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I have a Raspberry PI 3 B+, and when I activate the GPIO 4 as output and checking agains a multimeter I get its 3.3V as expected.

If I connect the GPIO4 to one end of a coil (http://www.electronicoscaldas.com/datasheet/JQC-3F-T73-Series_Liming.pdf) the voltage drops significantly, from 3.3V to 0.99V, and of course, the relay doesn't change it state.

I thought that the coil had a huge resistor and again checking with the multimeter I could verify that it has only 24Ω.

When using the pin 1 (3.3V continuosly) I see that the relay gets activated.

What's wrong with GPIO4? Is there another GPIO which I can replace this with?

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    DO NOT connect a GPIO directly to an inductive load like a relay coil or a motor. You will destroy the GPIO and then the Pi.
    – joan
    Commented Jun 16, 2019 at 20:30

3 Answers 3

Reset to default
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There is nothing wrong with the GPIO, it just can't supply enough current for your coil.

If the coil has just 24Ω, at a 3,3V the GPIO would need to supply 137mA.

This source at raspberrypi.org says that the maximum current it 51mA for all GPIOs together and 16mA per GPIO.

You can connect the GPIO to a transistor that can supply the necessary 137mA.

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@RalfFriedl is right, but I found some relays which are done for raspberry and arduino.

https://www.youtube.com/watch?v=My1BDB1ei0E

Now the circuit works.

@joan do you think that this kind of relay will destroy the board?

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Question

Why?

(1) It does not work when GPIO4 connects to a relay switch/coil, but

(2) It works when GPIO4 is connected to the input of another relay module?

Answer

There is a big difference between a "relay switch/coil", and a "relay module".

Case (1) GPIO4 is connected to directly a "relay coil", which needs 30mA to 70mA to switch on. In

Case (2) GPIO4 is connected to the input driver circuit (usually a transistor or a optical isolated transistor) This input driver needs only 3mA to 5mA to activate another power transistor which can output 30~70mA to drive the relay coil. So there is no problem.

In other words, you confuse between a "relay coil" and a "relay module" which consists of a input driver which indirectly drives a relay coil.

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