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FLAME SENSING
The Honeywell S89 series primary ignition controls utilize the flame current rectification principal for main
burner flame sensing.
The flame rectification phenomenon occurs as follows: The ignited gas flame causes the immediate
atmosphere around the flame to become ionized (gas atoms become electrically charged). The ionization
causes the atmosphere around the flame to become electrically conductive. An AC voltage output from the
control sensing circuit is routed through the flame sensor probe. When the sensor probe and the burner head
are both in contact with a properly adjusted flame, the burner head with its larger surface attracts more free
electrons, thus becoming negatively charged. The sensor probe with its small surface area gives up free
electrons, thus becoming positively charged. The free electrons from the AC voltage in the sensor probe flow
through the ionized gas flame to the grounded burner head. As the AC current passes through the gas flame, it
is rectified into a DC current flowing back to the grounded side of the sensing circuit. The flame in actuality is a
switch. When the flame is present, the switch is closed allowing current to flow through the sensing circuit of
the control. When no flame is present, the switch is open with no current flowing through the sensing circuit of
the control.
The DC current flow is measured in units called DC microamperes. A steady DC microampere current of 0.8
minimum (and steady) or higher through the sensing circuit of the primary ignition control is sufficient to keep
the burner running without a safety lockout. See Figure 1 for sensor probe and electrode dimensional settings,
Figure 12 for flame current measurement.
Honeywell S89F Flame Signal Measurement
Figure 12
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