RE: [TCML] 7.5V 52A Filament Transformer for 3CX3000 Needed

["Dr.Hankenstein" <dr.hankenstein@xxxxxxxxxxxxx>]

Normally, filament xfmrs should be chosen to approximate the max load.
The reasoning behind this is as follows:

Filament resistance is very low at start up, thus causing heavy inrush
current that could damage the filament.

By closely matching the xfmr impedance to the filaments' maximum
operating  current, the inrush can be greatly reduced, resulting in
longer filament (and tube)life.

One could, of course, use a variac (or other current limiting device)
and bring on the current slow and easy for even better results. 

Hank

-----Original Message-----
From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
Behalf Of jimlux
Sent: Friday, June 12, 2009 6:18 AM
To: Tesla Coil Mailing List
Subject: Re: [TCML] 7.5V 52A Filament Transformer for 3CX3000 Needed

Phillip Slawinski wrote:
> DC,
> 
> Pardon me, but that seems a little absurd.  I can think of many
> reasons not to do this.  To begin with the transformer would make a
> good power source for the tube in question.  Secondly if one were to
> do this, the transformer would be many times larger, and heavier than
> it needs to be.  If anyone has enough 7.2kV pigs to do this why
> wouldn't he just sell one to buy a real filament transformer?
> 
> -Phillip Slawinski


"real filament transformers" are pretty expensive, new, and, as noted by

the original poster, are hard to find used/surplus/cheap.  Oddly, it's 
probably easier to find an inexpensive pole transformer than a 
inexpensive filament transformer.

That said, using a 7.5kVA core for an application needing 400-500VA or 
so is somewhat overkill.  If one has them, a couple microwave oven 
transformers might be a better starting point (yes, a typical microwave 
is 600W, but the transformer gets pretty toasty.. I wouldn't use one at 
500VA for continuous duty).  And hey, the MOT already has a 5V or so 
winding on it.





> 
> On Thu, Jun 11, 2009 at 15:43, DC Cox<resonance@xxxxxxxxxxxx> wrote:
>> If you can't find one, you can do what I did:  remove the outer HV
winding
>> on a 7.2 kV pole xmfr, 5 kVA, then wind on some #6 AWG wire to make
your
>> own.
>>
>> Experiment with 10 turns of finer wire to get the turns ratio right,
then
>> wind on the big wire.
>>
>> Dr. Resonance
>>
>>
>>
>>
>>
>> On Thu, Jun 11, 2009 at 7:45 AM, Cameron B. Prince <
>> cplists@xxxxxxxxxxxxxxxxx> wrote:
>>
>>> Hey guys,
>>>
>>> Does anyone have a 7.5V 52A filament transformer they¹d be willing
to part
>>> with?
>>>
>>> We actually need two if available. Please contact me off-list.
>>>
>>> Thanks,
>>> Cameron
>>> _______________________________________________
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>>> Tesla@xxxxxxxxxxxxxx
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>>>
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