Here are two behaviors that I cannot explain:

  1. I have an ESP8266-12F chip that is connected to an USB-TTL adapter (FTDI) through a breadboard.

The adapter has TX and RX pins connected to RX and TX of the ESP chip directly. The adapter has GND and 3V pins connected to the breadboard so I have 2 rails available to pull up/down some pins of the ESP chip on top of VCC/GND.

Here is first issue: when I plug the black probe of my DMM somewhere in the ground rail, very often the LCD screen of my desktop PC goes black (like off, then on) for a second. Why so? (please remember the FTDI is powered via USB from that PC).

  1. When I then put the red probe of my DMM to GPIO2 of the ESP chip, it lights on the built-in led of the ESP chip.

Why so? I always thought a voltmeter is "passive", like it does not disturb the circuit I am testing.

I tried with an other DMM: this one does not trigger issue number 1 (screen never flickers), but does light the blue LED as soon as I put the red prob on the pin (eventhough less bright).

Both DMMs are on "V" (DC) position when measuring (not in diode mode or else), auto range.

Edit : https://www.youtube.com/watch?v=Tptn1OpsAoM

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    \$\begingroup\$ Is the first DMM powered from an AC outlet, or does it have any connections to other equipment? \$\endgroup\$ Commented Dec 17, 2021 at 19:15
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    \$\begingroup\$ Musa, in your first case (problem with one DMM but not the other one) it is likely something related to what @Peter hints towards. The second case that applies to both DMMs makes me wonder if you have your DMM set to measure Ohms (resistance.) If so, it does provide a small voltage for that purpose (which may auto-adjust) and this may, via protection diodes present on I/O pins, supply power enough that you can see an LED. \$\endgroup\$
    – jonk
    Commented Dec 17, 2021 at 19:33
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    \$\begingroup\$ Other modes can inject voltage. That's how the diode or the resistance or continuity features work. The voltage function shouldn't. \$\endgroup\$
    – Passerby
    Commented Dec 17, 2021 at 19:33
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    \$\begingroup\$ The DMM is on the "V" range but is the red probe in the "A" socket? \$\endgroup\$
    – user16324
    Commented Dec 17, 2021 at 19:49
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    \$\begingroup\$ I am thinking that shaking the probe in the air may charge the probe via tribo-electric effect. \$\endgroup\$ Commented Dec 18, 2021 at 3:54

1 Answer 1


Can a DMM inject voltage?

If your DMM is set to measure Ohm's, continuity, or "diode", then a small amount of current will be injected into the circuit under test. And, depending upon the impedance being measured (and the DMM) several volts may develop across the DMM leads.

For example, I am able to make a string of blue LEDs light up (dimly, but distinguishably on, in a dark room) simply by testing them for continuity with my DMM.

A DMM set to read current (i.e. mA etc.), on the other hand, can act essentially like a short circuit. Probing between a GPIO pin and ground or Vcc/Vdd with the DMM on a current scale can definitely change the state of the pin.

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    \$\begingroup\$ This is what I know of as well. Voltage amount injected in diode mode depends on DMM as well (some +2V, some even +3V, etc). You answered the title but that does not answer my problem, sorry I did not know how to title my post. \$\endgroup\$
    – Musa
    Commented Dec 17, 2021 at 20:35

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