Homework 2 - ECE 2100 Professor Alyosha Molnar Homework...

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ECE 2100: Professor Alyosha Molnar Homework Assignment #2 (rev 1) Due Feb 17, 2010 Areas covered: Node, mesh analysis. Thevenin, Norton equivalents. Maximum power transfer. Superposition. LEDs, Photodiodes, Photovoltaics. 1) Consider two LED lighting systems, one using amber LEDs ( λ = 600nm) and the other using blue ( λ = 450nm). a. What is the expected turn-on voltage of each? b. In each case you put 3 diodes in series, and place a resistor in series with them. In both cases you want to limit current to 50mA when 12V is placed across the whole string (Current sets the number of photons generated). In each case, choose an approximate resistor value to accomplish this. 2) Modeling an optical isolator: Optical isolators are used to pass signals between parts of a system that are operating at different voltages (for example in hospital rooms where everything is isolated on a floating ground to avoid discharges). The basic circuit is shown in Fig. 3a: it uses a resistor in series with an LED, whose light couples to a photodiode. Specifically, light intensity is proportional to LED current (L = k 1 I LED ), and photodiode current is proportional to absorbed light (I PD = k 2 L). 0 < k 1 , k 2 <1. An I-V curve of the input port looks like that shown below (Fig 2b). I PD , when plotted vs I LED gives the curve below it (Fig 2d). a. For Vin > V TO, the turn-on voltage of the LED, this circuit can be modeled as shown. Based on the curves shown, find values for V TO and A I and Rs.
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