Triac Phase angle control Heating method

Question

I purchased a soldering station (Product Link: Tenma 2064549. Just for give more idea. Not for taking about the product.) few years ago and replaced the heating element. After replacing heating control system get malfunctioned. When heating up the soldering iron, display shows deceasing temperature. Finally I was able to solve this issue by reversing polarity of the heating element.

As I found, there is no dedicated sensor for measure actual temperature and feedback to the control system. Simply it has BT136 Triac, MOC3041 zero-cross triac driver and Microchip PIC based control system. There is a transformer that can make 26V and 9V. 26V for heating element and 9V for control system. There is a Op-Amp based current measuring circuit also. But, I couldn't find any current detection point.

When I cool down the soldering iron tip by sinking it in water, system detect the heat loss and powering the heating element.(I measured heating element current using multimeter).

I am wondering how this system control the temperature without feedback. And How system detect heat loss by measuring the current through the heating element (If such system include)

I am expecting your idea and experience to understand this heating control system.

Answer 1

It is likely that temperature is being sensed by measuring the heating element resistance before the triac is energized during each half-cycle. This can be done by taking an ADC sample of the voltage on the heater before the phase firing occurs. For simplicity, my simulation shows a 50% duty cycle with a 1 Hz switching frequency. And for a heater, this is just as good as a phase-fired triac. Maybe even better.

^{simulate this circuit – Schematic created using CircuitLab}

The temperature is a voltage proportional to the resistance of the heater element, which is measured during the OFF interval, where the element is powered by a 200 ohm resistor fromnthe 24 VDC supply. The readings are multiplied by 10 for readability.

Answer 2

These devices use a thermocouple, as @Bryan suggests. Using heater resistance is theoretically possible but heaters have manufacturing variations and (worse) will increase in resistance as they age since NiCr is not a noble metal. Making the heater from platinum wire or individually calibrating the heaters is not an attractive proposition when there is an alternative that depends only on the alloy composition of the metals.

When there are only two wires to the heating element, the thermocouple is in series with the heater resistance, so the temperature can only be measured by briefly removing the current through the heater and measuring the mV that is coming out. There's typically another temperature sensor in the handle that measures the temperature of the cold junction(s). And a sensor to indicate the iron is in use so it can be allowed to cool when idle- to save energy and increase the tip life.