Relay switches but no power

Question

I am trying to connect my Raspberry Pi 3 to a relay and power a small pump with that.

The relay I use: 8-Relay. I can use the water pump with 3V to 12V and I want to connect a 5V external power supply to the relay.

I've connected the Raspberry Pi and the relay according to this tutorial (External Power Supply + to Relay JD-VCC / EPS - to Relay GND / Raspberry Pi 3,3V to Relay VCC / Connected one GPIO pin).

I can switch the relay on and off, so far so good. But when I connect the relay output to the water pump nothing happens. When I connect the water pump directly to the external power supply then the water pump is turned on (so no problem with the pump).

What am I doing wrong?

Edit: I've connected the relay with the pump like this:

Answer 1

Based on your wiring diagram, your wiring is incorrect, you are not providing any power to the motor. The relay does not supply a voltage itself, it is merely a switch. The VDD pin on the relay is only for the driver (coil), and not for the switched side.

You need to change your wiring to supply power to the motor as if the relay is not there.

You can either use it as "low-side" where the relay is connected to GND and motor directly to V+. Or a "high-side" switch where relay is connected to V+ and motor directly to GND

^{simulate this circuit – Schematic created using CircuitLab}

Note: Due to the isolating nature of a magnetic relay, you do not need to connect ground of water pump to ground raspberry pi, and you do not need to use the same power supply as raspberry pi, in fact for something like a water pump I would highly recommended not to do so!