What is a typical resistance value for a cad cell in darkness?

• A. 1000 ohms
• B. 10000 ohms
• C. 100000 ohms
• D. 1000000 ohms

Answer: (C)

Cad cells are light‑sensitive resistors whose resistance drops as they are illuminated. In darkness there are no photons to generate charge carriers, so the cell hardly conducts and presents a high resistance. A typical dark-resistance value used in flame-sensing circuits is about 100,000 ohms. When flame is present, the flame’s light hits the cad cell, resistance falls dramatically, changing the voltage at the sensor input and signaling the controller that a flame is detected. This high-dark-resistance value provides a clear baseline so the circuit can reliably detect the switch to a low-resistance state caused by light.