[sf-lug] SF-LUG meeting notes for Sunday December 3, 2017

Michael Paoli Michael.Paoli at cal.berkeley.edu
Wed Dec 6 21:11:20 PST 2017

Uhm ... 120VAC to the secondary of a 25V transformer ???!?!

Uhm, ... while it lasts.

That's more likely to give you a quickly overheated failed shorted
transformer with a bunch of quite unpleasant, if not downright
toxic fumes, and quite possibly an electrical fire, and very possibly
in more than one place.

Depending upon what you've got upstream on your circuitry you may
overload/overheat other components and potentially cause additional
problems and/or fire(s).

So, where you applied that 120 VAC to your transformer, what's
the first circuit breaker or fuse upstream of that?  15A?
Is everything from that transformer back to that breaker rated at
15A or more?  Probably not.  So your fried, and possibly on fire
transformer may not be your only problem.

Quick, do you know what extinguisher class(es) you can apply to a live
electrical fire?  Extra credit:  What if there's also data there that
you're hoping to not destroy?  Bonus (might be a trick question):
What if you may want to not asphyxiate everyone in the room?

Applying approximately 5x the nominal voltage rating to a transformer's
winding is generally a really bad idea.

Exercise:  What's the wattage rating of the transformer?
Can we figure out an approximate impedance/inductance of a 25VAC
secondary of a 120VAC/25VAC stepdown transformer?
What would nominal current in primary be at zero load (actual
practical, not some theoretical perfect transformer)?
If we apply 120VAC to the secondary (don't do that!!!), calculate
what the current would be at zero load - presuming our practical
non-ideal transformer.  If the dielectrics/insulation doesn't break
down, how much power would it consume, and does that exceed the
transformer's nominal rating, and by how large a factor?
If we presume 8% waste leakage power at 0 load, and we
put 120VAC to the 25VAC secondary, how much power in that scenario,
and does it exceed nominal max wattage, and if so by how large a

> From: jim <jim at well.com>
> Subject: Re: [sf-lug] SF-LUG meeting notes for Sunday December 3, 2017
> Date: Thu, 7 Dec 2017 04:46:57 +0000

> Maybe applying 120VAC to the secondary
> of a 25V transformer would present about
> 600 VAC (about 900V peak), for those of
> us without old color CRT teevees?

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