A Cad (Cadmium Sulfide) Cell is a photo conductive flame detector that changes its electrical resistance depending on how much light is available at the sensor. This is a common flame detection device used on fuel oil fired burners. The Cad Cell is used in conjunction with an Oil Primary Control mounted to the oil burner.
When the Cad Cell is mounted inside the oil burner and the oil burner is off, the location is dark and therefore the resistance reading between the two yellow wires connected to the Cad Cell are very high. These two yellow wires are connected to the F and F terminals on the side of the oil primary control. Usually the resistance reading will be 1 M ohm (1 MegaOhm) or higher between the two yellow leads.
When the oil burner ignites the oil, the chamber in which the Cad Cell is located becomes brightly lit due to the flame. This light changes the electrical resistance in the Cad Cell. With a good flame and a good Cad Cell, the resistance reading should usually be between 300 to 1000 ohms. The oil primary control is usually looking for an electrical resistance below 1600 ohms in order to verify the flame is present.
Before the oil primary control will allow the oil burner to turn on, there must be a connection between the T and T terminals on the oil primary control to show a call for the heat to turn on. The oil primary control must also have a resistance reading higher than 1600 ohms between the F and F terminals where the cad cell is connected to. The readings from the Cad Cell at the F and F terminals verify that there is no flame present when the resistance reading is higher than 1600 ohms. Then after the oil primary control turns the burner on, sending power to the spark transformer, the blower and pump motor, and to the oil solenoid, the oil primary control looks for a lowering resistance reading on the F and F terminals indicating the flame is present.
If the resistance value does not lower below 1600 ohms after roughly the first 10 seconds of operation, the manual reset button will pop and the burner will shut off, signaling a problem.
In order to troubleshoot the Cad Cell, disconnect the Cad Cell wires from the oil primary control and connect a multimeter to the two yellow leads from the Cad Cell. One multimeter probe is connected to each wire and the meter is set to resistance to measure ohms. Turn the burner on by connecting the T and T terminals. Since the Cad cell wires are disconnected from the oil primary control, the resistance reading between the F and F terminals will be immeasurable which is an acceptable measurement for the oil primary control to turn on. Measure the resistance value on the cad cell while the flame is present.
Resistance measurements while the Flame is Present:
OL= Bad Cad Cell
> 1600 ohms= Bad Cell or Weak Flame
<1600 ohms but >1000 ohms= Good Cell, Weak Flame
<1000 ohms= Good Cell, Good Flame
Also before turning the power to the oil burner on, check to make sure the Cad Cell sensor glass is clean so light can enter the glass of the Cad Cell Sensor.
Resistance measurements while the Flame is Not Present:
> 1,000,000 ohms= Good Cell, No Flame, No Light
> 1600 ohms but < 1,000,000 ohms= Bad Cell, Accidental Light Entering
<1600 ohms= Bad Cell, Possible Flame Present, Chamber Open to Light
<0.5 ohms= Bad Cell, Shorted Wires
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Published: 12/15/2021 Author: Craig Migliaccio
About the Author: Craig is the owner of AC Service Tech LLC and the Author of the book “Refrigerant Charging and Service Procedures for Air Conditioning”. Craig is a licensed Teacher of HVACR, Sheet Metal, and Building Maintenance in the State of New Jersey of the USA. He is also an HVACR Contracting Business owner of 15 years and holds an NJ HVACR Master License. Craig creates educational HVACR articles and videos which are posted at https://www.acservicetech.com & https://www.youtube.com/acservicetechchannel