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Raspberry Pi 4 GPIO Leakage Voltage (1.2V) When LOW on Relay Module

emreeyy

Hi everyone,

I am using a Raspberry Pi 4 Model B to control industrial 24V relay modules. However, I am not using an external 24V power supply; instead, I am routing the Pi's own 3.3V pin to the common power rail of the relays to use them as simple mechanical switches for 3.3V signals.

I have wired the Raspberry Pi GPIO pins directly to the relay signal inputs via a simple terminal block breakout board (no transistors or optocouplers are used yet).

The Problem:

 When the GPIO pin is ⁠HIGH⁠, I read exactly 3.3V on the line, which is correct.

 When the GPIO pin is set to ⁠LOW⁠, instead of 0V, I see a constant 1.2V leakage/floating voltage on the multimeter.

There is currently no common ground (GND) connection between the Raspberry Pi and the panel's internal structure. I suspect this 1.2V is due to a floating ground issue or feedback leakage through the relay's internal coil/status LED.

What is the best way to eliminate this 1.2V leakage and get a clean 0V when the pin is LOW? Should I use an optocoupler module (like PC817) or a transistor driver?

Thanks in advance!

  • 0

    Hi,

    You may need to sketch a diagram, and attach a photo showing your setup, and links to the relay module and any other bits you're using. Right now it's very unclear what your precise set-up is. 

  • 0

     reference the rpi common to the panel internal structures common and the rpi power input common. Make everyone's common , in common. 

  • 0

    If the Pi is going to power and external device they both have to have a common ground reference. 3.3V on the Pi is referenced to its own ground. If the Pi's ground is not the same as the equipment ground, you will have issues. Let's just say I have had issues until I create the common ground.

  • 0

    Hello,

    1. Do NOT connect a 24V relay directly to an output of the Raspberry Pi. This will damage the Pi's port.

    2. You need an external driver transistor to control the 24V relay. Alternatively, you can use an optocoupler such as the PC817.

    The GND - normally - is the base of each switched schematic, special circuits can have other base voltages.

    A relay has three operating states:

    a) Turn-on voltage: This is the minimum voltage at which the relay turns on.

    b) Hold voltage: This voltage is slightly lower than the turn-on voltage and can be even lower before the relay turns off. This voltage can help save power.

    c) Turn-off voltage: This voltage is so low that the relay turns off. A voltage may still be applied, but it will no longer be held.

    You can find the voltage states of your 24V relay in its datasheet.

    3. You can use an NPN transistor, for example. BC547 or BC546 or similar switching transistors (emitter to GND, collector to the relay) – the relay is connected to the supply voltage (+24 V).

    4. Don't forget the diode, e.g., 1N4007, in parallel with the relay coil; cathode to the positive supply voltage, anode to the switching transistor collector.

    Alternatively:

    There is another option where the relay is connected to ground (GND) and can be switched with a PNP transistor.

    However, this circuit is somewhat more complex and requires a few more components.

  • 0

    You need to supply a schematic of the connections. Typically a Pi output pin cannot drive a relay coil because the voltage is too high, the current is too high and the inductive flyback is too high. It sounds like you are trying to pull down a coil pin of a coil whose other pin is connected to 3.3V. The I/O pin cannot sink enough current to pull that pin to ground. A transistor would help, but the coil may need a higher voltage and it would definitely need a snubber diode.

  • 0

    Hi emreey.y.

    Please reference my spring clean 2026 project: The WrongCo Garden Guardian (a.k.a. Rube Frankenstein).

    I have schematics showing a raspberry pi pico 3v outputs driving 3v relay modules and 3v mosfet boards. The modules then drive 12v circuits. These output driver boards could just as easy pilot 24v devices. 

  • 0

    The RPI does some interesting things with the GPIO pins.

    You may have to set up some external termination resistors to get the right behavior.

    As I recall, there are some internal options you can set to the output configuration to use internal termination.

  • 0
    emreeyy said:

    Wenn der GPIO-Pin HOCH ist, messe ich genau 3,3 V auf der Leitung, was korrekt ist.

    Wenn der GPIO-Pin auf LOW statt 0V eingestellt ist, sehe ich eine konstante 1,2V-Leck-/Schwimmenspannung am Multimeter.

    Look here: https://www.raspberrypi.com/documentation/computers/raspberry-pi.html#gpio-pads-control

    most of the port pin are tri-stated pins.

    Yes, it means that the pin can have 1.2V - the middle of the supply voltage as well as meaning this state is in "high-Z".

    This is NOT a "leakage".  Wink

    Z = Impedance in (k)Ohm.

    This function is a very important function if you want to use that pins with different jobs.

    Such port can be driven in: "switched on" (sending high to the output), "h-Z" (the software let the port "swimming"), "switched-off (sending low to the output)

    Gerald
    ---