[APD] RE: Metal Halide lights on timers

[Bill Wichers <billw at waveform_net>]

> 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...

        -Bill



*****************************
Waveform Technology
UNIX Systems Administrator

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