Keysight Technologies U1231A Guia Del Usuario página 81

Making Measurements 2
Using the μA function to measure flame rectification circuits
The multimeter's μA measurement function can be used to measure flame
rectification circuits (flame sensors) down to 0.01 μA.
Flame sensors, whether they are used in a home furnace or on a large industrial
boiler, indicate the presence of a flame and are part of the safety circuit. These
types of flame sensors use a process of flame rectification to sense that the flame
is lit. Typically, these flame sensors must be engulfed in the burner flame to
function.
Flame rectification uses the fact that a flame will rectify an AC voltage to DC
voltage and allow the DC current to flow through a flame to detect a flame.
Normally, an AC voltage is applied to the flame sensor with a wire coming from
the ignition module. When the flame sensor is engulfed by a flame, the AC voltage
is rectified and a DC current, commonly 4 to 12 μA, flows from the ignition module
through the wire to the flame sensor, through the flame to the ground on the
furnace chassis.
The ignition control module has a circuit to detect the DC current, and it
commonly closes a relay when the DC current is detected, that verifies that the
burner is properly lit. If the burner fails to light or is extinguished for any reason,
that DC current disappears and the control module takes suitable action to turn
off the burner.
To measure flame rectification circuits:
1 Set up your multimeter to measure μA measurements as shown in Figure 2-25.
2 Connect the multimeter between the flame sensor probe (COM terminal) and
the ignition control module (mA terminal).
3 Probe the test points and read the display.
Keysight U1231A, U1232A, and U1233A User's Guide 81