American Standard 50A65-475 Manual page 7

Silicon Nitride Ignitor
The ignitor used with this series of White-Rodgers con-
trols is constructed of a tungsten heater element and
silicon nitride ceramic insulators. The voltage rating of
the ignitor is 80 volts R.M.S., which is less than the line
voltage applied to the furnace; therefore, the control
will reduce the number of cycles it supplies to the SiNi
ignitor per second. The control checks the line voltage
constantly and when a call for heat is received the con-
trol sets the number of cycles it is going to supply to the
ignitor based on the line voltage it is reading. This re-
duced number of AC sine wave cycles per second re-
duces the effective voltage R.M.S. applied to the
SiNi ignitor per second. If the line voltage is low, the
number of cycles will go up, or if the line voltage is
high, the number of cycles is reduced.
After each successful ignition, the number of AC sine
wave cycles is reduced. The lower number of AC sine
waves reduces the effective voltage applied to the
ignitor. After each successful ignition attempt, the
burner lights, then the number of cycles is reduced.
During this learning program the number of cycles
will become too low and the gas fails to ignite. A retry
is initiated by the IFC and the number of AC sine
wave cycles is raised slightly to increase the ignitor
temperature.
Once a successful ignition attempt occurs during a
retry, that cycle count will be maintained for the next
255 calls of heat, or until another retry is called. The
IFC repeats the learning process again after 255 calls for
heat have been completed, or whenever power to the
furnace is cycled off and then on again.
This learning process is employed to provide the
most reliable ignition system possible. By lowering the
ignitor temperature, you will get the longest possible
service life from the rugged Silicon Nitride Ignitor. This
control requires the correct polarity of the 120 V.A.C.
wiring and a ground connection for proper operation.
If the line voltage is low, or the line polarity is reversed,
or the ground is not properly connected, the IFC control
will lock out and cause the diagnostic status L.E.D.
to flash six times for control #50A61-605 CNT2536
only. Controls #50A65-474 CNT2854 and #50V65-495
CNT2871 will flash six times if the line polarity is
reversed and nine times if line voltage is low or
ground is not properly connected or the ignitor is open.
The maximum voltage permitted between line neutral
and ground is 10 volts; otherwise the IFC control will
not be able to properly vary the number of AC sine
wave cycles to the Silicon Nitride Ignitor.
This ignitor can only be checked by ohming in a cold
condition. A good ignitor reads 11 – 18 ohms. A bad
ignitor will have a much higher resistance (impedance).
60 Hertz AC Sine Wave
One Cycle
Time 1/60 of a Second
Number of cycles per second affects the effective
voltage or voltage R.M.S.
R.M.S. = Root – Mean – Square = The value assigned to
an alternating current or voltage that results in the
same power dissipation in a fixed resistance as DC
current or voltage of the same numerical value.
One or more cycles are turned off by the I.F.C. control to
control the R.M.S. voltage supplied to the silicon nitride
ignitor.
5
Cycle Turned Off