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Re: Microwave Xformer
In a message dated 97-01-13 00:42:11 EST, you write:
<<
> All this talk of microwave transformers and tube coils, etc.. has raised a
> few questions for me. I have some schematics of tube coils, etc, provided
by
> a few kind people on this list (thank you again). But I am wondering what
> the best tubes to use are... Also, are microwave oven transformers the best
>fitted transformer to operate a tube coil? They are fairly easy to get ahold
of.
>Thanks,
>Brad
>>
Microwave oven transformers (MOTs) are well suited for tube coils but they
have a number of advantages and disadvantages. They are generally shunted,
and therefore current-limited, and this right off the bat limits the amount
of spark length that can be obtained. Generally, a medium or large MOT can
give a 10" spark, 12" max. Also, they''re not particularily efficient. It
seems the manufacturers wanted to save money on copper and iron, or wanted to
keep the weight down--and I can't blame them. If you have two or more units
of the same voltage, they can be connected with outputs in parallel for more
current, but you may need a larger tube or multiple parallel tubes to handle
the power. In building these Tesla coils, the challenge is to obtain max
spark without melting your tubes. The voltage (about 2 kV ) is a little low
for larger tube TCs but is fine for small TCs. The shunts can be "whacked
out" of the MOT using a punch and hammer, this will increase the current
capability, but efficiency will drop like a rock.
For a really low-budget, or compact system, a little "trick" can be done with
the filament winding of the MOT: This winding usually has a few turns and
provides 2.5 volts at 15 amps. In many cases, there's enough room to add a
few more turns of wire in series with the original winding, thus giving the 5
volts that many tubes use. One disadvantage of this method is it makes it
harder to preheat the filaments, but the main disadvantage is that if you run
the system from a variac, when you turn down your high voltage, you also turn
down the filament voltage---hee hee ho!
If you want longer sparks (15 to 30") , it's a good idea to use one or more
plate transformers. I once used a 5 kVA pole pig to obtain 36" sparks. But
at this 5 kVA to 7 kVA power level, you'll need tubes that can withstand the
potent brunt of this electrical energy! I used (3) 833A's to obtain the 36"
sparks, the tubes were run at 6000 volts AC, which is way beyond their design
limit--but they did run cool, with just the slightest redness on the plates.
I don't really recommend using this high a voltage but it's interesting to
note that the tubes withstood this insult. A tube TC using a 1 kW, 3500
volt plate transformer with two 833As can give an 18" spark and draw 1100
watts. (It can give a longer spark, but efficiency will drop)
Here are some spark lengths that can be expected from various tubes: Two
graphite plate 811A's give an 11" spark, one 833A gives a 16" spark, two
833As give a 22" spark, one 4-250A gives a 12" spark, one 4-125A gives a 7"
spark, one 3-500 Z gives about a 16" spark. One 304 TL gives about a 13"
spark. There are many other tube types that work well. Other tube types
that work well are: 803, 845, 805, 8000, HF 300 , 211, 4-400, and many
others. Tubes with graphite plates are especially robust and can withstand
bruising current overloads. The 833C, which is the graphite plate version of
the 833A , is reported to be very good.
To obtain longer sparks or greater efficiency, pulsed tube coil operation may
be desirable.
John Freau