and then measure the LOW voltage that
at the primary. Use this to determine the voltage RATIO, and then use
to determine what the output would be if you put 120 into the primary.
the way, this is a good method to determine the output rating of ANY
voltage transformer that was designed for 60Hz operation. If you are
the method with a transformer that has ANY of the windings connected to
case, then make sure that the case is resting on an insulated surface
during the measurement, and DO NOT TOUCH THE CASE while the measurements
are being made. High voltage isn't the ONLY voltage that can kill!
can knock you for a loop if your skin is moist.
Most microwave transformers have one side of the high voltage winding
connected to the case or frame of the transformer. Removing this
is not always possible, and even if it was, doing so would be somewhat
dangerous, since the designers assumed that THAT side would be residing
ground potential. Although ONE microwave oven transformer will only
2.5KV, TWO of the suckers can be wired so as to produce 5KV at 250ma or
greater! I'll get to the importance of that current factor in a minute.
use two transformers you first connect the frames together. This will
as a frame-connected center tap. Note that all this means that this
arrangement is subject to all safety precautions pertinent to high
transformers with a center tap attached to their case or frame. (I have
posted a notice about this today for those who are interested). I prefer
mount both transformers on an INSULATING support made of plexiglass or
other insulating material. I place them about an inch apart, and then
connect the frames using a good heavy wire (I use #12 single strand
and crimp-on terminals (which I always solder after I crimp). We want to
connect the primaries of the two transformers in anti-parallel. For
example, if each primary consisted of a white wire and a black wire, we
want to connect the black of one to the white of the other, and the
of one to the black of the other. That is the opposite of a true
circuit... it is anti-parallel. The reason we do this is so that when
transformer's secondary is giving out a large Positive potential, the
transformer's secondary will be giving out a large Negative potential.
net result is that the voltage DIFFERENCE between the two high voltage
leads will be 5KV. What we have just done is create a 5KV at 250ma
transformer that is rated at 1.25 KiloWatts. Not too shabby! It draws
12 amps under full load, so use adequate sized wire, not some dinky line
cord that you stole off of a tv set.
Such a transformer has a relatively low impedance at its secondary. This
means that it can pump LOTS of current very efficiently into a storage
capacitor. This makes it dandy for charging those medium high voltage
larger capacitance pulse capacitors. Note to Q freaks: You can increase
Q of a capacitor by reducing the thickness of the insulation.. this also
reduces the voltage rating of the capacitor. But now we are only talking
an RMS voltage rating of something bigger than 5KV, not the usual 15KV.
terms of power rating, a 5KV transformer at 250ma is the same as 15KV at
83ma. And there is nothing to stop you from paralleling two of the above
5KV units to achieve even higher current (and thus POWER) ratings. There
a limit to how much current you can draw from a wall plug before you
When using a 5KV 250ma transformer as described above to build a Tesla
coil, be aware of the fact that the spark gap will have to be closer
is usual with 12KV transformers, and it will get very very hot. The
comprising the gap material must be robust! Remember that you want a
Disruptive Discharge, so avoid sharp edges if using a stationary gap
blast it with as much air as you can get. A rotary spark gap works well
with these designs, and it has less of a cooling problem.
When using relatively low High Voltages such as 5KV, you compensate by
using larger capacitances to keep the power level as high as possible.
the Tesla coil primary DC resistance as low as possible by using copper
tubing instead of wire. I like to use the larger diameter tubing, stuff
larger than 1/4 inch diameter. Cuts down on corona, and helps keep the Q
high. Keep ALL connections tight and solid. Solder or Bolt connections
wherever possible, and when making removeable connections, such as
taps, try to maximize the surface area, because the current flow is
a surface phenomenon at high frequencies. The weakest link in your coil
design will be the limiting factor! ONE bad connection in a series
limits the current throughout the entire circuit.
Another note of interest: The thickness of the insulation of a capacitor
affects both its voltage rating and its capacitance. Let's see what
if we take the same amount of physical material that would comprise a
.1 mfd capacitor and use it to build a 5KV capacitor. Assume the
is three sheets rated at 5KV per sheet. One sheet would therefore be
to insulate our 5KV. But now the distance between plates is 1/3. so the
capacitance of just this one sheet is now increased to .1*9=.9mfd Ahhh,
but we ain't done yet! Use all three sheets the same way and you now get
.9*3=2.7mfd. Energy storage is the product of the voltage times the
capacitance. The original 15KV .1mfd capacitor had an energy storage
of 1.5 millicoulombs. The 5KV 2.7mfd capacitor has an energy storage
of 13.5 millicoulombs, a factor of 9 better for a capacitor using the
amount of physical insulation!
