Page 9 - index

SELF-REGULATING 1 - Terminal threaded

2 - Electrode
SHEATHED HEATER 3 - Seal
4 - Plug body
5 - Seal ring

PLUGS 6 - Magnesium oxide
7 - Sheat resistite stem
8 - Control spiral
9 - Ignition spiral

Structure and properties of the sheathed heater plug
1
The modern sheathed heater plug consists in the main plug
body, the heating element with the heating and regulating
coils and the connecting bolt. The non-corrosive glow tube
is pressed into the housing in such a way that gases cannot
2 escape. The plug is also sealed by O-ring at the connecting
part.
The heater plug draws its electricity from the battery and is
3 controlled by an electronic heating-time control unit.

4 The heating coil and regulating coil

The fundamental principle of a modern sheathed heater plug
consists of the combination of a heating coil and regulating
coil in one joint resistor element. The heating coil and the
front part of the heating element comprise the heating zone.
5 The resistance of the regulating coil at the rear increases
with the temperature and thus the current and temperature
are regulated downwards. Or in other words: the heating coil
and the regulating coil are electrically switched in series
6 and welded together. Both coils have positive but varying
temperature-resistance factors. The regulating coil has a
high coeffcient and the heating coil a low one. The heating
coil is welded into the dome of the glow tube, the regulating
coil is fastened to the conducting connecting bolt.

Protection of the heating coil and regulating coil

The entire coil is packed tightly into a compressed, electrically
insulating, but very heat-conducive ceramic powder, which
is normally magnesium oxide. The powder is mechanically
compressed to such an extent that the coil is fxed as if cast
7
in cement. This makes it so sturdy that the heating coil’s and
regulating coil’s wires permanently withstand all vibrations.
Even though the individual windings are only separated from
each other by a few tenths of a millimetre, coil shorts cannot
8 occur and a short circuit with the glow tube, which would
destroy the plug, is inconceivable.

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