Solar-powered motor runs on 10 nA

-March 18, 2004

Designs for solar-powered applications with low-duty-cycle requirements can often rely on capacitors for energy storage in place of less reliable batteries. Typical applications include solar positioning, telemetry transmitters, chemical pumps, data loggers, and solar-powered toys. The circuit in Figure 1 can run a small pager motor from the output of a small calculator-type solar cell in near darkness. The circuit works by repeatedly charging a 4700-µF capacitor, C1, to 1.75V and then dumping the charge into the motor. Only the self-leakage current of the solar cell limits low-light operation. The circuit itself has such low leakage currents and trigger-current requirements that it can run the motor on 10 nA of current if you use a low-leakage energy-storage capacitor. Transistors Q1 and Q2 form a regenerative pair similar to a thyristor. The 1N4007 diodes take the place of pullup and pulldown resistors, and the diodes bypass the leakage current of the transistors and LED.

As the C1's charge approaches 1.75V, the green LED starts to conduct, causing Q1 to turn on and feed current to the base of Q2. The amplified base current appears as a disturbance at the collector of Q2. The emitter-base drop of output transistor Q4 isolates the collector of Q2 from the output transistor, and the emitter-base-drop of Q3 and the 10-nF capacitor, C2, isolate Q2 from the dc bias at the base of Q2. However, the nanoamp-magnitude ac disturbance at the collector of Q2 couples into the base of Q1 via C2, causing fierce regenerative action. You achieve nanoamp triggering and charging of C1 through the use of leakage diodes in place of pullup resistors, through isolation of the load at the start of regeneration, and through the dc isolation of Q1's bias voltage from the collector of Q2 at start of regeneration. As regenerative action continues, a dc latching path appears between the base of Q1 and the collector of Q2 through transistor Q3. At this point, output transistor Q4 also enters saturation, and the motor runs.

The high motor load quickly discharges C1 toward 1.1V, at which point Q1 can no longer sustain regenerative action because of the voltage loss in the emitter-collector junctions of Q1 and Q3. The 100Ω resistor and the reverse charge on C2 drive Q1 into cutoff and another energy-storage-capacitor charging cycle begins. Substitute a blue LED for the green one or add diodes in series with the LED to increase circuit-firing voltage beyond 1.75V. You can use 10-MΩ resistors in place of 1N4007 diodes to improve noise immunity if you don't need less-than-1-µA operation. Capacitors become leaky if you leave them in storage. You may need to condition such capacitors by applying a 9V battery to the capacitor for a few days. Use two solar panels in series to provide enough voltage for very-low-light operation.

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