For you REALLY high voltage freaks, note that if you took the three 5KV
.9mfd capacitors and put them in SERIES you would get a capacitor rated
15KV and .3mfd, a power increase by a factor of 3 over the original 15KV
.1mfd capacitor. This should tell you something about how to build
capacitors that are rated for either higher voltage or higher power
within a smaller physical size. Or, you can use the technique to make
capacitors of the same SIZE, but HIGHER POWER.
Another note about such capacitors: The Q of the capacitor increases
drastically as you put the plates closer together. The dielectric can
the stored charge faster. To keep the Q high, make the capacitors flat
rather than rolled. Rolled capacitors are OK, but there is inductance
that is NOT present in a flat capacitor design. Capacitors should be
immersed in oil and voided of all air possible. The oil must be "dry"
moisture such as water), and free of contaminants. When the insulation
thinner, you must be careful to avoid burrs on the metal sheets, as they
can puncture the insulation easily. A super-heavy duty industrial
Aluminum foil is adequate for the plates, and has the advantage of not
needing any deburring. Remember that in any style of homemade high
capacitor that is oil filled you must keep safety in mind and have
safety venting in the event of insulation breakdown. Somewhere there
be a clear plastic window so you can see if there is internal arcing.
Bottom line is that two microwave oven transformers CAN be used to
a quite powerful Tesla coil if you pay close attention to the spark gap,
capacitor, primary, secondary, and use a toroid on the secondary.
that the actual Voltage you apply across the Tesla Primary is not the
limiting factor, but the POWER you deliver, and the tuning of the
When all else fails, try something else!
Fr. Thomas McGahee
(Yep, I am a 50 year old priest that has taught electronics for 30
am a dyed-in-the-wool Amateur Scientist, having built numerous Tesla
X-ray machines, Lasers, and literally hundreds of other fun things. I
I can learn something from each of you, and contribute something in
Sources - UK
Get a catalogue from Linsay Publications - New York - lots of
interesting books that you can't get here.
No email but you can fax them on - USA 815 935-5477 (24hrs)
Ask for a free catalogue.
my stuff came very quickly and they take credit cards.
Tesla Coil by George Trinkaus 24 pages, very home brew but
cheap and well presented. $4.95
Tesla Man out of His time: Margaret Cheney - good biography of Tesla
Tesla Coil Handbook - Todd A.Pringle 60 A4 pages - well presented
should get you up and running. $9.95
TESLA COIL CONSTRUCTION GUIDE - 75 pages - $18.00 List
Contains plans for building 5 different coils with spark lengths from
one inch to 8 feet. All of the plans have been designed using the latest
Tesla coil engineering technology and are based on actual coils built
tested by the author and other coilers. A unique step by step design
procedure and graphs are shown for designing Tesla coils using manual
calculations for designers who do not have a computer. The Guide
Sections and 37 Illustrations.
Shipping - 2nd day air was reasonable - my orders came in 4 days!
Or International Tesla Society
FAX USA 719 475-0582
(airmal $7.25 first item $2.50 other items)
Duane Bylund: Modern Tesla Coil Theory 140 pages - good on theory and
practical - covers valve (tube) and solid state designs as well as
Spark Gaps. (poor quality print - good content) $19.95
Brent Turner: Tesla Coils - good on spark gap and valve designs.
"I have finished up a totally re-vamped book. Cleaned up some of the
text, added a BUNCH of stuff, clarified everything, and added TONS
of pictures, drawings, etc. The new book is entitled "The Tesla Coil
Book - How they work and how they are built". 198 pages, and a cover
price of only $23.95 now. (Better, cheaper, etc.) It's not really
geared for the Ph.D, but gives a good, solid foundation that the
novice/intermediate builder can grow from. (Available from me, but
not yet from the ITS though. I *just* got it finished...) "
RS Components (Corby)
Farnell Components (Leeds)
Vast catalogues but no high voltage transformers, or caps.
(These firms can be sniffy about dealing with individuals so style
yourself Joe Blogg services - ask RS for their catalogue on CD ROM.
Scientific Wire Co - wire.
oil ignition transformer can be found apparently from
yellow pages heating engineers. Not so many neon's though - planning
restrictions means we don't have so many here.
Someone got a pig pole from an Electricity Board.
Or try micro wave transformers in series.
Transformers are too heavy to consider importing.
I've tried Bull, Greenweld, Anchor with no luck on this stuff.
Haven't found any UK sources.
Could import. or build your own
beer bottles make a cheap first capacitor
Follow the waterboard as they tear up the streets for offcuts of
6" dia PVC or bigger. 4" diameter PVC is widely available at
Do It All have 4" flexable Aluminium ducting.
Aluminium foil tape is available from RS and Farnell
Look out for scrap - my daughters hair dryer just
blew but the motor and fan will do to quench a gap.