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1) I would recommend to de-install the dimmer switch and re-wire using a standard Single Pole Single Throw ON/OFF switch for the address sign.
2) You mention that the GFCI receptacle is controlled by a dimmer switch, therefore I assume that the dimmer switch is wired upstream of the GFCI receptacle. It is not recommended to wire a dimmer switch in your type of application. Dimmer switches control and vary the voltage levels and are only meant to be used to control a permanent light fixture and never a receptacle of any type or in any location.
In fact, it is a violation of the National Electrical Code to control any type of receptacle via a dimmer switch.
A switched receptacle can only be controlled by a standard Single Pole, or 3-way or 4-way wall (ON/OFF) switch and never a dimmer type of switch. Reason being, if a receptacle is controlled by a dimmer switch and you plug in any non-lighting load, you will destroy the plugged-in load in the receptacle due to the possibility of the varying voltage level originating from a dimmer switch.
Hope this helps.........If you have any additional questions, let me know and I'll be glad to answer them for you.
Otherwise, don't forget to rate me before you log Off.The next time you have an electrical question, you can also request for me at:http://www.justanswer.com/home-improvement/expert-your-electrician ..........Thanks..............Kevin!
Thank you. I will rate you highly. But one more question. For the last year, at the end of the driveway, there has been an exterior lamppost light plus the gfci receptacle, and both have worked fine on the dimmer (with or without any Xmas lights plugged into the receptacle). It was only when I plugged in the low-voltage new lighted address sign into the receptacle that any problems occurred. Does this make sense?
1) A dimmer switch is basically a variable resistor or rheostat and the internal electronics are what controls the lights to dim by varying the voltage levels. More than likely, the low voltage address sign is sensitive to the voltage on the line caused by the dimmer switch and will only work if a constant 120 volts is applied to the transformer.
Thank you for the excellent service rating..............much appreciated!
If you have any other questions or follow-up questions, just reply back to this question and I'll be happy to answer them for you. No need to create a new question.
Take care and have a great remainder of the weekend............Thanks again..........Kevin!