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I found on the internet and I built the schematic below. Then I tested with the oscilloscope and I found that when going from on to off and from off to on then some unwanted transitions appear on the screen. On the oscilloscope the yellow trace is the output on pin 1 of LM393 and the blue trace is the signal coming from pin 3 of LM393.

Please let me know what should I do in order to make the circuit work correct ? I think that the problem are the repeated transitions made on pin 1 of LM393 (the yellow trace on the scope).

Also, I tried the following: by using a 1M resistor between pin 1 and 3 of LM393 and by using a BY500-100 diode in parallel with the heater, and the screenshots were the same, so no good results. I used a 24Vdc/2.5A SMPS for powering the circuit.

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    \$\begingroup\$ Adding a 1-Meg resistor is the way to go but you need to let the (+) pin go up with the 1-Meg res. But the impedance at pin (+) is too low (seems you have a big cap.) then add a 10k or in series with the (+) and connect the 1-Meg at the junction of 10k and 1-Meg. See figure 3 in this doc. \$\endgroup\$ May 6, 2022 at 5:37
  • \$\begingroup\$ I made the recommended modifications, and the circuit is working like this: ibb.co/LrMQCfs ibb.co/5KG72Ny ibb.co/hVptpGK \$\endgroup\$
    – beard999
    May 7, 2022 at 3:31
  • \$\begingroup\$ Ok so it did solve the issue then? \$\endgroup\$ May 7, 2022 at 7:26
  • \$\begingroup\$ Yes, the soldering station works normally now @VerbalKint. \$\endgroup\$
    – beard999
    May 7, 2022 at 9:03
  • \$\begingroup\$ Perfect, I will document my comment as an answer and if you don't mind validating it, I would appreciate. Merci ! \$\endgroup\$ May 7, 2022 at 11:46

2 Answers 2

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The lack of hysteresis in a comparator is a typical issue for slowly-varying signals. You can solve the problem by bringing a bit of positive feedback via two resistors: when the output starts toggling, the resistor connected between the output and the (+) input will bring the output further up, speeding up the transition:

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However, it is important that the (+) pin can be shifted by the 1-Meg resistor. If that impedance is too low, as in your case with the big capacitor, then there is almost no hysteresis and the comparator hesitates. For that reason, you have to insert an extra resistor - \$R_2\$ in my example - so that the (+) pin can increase during the transition.

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This is a time-proportioning temperature controller, so multiple controller output transitions are expected within the proportional band. U1B is an oscillator that provides the timebase.

Assuming the yellow trace is heater power, it is what I would expect to see if the temperature overshoots (it should then return to a roughly constant on/off ratio after some time- drawing heat from the tip will increase the on/off ratio).

Perhaps you could explain what exactly the problem, and what exactly the two oscilloscope traces represent.

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  • \$\begingroup\$ I added some more information in the initial question. @Spehro Pefhany \$\endgroup\$
    – beard999
    May 6, 2022 at 2:45
  • \$\begingroup\$ The blue trace is the signal on pin 3 of LM393 and the yellow trace is the signal on pin 1 of LM393. The problem is that the signal is going on-off and off-on, like a oscillation when the comparator output is going from on to off and from off to on. \$\endgroup\$
    – beard999
    May 6, 2022 at 3:20

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