[Prev][Next][Index][Thread]
Microwave Transformers / Capacitors
[The following text is in the "ISO-8859-1" character set]
[Your display is set for the "US-ASCII" character set]
[Some characters may be displayed incorrectly]
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 with
the current varying from model to model. 250ma is what a small one is rated
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 your
power line to the SECONDARY, and then measure the LOW voltage that appears
at the primary. Use this to determine the voltage RATIO, and then use THAT
to determine what the output would be if you put 120 into the primary. By
the way, this is a good method to determine the output rating of ANY high
voltage transformer that was designed for 60Hz operation. If you are using
the method with a transformer that has ANY of the windings connected to the
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! 120VAC
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 connection
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 at
ground potential. Although ONE microwave oven transformer will only provide
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. To
use two transformers you first connect the frames together. This will act
as a frame-connected center tap. Note that all this means that this
arrangement is subject to all safety precautions pertinent to high voltage
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 to
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 copper)
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 white
of one to the black of the other. That is the opposite of a true parallel
circuit... it is anti-parallel. The reason we do this is so that when one
transformer's secondary is giving out a large Positive potential, the other
transformer's secondary will be giving out a large Negative potential. The
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 about
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 but
larger capacitance pulse capacitors. Note to Q freaks: You can increase the
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 of
an RMS voltage rating of something bigger than 5KV, not the usual 15KV. In
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 IS
a limit to how much current you can draw from a wall plug before you trip
the breaker.
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 than
is usual with 12KV transformers, and it will get very very hot. The metal
comprising the gap material must be robust! Remember that you want a
Disruptive Discharge, so avoid sharp edges if using a stationary gap (and
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. Keep
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 variable
taps, try to maximize the surface area, because the current flow is mostly
a surface phenomenon at high frequencies. The weakest link in your coil
design will be the limiting factor! ONE bad connection in a series circuit
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 happens
if we take the same amount of physical material that would comprise a 15KV
.1 mfd capacitor and use it to build a 5KV capacitor. Assume the insulation
is three sheets rated at 5KV per sheet. One sheet would therefore be enough
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 rating
of 1.5 millicoulombs. The 5KV 2.7mfd capacitor has an energy storage rating
of 13.5 millicoulombs, a factor of 9 better for a capacitor using the same
amount of physical insulation!
For you REALLY high voltage freaks, note that if you took the three 5KV at
.9mfd capacitors and put them in SERIES you would get a capacitor rated at
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 density
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 dump
the stored charge faster. To keep the Q high, make the capacitors flat
rather than rolled. Rolled capacitors are OK, but there is inductance there
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" (no
moisture such as water), and free of contaminants. When the insulation is
thinner, you must be careful to avoid burrs on the metal sheets, as they
can puncture the insulation easily. A super-heavy duty industrial strength
Aluminum foil is adequate for the plates, and has the advantage of not
needing any deburring. Remember that in any style of homemade high voltage
capacitor that is oil filled you must keep safety in mind and have adequate
safety venting in the event of insulation breakdown. Somewhere there should
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 produce
a quite powerful Tesla coil if you pay close attention to the spark gap,
capacitor, primary, secondary, and use a toroid on the secondary. Remember
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 circuit
to resonance.
When all else fails, try something else!
Fr. Thomas McGahee
(Yep, I am a 50 year old priest that has taught electronics for 30 years. I
am a dyed-in-the-wool Amateur Scientist, having built numerous Tesla Coils,
X-ray machines, Lasers, and literally hundreds of other fun things. I hope
I can learn something from each of you, and contribute something in return)