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= Sustainability Analysis of Design Idea =

The design idea considered here is a circuit that provides a constant current source over wide range of input voltages [1]. My intended primary application involves powering a string of LEDs for use as a light bulb.


 * How does this design idea, or applications related to it, affect sustainability?**

To help answer this question, the extent to which the design idea embodies the green engineering design principles will be explained.


 * Ecology**

The majority of natural resources required include silicon, gallium, copper, and oil. The silicon is used for diodes, BJT, and MOSFET. Gallium is the semiconductor used to create LEDs. Copper is used to create circuit board traces. And, finally, plastic is used in the fiber glass to make the circuit board and the housing for the bulb. Depending on the energy source used to power the circuit, oil may be used once again.

The circuit provides a relatively efficient way to power LEDs -- relative to using just a series resistor, that is. A higher efficiency equates to less energy drawn from the source for the same amount of light output. Therefore, it is less expensive to inject photons into the environment. This light pollution can adversely affect nearby plants and animals [2].


 * Energy**

The energy consumed by this LED driver circuit depends primarily on the amount of LEDs used and the amperage. For example, consider powering 5 LEDs at 20mA with a 12V source. Assuming the forward voltage drop of a single LED is 1.5V, all 5 LEDs consume 150mW. The series transistor would need to drop 12-7.5=4.5V at 20mA, which is 90mW. In total the circuit consumes 240mW. If this LED light bulb is used in an automobile, energy is derived from gasoline via the alternator and then the battery. Use of this non-renewable and polluting energy source results in a myriad of impacts to the environment and its inhabitants. However, if the light bulb is used within a building, it's possible to power the circuit from a clean energy source, such as wind or solar.


 * Economy**

The cost of this design idea include components, circuit board, and housing. Assuming the use of 5 LEDs, components cost about $6 according to single quantity pricing from Digikey. The circuit board price depends on volume and turn-around time. As an example, Advanced Circuit Board (4pcb.com) pricing for 5 PCBs in a week costs $33 per PCB. In higher volume, this price would drastically decrease since most of PCB cost comes from one-time setup expense incurred by the PCB manufacturer. After the initial setup costs, making copies of PCBs is much cheaper making the original. The cost of the light bulb housing and materials to enclose the circuit board are harder to predict since there is no off-the-shelf product.

The reoccuring cost for this design idea is energy cost. If powered by gasoline via a car battery, the cost depends on price of fuel. For example, in 2011 the price of one gallon of gas is about $4. The gallon holds 130MJ of energy. If an internal combustion engine can extract about 20% of that energy [3], 26MJ remains for moving the vehicle and powering the electrical system. For argument sake, if the vehicle is idle and the engine is only power the electrical system, this amounts to 11 cents per kWh.

The life of this circuit depends, in large part, to how long the LEDs last. That is, how many hours the LED can put out light before burning out. Estimates of LED life vary quite a bit from manufacturer to manufacturer. Ranges span from 25,000 (2.8 yrs) hours to about 75000 hours (8.5 yrs). Disposal of LEDs do not contain toxic substances so disposal is relatively low risk compared to CFLs for example, which contain mercury. After the LEDs burns out, the circuit may still function after new LEDs are implanted.


 * Social and Political Equity**

This design idea impacts the user and anyone, or anything in the vicinity of the light output. This may be good or bad, depending on the users intentions. For example, it may lead to light pollution as discussed above. It may be a good thing if one intends to light spaces in a building -- leading to lower energy costs. Lower energy usage may result in less oil purchased from hostile countries, depending on the type of energy used to power the circuit in this design idea.

[1] Donald Boughton, Circuit achieves constant current over wide range of terminal voltages, EDN,November 4, 2010, http://www.edn.com/article/ 511297-Circuit_achieves_ constant_current_over_wide_ range_of_terminal_voltages.php

[2] P. DEDA, I. ELBERTZHAGEN, M. KLUSSMANN, LIGHT POLLUTION AND THE IMPACTS ON BIODIVERSITY, SPECIES AND THEIR HABITATS, Secretariat of the Convention on the Conservation of Migratory Species of Wild Animals, UNEP-CMS, http://www.starlight2007.net/pdf/proceedings/P_Deda.pdf

[3] Internal combustion engine, []