Mercury lamps power supply

Immediately after ignition, mercury is still liquid and the discharge takes place in the fill gas (argon). At this point the voltage is low and the current is limited essentially by the short-circuit current delivered by the power supply. As the temperature within the lamp increases, mercury vaporizes. The lamp impedance increases, and this causes the lamp voltage to increase and the current to decrease. After about 1 min, the bum-in period is finished and the lamp reaches stationary conditions. 

The most common type of power supplies for mercury lamps with large arc lengths combine a step-up transformer and a "ballast" capacitor placed in the secondary circuit of the transformer (see Figure 3.10). Since the lamp power depends on its capacitance, the step-wise power can be simply varied by switching capacitors as long as the lamp is operated at appropriate cooling conditions. 

Resonance Lamp.—Such lamps (sometimes called low pressure lamps) are often used as line sources in photochemical studies. These usually contain a small amount of a metal vapor (e.g., mercury, cadmium, zinc, etc.) and several mm pressure of a rare gas. They operate at relatively low current (ca. 100 ma.) and high voltages (several thousand volts). This is in contrast to a typical medium pressure lamp which may operate off a 110-220 v. power supply delivering ca. 3-5 amp. The most common example in photochemistry is the mercury resonance lamp which has strong emission of the unreversed resonance lines at 2537 A. and 1849 A. (ca. 90% or more of the total) along with other, much weaker lines ( resonance lines are those which appear both in absorption and emission). There is little continuum. Sources of this type are widely used for photosensitized reactions. 

Medium-resolution absorption spectrometer emission spectrometer with red-sensitive photomultiplier or CCD detector laser excitation source such as listed in Table 1 (or medium pressure mercury arc such as described in earlier editions of this text) neon calibration lamp and power supply (available from, e.g.. Oriel Corp., Stratford, CT) reagent-grade iodine 100-mm glass cell with Teflon stoppers for absorption studies heating tape with controlling Variac 50-mm cell for emission studies vacuum system, preferably with a diffusion pump and cold trap, for pumping down emission cell. 

A widely used photometer used as a high-pressure liquid chromatographic (HPLC) detector uses the intense 254-nm resonance line produced by a mercury arc lamp (see Chapter 6). Others employ a miniature hollow cathode lamp as a very-narrow-wavelength intense source. For example, a zinc hollow cathode lamp gives a line at 214nm that is adequately close to the maximum wavelength of peptide bond absorption (206 nm) so that it can be used to measure peptides and proteins. Details on the hollow cathode lamp are found in the section on Atomic Absorption Spectrophotometry. The hollow cathode lamp also has a long, useful Hfetime if a lower-current, nonpulsed power supply is used. j... 

The most common type of power supplies for mercury lamps with large arc lengths combine a step-up transformer and a ballast capacitor placed in the secondary circuit of the transformer. 

Fig. 1. Irradiation apparatus. A, water cooling B, Ar inlet C, septum (rubber) fok withdrawing IR samples L = high-pressure mercury lamp (Philips HPK, 125-W), used in connection with a Philips VGIIHP 125-W power supply converter unit. Dimensions (in mm) a = 400, b = 240, d = 70, e =44, f = 28, g = 6, h = 10, i = 10.

A 550 watt Conrad/Hanovla medium pressure A.C. mercury lamp surrounded by a 1 cm thick pyrex water jacket, for cooling and Infrared absorption, provided the Irradlance, while an electrical heater and fan system was used to adjust the sample temperature. The lamp was allowed to operate from Its standard power supply with no attempt at regulation of the Irradlance. Lamp voltage and current were the only parameters monitored to characterize Its performance. The exposure region was a perforated aluminum cylinder 34 cm In diameter and 23 cm high. 

EDLs are very intense, stable emission sources. They provide better detection limits than HCLs for those elements that are intensity-limited either because they are volatile or because their primary resonance lines are in the low-UV region. Some elements like As, Se, and Cd suffer from both problems. For these types of elements, the use of an EDL can result in a limit of detection that is two to three times lower than that obtained with an HCL. EDLs are available for many elements, including antimony, arsenic, bismuth, cadmium, germanium, lead, mercury, phosphorus, selenium, thallium, tin, and zinc. Older EDLs required a separate power supply to operate the lamp. Modern systems are self-contained. EDL lamps cost slightly more than the comparable HCL. 

For the investigation into the Influence of u.v. radiation on the dielectric properties of polycarbonate, pressed sheets of the material were irradiated at room temperature in air by a standard Hanovia mercury lamp. The lamp provided a spectrum of wavelengths within the range 254 to 546 nm and sample intensities of 2.47 mW cm and 0.91 mW cm respectively at the principal wavelengths of 365 nm and 254 nm. A voltage-stabilized power supply assisted consistency of spectral output. 

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