240v - 12v conversion

Mar 14, 2005
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A few years ago I attemted to purchase a 12volt adapter to run a coolbox, in the end I made my own by adapting a standard battery charger by fitting a 12v socket from the usual cable which has the crocodile clips, it has worked brilliantly for around nine years now. I no longer have the coolbox but still have the adapted charger.

My question is: Is there any real problem with using this adapted charger for any lengthy period to run say a 12v fan or other 12v gadget? The maximum output is 4amps, could it overheat?
Mar 14, 2005
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Hi Tony,

There are significant differences between a battery chargers, and a power supplies.

A simple battery charger takes the 230V ac mains, and through a transformer converts it to approx 12V ac. This AC signal is then rectified to produce a DC output. However the rectified output swings from 0Vdc to a peak of about 22V dc. 50 or 100 times a second (depending on the type of rectifier used) This is like turning a switch on and off at the same speed. This is fine for charging batteries, but this type of power may cause other electronic equipment to malfunction or at worst may cause damage.

In general battery chargers are designed for continuous use and so will probably not suffer if left permanently on. Most will incorporate some for of thermal overload protection should it become too hot.

A power supply is designed to provide a true smoothed DC output through a wide range of currents. These can also be used to charge batteries, though the units fitted to most modern caravans only supply 13.8V where as you need nearer 14.5V to fully charge a caravan battery. The manufacture will specify the duty cycle (how long you can leave it on)of the supply.

Provided your power supply current capacity equals or exceeds the appliances need, and the voltages are compatible, you should not have any trouble powering your gadgets.


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