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

jim jim at well.com
Wed Dec 6 20:46:57 PST 2017

Thank you.

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?

On 12/07/2017 04:35 AM, Michael Paoli wrote:
> ~600 VAC, not DC.  Don't f*ck with sh*t like that unless you very much
> know what the heck you're doing - can be fatal if you f*ck up.
> I used a power transformer from an old not solid state color TV
> console has various higher output voltages to power tubes and B++
> voltage (go look it up - pro'lly ought be on Wikipedia).
> Anyway, I don't recall precisely what value resistor I used,
> I did calculate to be high enough on resistance to make the current
> quite low, and also to not overheat the resistor at a dead short
> at full voltage.  So ... let's see ... was probably 1/4 W resistor,
> AC voltage - I don't remember, but likely something in the ballpark
> of 600 VAC RMS.
> And AC would be *slightly* preferable - higher peaks (more likely
> to achieve ionization).
> And lowish (e.g. 60 Hz) frequency also relatively preferable - give
> contacts potentially half a chance or so to "rest" between half cycles.
> With 60 Hz, if there was anything especially "interesting" happening
> at the contacts (actively arcing / burning anything off), that would also
> be more likely to be detectable (seen/heard/felt) than DC - so better to
> be able to monitor progress/activity - at least if used/needed for more
> than some second(s) or less.
> So ... W=A*V, A=V/R,
> W=V^2/R
> R=V^2/W
> R=(600V)^2/(1/4)
> $ echo '600*600*4' | bc -l
> 1440000
> $
> Resistor tolerance probably 20%, so ... standard values ...
> $ echo '1.8*.8' | bc -l
> 1.44
> $
> Bare minimum would be 1.8M - but that's before we take into account
> accuracy of the voltage measurement - so I probably went with something
>>> 2.2M that was handy and kept the power low.  Figure max power at the
> switch contact points would be with 1/2 the voltage on the resistor and
> the other half on the switch (if/when it happened to match impedance - at
> the applied voltage) ... but I didn't want 1/4 or 1/8 W or anything near
> that possibly concentrated between the switch contacts - lest they
> potentially get spot welded together.  So I probably grabbed something
> quite a bit higher in resistance, that would still allow enough current
> to potentially and likely burn off whatever was preventing the contacts
> making electrical contact, with negligible risk of welding them
> together.  So I likely did something in the 5.7M to 22M range.
> I'll leave max power to the switch contact points and max current for
> those resistor values as an exercise.  ;-)
> The basic methodology was close the contacts (mechanically - but they 
> didn't
> close electrically), apply high voltage limited current,
> release the contacts, remove the voltage and retest the switch - if it
> worked, done, if not, maybe retry ... possibly slight more
> power/current/voltage if it didn't work ... and if it's stuck closed,
> already did too much power/current/voltage - and time to buy a new
> switch.
> Also, 600 VAC - that's what I recall, at least approximately (was
> about what I was aiming for - enough to arc a very small closed but not
> conducting gap with approximately air and some presumed metal oxides
> between).  I may have done something else to step up the voltage if I
> didn't get ~600 VAC straight from that transformer's output - using
> whatever highest voltage output taps it offered.  But I may be
> misremembering.  I may have possibly used an underdriven flyback
> transformer, or an ignition coil (yet another type of transformer).
> In any case, I recall having something around 600 VAC, and I'm about 80%
> certain it was straight off the highest voltage taps of power transformer
> from and old non-solid state color TV console chassis.
> Random mini-fact: rough rule of thumb for dilectric (electrical 
> breakdown)
> voltage of air: about 10,000 V/in.  This can however, be altered by
> various factors.  E.g. gas mixtures, temperature, humidity, dust.
> Firefighters actually have guidelines when using water on energized
> electrical equipment - what type of water delivery (fog/spray/stream),
> what voltage, and required safe distance.  F*ck up and you're dead.
> Hmm, maybe exactly what I did (or nearly so) might even be in my old
> hardcopy hardware logs.  Have about 100 or 200 pages of such "data" ...
> before I switched to going electronic (with regular backups accessible
> in various ways ... much more searchable, also much more convenient to
> record diagnostics and precise digital data information).
> Alas, ye olde hardcopy - not very searchable ... but does still function
> when (all the) computer equipment is down (like when I really only had
> exactly and only one computer available to me).
>> From: jim <jim at well.com>
>> Subject: Re: [sf-lug] SF-LUG meeting notes for Sunday December 3, 2017
>> Date: Thu, 7 Dec 2017 03:43:13 +0000
>>     600VDC? with a 10MOhm resistor. Great
>> tip!
>>     Where'd you get the 600VDC? Not from
>> an everyday receptacle.
>> On 12/07/2017 03:23 AM, Michael Paoli wrote:
>>> oh, and I did have one key on a Cromemco 3102 terminal which I'd 
>>> received
>>> which didn't work (magnetic reed relay switch ... and ... how'd I 
>>> repair it?
>>> switch physically closed contacts, but not electrically - sealed in 
>>> a glass
>>> envelope - apply a rather high voltage (about 600+ VAC) very limited 
>>> current
>>> (about a 10+M Ohm resister in series) ... burnt off whatever 
>>> (oxide?) was
>>> preventing electrical contact, without welding the contacts together 
>>> - has
>>> worked fine ever since)
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