November 16, 2009

The PTC Temperature Controllers have a feature that is meant to protect a laser diode (or other hardware you’re thermally stabilizing) in the event that the remote setpoint signal is lost. With NTC sensors, a 0 V setpoint equates to a “hot” temperature. If the setpoint signal is lost, the controller will drive heating current (at current limit) to match the sensor voltage to the 0 V setpoint. This can potentially cause excessive temperatures at the laser diode.

The failsafe circuit substitutes a “safe” default setpoint voltage when the setpoint voltage drops below a limit. In the factory default configuration, the minimum setpoint is approximately 0.3 V. The substitute setpoint is approximately 1.0 V. This correlates to a setpoint of about 25°C, assuming the system is using a 10 kΩ thermistor and a 100 µA bias current through the thermistor. Since room temperature is typically 25 °C, this is usually a “safe” temperature setpoint default. Both the trip point and the substitute setpoint can be modified to meet the specific requirements of your application.

Application Note AN-TC07 describes how to configure these parameters.

Please be aware that circuit modifications implemented by the customer will void the Wavelength Electronics warranty. Only factory implemented modifications are warrantied. For factory service please contact Wavelength or an authorized distributor.

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