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Why Is The 50Hz AC Mains Waveform Distorted?

By Leo Simpson

Everyone knows that the 50Hz AC mains waveform is a sinewave, right?

Well, in theory it is a sinewave but in practice it is distorted because the peaks have been clipped off. For years now our scope screen grabs have shown this but we have not dwelled on the reasons why.

Recently though, we have had emails from readers who have sent photos of their scope screens showing the classic flat-topping of the mains waveform. And they want to know why this is happening.

You can blame this gross distortion of the mains waveform on two factors: gas discharge lighting and switch-mode power supplies.

Gas discharge lighting refers to all lighting systems which use an electric current through a gas to generate light. It applies to all high and low-pressure sodium lamps, mercury vapour lamps and fluorescent lights.

Click for larger image
This screen grab shows the typical flat-topping of the 50Hz AC mains waveform (green trace) caused by the peak currents drawn by gas discharge lighting and switch-mode power supplies. The purple spikes show the relative amplitudes of the 50Hz fundamental and the odd harmonics up to 550Hz. In fact, the harmonics are significant up to at least the 19th, 950Hz.

In each of these cases, the gas discharge draws current from the AC mains supply only when the actual voltage across the lamp exceeds about 100V. So the current is only drawn from the peaks of the waveform and this inevitably loads down or clips off the peaks.

In recent years the situation has become much worse for the electricity generators and distributors with the widespread use of switch-mode power supplies in virtually all electronic appliances.

It more or less started with the advent of PCs and their adoption of the more efficient switch-mode rather than conventional mains transformer-driven power supplies which are much heavier, bulkier and more expensive.

Switch-mode power supplies were naturally also used in laptop supplies, then TV sets, DVD players etc. Now they are used in virtually all electronic equipment with the sole exception of high performance audio amplifiers (such as our own Ultra-LD amplifier series).

Naturally all those large power-hungry Plasma TVs (albeit these days not quite so power-hungry) and large screen LCD TV sets use switch-mode supplies.

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