# Driving a fluorescent tube

Posted on Feb 7, 2014

Even if a fluorescent tube it`s not a neon glow lamp and this page it`s not maybe at the right place, this circuit is very interesting because it uses an odd way to power a very common lamp. Like glow lamps, neon tubes are composed by two electrodes in a low pressure atmosphere composed by a mixture of gazes; usually argon and mercury producing UV

light that is converted into white light by a fluorescent layer on the internal surface of the tube. When the tube is off no current can flow through it when the voltage between its electrodes is lower than a threshold voltage (it varies with the length of the tube from about 300V for the smallest ones, up to more than 10kV for the longest ones). To reduce this voltage by a factor of 10 and make the tube start better the electrodes are often heated by a current flowing through them. Depending on the start circuit this current may (or may not) be shut off after the tube has started. Once the tube is lit, the voltage between its electrodes falls and remain roughly constant even if the current is varied (between 30 and 100V depending on the length of the tube and on the gas inside). Fluorescent tubes are often powered with AC current, but with this circuit we will drive it in DC. Basically this circuit is a voltage doubler (composed by the two diodes 1N4007 and the two high voltage electrolytic capacitors of 10uF - 350V) that generates, when the tube is off, a DC voltage of more than 600V from a 230Vac main. Unfortunately this circuit doesn`t work with a 110Vac main. If a low power tube is used (for example 4, 8 or 12W) these 600V are enough to strike the it. Once the tube lit, a current starts flowing through the two capacitors of 470nF - 630V that causes a voltage drop that ensures the service voltage of about 30V for the...

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