A temperature controlled CA3046
So, finally started work again on the heated CA3046. First, the results of the previous post regarding the CA3046 were reexaminated. Indeed, when building the schematic from that post: 
and heating that circuit with a hair dryer:
the base voltage of the NPN transistor changes along with the temperature.
The next step I took was connecting that output to an opamp comparator to be able to switch a heating circuit on and off. Passing a certain voltage level, should turn off the heater. Tested with LED’s and, once again, the hair dryer:
Ah, nice! The heater section is inspired by one of the (great!) electronotes from the past (S-019). The LED section of the circuit was given an own comparator because the heater and the temp sensor are in so close balance, that the opamp doesn’t switch to negative.
And the result, the schematic:
Nice touch (I think) is that the circuit contains 2 LED’s: one is lit when the CA3046 is not at temperature and the other lights up when the CA3046 is at temperature (because of the comparator U1).
Next step: combine this heater with the actual exponential converter circuit.
Oh don’t forget to GND the substrate of the CA3046 (pin 13). Update: or should this be the most negative potential (-15V?). Better be sure before connecting that CA3046…