I'm using the circuit below in a custom PCB to switch on and off an electrovalve.

I assembled partially the PCB, just to verify some voltage across the triac. I didn't connect the MOC3021 and R40 at the moment.

I plugged the 230V main AC and measured the voltage across the two terminals X10-1 X10-2, and strangely (for me) I measured 148 VAC! Note that GND230 is only a label to distinguish phase and neutral in the schematic. I was expecting that, since the gate is disconnected the A2-A1 was at high impedance and any voltage would have been measured between A2 and A1.

As extra test I connected a bulb lamp between X10-1 and X10-2 expecting that it would switch on with 148 VAC. Instead, when I plug the 230 main AC, the lamp is lighting for a very short time (<0.5 sec) and then it switch off. In this condition the voltage between X10-1 and X10-2 the value is zero as expected.

What happen that I don't understand?

As a secondary question, what could be a right value for R40?

enter image description here


It appears you have some signal capacitance on power up that drives with enough uA to trigger on power up. Since you show no prevention for this by design.

What you are seeing is Triac leakage current in your probe.

If that current *Rin=Vin exceeds spec with no load, then you have a test or design failure.

  • \$\begingroup\$ Could you please be more detailed in such a way to understand where and what I have to change the design? These uA are sinked or sourced by the gate? The optoisolator and R40 are not mounted, the circuit on the side of the optoisolator triac is disconnected. Is the capacitance between the pin A1 and the pin G of the triac? I'm using a tester on the two terminals X10. \$\endgroup\$
    – daigs
    Sep 29 '19 at 22:40
  • \$\begingroup\$ Did you measure with 10MOhm? \$\endgroup\$ Sep 29 '19 at 22:41
  • \$\begingroup\$ I meant I used the tester to measure the 148 VAC at the X10 terminals. As soon as the 230V reach the A2 terminal by the lamp filament, the voltage drop (correctly) to zero on X10. Should I modify the circuit, connecting the load on the A1 side? \$\endgroup\$
    – daigs
    Sep 29 '19 at 22:44
  • \$\begingroup\$ Not unless you can do as I asked and leakage current exceeds your spec \$\endgroup\$ Sep 30 '19 at 3:58
  • \$\begingroup\$ what do you asked? Is this a rebus game or a forum that help people? \$\endgroup\$
    – daigs
    Sep 30 '19 at 15:06

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