Alternating and Direct Current

So far, we’ve been using batteries in our circuits, and we know that with conventional current, the flow of electricity goes around the circuit one way, from positive to negative. We call this Direct Current or DC, where we have a positive and a negative (or ground).

Our mains electricity works differently: The current is reversed in cycles. So, the voltage goes from positive to negative, then negative to positive, then starts over again. We know this as Alternating Current or AC.

Ironically, we describe AC and DC in terms of voltage, but label them in current terms. We must remember that voltage and current are relative to each other, so as we reverse the voltage, we also reverse the current.

The frequency of these AC cycles is measured in units called Hertz, abbreviated as Hz, which relates to cycles per second. Our mains electricity is between 50Hz and 60Hz depending on the country. Aeroplanes use AC for electrical power, at a rate of 400Hz.

With mains power speeds alternating as fast as 50Hz, it would be impossible to use a multimeter to see the voltage changes over this time period, so we use a type of test equipment called an Oscilloscope providing a simple graph of what the voltage is doing over a specified time. Our Oscilloscope can measure DC and AC, with DC showing as a straight line, and AC as a sine wave.

Schematic Symbol Description Circuit Schematic Reference
Direct Current DC Schematic Symbol Direct Current (DC - IEC 60617)  
DC Schematic Symbol DC Voltage (Common use)  
Alternative Current AC Schematic Symbol Alternative Current (AC)  
Alternative and Direct Current Capable Schematic Symbol Equipment with AC and DC operation  

 Notes:

  • DC Symbol is a solid line with dashed line underneath, it seems there are various conventions on how many dashes the line should contain.
  • The symbols are sometimes seen next to the letter V, indicating the voltage being AC or DC.


DC Waveform

If we plot a graph of our voltage on a DC power supply, we just see a straight line going along as time passes. There’s not much else to report, not much happens.

DC Voltage Plotted on a graph

AC Waveform

Our AC waveform has a lot more going on! The voltage rises to a positive peak, then drops the other way to a negative peak, giving a sine wave for each cycle.

AC Voltage Sine Wave

This is a single cycle. The number of cycles in one second (the Frequency) is measured as Hertz or Hz. Mains electricity in Europe runs at 50Hz, meaning there are 50 of these cycles per second.

We can identify parts of the sine wave for a single cycle:

AC Voltage Sine Wave Anatomy

Amplitude or Amplitude Peak is the maximum height (or voltage) of the sine wave. In our diagram, this would be 5V.

Peak to Peak is the maximum height between the positive and negative peaks. In our diagram, this would be 10V, as the peaks are +5V and -5V respectively.

Period or time per cycle. Our sine wave shows 1 second, which would be low frequency.

Average Value - If we were to add up and average out all the values on the wave, we would arrive at the average value. To save us work, a simple calculation can be made by multiplying the peak value by 0.637. Our supply would give us 3.19V.

RMS Voltage - This represents the useable (Effective) voltage given the drop and rise of the wave, rather than looking only at the peak voltage. When measuring AC voltage, the RMS value is often given in the reading. This RMS Voltage is calculated from multiplying the Peak voltage by 0.7071 - Our supply would give 5 x 0.7071 = 3.54V. If the AC wave were to be converted to DC, the RMS would represent the ideal voltage that would be produced.

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