# grid frequency

Posted on Feb 5, 2014

Wall sockets supply power in the form of a 230 volt sine wave. The frequency of this sine wave is supposed to be 50 hertz. In other words, the voltage on the socket alternates between positive and negative at a rate of 50 full periods per second. (At least this is how it works in the Netherlands; many countries use different frequencies and/or voltages. ) There

are clocks, so-called synchronous clocks, that count voltage alternations to determine the passage of time. Is the grid frequency accurate enough to keep such clocks running right I used an Arduino to measure the frequency of the sine wave from a wall socket. To avoid working with dangerous high voltage, I used a transformer from an old halogen lamp. The transformer is plugged into the wall socket, and the low voltage side of the transformer is hooked up to the Arduino through a resistor network. The Arduino measures the sine wave at about 4386 samples/s. Whenever it detects a rising zero-crossing edge in the sine wave, it sends a timestamp to the PC. Some kind of reference clock is needed to measure the frequency of a signal. The Arduino has an on-board clock crystal but I was not sure about it`s accuracy. So I decided to use a GPS receiver as my reference clock. It generates a pulse per second (PPS) with very high accuracy. The GPS receiver is connected to the Arduino by means of LadyAda`s excellent GPSshield. When the Arduino detects a time pulse from the GPS receiver, it sends a timestamp from its on-board clock to the PC. Software on the PC uses these timestamps to detect variations in the rate of the Arduino board clock. The GPS signal is not strong enough for indoor reception unless the receiver is very close to the window, so I built the test setup in the window sill. I let it run for 8 days while recording data to...

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