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NSTs and The HEAT
Coilers,
The confusion comes from the multiple meanings we assign to the term
"burn out the transformer".
When you run a neon sign transformer un-loaded then you stand the
greatest chance of destroying the transformer *DUE TO OVER-VOLTAGE
ON THE SECONDARIES*. Recall (or learn for the first time) that neon sign
transformers are designed to run under load and are current-limited.
When you first turn one on, the quite high voltage of several
kilovolts will cause the neon gas in the neon sign to ionize and
begin conduction. Once the sign is 'on' then the voltage required to
maintain light production is reduced *greatly*. Typically less than
a kilovolt is necessary to *maintain* conduction, even though several
KV are needed to initiate conduction.
If the NST is run with no-load, then the transformer gets to
experience the full stress of the rated secondary voltage. But the
NST was not designed to experience this level of sustained voltage
for very long. Thus, running the NST un-loaded can cause a voltage-
related breakdown of the transformer.
When the NST is destroyed in this manner you might refer to it
as having been 'burned out' due to the HV breakdown, but this
terminology is somewhat misleading.
In a "regular" un-limited transformer you usually experience true
"burn out" when one of the secondaries gets shorted out and the
secondary begins to draw excessive current. A neon sign transformer
cannot experience this form of "burn out" since it is current-
limited.
If you short out the secondary of a neon sign transformer then the
current-limiting shunts will limit the current. The secondary has
a resistance. The limited current will flow through the secondary
and there will be ohmic heating of the secondary following the
familiar P=E*I or P=I^2*R power formula. You can run the neon
sign transformer in this shorted mode all day and it will *NOT*
"burn out", because it was DESIGNED to run in current-limited mode.
It *WILL* get warm/hot because of the above-mention power loss in
the secondary windings, and it is this heating that helps get the
tar next to the transformer loosened up after you have frozen
a neon sign transformer prior to final disassembly.
To re-cap:
(1) An un-loaded NST will run cool but can experience voltage-
related breakdown, since its output should normally be less than a
kilovolt once it has ignited the neon sign tube.
(2) If you short the secondary of a neon sign transformer then the
current-limiting kicks in. The transformer will run hot, but
it was designed to do so, and this will not damage the NST in any
way.
I hope that this explanation helps.
Fr. Tom McGahee
----------
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Remove tar from NST
> Date: Thursday, September 24, 1998 7:00 PM
>
> Original Poster: Daniel Hess <dhess1-at-us.ibm-dot-com>
>
> Malcolm, All;
>
> Well doggonnit! Now I'm going to have to dig out my "Neon Sign Techniques"
> manual and get to the bottom of this open/shorted secondary thing. As I
> recall,
> the fastest way to burn up a nst was to run it under loaded, something
you'ld
> want to avoid in the neon sign business. And an open secondary was
considered
> the ultimate underload albeit something desirable for the purpose of
depotting
> the core.
>
> I'll dig it out this weekend and post the results of my research and who
> knows;
> Maybe I'll get to eat some crow while I'm at it...
>
> Daniel
>