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[APD] RE: Metal Halide lights on timers

I wish I knew more about MH power supplies.  I could _not_ find a circuit
diagram on the web.  Try the 3 $5 timers, and see if you can isolate the
problem.  If its one of the MHs, we can always compare the internal parts to
see why the bad one doesn't work.

Basic MH ballast schematic:

277v -->||<---[cap]---,
240v -->||<           |
208v -->||<           |
120v -->||<          (o) MH Bulb
       >||<           |
       >||<           |
       >||<           |
com -o->||<--o--------'

The cap (capacitor) is also sometimes called the "ignitor". If it fails, your lamp won't start. The ballast is wired as an autotransformer, and there are usually several voltage taps available (I list the US standard voltages. 208 and 277 are the voltages of the legs of a 3 phase Y used in commercial power distribution for those that are curious). If the ballast goes bad you loose the ability to limit the current drawn by the bulb, but I would think that would cause the bulb to fail rather than welding the contacts of a timer...


Waveform Technology
UNIX Systems Administrator

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