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Op Amp function ?

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Wp100

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Got this soldering iron circuit which I think I can follow as to how the 555 operates when triggered via U2B and the control pot, but its what U2A is sensing via R4 and R11 that is the puzzle ?

Should add that W2 looks like it may have been a trim pot or thermistor /diode at one time and just had a resistor fitted for this lower cost solder station design with no sensor in the solder iron itself.
 

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hi,
R11 is half of a potential divider with R8, it sets the Vref on the +vinp on R7/R14, pin #3 of U2A.

The current flowing thru R3 develops a voltage across R3 proportional to that current.
The R4 resistor is the Rin of the U2A OPA amplifier, the output of U2A is fed back to the comparator U2B, which controls the 555 trigger rate

Is this what you are asking,?

E

EDIT:
If W2 was a thermistor located in the irons heated body, it would provide feedback to U2A to correct for heat loss, ie: increase the element drive current.
 
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U2A measures the current and the voltage to the heating element.
U2B compares the output of U2A to the control pot, you have already said that.
555 is one-shot that gives PWM relating to U2B output.
Together the temperature is controlled by controlling the power to it. It can work ok until you try to solder a large component. This isn't a temperature control as you have already noticed.
 
Thanks both..

I thought it was using R3 as a current sense but the position of those connections was throwing me.

So if I'm right, once the 555 is triggered U2A will sense the change of voltage /current across R3 and turn on U2B though that will be slowly due to C3 charging, which when meeting vref set by the control pot will again trigger the 555 .
 
Yes you are right.
When the heat is on C3 is charged to a voltage that follows the current. The current is lower when the iron is hotter hence higher voltage on C3. Higher voltage on C3 delays more the trigger to the 555 (while C3 discharge) means longer off periods. Once the 555 is on it stays on for a fixed period of about 5 seconds.
In this method the effect of the increase of filament resistance in temperature is used to regulate the temperature.
It saves temperature sensor which is good but you have to judge it by the performance.
 
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