I don't believe the recovery we are seeing here is the opamp recovering from saturation, more so it is the thermocouple recovering from the back emf from the heater coil,….
it is the thermocouple recovering from the "back emf" from the heater coil !!?The real physical phenomenon involved in the generation of emf (voltage) at a ThermoCouple Junction, is the
“SeeBeck effect”.
But in your comment, what does “recovering from back-emf means”? Are you referring to the opposite (
corollary) effect of “SeeBeck effect”, termed
“Peltier effect”?
“Peltier effect” states that
when a current is forced thru a Thermocouple junction, heat-energy is transferred from the “Hot-Junction” towards the “Cold Junction”. Mind you, this is not by joules heat transfer.If by
“back-emf” you are referring to the above
“Peltier effect”, you may well be correct, since the heat energy transfer from hot to cold junction by the Peltier effect, in effect reduces the thermal gradient, by raising the cold-junction temperature.
If indeed this is true and others here also agree to this, then it is all the more required that David or anybody’s firmware sampling the Thermocouple emf has to do it as late as possible, meaning just before putting ON heater.
Or at least well late in the cycle, when the thermal-imbalance of this “effect” dies down, significantly. In effect you are giving a delay for the normal conductive heat transfer to predominate over the Peltier result. The
”Peltier effect” is well documented and it is within the trio of
“ThermoElectric effect” (Seebeck, Peltier, & Johnson effects).
May be other’s here can comment whether this Peltier effect is relevant, when T12 heater coil is heated, and this is indeed what is happening.
https://en.wikipedia.org/wiki/Thermoelectric_effect#Peltier_effecthttps://en.wikipedia.org/wiki/Thermoelectric_effect#Seebeck_effect Location of EMF generated in a ThermoCoupleOne associated fact: It is a common misconception that the location of the EMF (voltage) generated by a Thermocouple is only at the Hot-Junction. But in reality, it is not so. As per the SeeBeck effect, this emf generation is spread out all over the entire length, of the two conductors (
with dissimilar materials), from Hot-Junction to Cold-Junction. That is, the entire portion of these two conductors which experiences different temperatures (
or a thermal gradient) contribute towards generating this ThermoCouple voltage. (
Ref: fig-1: Std ThermoCouple Circuit)
Many members here (
including me) had this misconception earlier, since I had no well defined notion about ThermoCouple Seebeck effect, until I refreshed the material and researched the topic in detail. But this misconception, screws up your rationale & logic, when thinking about the physical phenomena happening within the Tip.
In the above fig-1 (Std-Thermocouple), both the Red & Yellow conductors from Hot-Junction (@Tsense °C) to the Cold-Junction (@Tref °C), contribute towards these EMF generation. Finally the differential voltage is fed to the sense-amplifier at the instrumentation block (
typically opamp). In fact if there is a further temperature difference between the Cold-Junction (@ Tref °C), & the Instrumentation-amp (@Tmeter °C), (meaning Tref ≠ Tmeter) in the above figure, then both the (+ve & -ve) Copper wires connecting them will also generate an EMF, as well. But since both these wires are of same material “Copper” (
NOT dissimilar materials), the emf’s generated in both the +ve & -ve wires will of the same amplitude and, so will cancel out.
In the special case of T12 (
& similar) cartridges, one of this conductor (
+ve) is the one joining to the Heater-Coil in series. The other conductor (
with dissimilar material) is the
–ve conductor, the end of which goes directly to the hot-junction located at the tip of the cartridge. For T12 the
cold-junction is, the dual ”terminal-contacts” at the rear-end of the cartridge, where they meet the copper conductors from the handle-wiring.
I hope this discussion above puts to rest categorically, the confusion and questions about where the “Cold Junction” is located in the T12 unit (Handle or MCU etc). Refer to fig-2a & 2b below for clarity on the above discussion with regard to T12 & similar cartridges.
Fig-2a & 2b attached below.ThermoCouple Type of T12: K, N, C ?There was a question about what is the Thermocouple type (
“K”, “N”, or “C”), the T12 cartridge has? But the measured 21µV/°C is not matching neither of these std-types. The reason is that this is not a standard thermocouple. From fig-2a,b it is clear that, the conductor from the Cold-Junction (+ve terminal) to the Hot-Junction, is itself made up of two type of materials. Probably the conductor starting from +ve terminal is “chromel”, which joins to the heater coil, which is made of probably Nichrome. The conductor from the Cold-Junction (–ve terminal) to the Hot-junction may be made of “alumel”. So its thermal-EMF characteristics may be different.
This para (about materials) is fully speculation from my part, and not based on any hard facts, but it gives an answer for the possible mismatch from the known Std-Thermocouple types, in terms of EMF/°C characteristics.
I don't believe the recovery we are seeing here is the opamp recovering from saturation, more so it is the thermocouple recovering from the back emf from the heater coil,….
Now coming back to “Bozog’s” comment, the extended time (~400µs) for which a reduced voltage is seen at the opamp output may be the combined effect, of both opamp recovery & the above cooling by Peltier effect. We cannot separate out the effects in the visualized waveform.
Finally one more point, to see the full effects of the mosfet turn-off delay, opamp recovery delay etc, & characterize it more clearly, we may need all three following graphs in the same time-line. (1. Mosfet Turn-off command by controller (port output). 2. Heater Voltage & Opamp-in, 3. Opamp-out ). If not possible two each in two separate graphs.