DrDAQ is a versatile instrument that connects to the USB port of any PC. Using the supplied PicoScope software it can be used as an oscilloscope, spectrum analyzer and signal generator.
The purpose of this experiment is to monitor mains voltage and frequency fluctuations over a seven day period. This information should show the effects of loading of the National Grid.
The electricity generating companies have tolerances which they are required to meet.
The single phase mains tolerances in Europe are:
The ADC-42 reads voltages in the range of ±5 volts. The mains voltage has to be dropped to ±4 volts (leaves room for the voltage fluctuations).
One of the simplest and safest ways to measure mains voltages is to use a simple ‘plug in the wall’ power block to drop the mains voltage to a safer level. This has the advantage that it provides electrical isolation from the dangerous mains voltage.
There are two types of basic power block available:
These are most easily used since they simply give a scaled version of the mains voltage. They usually give an output in the range 6 Vac to 12 V AC, and therefore a simple potential divider circuit may still be required to attenuate the output to a suitable range (see below).
These are similar to the AC types above but they also have a built in rectifier to convert the AC voltage to DC and also usually a capacitor to provide some smoothing of the DC signal - some types may also have some kind of voltage regulator. To use this type, you need to remove the rectifier and capacitor (and regulator if fitted) — this should only be attempted by a qualified electrician.
Warning: if you are in any doubt then seek advice from a qualified electrician.
The output from the power block may need attenuating further to suit the ADC-42 input range. This is best done with a simple resistor voltage divider as shown with the formula below.
Vo = Vs x Rb / (Ra+Rb)
Vo / Vs = Rb / Ra+Rb
Vs / Vo = Ra+Rb / Rb
(Vs / Vo)Rb = Ra+Rb
Ra = (Vs / Vo) Rb - Rb
Vs = 20 volts peak
Vo = 4 volts peak
So Ra = 5Rb - Rb
Therefore Ra = 4Rb
With the equation above and Rb = 10l, Ra = 40k. A value of 40k is not a standard resistor value, so a 30k resistor and a 10k resistor in series can be used.
Using these power blocks with PicoLog and PicoScope:
The PicoScope spectrum analyzer can be used to investigate the harmonic content of the mains supply.
PicoLog can be used to trend the longer term changes in supply voltage and also supply.
The experiment was carried out over one week using the equipment above and PicoLog software. Samples were taken every 30 seconds.