When using a PTC Temperature Controller, if I set the temperature set point below approximately 0.3 V, the setpoint always reverts to approximately 1.0 V. What is happening and how can I implement a set point voltage below 0.3 V?
The PTC Temperature Controllers have a feature that is meant to protect a laser diode (or other hardware you’re trying to temperature stabilize) 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.
This failsafe circuit substitutes a “safe” default setpoint voltage when the setpoint voltage drops below a limit.
In the factory default configuration the minimum sensor voltage is approximately 0.3 Volts. The substitute setpoint is approximately 1.0 V. This correlates to a set point 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 usually around 25°C, this is usually a “safe” temperature setpoint default.
Both the trip point and the substitute set point can be modified to meet the specific requirements of your application.
Application Note AN-TC07 describes how to configure these parameters.