Extend Defective Fluorescent Tube-Light Life

I present here a non-standard scheme to extend the life of defective Fluorescent Tube-Lights.

As I will explain in the next steps this method is based on study of the the basic fluorescent tube and its operation using the wire-wound-ballast and electronic-ballast.

The tube I modified six months back is still working indicating that the normal life of the fluorescent tube can be extended by 20 to 30%.

This scheme came to mind based on experiments conducted on the electronic-ballast designed by me: https://www.instructables.com/id/Electronic-ballas...

I found that while fluorescent tubes were originally designed for wire-wound-ballasts which needed active filaments at both ends for their operation, electronic-ballasts are capable of operating with only one filament. In this mode the electronic-ballast is operated outside specified electrical limits but appears to have sufficient design margin for continuous operation.

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Pictures:

1. The burnt-out/fused filament and blackened tube end

2. The filament resistance shows open (OL)

3. The normal filament shows 20-30 Ohms resistance

4. The copper wire to be used for shorting the terminals at the blackened end. One strand from multi-strand household wiring cable.

5. The terminals shorted out

6. The resistance is now Zero

Fluorescent tube-lights were designed to operate with wire-wound-ballast chokes. A more efficient electronic-ballast is now commonly used with these tubes.

Looking at the figure the following is the sequence of operations leading to the start up of the tube:

1. When the AC-Mains is switched ON, the circuit is completed through the ballast, the starter and the two filaments. The starter which is a mini-glow tube starts burning and heats up a bi-metallic contact within the starter. This causes the contact to close and a starting current now flows through the ballast and filaments.

2. This starting current heats up the filaments and electrons are emitted from them. With the starter contact closed the glow subsides and the bi-metallic strip cools down. After a short delay the starter contact opens.

3. Sudden opening of the contact while a current is passing through the inductive-ballast causes a high voltage to be generated based on the inductance value L of the ballast and the rate of change in current di/dt.

4. This high voltage L x di/dt across the tube along with the electron-emission from the filaments ionizes the gas within the tube and a discharge is setup between both ends.

5. This discharge and the coating on the inner side of the tube creates the fluorescent glow.

6. At this time the inductive ballast limits the current flow through the tube to designed limits and the voltage across the tube is not sufficient to make the starter glow and the starter contact remains open.

With continuous use the filaments deteriorate and usually one filament burns-out/fuses and that end of the tube is blackened. A continuity test on the fused filament would show open-circuit while that on a good filament would show 20 to 30 Ohms.

The electronic-ballast is designed usually as a half-bridge resonant-converter: https://www.instructables.com/id/Electronic-ballast/

Looking at the figure:

1. The input AC Mains voltage is rectified using a bridge-rectifier

2. This voltage is fed to a half-bridge resonant-converter operating at a typical frequency of 60 kHz.

3. The output of the half-bridge is connected via an inductor (L1), one filament, a capacitor (C1), through the other filament to the center point of C2/C3.

4. When the AC Mains is switched ON a starting circuit immediately energizes the half-bridge converter and a high voltage is developed across C1 which is the same as across the two filaments.

5. The mechanism for Ionization and setting up of the discharge is quite different as it depends on the frequency, the high-voltage and to some extent on the emissions from the filaments.

6. After start up the current is limited by the impedance of the inductor L1 and the resistance of the filaments.

While experimenting with the electronic-ballast I found that the tube-light started up and worked when one filament was by-passed and it worked intermittently when both filaments were by-passed. As I was working with my own design it was possible to monitor the tube current, the input currents and the dissipation of the active components. The conclusion of these experiments was that the electronic-ballast was capable of driving a defective tube-light with one filament by-passed.

I was able to confirm this with a commercial electronic-ballast as presented in the video.

I made some initial measurements of the lamp current using a digital multi-meter. However, as the frequency of the AC waveforms is ~60 kHz these values are not reliable being outside the frequency-response range of the multi-meter.

After setting up an isolated AC Mains power supply using an isolation transformer I was able to take more accurate measurements using an Oscilloscope with a bandwidth of 15 MHz.

The oscilloscope probe is 10:1 and current measurements are taken across a 10 Ohm resistor.

The following is the summary:

1. The oscillation frequency is ~ 62 kHz
2. Under normal conditions the Voltage across the tube is 106 V and the current is 28 mA indicating a power of 29 Watts for this 36 Watt Fluorescent tube.
3. The voltage across one of the filaments is 2.4 V and the dissipation would be 0.7 Watts at 24 mA.
4. With one filament shorted the voltage across the tube is 159 V, the frequency is not so steady and the current is 42 mA indicating a power of 66 Watts.
5. The current increase is 42/24 or 1.5 times
6. The power increase is 66/29 or 2.2 times