LEDs driven by AC without transformers or rectifiers.

We explore the driving of LEDs by untransformed AC. An extreme case is driving 1.9 V threshold (red) LEDs with UK mains, peak voltage 325 V. Commonly, driving is by transformed, rectified (DC) supply with a series resistor (where a significant fraction of the power is wasted) to limit current in the LED. With AC, one can instead reactively limit to a maximum current safe for an LED by employing a series capacitive impedance. Cheaper and simpler supplies can thus be employed in some cases. We analyse such non-linear circuits, and also explore questions of duty cycle and power experimentally.

Effect of Sulfur on the Reduction of Ilmenite by Syngas in Chemical Looping Combustion.

The reactivity of ilmenite as an oxygen carrier (OC) in the presence of H2S was studied. A simulated syngas (66% CO, 34% H2) was used as the fuel. H2S concentrations were set to 4700 and 6580 ppm. The effect of the presence of CO2 was also investigated. The experiments were carried out using a thermogravimetric analyzer (TGA) at atmospheric pressure, with temperatures varying from 1073 to 1223 K. The results showed that the presence of H2S had no effect on the reduction kinetics of ilmenite. With the presence of only CO2 in the syngas, deposition on ilmenite samples was not observed. In the presence of H2S, deposition was observed regardless of the presence of CO2. Higher H2S concentration led to more pronounced deposition. It was shown that deposition only occurred after the ilmenite sample was sufficiently reduced. For ilmenite oxidation, the mass change curves display a distinct peak followed by a valley when the sample was previously reduced in the presence of H2S, indicating reactions between the sulfur deposit and air. The amount of the sulfur deposit could be calculated using the oxidation curves. Scanning electron microscope-energy dispersive X-ray (SEM-EDX) and X-ray diffraction (XRD) analyses were conducted to examine the surface of the reduced samples and the results from these analyses confirmed the presence of the sulfur deposit on the surface of the samples that were reduced in H2S-containing atmospheres.