Transformer question (only slightly OT)

Tony Duell ard at p850ug1.demon.co.uk
Tue Sep 5 18:39:22 CDT 2006


> Wall warts, least here in USA must be fused internally or have a thermal
> interupter (usually the blow open).  I've forcably opened a number of 
> warts to replace protective devices.

I've opened up many wall warts over here and found the the transformer 
primary winge goes straigh to the tags on the transformer (no obvious 
connections to a fuse or thermal fuse) and those tags are wired straight 
to the mains plug pins. And yes, said wall wards have various approvals 
written on them.

I was told that the transfromer primary is supposed to fuse safely in the 
event of a fault. My experience is that it doesn't (I've had shorts on 
the output side cause the plastic case to deform and much magic smoke to 
come out, but the transformer primary was still continuos after I turned 
the power off. I did not feel like seeing if it would ever fail).


> 
> For many projects I use a standard 12V regulated DC wart and if higher
> voltages are required a simple multivibrator or switching regulator driving
> a suitable hunk of ferrite does nicely.  It allows me to work inside
> without scary high potentials floating around and also solves the problem 
> of "the right transformer".  If warrented I bury the HV system in a 

I'd much rathere make my own supply starting with a suitable transformer. 
Then I know it's correctly fused, It also means I can fit a real on/off 
switch (in the mains circuit).

Of course for experimental stuff, I just run it off the adjustable bench 
supply. 

-tony




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