Re: De-Gauss coil thermistors in TVs

"N Cook" <diverse8@xxxxxxxxx> writes:

Arfa Daily <arfa.daily@xxxxxxxxxxxx> wrote in message
news:dCPCi.27807$Db6.22320@xxxxxxxxxxxxxxxxxxxxxxx

"N Cook" <diverse8@xxxxxxxxx> wrote in message
news:fbg8sc$671$1@xxxxxxxxxxxxxxxxxx
Those usually 3-legged black blocks containing 2 PTC disc thermistors.
One of a few tens of ohms (cold) feeding to the coil and one of a few
hundred ohms (cold) connected between live and neutral , what is the
function of the one across the mains ?
As that one is separated from the other by 2 sets of spring clips, it
cannot
be there for indirectly heating the one feeding the coil for quick
completion.

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/


Isn't the second one across the back side of the first, rather than
directly
across the mains? Sort of a two thermistor series / shunt arrangement. I
seemed to recall from my long-ago theory days, that they were the inverse
of
each other, and that the one that's across is initially high resistance,
and
gets lower as it heats up, to shunt the degausing coils, mitigating their
effect, and keeping the series thermistor hot - and hence in the high
resistance condition - by continuing to draw just enough current through
it
to do that. Could be wrong - it's been a long time ago - but it 'feels'
right, and seems to work, in my head at least ... !!

Mind you, I'm going back to when they were two (definitely) different
individual components. The series one was a granular grey, and the shunt
was
a little smaller diameter and a little thicker, with a mainly blue
(painted)
body, with a yellow stripe at the top. I've probably still got some
somewhere. But perhaps they've come up with some other way of doing it
since
they've encapsulated the two elements together.

Arfa



This is the basic circuit from the schematic and values from an actual TV.
The 16 ohm one is PTC but the 470 one across the mains is NTC heating each
in turn with a soldering iron. The schematic states PTC for both so more
mystery
On closer inspection the two discs are in good thermal contact to the
central pin extended to a dividing metal disc but good thermal contact, the
spring plate connections are on the outer contacts.

ASCII , view in Courier or perhaps Properties/ Message Source

|-------------
\.-. |
|\| C|
16 | |\ C| degauss coil
'-' C|
| |
L ------------| |
\.-. |
|\| |
240V 470| |\ |
'-' |
| |
N -------------------------
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

Geez, there's a program to draw mediocre ASCII schematics? :)

I believe your discarded explanation is correct. The second one
is there to keep the series thermistor hot after the initial surge
through the degauss coil.

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