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

jim jim at well.com
Wed Dec 6 21:35:50 PST 2017

```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
> 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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