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Mains Moderator: Stepping Down The Volts

Do you have excessive mains voltage at your place? In some parts of Australia the mains voltage can be above 250VAC and that can spell early death for imported appliances rated to work on only 220VAC. In this article we show you how to reduce the mains voltage to a safe level for your equipment.

By Leo Simpson

Despite most people still believing Australia’s mains voltage is 240VAC (we get emails and letters telling us we’re wrong!) according to Australian Standard AS:60038 the “official” mains voltage is 230VAC – and has been for more than a decade.

Click for larger image
Fig.1: the circuit is pretty simple but is capable of taming a 250V AC supply down to a much more healthy 220V AC. Because it is so simple, don’t be complacent or sloppy when it comes to wiring. Even 220V can ruin your whole day!

However, in many parts of Australia the supply is still nominally 240VAC; indeed in parts of Western Australia and many mining towns it is even higher at around 250VAC – and sometimes even more.

There are several reasons for this but primarily it’s because a higher voltage is fed into the network at the substation so that at the end of (often very long) supply lines, with their inevitable I2R losses, there is sufficient voltage to do the work required.

That’s fine if you are at the far end of the line but if you’re at the near end, you cop that excess voltage. It’s long been the bane of people with incandescent light globes (the globe life is dramatically reduced) but now that the government has banned those globes, that problem obviously would have gone away. (Oh yeah? Believe that and I have this big steel bridge over Sydney harbour that I can sell you real cheap!).

220V equipment

Problems caused by excess voltage have been the case for many years but with so much imported electronic equipment, rated for only 220VAC, there is a big problem of reduced reliability at those elevated voltages.

The higher voltage can be a problem for two reasons: higher power dissipation and greater risk of catastrophic breakdown.

For example, if your equipment is rated for an input voltage of 220VAC and it is actually being fed 250VAC, that amounts to a 13.6% increase in voltage. But because power, for typical loads, is proportional to the square of the voltage, the increase in power dissipation over the rated voltage is up to 29%.

That’s a very large increase and if you are in an area where the ambient temperature regularly tops 40°C or more, that can mean that your equipment could easily expire.

Even if your mains voltage is around 240VAC, that still represents an increase in power dissipation of 19% (compared to running at 220VAC) which is still pretty significant, especially in equipment which has a tendency to run hot even at its rated input voltage.

Reducing dissipation

Maybe you don’t have a problem with the level of the mains voltage itself. But there is another good reason to run a key (expensive!) appliance such as a large screen TV or such like from a lower voltage: it reduces the internal power dissipation and therefore causes it less stress – which in turn should prolong its life.

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