[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [TCML] Self ballasting transformers
These small transformers are typically 75 watts or less, with maximum
voltage outputs of 30 volts RMS (Appliance Class 2) or 150 volts RMS
(Appliance Class 3). They may be either non-limiting (requiring an
external current limiting element, fuse, or circuit breaker), or are
inherently limiting by intentionally introducing a leaky, or partially
bypassed, flux path between primary and secondary windings. Non-limiting
small appliance transformers will typically use a circuit board fuse,
circuit breaker, power resistor, or a Positive Temperature Coefficient
(PTC) devices to provide overcurrent protection. The IEC appliance
protection classes (0 - 3) are described here:
http://en.wikipedia.org/wiki/Double_insulated#Class_II
Inherently current-limiting transformers add one or more magnetic shunts
between the primary and secondary windings, or one or more air gap(s) in
the flux path(s) that link the primary and secondary windings.
Transformers using magnetic shunts will have the primary and secondary
windings physically located side by side (using a single split bobbin or
two or more separate bobbins. The magnetic shunts then bypass some of
the flux between windings - this is used in non-electronic NST's,
OBIT's, and many doorbell transformers. Alternatively, some manufacturer
may locate windings on opposite legs of the core and add one or more air
gaps in the flux path between windings.
Only inherent current-limiting transformers can safely be used without
external current limiting/fusing. Class 2 and class 3 transformers are
defined within the National Electric Code for "power limited circuits",
and were previously governed by ANSI/UL 1585. They are now covered by
ANSI/UL 5085-2 or -3 for US, Canada (as identical standard CSA C22.2 No.
66.3), and the European Union. Unfortunately, copies of UL-standards are
only available by purchase if you really want all the gory details.
However, a brief summary of the salient points and scope can be found here:
http://tinyurl.com/6xxn3j
http://ulstandardsinfonet.ul.com/scopes/scopes.asp?fn=5085-1.html
http://ulstandardsinfonet.ul.com/scopes/scopes.asp?fn=5085-3.html
Bert
--
***************************************************
We specialize in UNIQUE items! Coins shrunk by huge
magnetic fields, Lichtenberg Figures (our "Captured
Lightning") and out of print technical Books. Visit
Stoneridge Engineering at http://www.teslamania.com
***************************************************
bsneath@xxxxxxxxxxxxxxxx wrote:
hi all,
I have a question regarding the self-current-limitting transformers
that are used in most old appliances. It would seem that they have no
shunts at all, and the only way they ballast themselves is with a
large number of primary turns. To clarify, the transformers i'm
talking about are those little <100watt ones that powered stuff like
DVD players etc. before switching psu's took over. But how does it
work? Another thing, when i put a new secondry on one such xfrmr
after extending the core (by using 2 E sections back to back instead
of the usual E I construction), the current limiting no longer worked
as well and i could draw more power out of it than it was rated for.
so, how do these xfrmrs self-limit, and how does the core extension
affect it?
_______________________________________________ Tesla mailing list
Tesla@xxxxxxxxxxxxxx http://www.pupman.com/mailman/listinfo/tesla
_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxxxxxx
http://www.pupman.com/mailman/listinfo/tesla