Carlin Component Testing - Manual (Page 6)

Carlin U – Residential Oil Burners – Page 78
What does cadmium sulfi de do?
• Cadmium sulfi de is an electrical conduc-
tor.
• When cadmium sulfi de is exposed to
visible light, its resistance changes. The
stronger the light source, the lower the
resistance.
• When the burner isn’t fi ring, with the
cad cell inside and not exposed to any
light, the cad cell has a high resistance
(over 50,000 ohms).
• When the burner fl ame starts, the cad
cell sees the light from the fl ame, and its
resistance drops.
• The primary control (48245, 60200, for
examples) knows fl ame is present be-
cause it senses this resistance change.
Fixed resistance primary test
Use this method to test the performance of
a primary control, removing the cad
cell as a factor.
• Remove the cad cell wires
from the primary control’s
“F-F” terminals.
• Connect one end of a
¼-watt, 1500 ohm (1.5 k
Ω)
resistor to one of the “F-F”
terminals.
• Start the burner. AFTER
fl ame has been established,
connect the other end of the 1.5
k Ω resistor to the other “F-F” terminal.
• If the primary control is operating cor-
rectly, it should accept this resistance
and operate normally. If the control does
not operate correctly, replace the con-
trol.
• Remove the resistor and replace the cad
cel leads on the “F-F” terminals.
Cad
Cells
Cad cell construction
A cad cell contains:
• a sensing plate coated with cadmium
sulfi de.
• a metal grid on the cadmium sulfi de
to supply the two connections from
the sensor.
• a clear coating over the cadmium
sulfi de and grid that prevents con-
tamination from moisture, dirt and
oil.
• a housing to mount the sensor as-
sembly.