Muscle power drives battery-free electronics

Recent developments in electric-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 practically unlimited number of charge/discharge cycles, survival of short circuits, and simple charging circuits that require only overvoltage protection. In addition, storage capacitors recharge quickly and pose no toxic-waste-disposal problems when the product reaches the end of its service life.

This Design Idea extends an earlier one by describing a muscle-power-driven capacitor charger. The combination of a muscle-powered electrical generator and a high-value capacitor provides a highly autonomous and environmentally clean power approach for emergency equipment and survival kits. Applications of such an alternative "renewable" energy source span a range of modern portable electrical and electronic devices, including cellular phones, MP3 players, AM/FM radios, PDAs, handheld PCs, and flashlights.

A muscle-powered capacitor charger contains only a few components: a storage capacitor, a bridge rectifier, and a voltage-limiting zener diode that protects the capacitor from excessive voltages (Figure 1). For practical energy-storage experiments, you can use 1 or 0.47F capacitors with 5.5V maximum ratings, such as those available from NEC-Tokin America (www.nec-tokin.com, Figure 2). For more storage, you can use higher capacitance capacitors, such as Elna's (www.elna.co.jp) 100F, 2.5V Dynacaps (Figure 3).

You can remove the lamp from an inexpensive, hand-powered flashlight and use its generator as a capacitor charger (Figure 4). Also, a variety of manually powered products now appearing on the market offer possibilities for experimentation. For higher outputs, you can use a stationary-bicycle-powered generator. Depending on the individual providing pedaling power, these generators can deliver average powers ranging from 20 to 100W. The hand-cranked flashlight in Figure 4 originally lit a 2.5V, 0.15A, filament-type bulb, which consumes approximately 0.4W at full brightness. However, measurements show that the generator could deliver more power and could charge a 1F capacitor to 5V in approximately 10 sec. Thus, the following equation calculates the energy, E, stored in the capacitor of value C: E=½C×V_MAX2=12.5J, and the following equation calculates the average maximum muscle-generated electrical power over time, T: T_MAX=E/T=12.5/10=1.25W.

You can use the following equation to calculate the effective energy, E_EFF, that the capacitor can deliver during its discharge cycle while its terminal voltage changes from maximum to minimum voltage: E_EFF=½C(V_MAX2–V_MIN2), where V_MAX2 and V_MIN2 represent the maximum and minimum operating voltages, respectively, applied to the powered devices. You can connect storage capacitors in parallel or in series. In both cases, make sure that the circuit includes proper overvoltage protection for the capacitors. To obtain additional voltages, you can add a dc/dc switched regulator to produce stable output voltages.

Important design considerations relate to the maximum voltage and current ratings of the diode-bridge rectifier and the zener diode, D_Z. Experimental measurements on the hand-cranked generator yield the following approximate values for its open-circuit voltage: maximum voltage of 10V rms, peak voltage of 14V, and maximum short-circuit current of 200 mA rms. For this application, an inexpensive bridge rectifier with 20V minimum peak-inverse voltage and 0.5A minimum forward current provide adequate margins. D_Z's breakdown-voltage rating should be slightly lower than the storage capacitor's maximum working voltage, and the diode's power rating—2W in this application—should exceed the product of the generator's maximum output current and the zener's conduction voltage.