Cement-Encased Resistor

Cement-encased resistors (often called “cement resistors” or “SQM” / “SQP” types) consist of a wirewound or thick-film resistive element placed inside a rectangular ceramic shell, with the gap between element and shell filled with a setting cement based on aluminium silicate and sodium silicate. The combination delivers excellent flame-retardancy, vibration resistance and short-time overload capability — typically 5× rated power for 5 s.

The cement provides a large thermal mass that buffers brief overloads, making cement resistors ideal for in-rush limiting, surge protection in switching power supplies, dummy loads for power-supply testing, dynamic braking in low-power VFDs and LED drivers, and bleeder discharge resistors. Standard power ratings span 1 W to 100 W per part; common form factors are SQP-5 W, SQP-10 W, SQM-20 W up to large 50 W and 100 W parts with high-temperature ceramic cement.

Limitations: cement resistors do not dissipate continuously as efficiently as aluminum-housed types (the cement is a poorer thermal conductor than the silica gel inside an alu housing), so their continuous power rating per unit volume is lower. They are also less suitable above 200 °C ambient. For cost-sensitive consumer and industrial PSU applications, however, they remain the dominant choice in the 1 – 50 W range.