I am trying to determine the frequency characteristics of an 8W fluorescent tube. For this purpose, I am driving it with a commercial signal generator via a 12W audio amp and step up transformer.
Before the tube ignites, I get get a nice clean sinewave. After it ignites, the waveform diminishes in amplitude by half and exhibits distortion proportionate with the brightness of the tube (as I increase the signal voltage).
I assume this is due to loading, but I am unsure of what approach to take, since the amp seems suitably rated. Series resistance does nothing.
** I suggest you are just about to destroy that fluoro tube or your amp or both.
A fluoro lamp must not be operated without some form of current limiting or it will soon fail - seems that the limited power output ability if your
12W amp and the winding resistance and inductance of the "step up " transformer are providing this now - by accident.
If you care to reveal details of what you are actually doing - ie what amp, what "step up" transformer & what frequencies you are trying - then your Q will make more sense.
First of all, you cannot run a fluorescent tube from a constant voltage source! You must have some resistance present, or else the tube will explode or the amplifier will be shorted.
A gas discharge tube is best measured in the time domain. The two characteristics are ionization time and deionization time. This is comparable to a bipolar transistor without any negative base current turning it off. It turns off eventually as charges recombine, but it doesn't go very fast. Most tubes ionize (turn on) pretty fast compared to turn-off time, with typical time for a xenon or mercury tube around 10us ionization and 1000us deionization. The fastest controlled gas tubes are hydrogen thyratrons, used for radar pulse generators (switching near megawatts in several nanoseconds).
As long as the tube has ions in it, it'll conduct, given voltage. Over a short period (a few microseconds perhaps), a slow tube might behave resistively. For longer times, the plasma will respond by producing more charge carriers (ionization), and longer still, deionization will take place inbetween.
A glow discharge's V-I characteristic isn't very nice. Terminal voltage can be quite high before breakdown (i.e., near zero current), while it maintains itself pretty well at some voltage when conducting. That alone is negative resistance, but the discharge itself also has a negative resistance characteristic (voltage falls off by a few volts as current is increased). This is why you cannot run a discharge tube from a constant voltage source.
Tim
-- Deep Fryer: a very philosophical monk. Website:
Linear Technology LTC publish some excellent application notes on flourescent drivers using ICs and transformers. As sais time domain is the better way.
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