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Registered Member #621
Joined: Sun Apr 01 2007, 12:37AM
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Posts: 119
I have long wondered this. In say a 175 watt Metal Halide HID lamp that they use above a saltwater reef aquarium or whatnot, when the lamp is fully warmed up and operating at its full and proper intensity, what kind of voltage is being applied to the arc tube, and how much current is it flowing through it? I am unsure if the ballast or power supply actually steps up the voltage into the thousands like an NST does for a neon tube, and/or if it gives it in the tens of amps to get such a brilliantly blinding white arc. Or am I wrong and they just operate at line level voltage and the ballast transformer just limits current under the 15 amps (our outlets are generally 120v 15a here).
And second question regards a standard fluorescent tube like a 48" 40 watt, I have the same question. During mid operation at full warm up and intensity, what kind of voltage is being applied to it and what kind of current is it flowing....I am guessing only in the mA range for the current....and again I am unsure if the transformer or ballast/power supply serves to raise the voltage or just limit current at line voltage level.
PS - I know the halides seem to get a strong "hit" on startup......I can hear the ballast transformer enclosure "knock" or "ping" sometimes a few times as if it is under very high load and hum louder upon startup.
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
I came over a a broken overhead projector and it had a 90V PSU. The igniter circuit was labeled 10kV 30A. It arced over when I tested it without a bulb and it blew my mains fuse with a huge bang.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Metal halide, sodium and fluorescent lamps all typically have arc voltages around 100V. The arc current is just roughly what the wattage would suggest, so about 0.4A for a 40 watt fluorescent and 1.75A for a 175W metal halide. (though not exactly because of power factor, harmonics and suchlike)
In countries with 240V line, the ballast is just a choke, but in 120V land it's often a high impedance transformer, similar to a NST but with only about 200V open circuit output. The arc would be unstable if you tried to run it straight off a 120V line.
The igniter often works by shorting across the lamp and allowing the ballast to flow high current, then suddenly breaking the circuit. The resulting inductive kickback from the ballast is what lights the lamp. This is also the reason for the buzzing and pinging you hear on startup.
Also, the current is higher and the voltage lower for the first minute or two, until the lamp warms up and builds up the proper internal gas pressure and electrode temperatures.
Some lamps, like xenon and UHP mercury, need a lot of voltage to start them. The igniter for these is more like a small Tesla coil, and as Bjoern found out, it often packs enough voltage to cause total chaos if the lamp goes missing or faulty.
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2 HID and Fluorescent Lighting Questions
