Tuesday, April 21, 2009

Power Supply/Charger Energy Waste Explained

I've had a few people recently ask me about power consumption by vampire devices in their homes - most specifically about power adapters for devices with rechargeable batteries like laptop computers, cell phones and iPods. The typical question is along the lines of:

I hear these things still draw power when plugged into a wall outlet even though the device (iPod, cell phone, etc) is not attached. Is that really true?

My answer is typically YES! These external supplies have transformers and some rectifier circuitry in them that convert alternating current (AC) voltage to direct current (DC) voltage. A schematic of a simple DC power supply/charger is diagrammed below (click image to see larger and clearer version).Here's how they work:
  1. The power supply/charger is plugged in to wall.
  2. AC current flows through the primary coil in the transformer and creates a magnetic field. This magnetic field from the primary coil is coupled into the secondary coil. The transformer is used to step down (reduce) the AC voltage by adjusting the number of coil wire turns (turns ratio) in the primary and secondary coils.
  3. The rectifier circuitry takes the AC voltage from the secondary coil and flattens it out into a non-oscillating DC voltage that portable devices can use for power and battery charging.
If you take a close look at the diagram you will notice AC current flows through the primary coil as long as the power supply/charger is plugged into the wall. Notice the device (laptop computer, cell phone, iPod, etc) does not have to be attached for power to be consumed and effectively wasted.

How much power is wasted by vampire devices like these? The U.S. Department of Energy estimates 5% of all electricity used in the U.S. is consumed by devices in standby mode and predicts this will increase to 20% by 2010!

What can be done? The simplest thing to do is unplug your charger when it is not actually being used to charge the device it was designed for. Another option is to plug power supply/charger devices into a switched power strip or electrical outlet, only turning power on when devices are attached.

We're also starting to see "smart chargers" for some devices that use some simple circuitry and only power up when a device is actually attached.

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