I have a 24V / 80W desoldering gun which turned out to have a PTC heating element. I'm not sure why they chose that, as it is intended to be temperature controlled. The resistance of the heating element is about 1.6Ω at room temperature and increases when heated externally.

I want to build a temperature controller and would either go for PWM or a simple bang-bang controller with TRIAC on AC. Since the resistance goes up pretty quickly I have no doubt to be able to control that.

But I'm a bit concerned over the cold state. Currently I would draw around 360W and that would be even worse in winter.

What would be the best strategy to go about doing this? Would it be possible to use a small-ish NTC in line, or would that interfere with the control side too much?

  • \$\begingroup\$ I doubt they chose to have a PTC heating element - rather they had little choice as most metals and alloys have a positive temperature coefficient. \$\endgroup\$ Aug 3 at 23:30
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    \$\begingroup\$ There are so called resistive heating elements, which are more common for temperature controlled systems. PTC heating elements are designed to self regulate, a resistive heat element is not. \$\endgroup\$ Aug 3 at 23:59
  • \$\begingroup\$ you could delay the triac turn-on until 75% of the wave has passed; trailing edge phase control. Really, you could use phase control to regulate it as well, just changing the phase delay to throttle. A 555 and a thermistor could handle it. If you went DC, you could put a series resistor coupled to a FET that can bypass the resistor when turned on. \$\endgroup\$
    – dandavis
    Aug 4 at 3:58

If you can characterize the resistance vs temperature of the heating element, your controller could have a separate mode where it initially limits power on startup until you reach some temperature where you know that the resistance is such that the controller turning full on or 100% duty cycle on your PWM will not draw more power than is acceptable to you. It would be helpful to have an indicator that shows the user whether the controller is in the initial "constant power" mode or in the "temperature control" mode.

  • \$\begingroup\$ The issue is more that I don't know how to limit the power. I could go for a low duty PWM, but then I would still have HUGE spikes in the on times... I wanted to go for a TRIAC to not having to deal with rectifying and smoothing 80W \$\endgroup\$ Aug 3 at 23:54
  • \$\begingroup\$ Huge spikes are handled with huge snubbers :D seriously, just start at like 5% duty and monitor the current to feed back and heat it gradually \$\endgroup\$ Aug 4 at 6:29

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