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I want to make myself a reflow oven following more or less this guide: http://andybrown.me.uk/2015/07/12/awreflow2/

Here is the zero-crossing detection circuit diagram: enter image description here

Thing is I would like to cut costs on BOM and workload. As I have a few spare 5V wall socket chargers lying around I went ahead and popped one of them open. It looks quite nicely built. I want to use this one for the power supply, but I now have this issue: where do I tap into it for connecting Q2 transistor base?

The main issue here is that the rectifier bridge (the magenta encircled IC) operates at 220V/ I.e. it comes in front of the transformer, not downstream as I need it. I figured that then perhaps I could take my signal from one of the secondary circuit pins of the transformer. I am not sure.

enter image description here

enter image description here

DMM readings (DC):

  • between any of the pins from the brown square and any of the pins from the blue square: 230 - 235 V
  • between pins 6 and 7 in the yellow square: 0 V

DMM readings (AC):

  • between any of the pins from the brown square and any of the pins from the blue square: <100V, rapidly stabilizing at 0V in a second or so
  • between the pins 6 and 7 in the yellow square: 1-2 V, rapidly stabilizing at 0V in a second or so

Any tips?

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You don't tap into the AC anywhere.

That type of power supply doesn't have the AC signal from the line voltage at a lower level.

It converts the line voltage AC to high voltage DC, and then uses a high frequency signal to make pulsing DC which that little transformer can bring down to a lower voltage.

There is no low voltage point where you can get a zero crossing related to the line voltage zero crossing.

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  • \$\begingroup\$ Aw... Ok. What if I tapped into the high voltage output of the rectifier bridge ? Any way to modify the zero-crossing detecting scheme to accomodate for the HV ? Maybe use some sort of heavy duty Q2 transistor ? \$\endgroup\$ – kellogs Feb 26 at 19:30
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    \$\begingroup\$ Tapping into the high voltage side gets you into all kinds of hazards. Don't. You could use a small transformer, or an optocoupler. There are circuits for both on the internet. \$\endgroup\$ – JRE Feb 26 at 19:44

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