Heart of a Thief
Taking the Joule-Thief apart exposes the proprietary RLP (Ruggedized Laminate Piezo) technology at the heart of the product's power-scavenging ability. With all PE material, the power output is proportional to the volume of ceramic under strain. So, if more ceramic could be compressed into the same area, the voltage per unit area under strain rises. That's the underlying principal behind the RLP's novel design.
The RLP is essentially a laminated stack comprising a stainless steel shim with a PE layer bonded to both sides using a proprietary polyimide bonding material. On the other side of the PE material is another bonding layer, that holds the copper layers to the ceramic.
The secret sauce here is the high-strength polyimide, which has a controllable melt temperature (the temperature at which it bonds). This temperature is set to take advantage of the fact that the stainless steel and copper layers shrink faster than the PE material as they cool. As a result, the PE material is compressed. "This gives it the greater density plus greater flexibility for displacement range," said Tanner. Not only does this prestressing provide more output voltage, but it allows more PE beam displacement before it goes into tension. This is important, as power output is also proportional to the square of the displacement. "A small displacement gives a large power output," said Tanner.
A calibrated mass at the end of the RLP provides the beam displacement in response to the vibration, like a spring mass, and while the Joule-Thief kit example is tuned to 60 Hz, there is sufficient play in the beam-fastening mount to adjust the length of the beam to allow a 2-Hz or 3-Hz authority, or tuning tolerance.
The generated PE voltage is a sine wave that can reach 150 V to 200 V from 1.5- to 2-g vibration. "That's why we include the electronics as it gives a standard DC voltage output, as most people don't want to deal with 150-V AC," said Tanner. Instead, the PE sine-wave voltage is fed to the circuitry via four copper tabs--two for ground and two for power--and the Energy Key circuitry provides the standard 3.6-V DC output via two low-loss 220-µF tantalum capacitors connected in series.