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I'm working with a camera with I/O pins, which I use to turn on lights.

enter image description here The diagram, provided by the camera producer, shows two types of diodes. But what are they, and what are their purposes here?

I believe the one marked with blue is a Zener diode, and it allows current to flow from pin 6 to pin 4, but why?

The diode, which is marked with purple, I have no idea about.

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  • \$\begingroup\$ The one with an "S" is a Schottky and the one with a "Z" is a Zener. Zeners disappear more voltage (higher Vf), Schottkys disappear less voltage (lower Vf) than a "normal" silicon rectifier diode. The Schottky's used here to keep the output close to 3.3v as possible while providine reverse polarity protection. The Zener is used here to clamp the maximum output voltage. \$\endgroup\$
    – dandavis
    Jan 12 at 8:29
  • \$\begingroup\$ Zeners are used reverse biased, which basically means they act like an insulator under their Vf, and like a magic resistor above their Vf that clamps the voltage to the Vf. You can think of them as several diodes in series if that helps. \$\endgroup\$
    – dandavis
    Jan 12 at 8:37
  • \$\begingroup\$ It's very common to see this kind of circuitry around inputs and outputs and it's usually there to protect the main circuit against mistakes like shorted outputs, outputs connected to other outputs, outputs connected to power, etc. and perhaps also ESD. \$\endgroup\$
    – user253751
    Jan 12 at 10:39
  • \$\begingroup\$ Is it wrong to assume that the blue one is specifically a TVS diode? I have never seen a diode with that symbol in that position that wasn't one. \$\endgroup\$
    – Hatman
    Jan 12 at 21:51

2 Answers 2

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The Schottky diode (purple) prevents current flow from (what seems to be) the external power supply 3.3-24 VDC to the internal 3.3 VDC.

The Zener diode (blue) could act as overvoltage protection. For example when the external power supply is 24V, it would limit the voltage applied to the transistor.

You'd have to have the component specs to be 100% sure.

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Purple is a Schottky diode.

Blue is a Zener.

They may be there to protect the circut from overvoltage, but as the component types and values are unknown, the details how the circuit works can't be detailedly explained.

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