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Experiment: Severe underdriving in a preheat circuit

Experiment: Severe underdriving in a preheat circuit

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This was a product of boredom more than anything else. :P

This is my Sylvania F15T12/D, running on a 4 watt incandescent night light bulb as the ballast. The lamp was started using a 40 watt bulb, then switched to the 4 watt unit. Lamp power was approximately half a watt, and believe it or not, the lamp was able to sustain a hot cathode discharge, without rectifying, like this.

The purpose of the experiment was to see if, as long as a lamp is started with adequate preheating, it could be severely underdriven without sustaining wear, as long as it was enough current to sustain a hot cathode discharge with no rectifying. I had already figured the answer was probably no, and this experiment concluded as such. My lamp is a little worse for wear now. :P

I might repeat this experiment with a more sensible underdrive, say, 6 or 7 watts. Then the results might not be as easy to guess.

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Album name:themaritimegirl / Experiments, Projects, & Mods
Keywords:Lamps
File Size:65 KB
Date added:Jun 10, 2014
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sol
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Jun 10, 2014 at 06:50 PM Author: sol
If your experiment is successful, then you've made yourself a fluorescent dimmer.
Peach_Lover
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Jun 10, 2014 at 07:20 PM Author: Peach_Lover
Yep, this is the principle that fluorescent dimmers work on. Cool experiment.
themaritimegirl
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Jun 10, 2014 at 07:33 PM Author: themaritimegirl
Explicit cathode heating definitely helps, though. But yeah, it seems fluorescent lamps can tolerate a small amount of dimming, without cathode heating, without sustaining damage. My F15T8 desk lamp only runs the lamp at 11 watts. Thanks guys.

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Medved
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Jun 10, 2014 at 09:52 PM Author: Medved
Lower current does mean less heat for the cathodes, so their temperature goes down and cause higher cathode fall voltage. This then recovers some of the dissipated power, but accelerates the ions more, so more sputtering. The damaging effect is somewhat compensated by the lower current required from the cathodes, but still net effect is worse sputtering.
As normally the cathode fall is just about 15V, 30% current means it could rise to 45V to reach the same dissipation and the 45V is still within the reach of the ballast. The difference will be smaller in reality, as at first the current to handle is lower, so lower emission is sufficient.
I would expect the 30% to be about the minimum for the arc to remain stable, that is even the level used in many T5 emergency fixtures when operated from the battery...

No more selfballasted c***

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Jun 11, 2014 at 05:23 AM Author: rjluna2
So, what is the optimal voltage/current for dimming purpose with minimal cathode wear on this fluorescent bulb here Medved?

Pretty, please no more Chinese failure.

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Jun 11, 2014 at 06:48 AM Author: Medved
I would expect the lamp design engineers did a good job in optimizing the cathodes for the longest life at the rated current (of course, respecting the manufacturing tolerances and cost limits for a given product). So the minimum wear would then be exactly at the rated current, so no dimming...

No more selfballasted c***

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Jun 11, 2014 at 08:01 AM Author: rjluna2
I see, Medved

Pretty, please no more Chinese failure.

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