Fluorescent starter

Came across this today while searching for something else. Wouldn't the SCR used in the starter saturate the lamp's current limiting inductor and blow the filaments? Then again, it's only half wave so the filaments would probably be OK...

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Reply to
Oppie
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Meh, the filaments obviously are quite a bit of resistance, plus the inductor has quite a bit of DCR itself. It probably decays fairly quickly.

Looks like it manifests in the scope shots. Still an outside chance that could be their probes (I don't see mention of what probes were used?).

Tim

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Reply to
Tim Williams

Not a problem. The SCR circuit starts the lamp a lot faster than the glow starter, so there is less time spent in saturation & less "wear and tear" on the filaments. Once started, the voltage drop created by the arc brings the peak down below which the SCR circuit can conduct.

Ed

Reply to
ehsjr

If it did, then the glow starter would have the same problem. During the starting cycle, the filaments and ballast inductor are in series, directly across the AC voltage, and (as the app note points out) they remain so intermittently for several seconds while the lamp is starting up.

As I understand it, the DC resistance of the filaments is quite high. Two of them, in series, across the AC mains, have a resistance high enough that the current heats them to a red glow, but not hotter than that. They don't need to be heated enough to emit a great deal of light, like an incandescent bulb filament does... just hot enough to start emitting electrons.

The SCR and clipper is simply doing what the glow starter did... connecting the filaments together through enough power-line cycles to heat them up, and then disconnect them at a point where the inductive kickback from the ballast will ionize the mercury in the tube and start current flowing through the tube itself. The filaments will then remain warm enough to emit electrons, through the current flowing through them and into the ionized gas.

Think of them as being a bit like vacuum tube (thermionic valve) filaments, of the "directly heated cathode" variety.

Television sets and table radios used to use a somewhat similar technique for their filaments. Rather than using a high-current 6.3- or 12.6-volt transformer, and driving the filaments in parallel, the set would be wired with all of the filaments strung together in series, and the series string was connected to the AC mains (after the power switch). Many of the tubes in the string would have rather odd-looking filament voltage prefix numbers (15, 17, 34)... these were picked so that the sum of the necessary tubes' voltages summed up to

120 or so.

In many cases, the warmup characteristics of the filaments were explicitly specified and controlled, so that the whole string would warm up (DC resistance increase) at the same rate, and no one filament would hog the voltage and burn out.

Reply to
David Platt

"Oppie"

** The inductor can take 120VAC (or 240VAC) from end to end and not saturate.

So half cycle pulses at 50/60Hz will likely not bother it too much.

... Phil

Reply to
Phil Allison

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