Sense Ac voltage drop.

Hi to all,

I'm looking of a way to sense Arduino an ac voltage drop from 13.7v to 11.0v.

Typically, I'm connecting a heavy load >7amps to a transformer via relay + transistor from an Arduino pin. The circuit will light an led if load is faulty. I tried using a voltage divider 10K/ 1K + rectifier to drop the voltage down. It didn't work. Voltmeter reads ok but Arduino's analog pins varies between 1023 and 200 giving funny readouts.

Any suggestions

Best regards Kenneth

Did you use a smoothing capacitor as well as the rectifier?

No I didn't. So you think it's reading the peak ac values?

Without a smoothing capacitor, the reading you get will vary depending on when in the AC mains cycle you sample it. With a smoothing capacitor, you will read either the peak value or half the average of each half cycle, depending on where you put it. The attached circuit reads half the average value of a half cycle, reduced by a further factor of 2 by of the potential divider.

Thanks for the tip.

A smoothing cap is a must! I tried 10 ,100 & 2200 micro farads. 100 was the best. 2200 readings were a bit slow.

Thanks again, Kenneth