Photo Detectors – CdS Cells

CdS cells (cadmium sulfide photoresistors) are semiconductor-based photoelectric sensors whose resistance varies nonlinearly with light intensity.

 

1. What are the Working Principles of CdS Cells?

Photoelectric Effect: When a CdS material absorbs photon energy, electrons in the valence band transition to the conduction band, generating electron-hole pairs and causing a decrease in resistance.

 

Spectral Response: Typical peak sensitivity is 540nm (yellow-green light). They are sensitive to visible light, with a weaker infrared/ultraviolet response.

 

Dark and Light Resistance: In the absence of light, the resistance can reach 1MΩ (dark resistance) and drop to below 10Ω (light resistance) in strong light.

 

2. What are the Key Features of CdS Cells?

Parameter	Typical Value	Description
Response Time	20-100ms	Time required for resistance to stabilize under changing light conditions
Temperature Coefficient	-0.5%/°C (25°C reference)	Sensitivity decreases at high temperatures
Power Dissipation	≤100mW	Avoid overload and thermal runaway

 

3. What are the Typical Application Scenarios of CdS Cells?

Light Control Circuits: Automatically turning streetlights on and off, and camera exposure adjustment.

Security Systems: Smoke detection in fire alarms.

Consumer Electronics: LCD backlight brightness adjustment.

 

4. Selection Considerations for CdS Cells

Spectral Matching: Select the appropriate CdS model based on the light source type (e.g., LED/incandescent).

Environmental Tolerance: Epoxy-encapsulated models are recommended for humid environments.

Aging Issues: Long-term UV exposure may cause sensitivity degradation.