I have the below input protection and filter circuit. I want to understand how it works.

enter image description here

Can someone tell me the purpose of R0101 and D0102 and D0104?

As far as my understanding level, when the voltage at the emitter of Q0102 exceeds the voltage at the base of Q0102 by its base emitter voltage threshold (assumed to be 0.6V), the Q0102 transistor will short the gate and source of the MOSFET and turn it OFF. So, when the input voltage goes low, the input gets cut off. Am I correct? Is that the intended purpose?

The Zener diode is to protect the gate-source of the MOSFET not to exceed the maximum GS Voltage. The Pi Filter, C0102, L0101 and C0103 is there to reduce the conducted emission from the U12V to the input rail.

Can someone explain the purpose of R010, D0102 & D0104, please?


1 Answer 1


I’d say you’re right about Q0102, if U12V goes higher than the 12V input then Q0102 will switch off Q0101. I’d guess that D0102 is there to catch any inductive voltage spike at switch-off, and possibly to provide an inrush path that doesn’t go through L0101 although I’m not immediately sure why that would be necessary. It will certainly limit ringing of the Pi circuit. I’m not sure about D0104 - the way the schematic is drawn hints that there may be more to the circuit than is shown - that little number 8 above R0104...

  • \$\begingroup\$ Thank you for the answer. But I am not sure why the D0104 is necessary. The circuit would work the same without that D0104, right? \$\endgroup\$
    – user220456
    Feb 2, 2021 at 10:55
  • \$\begingroup\$ The two reasons I can think of are 1. To prevent reverse-biasing, although that doesn’t make sense because there’s nothing to protect C0105 if a negative voltage were applied to U12V, or 2. to add a diode-drop to the voltage on the emitter of Q0102, although I can’t see any reason why that would be desirable. However, if some other circuitry were added at the top of R0104 then it might make sense. That little number 8 makes me think that there might be an option for that on the PCB. \$\endgroup\$
    – Frog
    Feb 2, 2021 at 18:55
  • \$\begingroup\$ Sorry, mad bad. There was a test point number 1008 at that position and I had erased the 100 and not the 8. \$\endgroup\$
    – user220456
    Feb 3, 2021 at 2:56

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