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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!

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  • 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.

Reply
  • 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.

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