emonTx arduino shield

Hello, if I want to use the shield attached to arduino, do I need two power supplies, one for the shield(9v) and another for the arduino(9v).

Yes and No !

The emonTx Shield is powered via the Arduino so only one DC power supply is needed, however the voltage sensing is done via a 9v AC adapter which despite being a “power supply” it is only used as a voltage sensor and is not mandatory, but it is required if you wish to measure real power.

So no, only one source of power needed, but a 2nd power supply unit can be used as a voltage sensor (and is generally recommended).

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