How to operate 120V appliances off a 12V battery

Tuesday, October 21, 2014

Q. I would like to operate 120V appliances off a 12 battery. I have heard that there are devices that let you do this. What are they and how do they work?

A. They are called “inverters” They have a 60 HZ pulse generator that drives two power transistors in push-pull. The collectors ofthe power transistors are connected to the center-tapped primary winding of a step-up transformer. The power transistors take turns conducting, and alternately apply pulses of current to the primary winding. This induces 120V pulses in the secondary.

Since the step-up transformer has a 10-to-1 step-up ratio, when current is drawn from the secondary, about ten times as much current must be drawn from the primary. This current, of course, must come from the 12 V battery. Consequently, if you connect something to the output of the inverter that draws much current, it will discharge the 12 V battery faster.

For example, a coffee maker draws about 10 A or more from the 120 V line. This would demand about 100 A from the battery, which would quickly discharge it. So an inverter should only be used with appliances that do not draw much current. While the current rating is usually not listed on an appliance, the power rating usually is. To the current the appliance will draw, just use the equation I = P / E, where P is the power drawn and E is 120 V. Multiply this figure by ten to determine how much current will be drawn from the battery.

You can usually inverters at places that sell camping supplies. They are usually rated in terms of their power-handling capacity; that is, how many watts they can handle.

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