RE: PT CURRENT LIMITING QUESTION!

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "David Dean by way of Terry Fritz <twftesla-at-uswest-dot-net>" <deano-at-corridor-dot-net>

Hi Drew,

In order to limit the current from the potential transformer to 60mA (a
reasonable value, given as an example only) you need to limit the input
current to 7.2 Amps. This is assuming input of 120V and output of 14400
Volts, or a 120 to 1 turns ratio. A resistor of 16.66 Ohms with a power
rating of about 900 Watts would do the trick. An inductor of 44mH capable of
passing a current of 10 Amps would be ideal. A single MOT with the secondary
shorted would get you close.

The easy way would be to parallel two or three of the blown OBITs and hook
that combination in series with the low voltage winding of the PT. Just be
sure to short the secondaries of the OBITs so that they cannot make any
unwanted sparks just in case one or more happened to decide to "come back to
life" all of a sudden. Also you will want to check the primaries with a mete
to make sure they have continuity, and plug each one in by itself to make
sure the primaries are not shorted (it will pop the breaker if they are).

As to the fluorescent ballasts, they will pass about an Amp if they are
supposed to light two four foot bulbs. (the most common size) If you short
the output leads to a fluorescent ballast they will overheat and start
leaking in short order. I don't know of anyone who has used them for ballast
for a TC, I'm sure someone somewhere at sometime probably has. The question
comes up from time to time, the standard answer is that they are really
unsuitable for the intended use.

As for the 12V transformers, if you have some large ones, the 6kg size might
be OK, they could work if the primary current rating is high enough. The
winding ratio is about 10 to 1, so to get an Amp on the primary the
secondary would have to be able to put out 10 Amps. It would take 8 10A
transformers in parallel to do the job. These are not current limited
transformers (usually, unless specially designed for battery charging or
something like that) so the secondaries will have to be left open. 12V
floating "in space" is not too problematic.

The transformers from the tube type oscilloscope may offer some interesting
possibilities. You may have one or more high current filament transformers,
a couple of B+ transformers of the 450 - 0 - 450 volt or 600 - 0 - 600 volt
variety, as well as the possibility of a high voltage transformer which
could be used as a power supply for the TC in its own right. In the old
days, the acceleration voltage for the CRT was often supplied by a high
voltage transformer. I have an old Dumont Dual Beam Oscillograph from 1952
originally used as a radar gun sight on a Navy vessel which is made that
way. It has two large square cans that look almost like potted lunch meat
came in them (also something from the old days) which are the acceleration
voltage transformers. They are 4.5KV. If your scope has one or two
acceleration voltage transformers, they could be used as a TC supply, albeit
a small one. I'd say these parts would be of greater value if properly
tested and identified and sold on Ebay or something like that, and used as a
fund raiser for getting some more usable and needed parts for your project.

My money is on the OBITs.

later
deano

> -----Original Message-----
> From: Tesla list [mailto:tesla-at-pupman-dot-com]
> Sent: Saturday, February 03, 2001 10:00 PM
> To: tesla-at-pupman-dot-com
> Subject: PT CURRENT LIMITING QUESTION!
>
>
> Original poster: "Drew Murray by way of Terry Fritz
> <twftesla-at-uswest-dot-net>" <drewallmighty-at-hotmail-dot-com>
>
> Hello all,
> I have a big question about potential transformers, but first I will tell
> you my story. My first transformer was one of five oil burner
> ignitors that
> i paid ten dollars for. I was running my coil with no safety gaps or
> anything. I have since blown every one of the OBITs. All that is probably
> wrong with them is that they need to be depotted, but i don't
> feel that it
> is worth it. They are now serving as nice heavy boat anchors. I
> am getting a
> potential transformer on tuesday and i will need a way to limit
> the current
> flowing to it. The word ballast has also popped up in documents
> about this
> subject which i have read, and it makes me wonder. Can flourescent lamp
> ballasts be configured to serve as ballasts for PTs? I have also heard
> mention of MOTs being used to limit current to PTs. I don't have any MOTs
> nor is there anywhere local that i can get them, (unless i pay a
> rediculous
> amount of $$$ for new ones). I do have 312kg of small to medium
> sized step
> down transformers ranging from 20g to 6kg in weight. Most are 115v - 12v
> transformers from radios and stereos. I have a few really big
> ones that were
> from a huge, i mean monsterous, vacuume tube oscilloscope.
>      I have thought about the similarities between these transformers and
> MOTs. I think that they are built the same but just use different winding
> sizes and guages of wire. Is it possible that i can use these
> transformers
> to ballast a small Potential Transformer? If so, how would i
> configure them?
> Please help.
>                                                       Up here in Canada,
>                                                       Drew Murray
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