Taking the lead

Posted on 13 Sep 2007 at 16:27

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Powercast's Powercaster is essentially a low-power, ultra-high frequency (UHF) radio transmitter. One or more Powerharvester units placed within range are able to convert the electromagnetic waves they receive into a small electric current. While this power supply is only in the milliwatt range, it is enough to power or charge small devices such as flash-based MP3 players or even mobile phones.

Powercast's system is a novel way to power gadgets in the home but it's not the first practical example of a system powered by radio signals. Radio-frequency identification (RFID) tags are used, among other things, to track stock within distribution centres and shops, and have been incorporated into smart cards such as Transport for London's Oyster. Many RFID chips have no means of powering themselves. When subject to a radio broadcast at the correct frequency, such as when an Oyster card is placed on a reader in a station, they can extract enough power from the interrogating signal to transmit a response.

One problem that all forms of electromagnetic energy transmissions share is that there is little inherent feedback between the receiver and transmitter. If the power use drops to zero at the receiving end, the transmitter will continue sending power unless some kind of signal is sent back to tell it to stop.

Following Induction

Of all the ways to transfer power wirelessly, electromagnetic induction is by far the most widespread and the longest established. In 1825, a British scientist, William Sturgeon, invented the first practical electromagnet by passing a current through an insulated coil of wire wrapped around an iron core. This created a magnetic field that disappeared when the current was stopped. Six years later, Michael Faraday discovered a process that appeared to be the reverse of Sturgeon's experiment. By subjecting a conductor to a changing magnetic field, he was able to create, or induce, a current within it. He built a device in which two insulated wire coils shared a single iron core, and found that passing an alternating current through one caused a current to flow in the other, unconnected coil - a process known as inductive coupling.

This discovery formed the basis of the electrical transformer. It was subsequently found that the ratio between the number of windings in the primary and secondary coils dictates the ratio of the input and output voltages. This is how the National Grid runs at up to 400,000V but is stepped down at local substations to feed homes at a relatively safe 240V. Transformers can be very efficient, with those in the national grid transferring as much as 95 per cent of their input power. Some energy is inevitably lost, though, due to resistance in the coils and because the core's magnetic field creates undesirable electricity inside it, producing eddy currents that generate heat.

Most electrical devices in the home rely on induction, too. Modern electronic chips and components typically require a power supply of five to 15 volts, so transformers are essential to the gadgets and computers that contain them. In the kitchen, induction hobs use a powerful magnetic field to deliberately induce eddy currents in the base of a compatible saucepan, which produce heat as they dissipate.

Something in the air

More recently, scientists have found ways to exploit the fact that there is no physical contact between the coils in inductive-coupling circuits. The most widespread example is the electric toothbrush. Water and electricity are best kept apart, and induction enables the toothbrush and its charger to be sealed in plastic. The charger contains a primary coil powered by mains electricity, while the toothbrush contains a secondary coil. Placing the toothbrush on the base completes a step-down transformer, charging the battery with a suitable voltage.