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Reverse Engineering help — Parallax Forums

Reverse Engineering help

TCTC Posts: 1,019
in General Discussion
Hello all,

I am reverse engineering a circuit, and I am trying to understand one part of it. I cant give any real details about what I am working on, so please bear with me.

This circuit is coming from the secondary side of a transformer. I am having trouble understanding what could be the voltage after the 2 series zener diodes? And wouldn't transistor "Q402-TR1" just short out when the voltage on the base is high enough?

Could someone please help me understand what the engineer thinking.

Thanks
TC
2-3-2017 2-20-26 PM.jpg
1336 x 357 - 61K

Comments

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2017-02-03 19:48
    The two Zener diodes in series drop the rectified voltage from the transformer by the sum of their reverse breakdown voltages (4.3V + 3.6V = 7.9V).

    Q403 is a current mirror. It sets the maximum current that Q402 can pass when it's turned on, in this case:

    (5V - 0.6V) / 2K

    It would be helpful to see where that vertical wire goes, though, to facilitate a more complete answer.

    -Phil
  • TCTC Posts: 1,019
    Phil Pilgrim (PhiPi) wrote: »
    The two Zener diodes in series drop the rectified voltage from the transformer by the sum of their reverse breakdown voltages (4.3V + 3.6V = 7.9V).

    Ah... Now I understand.
    Q403 is a current mirror. It sets the maximum current that Q402 can pass when it's turned on, in this case:

    (5V - 0.6V) / 2K

    It would be helpful to see where that vertical wire goes, though, to facilitate a more complete answer.

    -Phil

    I was wondering if it was a current mirror circuit, but I did not know for sure.

    Here is the circuit that controls the transformer. Some of the values/parts could be wrong. The board is all SMD, and trying to figure out what the marking codes can be a pain. The input is coming from a IO pin from a micro.

    2-3-2017 2-47-42 PM.jpg
    1589 x 581 - 105K
  • DigitalBobDigitalBob Posts: 1,513
    Is this a part of some power supply or Amplifier? what did it come out of?
  • TCTC Posts: 1,019
    DigitalBob wrote: »
    Is this a part of some power supply or Amplifier? what did it come out of?

    I'm sorry, I can't give that information out. It is a prototype item. I am only reverse engineering it to learn what and why engineers do what they do.

  • ercoerco Posts: 20,256
    TC wrote: »
    I'm sorry, I can't give that information out. It is a prototype item.

    Sure sounds like a Death Ray.

  • TCTC Posts: 1,019
    erco wrote: »
    TC wrote: »
    I'm sorry, I can't give that information out. It is a prototype item.

    Sure sounds like a Death Ray.

    You figured it out. Muhahaha
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