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I am trying to understand the soft power switch operation of a LED lamp I dissembled. The circuit of the lamp is depicted below.

The switch S1 can be considered as a momentary on switch (SPST). The power P-mosfet Q1 is used to drive the LED. Q1's base is controlled from NPN (Q3) transistor which when in on state pulls it down. The values of resistors are accurate as indicated in actual circuit. The values of capacitors are not valid as I am not able to measure them, but yet I have difficulty understanding the operation.

As opposed to other soft latch circuits, this one uses just two transistors. And there is no feedback from drain of Q1 to base of Q3. If I have transfer accurately (*with the exception of capacitances) the circuit on diagram it seems a very simple design.

In simulation I can't find the exact capacitor values to make it work as the actual one.

The lamp actually works, as when pushing momentarily switch 1 the lamp turns on and when pushing again lamp turns off.

enter image description here

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  • \$\begingroup\$ Your circuit diagram is not complete. The lamp CANNOT latch on without some path for base drive to Q3 that doesn't involve the switch. \$\endgroup\$
    – Dave Tweed
    Aug 6, 2019 at 14:48
  • \$\begingroup\$ Are you sure S1 is a momentary switch and not a "click-on/click-off" switch? \$\endgroup\$
    – Justin
    Aug 6, 2019 at 14:49
  • \$\begingroup\$ Yes, @Justin that's for sure. \$\endgroup\$
    – Curious_Dim
    Aug 6, 2019 at 14:51
  • \$\begingroup\$ @DaveTweed I am afraid so, but I cannot find any other path from Q3's base to any other component than those depicted, using continuity test in multimeter \$\endgroup\$
    – Curious_Dim
    Aug 6, 2019 at 14:55
  • \$\begingroup\$ @DaveTweed are you aware of any soft latch circuit of just two transistors like the one I provided \$\endgroup\$
    – Curious_Dim
    Aug 6, 2019 at 14:56

1 Answer 1

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Your circuit diagram is not complete. The lamp CANNOT latch on without some path for base drive to Q3 that doesn't involve the switch.

As you said yourself, a connection from the drain of Q1 to, say, the node between R4 and R6 would do the trick. Otherwise, those two resistors serve no purpose at all!

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