A circuit for detecting an input voltage of a transformer is provided. The circuit includes a current source circuit coupled to a winding of a transformer. A current-to-voltage circuit is coupled to the current source circuit to generate a first voltage in response to a current outputted from the current source circuit. A sample-and-hold circuit generates an output voltage by sampling the first voltage. The transformer is coupled to the input voltage, and the output voltage is correlated to the input voltage of the transformer.