[XCSSA] Can you break transformers?

[X-otic Computer Systems of San Antonio]

On Sep 4, 2008, at 10:47 PM, X-otic Computer Systems of San Antonio  
wrote:

> My thoughts:
>
> 1. AC voltages are generally not regulated.  They are rectified to
> DC, then the DC is regulated.
Right, which is exactly what I'm doing on my board using a bridge  
rectifier, a few caps, and some voltage regulators. I do it this way  
because of the "noise" DC wall warts tend to have when using analog  
components (such as the SID). For lower noise, some people talked  
about using multiple output transformers and things like that, but I  
think the voltage regulators should be good enough, at least for now.

> 2. The rectified DC voltage you get from 12VAC depends on type of
> rectifier, load, regulation, and other factors.
Yep. I have a 7812 and a 7809 (in series after the 7812). So I end up  
with 12V and 9V. The 12V goes to my SID boards as they have their own  
7809 rectifiers. Then 9V goes to my CORE board which has it's own 7805  
rectifier. Might be a bit wasteful since basically every board has a  
regulator/rectifier, but that is because the design is meant to be  
module based.
>
> 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.

*OH* ok that makes a good bit of sense. I didn't know that!
>
> 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!

Agreed. Problem with the C64 PSU was I didn't have a good way of  
testing my boards since I don't have anything else that outputs 9VAC  
and 5DC, although I guess I could have made a test board or something.  
For my 12VAC adapter, I have multiple 12VDC adapters I could have used  
so yeah, good point :)

> 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.

Secondary meaning the 12V side? I was thinking about that. Question  
is, do I put it before or after the regulator and/or caps? Seems like  
a requirement at this point :)

> Are you destroying vintage Commodore PSU's?!?

Alas yes. Actually the C64 PSU does have a fuse that is probably  
blown, but those supplies have been known for being notoriously  
difficult to get into. So even though it's probably just a fuse, I  
can't get to it to replace it.

> 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.

On that note, what can actually blow these things while testing? Just  
shorting +V to GND or something like that? I must've done something  
bad since the wall wart I got was a 3amp and I was drawing maybe 1/4  
amp at the most when I tested it with my CORE board (it houses the PIC  
and powers my LCD display). I checked and the wall wart doesn't appear  
to have a fuse I can see. The only thing in the case appears to be  
just the transformer :/

> 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".

Well, I'm thinking of just buying a new one of Allied and being done  
with it. Not sure of those are fused or not though, but I'll go ahead  
and just add a fuse the next time around and be done with it :)

Thanks for all the helpful input Charles!