星期四, 十二月 21, 2006

Electrical Requirements

The electrical requirements of D2 type lamps are nasty. They require ballasts which are more difficult to homebrew than other ballasts. I strongly encourage hobbyists, do-it-yourselfers, and hackers to *NOT* try this. Try homebrewing a D2 ballast only if you have the patience of two saints, lots of electrical and electronic project skills including high voltage skills and skill in homebrewing high voltage transformers with the combined difficulties of flyback transformers and xenon trigger transformers, and a budget for replacing lots of blown parts before you get it working. You are better off buying ballasts from Osram, Bosch, or Aromat (a division of Matsushita) or others. For one thing, these lamps require special sockets made by few manufacturers and mostly sold only to ballast manufacturers.
The D2 types require a starting pulse. 7 kilovolts may on an average spark through these bulbs, but for reliability you need more, maybe 10 or possibly 12 kilovolts. Automotive use requires ability to restart a hot bulb with the mercury vapor pressure high, and this requires even more voltage - 12 to 15 kilovolts and maybe even more for good reliability. The usual ballasts supposedly produce starting pulse voltages like 18 kilovolts minimum, 20 kilovolts typical.
D1 types have an integral ignitor which the ballast has to work with.
Starting pulses must be repeated frequently until the arc is established.
The ballast must supply an open circuit output voltage - other than the starting pulses - of over 300 volts, preferably 400 or maybe preferably 450 volts - to force the arc to establish.
D1 and D2 type lamps are 35 watt lamps. Once the arc is established, the ballast must supply limited current or else the arc will draw extreme current and this will be bad for the bulb and/or other parts. The voltage across the lamp is normally around 80-90 volts when it is warmed up, but will be less during warmup. The ballast must handle a lamp voltage possibly as low as 16 volts early in warmup, although this voltage usually bottoms out higher - probably at least in the 20's of volts.
The ballast must deliver 35 watts to the lamp when the voltage across the lamp is between 70 and 110 volts. When this voltage is lower, the ballast must deliver at least .5 amp but generally no more than 2 amps and preferably as close to 35 watts as possible. Higher currents are preferred - a partially warmed up metal halide lamp sometimes has an unstable arc at lower current.
An automotive grade ballast often delivers boosted power (above 35 watts) at some times during warmup to give near-full light output. Note that a xenon arc or a mercury vapor arc does not produce visible light as efficiently as a metal halide arc does. Automotive grade ballasts with boosted power at some points of warmup have circuitry that models the thermal characteristics of the bulb. The maximum safe current for the bulb's electrodes must not be exceeded during a power boost during warmup.
A voltage across the bulb higher than 110 volts only occurs in the early stage of establishing the arc or if the bulb is failing. The ballast should deliver enough power to heat up the electrode tips enough for the arc to establish - more is better and over 35 watts is OK as long as the current is not excessive. But excessive power delivered to an aging bulb can cause the bulb to explode.
D1 and D2 lamps and most other metal halide lamps require AC. DC is tolerable briefly, and then preferably only if the bulb is cold. A DC electric field, hot quartz or hot glass, and salts or alkalis is not a good combination - electrolysis effects can occur which can create weak spots or cracks in the arc tube.
The AC delivered to a D1 or D2 type bulb usually has a frequency of a couple hundred to a few hundred Hz. Higher frequencies are probably OK with D2 types but the ignitors in D1 types may only work correctly or even be adequately conductive in a certain range of frequencies.
The AC current waveform in a D1 or D2 type lamp is traditionally a squarewave or close to a squarewave. Other waveforms have higher peak current for a given average current or RMS current, and the higher peak current is harder on the electrodes and may shorten the life or cause problems with the use of higher currents during warmup.
Metal halide lamps should not be overpowered, except where permissible for accelerated warmup and near-full light output during warmup. Overpowering one will shorten its life and increase the risk of the lamp exploding.Underpowering a metal halide lamp is also bad. If the electrodes are not hot enough, they do not do a good job of conducting electrons into the arc and voltage drop in this process (known as the "cathode fall") is excessive. Excessive cathode fall causes positive ions in the arc to hit the electrode at excessive speed which "sputters" electrode material onto the inner surface of the arc tube. For more info on discharge lamp mechanics, look in my
Discharge Lamp Mechanics File.It is not recommended to experimentally operate metal halide lamps at reduced power. Besides the bad effects of high cathode fall on hot electrodes, an unusual temperature pattern can have the chemicals in the arc tube condense in locations that can block some of the light. And if the electrode cathode falls are excessive and unequally so, a DC electric field can result, which can cause destructive electrolysis effects on hot salts on hot quartz. This can cause the arc tube to crack.
Metal halide lamps should have power input within 10 percent of their rated wattage.

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