Tuesday, 22 December 2009

Inverted Thinking

Many boaters ask what the technical difference is between the more expensive Pure Sine Wave inverters and the basic Modified or Quasi Sine Wave inverters.

Of course, many know that a lot of the fussier mains powered domestic equipment on boats needs pure sine wave and won't run properly on MSW/QSW inverters.

However, few know why...

These are the waveforms that I captured from my two inverters.

This one is the output from the Mastervolt pure sine wave inverter on Willawaw:

This one is the output from a Maplins cheapy modified sine wave inverter that I use in the car sometimes:

Even without any electronics expertise, you can see that they are very different and the distorted (almost squared) waveform of the MSW inverter is a compromise between a reduced technical complexity, which allows it to meet the price target and its ability to supply many, although not all, mains appliances.

The pure sine wave from the Mastervolt is an emulation of the mains that you get from your household socket. It is achieved with a lot more circuitry, which results in the increased cost.

Most devices with motors or microprocessors, digital clocks, etc struggle with the outputs of MSW/QSW inverters.

As is usual with electronics, you get what you pay for.

If you don't want to spend all that money on a pure sine wave inverter, then the answer is to run as many appliances as possible from 12 or 24VDC and then just use a small Maplins type MSW inverter to charge your phone, camera, etc.


  1. A question
    My only inverter is one of the very small Sterling inverters - I think it cost all of £35! The plan was to use it for charging i-pods and mobile phones but I have also used it to power my laptop, which works ok so long as the unit starts with a full charge in its battery. Sometimes it keeps cutting out and the only answer is to run the engine to get it going and then its ok drawing from the boats battery bank alone.
    Do you think this a problem with the inverter or the battery bank (4 years old).
    Captain Ahab

  2. Post the question on www.justcanals.com/forum and I'll have a go at solving the problem.


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