Making sense of this Circuit?

Discussion in 'General Electronics Chat' started by SimplyElectronics, Aug 28, 2016.

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  1. SimplyElectronics

    Thread Starter New Member

    Jul 14, 2016
    Hi Guys,

    I am currently practicing my reverse engineering skills starting at creating a schematic from an existing circuit board.
    I have been able to draw the schematic based on a simple led circuit. (This is simplified and first part removed to meet forum restricted topic rules)

    I am struggling to figure out what the electrolytic cap is doing (smoothing?) and why R1 is there.

    I know what a zener diode does and how they work, but not sure what it is needed for here?

    If someone could help me understand this I would be ever so grateful. I'm sure something will click with sudden realisation as soon as someone explains.
    Thank You in advance :)

    Circuit Help 2-01.gif
  2. ericgibbs


    Jan 29, 2010
    hi SE,
    I would say your simplified DC supply is really an AC Source.??
    The LED conducts on the positive half wave of the supply and the R1 reduces the reverse voltage to lower than the breakdown voltage of
    the LED.
  3. dl324

    AAC Fanatic!

    Mar 30, 2015
    I saw the schematic before the thread was blocked and you replaced the line AC/rectifier with the supply block. Neither made much sense. The capacitors in both look strange.

    My suggestions:
    1. Draw the schematic so conventional current flows from top to bottom.
    2. Use conventional component symbols or label for clarity. I assume the hollow plate on the cap is the positive terminal, but a well drawn schematic minimizes the need to make assumptions.
    3. The zener diode is using the wrong symbol and makes no sense as drawn.
    4. Retrace the circuit to make sure the schematic is correct.
    This makes more sense to me:
  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
    The 100uF Capacitor is a short circuit in a DC power supply. This circuit will do nothing.
  5. ci139


    Jul 11, 2016
    . . . suppose your 25mA label is the manufacturers marking of whatever

    since the LED requires some 1.6V drop to be noticed and 500µA or more we get about 1V drop on R2 totaling 2.6V on "terminals" (assuming the cap holds no charge)

    2.6V sets 55mA through R1 that is about 1/(1%) of what goes through LED - to notice a light flash takes (not in there) . . . if i recall right then 8ms or 1 eight of a second (depend but is what i recall) . . . . . . . . . so the forehead to that 2.6V is not much -- or -- dt = -1·ln(1-U.c/U.term)RC , since 47<<2000+ Uc=U(1-1/exp(t/(RC))) and Uc <= U - 2.6V thus U.min-sense = U = 2.6 · exp(t/(RC)) -- Excelling :D -- for 8ms U=14.3V and for 125ms it's :eek:about 1TV (tera-volt) - from what we conclude it's a random object insertet to high current path to be vaporized o_O
    Last edited: Aug 28, 2016
    GopherT likes this.
  6. JohnInTX


    Jun 26, 2012
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