For a few simulations more Part 5 - NTC and optocoupler

For a few simulations more: Part 5. NTC thermistors, optocouplers and temperature control.

I wanted to illustrate one more simulation of temperature control with NTC thermistor, using an optocoupler as a modern switch, and to show once more how we can combine electronic simulation and thermal engineering with the wonderful simulation software offered by LTspice. Also I found a very relevant spice model for the digital 6N137 optocoupler commercialized by Vishay. I was surprised that the importation of the Pspice model ( pspicevo.pdf (vishay.com)) was not as straightforward as it used to be. The simulation in LTspice crashed because the Pspice model contained features not supported by LTspice, Fortunately, I found a fix for that at the following address.

Using Spice/PSpice (Vishay optocoupler) models in LTSpice - Electrical Engineering Stack Exchange

The new subckt file can be found in appendix in the zip simulation file and also in the "code section", together with usable symbol. In order to be 100% sure that the LTspice version of the 6N137 was as relevant as the original Pspice model, I reproduced a circuit present in the data sheet and computed the switch time announced in the spec. (Low to high and inversely)

Conforted in this conversion, I wanted to test the original model in PSPICE , and more particularily PSPICE for TI which is the only Orcad software installed on my machine. I built up the same simulation and the following pop up screen warned me with a message that the original Pspice model cannot be used in Pspice for TI...... which seems a bit strange to me.... I reprint hereunder the message. Nevertheless, it's been tried and we can go on without looking back.....

The principal thermostat circuit I simulated in this part 5 was not found on the web, but who knows, as nothing is never new by today's standards, I will be very happy to be warned if this circuit is to be found somewhere else (I will include the original reference if needed).

The structure of this circuit is completely classical : a Wheatstone bridge compares the voltage provided by the temperature sensor, to a set voltage. A comparator switches an Vishay 6N137 optocoupler which relays the switch pulses to a mosfet, commanding an heater element. Looking at the waveforms, it turns that this circuit is doing exactly what it has to do.

As always, the schematics parameters are adjustable at will : thermal capacity, thermal resistance, response time of the sensor, ambient temperature, set temperature......all these temperature sensing crucial parameters can be adapted to the practical measurements, all this for an optimal temperature sensing.