We are going to end our latest trilogy of LTSpice simulations with another demonstration of the powerful computing capabilities of this SPICE software in the field of NTC thermistor temperature control.
First, we will simulate a LT1040 fully electronic room thermostat. After that, we will go back to a basic pulse width modulation (PWM) circuit. We will conclude with an example of LED current control, where the temperature control will be applied to the LT8391 (Linear Technology, release date April 2016) using the LTSpice XVII 64-bit program.
The first circuit will use a LTC10401 electronic thermostat with an NTC temperature sensor. The circuit is shown in Figure 1
If you want to see the live emulation of this circuit simulation, a five-minute video can be viewed here.
In a nutshell, the simulation (performed according to a Monte Carlo analysis) shows how the LTC1040 will regulate the temperature of a system between two separate temperatures (24.5oC for the heating and around 25.5oC, with a hysteresis of 0.25oC around these two temperatures), for an external temperature first above, and then under, the target temperature.
In order to avoid duplicating the information available online, I will provide some complementary simulations not performed in the aforementioned video. Take, for example, the effect of the sensor response time. In stirred air, the response time of the thermistor NTCLE203 is 7 s. But if you build a housing around this component, the thermal response could easily be 15 s.
In Figure 2, the responses of the system in these two situations — different overshoots and duty times — are visible.
With the help of measurements recorded in the “spice log file” (Figure 3), one can compute an increase in overshoot of about 0.32oC when going from 7 s to 15 s. Also, the cooler will be on for 126 s for tau = 7 s and 169 s for tau = 15 s.