What resistance range should be established upon firing a fuel oil burner, when testing a cad cell?

What resistance range should be established upon firing a fuel oil burner, when testing a cad cell?

• A. 100-300 ohms
• B. 200-600 ohms
• C. 300-900 ohms
• D. 900-1200 ohms

Answer: (C)

Cad cell acts as a flame sensor in the burner safety circuit. When the burner is firing, light from the flame lowers the cad cell resistance, and a healthy sensor should read in a mid-range window that reliably signals the control to keep the flame burning. The established target is about 300 to 900 ohms. If the reading is much lower, it could indicate a short or overly strong illumination giving a false flame signal; if it’s much higher, the flame signal is too weak and the control may shut down or fail to keep the burner running. So, during testing with the flame present, you should see the cad cell resistance fall within roughly 300–900 ohms. If it isn’t, inspect or replace the cad cell, wiring, or the circuit connections to restore proper sensing.

Cad cell acts as a flame sensor in the burner safety circuit. When the burner is firing, light from the flame lowers the cad cell resistance, and a healthy sensor should read in a mid-range window that reliably signals the control to keep the flame burning. The established target is about 300 to 900 ohms. If the reading is much lower, it could indicate a short or overly strong illumination giving a false flame signal; if it’s much higher, the flame signal is too weak and the control may shut down or fail to keep the burner running. So, during testing with the flame present, you should see the cad cell resistance fall within roughly 300–900 ohms. If it isn’t, inspect or replace the cad cell, wiring, or the circuit connections to restore proper sensing.