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I am trying to use an old phone charger with 5V DC output. I measured the output to be 6.4V.

It works for powering an ESP8266 through VIN but does not work to power an IR receiver I want to use.

The receiver works when I power it through a different source (5V or 3V and ground from an Arduino.) It also does not work when I use one of the pins of the ESP8266 which gives me 3.3V when I measure it. It is also not working when I use 5V from the Arduino and ground from the phone charger (or vice versa.) This last bit confuses me most because I assumed which ground I use does not make any difference but perhaps I lack some basic knowledge here.

Finally, when I try to measure the current across the Arduino V5 out and the phone charger ground, it gives me 7V AC, and some really low DC voltage. I am rather convinced I am clearly lacking some understanding here. Can anyone explain this behavior to me?

Finally, if there is an easy fix to this to get the IR receiver working, I would be more than interested.

EDIT: I finally got the IR receiver working. It actually works with both 6.4V and 3.3V but I went for 3.3V sourced directly from the LDO on the board. The problem I had before with this was that the GPIO I used to receive the signal is not working. I might have damaged it previously somehow. As far as I understand, the ESP8266 GPIOs can handle 5V and perhaps also 6.4V (?) input. The board I am using does not have a pin for 3.3V out so I soldered I wire directly to the LDO, which I am not sure is a good idea but it is working (perhaps there should be a capacitor in-between).

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    \$\begingroup\$ Draw a diagram. Right now it appears you would also expect things to work when connecting one wire to one battery, and another wire to another battery, which obviously does not work as circuits need a return path as well. \$\endgroup\$
    – Justme
    Nov 13, 2021 at 16:59

2 Answers 2

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Can anyone explain this behavior to me?

Ground on one charger does not necessarily mean it connects to ground on another charger or "system". Ground is not "earth" so there is almost certainly a massive galvanic isolation between the two. Earth is different; earths are meant to be galvanically connected but, by no means is using earths as a common line for electronic circuits at all guaranteed to be successful either. They may allow power to be applied but they will also transfer fault currents from other appliances and circuits (i.e. wholly not recommended).

The AC voltage you measure between two different grounds from two chargers is due to capacitive coupling across the internal transformers inside said devices.

Finally, if there is an easy fix to this to get the IR receiver working, I would be more than interested.

Use the correct supply for the device noting that when you fed it with 6.4 volts, you may have damaged it.

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    \$\begingroup\$ If his ESP8266 has a "VIN" pin, it likely leads to an LDO. Therefore, the ESP8266 is likely to have survived the 6.4V unscathed. The IR receiver may have sustained damage, but this could easily be determined by powering it from the Arduino again and seeing if it works. \$\endgroup\$ Nov 14, 2021 at 1:41
  • \$\begingroup\$ I'm wondering why the IR receiver wasn't working when powered from the ESP8266's 3.3V pin. I can't think of an obvious cause for that. \$\endgroup\$ Nov 14, 2021 at 1:41
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    \$\begingroup\$ Actually, I reckon I know why. I assumed he meant he was powering it from the 3.3V LDO output of the ESP8266 module; but he actually says "one of the pins which [happens to measure] 3.3V". So probably an I/O pin with an active pullup, resulting in a very low current output. \$\endgroup\$ Nov 14, 2021 at 1:44
  • \$\begingroup\$ @SodAlmighty thank you for your discussion. Your assumptions are correct. VIN leads to an LDO (AMS1117). The IR receiver is still working (tested with Arduino). \$\endgroup\$
    – Mike
    Nov 14, 2021 at 12:23
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IR Rx's require low ripple Vdc to function due to the AGC sensitivity to supply noise.

Your charger is unregulated and noisy, and therefore the IR Rx needs a linear regulator to remove the noise within it's DC operating range.

Ground is just a local 0V reference. PE or protective Earth is ground somewhere connected to earth. Every floating system may become common 0V ground as long as there are no excessive CM noise ripple or other line filter to PE ground noise that create Common Mode (CM) noise. This can interfere with some lines of communication like USB with two separate SMPS, but is likely your problem here. Just a warning for future.

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