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tl;dr: The ESP32 DevkitC v4 has a BAT760-7 diode between the USB VBUS and the EXT_5V. What is the purpose of it?

I am planning to make a hobbyist project where I need 4.9-5.1V to an external sensor (a MQ3 gas sensor), and use a ESP32 Devkit (non-branded make, uncertain of which generation it is) to do some data logging. The USB power from my laptop is 5.1V, and I can read that voltage on the Devkit USB connector. That should be fine to power the sensor and the ESP32. However, the voltage on the VIN pin on the Devkit is merely 4.8V. I do get a signal, but since it is out of spec, I don't accept the readings as reliable. The sensor has a heating coil that draws 750mA, but switching my power source to a 2A USB charger did not change the number, so I assume it is not a problem that my powers ource is too weak.

Trying to understand this voltage drop, I found the schematis for the DevkitC v4: link. The relevant part are copied below.

esp 32 devkit schematic for usb connector esp 32 devkit schematic for linear voltage regulator

From these schematics, I see that there is a BAT760-7 diode between the VBUS from the USB connector and the EXT_5V pin, that I try to pull current from. My voltage drop is in practise 300mV, and that is not consistent with the spec for the BAT760-7 see image below, but I guess I have a pirate copy that has used a replacement part... I still have a diode sitting there, so I guess that by learning about the documented version of the ESP32 devkit, I could learn something about my own kit.

current-forward vootage for BAT-760-7

My question: What is the purpose of the BAT760-7 in this schematic? What risks and problem would I get if I shorted it to get a 5.1V connection straight from the USB? Is there some other way to pull current off the USB witout getting this voltage reduction?

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    \$\begingroup\$ although depicted as a zener diode, I believe the intention of who drew the schematic wanted to put the symbol of a schottky diode. \$\endgroup\$ Commented Mar 24, 2021 at 11:25
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    \$\begingroup\$ @PauComaRamirez Probably just confusion due to regional differences in schematic symbols. For example, the standard European symbol for a Zener diode is different, while the symbol for a Schottky is very similar to the American Zener symbol. \$\endgroup\$
    – TooTea
    Commented Mar 24, 2021 at 11:33
  • \$\begingroup\$ @TooTea xkcd.com/927 xDD \$\endgroup\$ Commented Mar 24, 2021 at 12:06

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It appears as though the diode D3 is to protect the VBUS from "feedback" from the case scenario where you would be connecting an external 5V supply to the EXT_5on connector J2.

External supply still will need to share/be referenced to the same Ground GND

Supply current from USB devices is "normally" limited to 500mA max but depending on how many devices are hanging on the hub you may have a lower limit.

If you have higher current requirements than are available from your host USB connection, you can provide power externally via this pin.

Note that the devices on the board are powered via 3V3 behind the regulator U2 which can be powered via VBUS from your USB connection or from externally provided power via EXT_5

If using externally powered devices make sure that the outputs that you connect to the Inputs of the microcontroller are within the voltage range of these inputs.

Although not recommended, since you could damage you usb hosts port, iif you are not going to use any external power source, you could bypass the diode with a short.

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  • \$\begingroup\$ Thanks! Confirmed many suspicions. I have a strong feeling I won't short that diode... \$\endgroup\$
    – LudvigH
    Commented Mar 24, 2021 at 14:05

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