Understanding The Gas Valve And Ignition System - Maytag MDE17CS Service Manual

uNDErSTaNDiNG THE GaS valvE aND iGNiTiON SYSTEM

The sensor, ignitor and gas valve are
interrelated and function as the ignition and
heat source for gas dryers. At the start of the
cycle the radiant sensor contacts are closed,
the ignitor is at room temperature and the gas
valve is closed, blocking the flow of gas. In
the wiring diagram below, the radiant sensor
contacts are wired in parallel with the gas
valve secondary coil. This bypasses current
around the secondary coil when the radiant
sensor contacts are closed. Therefore valve
#2 cannot open as long as the radiant sensor
contacts are closed.
The booster coil and the ignitor are wired in
parallel, although they are still wired in series
with both the radiant sensor and secondary
coil combination.
When the radiant sensor contacts are closed,
full line voltage is available to the booster coil
and to the ignitor. When the radiant sensor
contacts open, the current has to flow through
the secondary coil on gas valve #2 in order to
get to the booster coil and the ignitor.
A significant voltage drop develops across
the secondary coil, which renders both the
booster coil and the ignitor ineffective due to
their low resistance and the reduced voltage
available, even though they are still in the
circuit.
Radiant Sensor Contacts Closed
The holding coil, the booster coil, and the
ignitor all receive line voltage. The holding coil
and booster coil open gas valve #1. Gas valve
#2 is still closed, prohibiting gas to flow to the
burner. The ignitor, operating at line voltage,
begins to get very hot. (As the ignitor gets
hotter the resistance of the ignitor drops) The
ignitor glow radiates heat to the radiant sensor
and as long as the radiant sensor contacts are
closed the secondary coil of the gas valve #2
are bypassed.
Radiant Sensor Contact Open
The ignitor is now very hot and valve #1 is
open. The radiant heat from the ignitor causes
the contacts in the radiant sensor to open.
With the radiant sensor contacts open the
secondary coil of the gas valve is no longer
bypassed. The secondary coil is now in series
with the parallel circuit combination of booster
coil and ignitor.
Because of the relatively low resistance of
the hot glowing ignitor, most of line voltage
is dropped across the secondary coil. The
remaining voltage is dropped across the
booster coil and ignitor. The secondary coil
now opens gas valve #2 allowing the gas to
flow through both valve #1 and valve #2 to the
burner where it comes in contact with the still
very hot ignitor and the flame is ignited.
NOTE: Since the ignitor will begin to cool, the
tip of the ignitor is left in the flame to keep the
resistance low enough to keep the secondary
coil powered, which keeps the gas flowing and
the flame burning.
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