Solar Engine

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[edit] Why Would I Need A Solar Engine

A very good solar cell might be able to provide a milliamp or so under good light. But even an efficient motor will draw tens of milliamps or more. So how can a solar cell be used to provide power for a typical motor.

The secret is to put a power storage device, typically a high value (2200 uf to 4700uf) capacitor, and conditional switching circuit between the solar cell and the motor. The current produced by the solar cell would then continually be collected and saved on the capacitor. The switching circuit would only allow the motor to access the power stored on the capacitor when a there was sufficient power available. This is the basic description of what a solar engine, is and what it does.

Solar engines are often an integral part of a greater robot project, but can also built as a distinct and individual assembly or module, and then attached to an separately constructed robot. This can make it easier to incrementally build and test a robot

There are three main types, plus one special class of solar engine. These are listed below...

[edit] Type 1 Solar Engines

AKA Voltage Triggered Solar Engine

A type 1 solar engine is designed to trigger (I.E. "turn on the switching circuit") when the voltage on the storage cap reaches or exceeds a certain value. Once this happens, the motor will be allowed to draw current from the capacitor until the voltage drops enough to turn the switching circuit off again. Once the switching circuit is reset (turned off), the current produced by the solar cell will begin to charge the storage capacitor until it again reaches the trigger voltage. This process will cycle repeatedly, as long as there is enough light available for the solar cell to charge the capacitor sufficiently.

Charge & Fire Solar Engines Fire & Hold

Solar Engine circuits with hysteresis, that hold a
portion of the charge on the storage cap in reserve.

[edit] Type 2 Solar Engines

AKA Time Triggered Solar Engine

Rather than being triggered in response to a particular voltage on the storage capacitor, the switching circuit in a type 2 solar engine is triggered (turned on) after a given interval of time, selected to suit the needs of the particular application.

[edit] Type 3 Solar Engines

AKA Charge Curve Differentiated Solar Engines -or- Charge Rate Triggered Solar Engine

Theoretically, a type 3 solar engine is based on the idea that the solar engine should trigger as soon as the charge-curve slope falls below a certain angle. Put more simply, when the level of charge on the storage capacitor starts to "top off", the circuit would fire.

The circuit would basically monitor the charging current coming from the solar cell, until it begins to taper off (indicating that the solar cell has done nearly all the charging it can do), at which point the switching circuit would be turned on, allowing the motor to draw current from the storage cap until the voltage drops enough to turn the switching circuit off again, and the cycle repeats itself.

In bright light the charge on the storage cap may reach nearly 3.5V. In dim light it may only reach 1.2V. In any case, it would be the optimum in triggering point for the available light, makiking the type 3 the most efficient kind of solar engine.

[edit] Nocturnal Solar Engines

This is a special class of solar engine, which is intended to collect energy while it is light, and to make that energy available for use when it gets dark.

[edit] External References

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