[XCSSA] Can you break transformers?

[X-otic Computer Systems of San Antonio]

My thoughts:

1. AC voltages are generally not regulated.  They are rectified to  
DC, then the DC is regulated.

2. The rectified DC voltage you get from 12VAC depends on type of  
rectifier, load, regulation, and other factors.

3.  When you measure the AC voltage output from a transformer, there  
are two kinds of errors:
      a) Voltmeter may not be "true RMS" and AC will not be perfect  
sine wave.
      b) Measuring unloaded, output will be higher because  
transformer secondary (output) has some
          resistance.  It is designed to deliver rated voltage into a  
rated load, not open, and low
          current transformer will have a fair amount of resistance  
so open output voltage will be
          considerably higher.

Mainly because of 3b, your measurement of 15VAC from 12VAC  
transformer is very
plausible.

I would recommend one of the following choices to avoid destroying  
transformers:

1) do experimentation with a separate supply.  I keep a 0-50VDC lab  
surplus supply for experimentation.  Do not use irreplaceable vintage  
supply or transformer for experimentation!

2) Use appropriate fuse on transformer secondary.  Generally slow- 
blow is OK for big transformers, but little transformers might need  
fast-blow and even that might not be fast enough.

Are you destroying vintage Commodore PSU's?!?

Little generic wall-warts are notoriously easy to blow.  The current  
rating is often a "max" rating
and anything beyond that will blow it.  For safety purposes (UL and  
all that) there may be a fuseable link built into the transformer  
itself.  Better to blow wall wart than burn down house.  For  
experimentation, get a wall wart about 3x larger than needed.

If you buy a "real" transformer part it won't blow until it actually  
overheats and melts insulation.  Intended for use inside fused box of  
some kind, so transformer itself isn't "internally fused".

Charles Peterson


On Sep 4, 2008, at 10:04 PM, X-otic Computer Systems of San Antonio  
wrote:

> I'm not sure how I'm doing it but it appears as though I've killed
> both my C64 PSU and now another 12VAC wall-wart. The thing is, I was
> able to get inside the wall-wart, and all I found was a transformer.
> No fuse or anything. However, before I started tinkering with things,
> my multimeter showed about 15VAC from the thing (I guess 12VAC assumes
> it's going into some sort of rectification/regulation? Well, my
> initial tests with my new power board worked great - I was getting the
> DC voltages I was expecting (namely 12 and 9VDC). Well, I hooked up my
> PIC board without the PIC itself and got the proper voltages. Then I
> stuff it with chips and, after that, now my wall-wart reads 3VAC with
> no load.
>
> It's just spools of wire! How in the heck am I so good at nuking these
> things? :P Any thoughts?
>
> Good news is that I have other adapters, but they are DC and while
> it's not a big deal it's likely going to be fantastically noisy for my
> SIDs :/
>
> Help would be appreciated!
>
> Tim
>
> P.S. Frederick, if I can figure out how to keep my MidiBox SID powered
> up, I'll go ahead and test your SIDs next :)
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