An electromagnetic field source (EFS) for providing electromagnetic energy to a secondary coil, including two or more primary coils that each carry a time-varying current to produce an electromagnetic field, and a controller that selectively provides current to one or more primary coils based on their position with respect to the secondary coil. The secondary coil may be implanted in a human recipient and used to provide power for the operation of a medical device, such as an artificial heart or ventricular assist device. The invention also provides such a secondary coil and EFS, collectively referred to as a transcutaneous energy transfer (TET) device. The primary coils of the EFS or TET may be housed in furniture. For example, they may be housed in a bed mattress or mattress pad on which the recipient rests, or in a blanket for covering the recipient. The controller includes a proximity detector that identifies those primary coils that are closest to the secondary coil, and a current director that, responsive to the proximity detector, selectively directs time-varying currents through the closest primary coils. The controller may also include an orientation detector, coupled to the current director, that determines an orientation of the secondary coil with respect to the closest primary coils. In one implementation, the proximity detector identifies the quantity of closest primary coils utilizing a resonance frequency shift detector that detects a shift in inductance of one or more primary coils due to the proximity of the secondary coil.