ST201 Safety Thermostat

The Fuji Frontier SP-1500 scanner includes a safety thermostat designated ST201 in the light source section. This component forms part of the scanner’s protection system for the lamp house and cooling system.

ST201 is mounted inside the lamp house assembly, close to the light source and cooling airflow path. Its purpose is to monitor the temperature inside the lamp compartment and protect the scanner from excessive heat.

Function

ST201 is a normally-closed thermal safety switch. Under normal operating temperatures the switch remains closed, allowing the scanner’s light source safety circuit to remain complete.

If the temperature inside the lamp house exceeds the thermostat’s rated threshold, the switch opens and interrupts the safety circuit. This immediately prevents the light source system from continuing to operate under unsafe thermal conditions.

This protection helps prevent damage to the optical system and surrounding components.

Temperature Rating

The original thermostat used in many Frontier scanners is typically rated at approximately:

85 °C ± 4.4 °C

Many machines use a device from the Honeywell 2455RMG series or an equivalent normally-closed thermal protector.

A commonly available modern replacement that has proven suitable is the KSD301 thermal switch, provided that the 85 °C normally-closed version is used.

Cooling Airflow Monitoring

While ST201 monitors the temperature in the lamp house, the scanner also verifies that cooling airflow is present.

The airflow sensor used for this purpose is not located in the lamp house. Instead it is installed inside the scanner body, within the main scanner section.

The cooling system includes lamp house cooling fans and airflow ducts that move air through the light source section. The airflow sensor ensures that this airflow is present so heat generated by the lamp can be safely removed.

Electrical Connection

The ST201 thermostat is part of the scanner’s hardware safety loop and connects through the wiring harness to the PWR20 power control board.

On the PWR20 board this circuit connects through the PWR5 connector. In the SP-1500 wiring harness the thermostat circuit is typically associated with the black and grey wires entering the PWR5 connector.

Because ST201 is normally closed, continuity across this circuit indicates that the thermal protection path is intact. If the thermostat opens due to excessive temperature, or if the wiring path is interrupted, the safety circuit is broken and the light source system cannot operate.

Design Overview

The Frontier SP-1500 uses a simple hardware protection design for the light source system. ST201 protects against excessive temperature in the lamp house, while the scanner also verifies that adequate cooling airflow is present within the scanner body.

Together these mechanisms help ensure that the light source operates only under safe thermal conditions.