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Re: use of a MA meter

>I would like some feedback on the use of the availability of a good but 
>reasonable ma meter.  I have always used the standard chart for sizing 
>transformers to units.  Is this not a reliable way any longer?  
>Howie Cohen
>

It's not so much that the method is no longer reliable - The question is how
reliable was your job?  Consider all of the things that can effect an
"ideal" unit.  True, the chart is pretty darn close and in general will
work. But it can't for example tell you the status of a tube that's been in
service for either a specified or unspecified period.

A transformer is supposed (note I say 'supposed') to have a 30mA rating
(Open circuit) and it's supposed to run at 24mA (closed circuit) which
represents 80% of its maximum rating.  The physical design of the
transformer is such that at 80% you have  the "correct" amount of heat,
voltage, current, etc.  That is, once breakover voltage is reached and a
large amount of current starts to flow -  the transformer is designed for a
specific amount of current to "leak" or shunt back to the primary and
thereby keep it at the proper level. 

The transformer attempts to put out whatever breakover voltage the tube
requires.
If your transformer is under-rated it will generate spikes or square waves
that break through the insulation. If your transformer is over-rated the
amount of current it allows once break over is achieved is will be too high
and the insulation will break down thermally.

You can try the "old timer" method which is fairly accurate for neon but not
argon-merc, where you place a voltmeter across the secondary of an
autodimmer attached to the primary of a large transformer.  Bring the
voltage down and then up to what is called the "flicker point" and note the
dimmer's output voltage.  Divide this by the full line voltage (this can
fluctuate) and you will likely be somewhere in the 70 to 80% range.

You will find that just about any AC (magnetic) ammeter with a range of
0-50mA or more will work fine in line with the high voltage.  It seems
strange but you have to realize that there is not a large voltage drop when
you are in series.  Usually (that's usually) an Ammeter has a tiny shunt
resistor that limits the current to the magnet coil's requirement. But don't
mess with this unless it is a cheap used one that you don't care about
blowing off in case I'm wrong. 

Oh yes, one more method is the volts per foot method. I'll have to dig my
chart out and put it on the web site.  12mm neon for example uses 300volts
per foot and the same for an electrode pair.  Arg-merc 12mm is 225volts per
foot.  You add this up and choose the next highest transformer.  

Kenny


  
Kenny Greenberg   Neon Artist, Scenic Specialist, Columnist - Internet World
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