Method for driving battery-free electronic device

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Method for driving battery-free electronic device [Copy link]

Recent developments in double-layer capacitor technology have made it possible to replace rechargeable batteries in certain secondary power storage applications (Reference 1). Capacitors offer significant advantages over rechargeable batteries, including a virtually unlimited number of charge/discharge cycles, survivability in short-circuit conditions, and simple charging circuits that require only overvoltage protection. In addition, storage capacitors can be charged quickly and do not raise toxic waste disposal issues when the product reaches the end of its operating life.

　　This Design Idea extends the earlier idea by describing a powerfully driven capacitor charger. The combination of a powerfully driven generator and high-value capacitors provides a highly autonomous and environmentally clean method of powering emergency equipment and survival kits. Applications for this alternative "renewable" energy source cover a wide range of modern portable electrical and electronic devices, including cell phones, MP3 players, AM/FM radios, PDAs, handheld PCs, flashlights.

　　A powerful capacitor charger consists of only a few components: a storage capacitor, a bridge rectifier, and a voltage-limiting Zener diode to protect the capacitor from excessive voltage (Figure 1). For actual energy storage experiments, you can use 1 F or 0.47 F capacitors rated up to 5.5 V, such as those from NEC-Tokin America (www.nec-tokin.com) (Figure 2). If you need more storage capacity, you can use a capacitor with higher capacitance, such as the 100 F 2.5 V Dynacap from Elna (www.elna.co.jp) (Figure 3).

　　You can remove the bulb from an inexpensive artificially powered flashlight and use its generator as a capacitor charger (Figure 4). In addition, the various artificially powered products now on the market offer the possibility of experimentation. For higher output, you can use a stationary bicycle-driven generator. These generators can provide an average power of 20W to 100W, depending on the amount of pedal effort provided by the individual. The crank-start flashlight in Figure 4 initially lights a 2.5V, 0.15A tungsten bulb, which consumes about 0.4W at full brightness. However, measurements show that the generator can provide more power and can charge a 1F capacitor to 5V in about 10 seconds. The following formula calculates the energy stored in the capacitor (capacitance C): E = 1/2C × V _MAX2 = 12.5J, and the following formula calculates the average maximum power generation T for various times: T _MAX = E/T = 12.5/10 = 1.25W.

_{You can calculate the effective energy E EFF that}　　a capacitor can provide during its discharge cycle while the voltage across its terminals changes from maximum to minimum using the following formula : E _EFF =1/2C(V _MAX2 -V _MIN2 ), where V _MAX2 and V _MIN2 represent the maximum and minimum operating voltages applied to the device being powered, respectively. You can connect storage capacitors in series or in parallel. In both cases, make sure that the circuit includes appropriate overvoltage protection for the capacitors. To get the added voltage, you can add a DC/DC switching regulator to produce a regulated output voltage.

　　Some important design considerations are the maximum voltage and current ratings of the diode bridge rectifier and Zener diode D _Z. Experimental measurements of crank-start generators yielded the following approximate values for their open-circuit voltages: maximum voltage 10 Vrms, peak voltage 14 V, and maximum short-circuit current 200 mArms. For this application, an inexpensive bridge rectifier with a minimum peak reverse voltage of 20 V and a minimum forward current of 0.5 A provides sufficient margin. The breakdown voltage rating of D _Z should be slightly lower than the maximum operating voltage of the storage capacitor, while the power rating of the diode—2 W in this application—should exceed the product of the maximum output current of the generator and the conduction voltage of the Zener diode.