Hi there. I'll appreciate if u could help me with this circuit. It's been used in a video by Long Technical (

) The 1st stage U1.1 is a noninverting amplifier. Its output is fed into the inverting input of the 2nd stage U1.2 which is a square wave oscillator. And I'm having trouble understanding this oscillator. The RC positive feedback taps into a voltage divider connected to Vcc. The 10k-pot to vary temperature is in this voltage divider. Also tell me about the thermocouple sensor on the noninverting input of U1.1 Measured cold, it's a short. Does its resistance change as the hot air is turned on? Thanks, Etonam.

 

Hi there. I'll appreciate if u could help me with this circuit. It's been used in a video by Long Technical (

) The 1st stage U1.1 is a noninverting amplifier. Its output is fed into the inverting input of the 2nd stage U1.2 which is a square wave oscillator. And I'm having trouble understanding this oscillator. The RC positive feedback taps into a voltage divider connected to Vcc. The 10k-pot to vary temperature is in this voltage divider. Also tell me about the thermocouple sensor on the noninverting input of U1.1 Measured cold, it's a short. Does its resistance change as the hot air is turned on? Thanks, Etonam.

U1.1 (LM358) is a non-inverting amplifier that boosts the signal from the thermocouple, and its output drives U1.2, which is wired as a Schmitt-trigger style comparator/oscillator, with the RC network (R8, R10, C1) and the 10 k temp pot set the oscillation threshold, effectively controlling heater duty cycle. And with a thermocouple, we are actually measuring the voltage generated at the thermocouple junction, which will be proportional to the temperature.

 

Thanks for the answer. The oscillation frequency should be fo = 1/2piR9C1. am I right? Could you direct me to litterature where I can learn in detail about this type of oscillator in U1.2? Once again, thanks for your time. Etonam.