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Re: [TCML] Self ballasting transformers



B,


The transformers used on regular appliance power supplies (linear supplies, not switching supplies) do not "self-limit" the current passing through them. It is up to the circuit designer to limit the amount of current that will be drawn from them by selecting an appropriate power supply design.

Given the opportunity, the ordinary transformers would attempt to provide infinite current, or at least an amount limited by the resistance of the primary and secondary windings, and the magnetic properties of the core. However, this is not a desirable operating mode, as there will be major heating losses in the windings that will also degrade the wire insulation and eventually lead to smoke, flames, and destruction of the transformer if you are persistent. Just as ballasts are required to limit the current draw of pole pigs, proper linear power supply design limits the current draw through appliance transformers.

Transformers like neon sign transformers and, to a lesser degree, microwave oven transformers, do have magnetic shunts that limit their short circuit current output, but such transformers are intended for specialized apps, and such designs are are definitely more an exception than the rule.

As I posted recently, you can estimate the amount of current a standard laminated silicon steel transformer core is rated for with the following equations based on the cross sectional area of the transformer core. Your ordinary E-I transformer was not current limited in the first place, so when you changed / added windings, you changed the resistance of the windings, and thus you may be able to pull more power through the transformer. Switching to two E cores did not increase the core cross section, so any differences in power output were due to changes in the wire characteristics.

However, you might want to scope the output`of your modified transformer and see if you are having significant deformation of the sine wave shape of the output voltage, which would indicate that you are driving the transformer core into saturation, which is undesirable.


Area [sq in] = 0.16 * SQRT(VA)


otherwise expressed as:


(Area [sq In.] / 0.16) ^ 2 = VA (volt-amps)


HTH,


Dave


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?
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