In the classic spice model of an NTC thermistor available on the market now, the temperature is simulated by the embedded TEMP variable. This is ideal for the examination of a circuit’s general response when external ambient temperature is swept, but doesn’t work anymore for an evaluation of a sensor’s response to a defined dynamic temperature profile. In temperature regulation, the transient state plays a big role in the circuit design. For example, the behavior of PID regulators can be strongly dependent on the sensor’s thermal inertia or response time.
To address this issue, we present a new SPICE model that uses the thermistor temperature at a third virtual pin connected to an external voltage. For the simulations, this external voltage represents the dynamic temperature of the thermistor according to the user’s application needs. The user can thus modify the thermistor temperature at will by changing this external voltage.
Take, for example in Figure 1, the exponentially changing voltage of a capacitor C charging through a fixed resistor R2 connected to a fixed voltage V2. When we connect such a voltage to the third pin of the thermistor model Tin, the simulation of Figure 2, represents the temperature variation of a thermistor submitted to a temperature step. The fixed resistor R2 value represents the response time of the thermistor, and the initial voltage defined for the capacitor represents the initial thermistor temperature. Both are user adjustable, R2 is swept from 1 to 10 s.
Voltage divider bridge circuit with NTC thermistor temperature driven (temperature step from 25 to 85oC)
Result of simulation: Upper pane shows thermistor voltage V(NTC)/ lower pane shows Thermistor temperature V(Tin)
For increased complexity, the fixed voltage of this first example can be replaced by a sinusoidal wave or a piecewise linear voltage (with file) describing a temperature profile measured in the application. The thermistor will follow this profile, with a delay fixed by an RC network.
Developing the application even further in the thermoregulation field, the temperature-/voltage-driven model can be connected to a voltage produced by the application circuit itself. This voltage must represent the equivalent temperature produced by the application. In this case, a temperature feedback loop is built to regulate temperature in the application.
An example of a useful application for this model would the simulation of a thermoelectric cooler controller, where the NTC feeds back to the power source to regulate the temperature. Using the voltage-controlled thermistor, a transfer function can be used to simulate the cooling/heatsink and load combination and feed the temperature to the NTC via a voltage.
Another example is a thermo-velocimetric (temperature speed measuring) fire detector, where the rate of rise of the thermistor temperature is used for switching a Schmitt trigger op-amp controlling a thyristor. The critical temperature profile (speed rise) can be recorded in a file, integrated in the simulation as a text file, and applied to the virtual temperature pin of the thermistor.
In general, the model provided can be used in any temperature regulation detection, control, or regulation process where the final temperature can be simulated and fed back to the NTC thermistor in order to regulate the temperature. As an example, adapting the proportional, derivative, and integrative constants of a PID temperature controller is now possible in real time based on the thermal response of the temperature sensor.
The thermistor model provided is supplied in six different electronic simulators as SPICE language syntaxes differ from one simulator to the other. These simulators are in alphabetic order:
- Cadence OrCAD 16.6 (also tested in the 17.2 version)
- LTspice IV (also tested in LTspice XVII 64 bits; LTspice XVII 32 bits is not recommended)
- NI’s Multisim 14.0 (a separate version exists for Multisim Blue)
The simulations are based on the same principal in each of these simulators and are ready for immediate use. The three-pin thermistor model contains a typical sensing circuit, including a voltage divider bridge. The third (simulation only) pin is connected to fixed voltage source via an RC circuit whose RC constant is the response time of the thermistor.
Further development of the circuit may be performed depending on the features available with each software (piecewise linear voltage source, piece linear voltage file, etc.). It’s important to note that all the import problems linked to the voltage-/temperature-driven thermistor model are already solved and require no additional effort from the users, who will be able to focus completely on their own application.
The original modelling of the NTC thermistor SPICE model was performed in LTSpice IV. More elaborate models are available that include Monte Carlo tolerances and worst-case analysis, and addition to a more complex thermal transfer function for the thermistor. To request more information on the model and simulations described in this article, please send an email to firstname.lastname@example.org.