Part one: NE555 Maintains Dynamically SPICE-Generated System Temperature Within the Limits
A few months back, a dynamic voltage-controlled thermistor SPICE model was presented on planet analog, A Multi-Simulator NTC Thermistor SPICE Model With Temperature Driven By a Voltage
With the help of this model, we intend to publish a series of technical notes about the LTSpice modelling of several old-school temperature control analog circuits. Now this might not seem that innovative, but the groundbreaking aspect is that the temperature generated by these circuits is going to be modelled in a live, time-dependent way. In these circuits, starting with the ON / OFF control and going to the more sophisticated PID temperature control, the temperature of a system (room / oven / fridge) is generated in the form of a simple circuit, and will then be sensed dynamically by a thermistor and regulated by analog devices like timers or analog PID controllers.
The first example in Figure 1 involves a classical heater based on a 555 timer, as found in Electro Schematics 1
A classical heater based on a 555 timer (Image courtesy of Electro Schematics)
Now we are going to complete this circuit with a heated system (Figure 2) behaving like a couple of capacitors / resistors, and whose temperature V(Tsystem) must be regulated. Thus we apply to this system the equivalent electrical power generated by the load of Figure 1 (an analog behavior V=F() source), partly dissipated to an ambient temperature (a pulse source or a piecewise linear, as we are going to evaluate the influence of variation of the ambient temperature on the temperature control of our system). And to add a bit more spice to the modelling, this ambient temperature will present some noise, modelled here with a PWL source with file (a piecewise linear text file where some noise can be generated).
A heated system whose temperature V(Tsystem) must be regulated. We apply to this system the equivalent electrical power generated by the load of Figure 1
We are thus ready to build up our LTSpice simulation, adding the 555 circuit and the voltage- / temperature-driven thermistor to our new system to control its temperature. The complete circuit and the LTSpice directives are presented in Figure 3.
Among the parameters declared, we have the heating element value Rheat, dT to induce variations in the ambient temperature, and the different times t1 to t4 representing the moment in time where these variations will be applied. This particular simulation intends to see if LTSpice describes the well-known circuit behavior realistically, and we will emphasize the effect of the ambient temperature (see the lower pane in Figure 5) on the heating capability of the circuit.
Note that the voltage at the Tsystem node is linked to the input of the temperature of the NTC thermistor U3. We have completed the loop of the temperature control and are ready to simulate.