Cold Heat soldering iron PCB circuit.

Thread Starter

Daniel Armstrong

Joined Jan 25, 2019
I had to create an account just to post this.

OK so I have no idea if this thing is genuine or a clone but I got it from the pay by the weight thrift store bins so effectively free. When I tried to use it I promptly melted the plastic around the tip which makes me doubt its authenticity.

Anyway here is my poorly drawn reverse engineered schematic of the PCB circuit from it. The entire job of this circuit is to do only 1 thing, turn on the 2nd LED (D3 here) when the supply voltage drops below a certain point, which it does tend to do when you effectively short circuit the battery through the tip. D4 is the power LED. None of the component values are correct here.

It consists of an LM324M quad op-amp with 3 sections used. What is marked U2 is obviously just a unity gain buffer for the reference voltage, U3 is the comparator, the part that is still above my pay grade is U1, I think it is forming a sort of constant current supply for the diodes D1 and D2 to forward bias them at a known current and use their combined forward voltage drop as the reference voltage for both itself and the comparator.

My questions are thus, does the U1 circuit have a specific name?, Does this make R6 a form of bootstrap resistor?

Either way it seems like an over-complicated way to turn on an LED below a voltage threshold.