Making a DIY System Frequency Meter

It is well known that the Irish grid transmits AC power with a frequency of 50 Hz, but does it really? Without getting too philosophical about frames of reference and the fickle nature of measurement, the actual system frequency fluctuates tightly around a 50 Hz average. Previous posts discuss the fascinating effects that significant demand events can have on grid frequency, and how the lights went off in parts of Britain last week after a drop in frequency.

In light of such frequency disturbance events, it can be interesting to watch the grid frequency rise and fall in response to system generation and demand dynamics. To do so, one can easily purchase a ready to go plug-in mains frequency meter, but what if you ultimately want to display it on a large screen on the office wall?  Well, you can probably purchase one that will do that too – the point is, we wanted an excuse to make one.

Before discussing the design and assembly of our frequency meter, it is important to mention that this was carried out in a controlled environment in the Electrical Machine lab at University College Dublin and we would like to extend our sincere gratitude to the staff at the School of Electrical and Electronic Engineering who facilitated our brief foray into the physical side of energy systems. Credit is also due to electroniclinic.com who’s similar project was useful and informative in adapting and designing our own frequency meter. [1]

In brief, AC power from the mains is ‘stepped-down’ from 230V to 9V using a transformer, this lower voltage signal passed through a bipolar junction transistor or ‘BJT’. The effect of the BJT is essentially to convert the signal from AC current to a DC current pulsing from 0 to 9V at the same frequency as the mains (a traditional AC/DC converter consists of a BJT coupled with capacitors to ‘smoothen’ out the pulsing behaviour into a constant direct current.) It is these pulses that we wish to count so as to calculate the system frequency.

The resulting pulsing DC current that emerges from the BJT is then reduced in voltage to comply with our components’ voltage ratings. The signal is reduced to approx. 3V using a voltage divider made out of 3x 10k ohm resistors.

Following voltage division, the signal is connected to the base plate of a 2N2222 NPN transistor which amplifies the signal and protects the Arduino Uno microcontroller to which it is connected via its collector leg. The microcontroller is programmed to count the number of ‘pulses’, and therefore corresponding AC cycles. The time taken for 50 cycles to be counted is recorded, and from this, the average frequency over this period (of approximately 5 seconds) is calculated.

You want one now, don’t you?

References

[1]. electroniclinic.com, “DIY Mains frequency meter,” 12 August 2019. [Online]. Available: https://www.electroniclinic.com/diy-mains-frequency-meter-for-110-220vac-using-arduino/