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

Michael Paoli Michael.Paoli at cal.berkeley.edu
Wed Dec 6 22:36:08 PST 2017

Seriously *not* recommended.

Would you connect 600VAC line input to your 120VAC transformer primary?
Not a good idea, right?

Well, connecting 120VAC mains to 25VAC secondary is about the same
overvoltage ratio.  Oh, and power, for fixed impedance?
It's proportional to the *square* of the voltage.
So ... 5x the voltage, 25x the power.
So ... gee, maybe your 5W transformer nominally wastes 1W - 10%, at 0 load.
Up the voltage 5x, now it's wasting 25W at 0 output load,
and that's 5x it's maximum rated power.  You can likely kiss the
transformer bye-bye in relatively short order ... and likely your
fuse too.

Even if you don't destroy the transformer (e.g. very brief), one
may still damage it - the overvoltage on the windings may cause
internal shorts - leaving one with a bad transformer ... though some
of the signs and symptoms of that may not be immediately obvious
(e.g. performs highly poorly on a ring test, runs
hot/wasteful/inefficient, etc.).

But hey, nobody's required to follow what I say regarding what I do
and don't recommend.

> From: jim <jim at well.com>
> Subject: Re: [sf-lug] SF-LUG meeting notes for Sunday December 3, 2017
> Date: Thu, 7 Dec 2017 05:35:50 +0000

> I'll try it. with a 5A or 10A fuse in line.
> On 12/07/2017 05:11 AM, Michael Paoli wrote:
>> 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
>> factor?
>>> 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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