# Help understanding how to best detect a small AC voltage...

#### KiwiSteve

Apr 11, 2014
28
Hello,

This is my first post to the forum and I'm a new beginner to electronics, so please go easy with me

I'm building a circuit that will monitor a small AC voltage coming out of a ultraviolet lamp ballast and provide a logic signal (>2.5v) to an Arduino digital input.

The ballast produces a small voltage of about 0.3v RMS (the schematic is incorrect). I pass this through a bridge rectifier which gives me a DC voltage. Then, I have a 10uf capacitor and 60k resistor in parallel.

This design has come about through various trials and tests of what will hopefully give me the desired result of a voltage > 2.5v < 5.0v.

With a multimeter placed across the resistor when the ballast is turned on, I see that the voltage increases quickly and then settles at about 3.5v, enough to make the Arduino register a HIGH logic state.

I've attached a schematic of the basic circuit.

Question: Is this the best way to do this or is this a complete fluke/hack?

Much appreciated!

Steve

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#### duke37

Jan 9, 2011
5,364
If your signal is 0.3V rms, then you will get nothing out of the rectifier due to the voltage drop. You will need to amplify before rectification.
The fact that you are getting an output may be due to wrong measurement or ground errors.
Is your input floating relative to the measurement circuit?

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
0.3V RMS does not describe the peak voltage unless the waveform is also known. If the waveform is significantly non-sinusoidal, you may get an output voltage, and that voltage is hard to predict. Your trial and error process is a practical one.

My question would also be "is this input floating?" "Floating" means that your 0.3VRMS source doesn't connect to anything else, be it ground/earth, mains live or neutral, or some other power source.

I would also be cautious about voltage spikes. You could protect your arduino somewhat by placing a zener diode across the output. To do this safely, you also need to know the impedance of your power source. I'm not sure if this happens to be a spare winding on an inductive ballast, or something else. Some series resistance will help in protecting both the source and the zener.

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