What is the Normally Open (NO) Thermostat Switch KSD301?

The Thermostat Switch KSD301 bimetallic disc operates in a Normally Open (NO) configuration. This means the circuit is initially open (disconnected) at normal temperatures. When the temperature rises to a predefined threshold, the bimetallic disc bends, closing the circuit and allowing current flow.

Normally open THERM-O-DISC T-O-D THERMAL LIMIT SWITCH L160-40F 60TX01

UL/CUL KSD301 Shutoff Thermostat,Normally Open NO Bimetal Snap Disc 125V 16A Auto Reset

uxcell KSD301 Thermostat 40°C/104°F 10A Normally Open N.O Adjust Snap Disc Temperature Switch

I. Core Features
‌Operation Principle‌
When the temperature rises to the set value, the contact automatically connects the circuit; when the temperature drops to the reset value, the contact automatically disconnects the circuit, which is the opposite of the normally closed type (disconnected when the temperature rises and connected when the temperature drops).

The bimetallic strip is heated and deformed to trigger the mechanism to achieve fast switching.

‌Technical Parameters‌
‌Electrical Specifications‌:
Rated voltage: AC 250V (resistive load) or AC 125V (UL standard)
Rated current: 5A/10A/15A (resistive load)
‌Temperature performance‌:
Action range: 0℃~300℃ (customizable), accuracy ±2℃~±10℃17
Reset and action temperature difference: 8℃~100℃ (adjustable)
‌Mechanical characteristics‌:
Contact resistance: ≤50mΩ, insulation resistance: ≥100MΩ13
Electrical strength: AC 1800V/1min without breakdown
Service life: ≥100,000 times17 (some models up to 150,000 times2)

‌Physical characteristics‌
Protection level: IP00 (no dust and water resistance)
Wiring method: plug-in terminal 250# or 187# (angle 0°~90° optional)
Installation method: supports various structures such as movable ring, flat ring, aluminum cover with holes, etc.

Normally Open State:
The KSD301 is designed to have its contacts in an open state when the surrounding temperature is below the specified operating temperature. This means the circuit connected to the KSD301 is broken, preventing current flow.

Temperature Trigger:
As the temperature increases, the bimetallic disc (made of two different metals bonded together) responds to the temperature change. The two metals expand at different rates, causing the disc to bend.

Circuit Closure:
When the temperature reaches the set point, the disc bends enough to close the circuit, establishing a connection between the contacts. This allows current to flow through the circuit.

Automatic Reset:
Once the temperature drops below a certain level, the bimetallic disc returns to its original shape, opening the circuit again and cutting off the current flow. This is an automatic reset feature.
In essence, the KSD301 is used as a thermal switch to control current flow based on temperature, acting as a temperature sensor and protector for various applications.

II. Application scenarios
‌Household appliances‌: water dispensers, electric thermos, microwave ovens, disinfection cabinets, electric heaters, coffee machines, etc.
‌Industrial equipment‌: motor overheat protection, rectifier equipment, laminating machines, car heated seats, etc.57.

III. Detection method (determine whether it is normally open)
‌Multimeter detection‌:
At room temperature (‌below the action temperature‌): the resistance of the two pins should be infinite (open circuit).
After heating (‌above the action temperature‌): the resistance value drops to ‌tens of ohms‌ (open circuit).
‌Marking distinction‌: The product body is usually marked with “NO” (Normally Open) or “Normally Open”.

IV. Product advantages
Fast response speed, clean contact action without arcing57;
High temperature control accuracy, strong reliability7;
Compact structure, strong adaptability17.

⚠️ Note: The actual selection needs to confirm the specific temperature threshold (such as 85℃, 95℃, etc.) and the reset temperature difference. The parameters of different models may vary.