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start surge of old television - Page 2

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Posted by Johnny B Good on July 21, 2009, 9:24 pm
 


====snip====


 _Replacing_ the PTC with a switch will turn it _into_ a gausser! You
need to keep the PTC in the circuit and _add_ the momentary contact
switch in series (always remembering the need to hold it in for the
several seconds it takes for the degaussing current to fade away - a
brief press will gauss up the tube otherwise).

 Modern monitors which offer a degauss option in a setup menu use a
relay with a timeout delay to allow the PTC to do its job properly.

--
Regards, John.

 Please remove the "ohggcyht" before replying.
The address has been munged to reject Spam-bots.


Posted by vaughn on July 21, 2009, 9:25 pm
 


   Yep, I knew it as soon as I read his post.  Unfortunately he must have
been posting while I was writing.

Vaughn



Posted by richardson on July 22, 2009, 9:05 am
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Posted by Ken on July 21, 2009, 8:55 pm
 

etc.

settles to

That's normal for starting.  
 

Posted by Johnny B Good on July 22, 2009, 12:43 am
 


 If anything, that LCD readout is grossly under-reading the true initial
surge. The degausing coil is not likely to be more than 10 ohms at most
(with an inductance no greater than 10mH)[1] and the typical cold
resistance of the PTC employed is typically a mere 10 to 12 ohms (this
is on monitors designed for 100 to 240v AC mains supplies).

 On 240v mains, the initial surge could well peak at over 15 amps for
the first millisecond or so if the switch contact closure event
coincides with the crest of the mains voltage waveform. That sort of
abuse could trip out a small (say 700VA) UPS so it is not a good idea to
switch an auto-degaussing colour monitor off to reduce the on-battery
loading to allow a protracted process to complete if there happens to be
a need to switch it back on to check progress or close down the PC.


[1] Unfortunately, I've never tested a degausing coil's resistance so
can only make an educated guess from my recollection of the wire guage
and number of turns used on a typical coil. AFAICR, the wire guage
seemed to correspond to 25AWG (about 0.5mm diameter) and I'd estimate
not much more than 100 turns.

 A visit to the Multilayer Air Core Inductor Calculator web page at this
address:

<http://www.pronine.ca/multind.htm>

 suggests that a coil with a 15 inch inner diameter and coil length of
0.25 inches would require 94 turns of 25 AWG wire for a test inductance
value of 7mH producing an outer diameter of 15.31 inches requiring some
372 feet of wire which results in a total resistance value of 12.04
ohms.

 Short of removing a degaussing coil from the next CRT monitor I decide
to scrap in order to test its resistance and count turns and make other
measurements to allow its inductance to be estimated, that 'Ball Park'
figure will have to stand for the time being. However, if anyone has
some actual figures for such degaussing coils, please feel free to chip
in.

--
Regards, John.

 Please remove the "ohggcyht" before replying.
The address has been munged to reject Spam-bots.


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