"N_Cook" wrote in message
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Arfa Daily wrote in message
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"Franc Zabkar" wrote in message
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On Fri, 16 Sep 2011 15:49:49 -0700, "Robbie Hatley"
put finger to keyboard and composed:




Just the other day, I had a case of a mis-drawn device on a schematic. It
was in the ignitor circuit for a short-arc discharge lamp. It was shown
on
the schematic as a thyristor with the gate left unconnected.


Is that schematic out there in wwwland? I was recently trying to find any
schematic of the ballast part of such a supply but could only find patent
references or nulled-out block diagrams. I was trying to get an idea of
the
post-trigger start up regime while the lamp is warming up.



The manuals are on the manufacturer's website, but that doesn't actually
help, as the schematic for the version of ballast that shows the ignitor, is
not included in any of the models' manuals. We had to get that one direct
from them. I have it printed out, but for some reason, I don't seem to have
it saved away anywhere. I have been through the rest of my manual versions
that I have got saved, and have found another schematic that has the same
ignitor on it, but with the SIDAC (almost) correctly shown as a diac. If you
care to mail me direct off-group with a current valid address that you
receive and read mail on, I'll send it to you.

I have discovered by being involved in repairing them, that there is
actually no great mystery to how the lamp starts up. I too initially thought
that it was a carefully controlled start-up procedure, but it's actually
not, and seems to rely totally on the characteristics of the lamp itself.
Basically, a current-limited AC voltage of something in excess of 100v RMS
is applied across the lamp, and at the same time, a series of striking
pulses of several kV are superimposed on top of that. This causes the basic
arc to strike, and the AC voltage drops to around 25 volts, which seems to
be enough to maintain the arc through the gas in the discharge tube. By
whatever means - in the case of this particular one by use of a thermal
switch heated by a power resistor in the RC network of the pulse generator -
the ignition pulses are stopped, leaving just the AC across the lamp. As the
lamp starts to warm up, the full plasma discharge starts to form in the arc
tube, and as this builds, the impedance of the lamp increases, resulting in
the AC voltage across the lamp increasing, until it settles at around 90 to
100 V with the lamp at full operating temperature. This appears to be purely
down to the lamp, and nothing to do with the ballast, whose sole job appears
to be to limit / control the current in the lamp so that it burns within
its rating. The max hot running voltage does tend to drop as the lamp ages,
and does vary a little between manufacturers. I have seen voltages ranging
between about 80 and 100 volts. I guess it's a bit like a neon bulb where
you get a fixed voltage across it of around 90 volts, irrespective of the
applied voltage on the back side of the current limiting resistor.

A couple of weeks ago, I had a similar lighting fixture from a different
manufacturer on the bench. It used exactly the same short-arc discharge
lamp, but had no electronic ballast. Instead, it had a simple passive series
inductive ballast, fed directly from the mains. The ignitor for this one was
a sealed unit that 'floated' on the lamp side of the choke, so this further
reinforced my belief that the electronic ballasts do nothing 'magic' and
it's just a case of arcing up the lamp initially, and then limiting the
current through it, leaving it to otherwise just 'do its own thing'.

I have a very good pdf on metal halide discharge lamps and how they work,
which I could also send you, if you are interested.

Arfa