CIRCUITS AND ELECTRICITY

Alternating Current and Oscilloscopes

Alternating Current and Oscilloscopes

Direct current

Batteries and cells provide an electrical current which flows around a circuit in only one direction. This is known as direct current (DC).

Alternating current

In comparison, mains electricity in the UK is supplied as an alternating current (AC) at approximately 230 volts. In this case, the current flows first in one direction around the circuit and then in the other direction. In other words, it alternates direction. The frequency of the mains electricity in the UK is 50 Hz which means 50 cycles per second.

AC signals

It is possible to visually represent an AC signal using an oscilloscope.

If you know the setting of the y-gain it is possible to use the trace as a voltmeter. For example, if the y-gain is set to 10V/div then you can figure out:

  • the peak to peak voltage: 60V
  • the peak voltage (V0): this is half the peak to peak voltage and so 30V in this example

If you know the peak voltage (V0) and the resistance (R) it is possible to work out the peak current (I0) by using the following equation:

V = IR

Root mean square (RMS) values

Due to the fact that the potential difference and current vary constantly in an AC signal, the average should be represented instead.

The effective value of the potential difference and current in an AC circuit can be worked out using their root mean square values: Vrms and Irms respectively.

Vrms = V0 / ? 2

In which:

  • Vrms = root mean square potential difference in volts (V)
  • V0 = peak voltage in volts (V)

Irms = I0 / ?2

In which:

  • I = root mean square current in amperes (A)
  • I0 = peak current in amperes (A)

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Oscilloscopes

In an oscilloscope there are two controls in particular which are of interest:

  • the y-gain in volts/division (1v/div)
  • timebase in time/division (e.g. 0.002s/div)

When the input is connected to the ground (zero volts) and the timebase switched off then the oscilloscope trace is spot in the centre of the screen.

At 3 volts from a DC battery the oscilloscope trace moves up three divisions. The y-gain is set to 1V/div and the timebase is off.

The trace becomes a horizontal line when the input is connected to the ground, or zero volts, and the timebase is switched on.

The line moves up three divisions when 3 volts are supplied from a DC battery. The timebase is on and the y-gain set to 1V/div.

The trace appears as a vertical line covering six divisions when 3 volts are supplied from an AC power supply. The y-gain is set to 1V/div and the timebase is off.

If the timebase is then set on 0.002s/div then a sine-like trace appears.

You can calculate the frequency of the trace by using the following equation:

f = 1 / T

In which:

  • f = frequency in hertz (Hz)
  • T = time period (T)