Flame Rectification Principle - Measurement - American Standard *UD Series Operation Manual

Flame Rectification Principle – Measurement
The White-Rodgers 50A50 and 50A51 integrated
controls use the flame rectification principle to prove
that flame is present after the gas valve has been
energized.
The flame rectification principle is based on the fact
that a flame can conduct electrical current. When a
positive charge is placed on the flame sensor and a
flame is present to complete the circuit from the
burner surface or ground, current will flow from the
burner ground (zero potential) through the flame
to the flame sensor which has a greater positive charge.
The ignition control will sense the current flow and
allow the gas valve to remain open and the heating
cycle to continue. The flame current is monitored by
the ignition control and will shut down the gas valve
if the minimum flame sense current is not present for
more than 0.7 seconds.
The 50A50 and 50A51 White-Rodgers integrated
furnace controls require a minimum of 1.0 micro amps
DC to prove flame.
The flame current microamp signal must be checked
as part of regular maintenance and during normal
service checks in order to properly diagnose the
ignition system.
The flame current microamp signal can be measured
with many of the new digital volt OHM meters.
However, there are many digital meters which do
not have microamp scales but can read DC volts.
The flame current is measured by removing the
flame sensor wire at the flame sensor and connecting
it to one of the meter leads. The other meter lead is
connected to the flame sensor.
A flame current adapter for digital volt meters, Pub.
No. 34-4816-01, see next page, is available which
allows flame current microamps to be measured on
the DC volt scale. The adapter has two leads with a
male and female quick connect for easy hook up to
the flame sensor.
There is also a flame current tester and simulator kit,
Pub. No. 34-4817-01, that provides capability to
measure flame current with a digital DC volt meter
and the ability to simulate flame current, check for
adequate ground and confirm gas valve operation.
See next page for additional information.
Flame Current Adapter Kit
Pub. No. 34-4816-01
25