Re: [TCML] Self ballasting transformers

[David Speck <dave@xxxxxxxxxxxxxxxx>]

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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