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I'm not experienced at electronics, so please forgive the simplicity of my question. But what is this diode in the picture below doing? The circuit is made of 3 in-series leds wired in parallel (3x3). Each path has a 150 ohm resistor. There is also a path from power source to ground with a diode (I think) which I can't figure out the purpose of. When I apply 12 volts to the circuit, the diode burns up, and if I bypass it, the circuit lights up fine. If it maters, these lights used to be powered by AC to 12V adapter, and I'm now trying to power directly with 12 V.

enter image description here

enter image description here

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  • \$\begingroup\$ "When I apply 12 volts to the circuit, the diode burns up, and if I bypass it, the circuit lights up fine." How are you ensuring that the 12 V is applied with the correct polarity ? May be it was connected backwards ? \$\endgroup\$
    – AJN
    Commented Nov 1, 2020 at 19:44
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    \$\begingroup\$ Good question. Only thing I can think of that would make any sense is that it's a Zener for over-voltage protection, relying on some sort of over-current shutdown to be present in the power supply. \$\endgroup\$
    – Unimportant
    Commented Nov 1, 2020 at 19:45
  • \$\begingroup\$ Also the "diode" is labelled on the backside as R4 not Dx. \$\endgroup\$
    – AJN
    Commented Nov 1, 2020 at 19:46
  • \$\begingroup\$ That diode looks "burned" to me. \$\endgroup\$
    – jonk
    Commented Nov 1, 2020 at 22:13

1 Answer 1

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Totally agree with the comments given. By looking at the photos given, I could draw the circuit below.

schematic

simulate this circuit – Schematic created using CircuitLab

The "mystery" component 'X' is definitely a Zener diode. Also, from the photo you can see the darker portion typical from common Zener diodes.

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    \$\begingroup\$ Given your work in extracting the schematic, I'd imagine that the OP trying to supply a 12V rail may have blown the zener. There's no current-limit resistor for the zener in the schematic you show and it's unlikely the OP knew enough (given your schematic here) to include one. Unless the 12 V supply matches VERY closely to the zener, it's likely it over-heated -- which would explain what I imagine from the photo to be evidence of it blowing out. +1 for the work in getting that schematic sussed-out. \$\endgroup\$
    – jonk
    Commented Nov 1, 2020 at 22:15
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    \$\begingroup\$ Maybe its just me, but without the specific comment saying that it is probably used for over voltage protection, this does not include an answer to what the purpose of the diode is, only that it is a Zener diode. To avoid confusion you might want to include that part of the comment? \$\endgroup\$
    – Yanick Salzmann
    Commented Nov 2, 2020 at 11:45
  • \$\begingroup\$ @YanickSalzmann you are right, better explain the reason why that Zener diode is in the circuit. As jonk mentions this is for circuit protection, since the supply is external the user can connect any voltage, even accidentally. The problem, as also jonk mentioned is no one protects the Zener diode, a current-limiter resistor in series would be convenient \$\endgroup\$
    – Eloy Calatrava
    Commented Nov 3, 2020 at 7:29

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