TL-Starters

During measurements are interesting phenomena come to light which never been thought of. It began with a measurement of the power during the start of a fluorescent lamp. An inexplicable part of the start-up sequence led to the discovery that light radioactive material is used in fluorescent tube light starters. Here the story:

Start Behaviour of Fluorescent Lamps

Fig. 1: Test circuit for measuring behavior of the starting TL.

The reason of this measurement was the question of how much energy is used during the start-up of a fluorescent tube light. For this measurement, using a 36 W fluorescent lamp set. The current and voltage were measured with a digital oscilloscope using a DC-current probe and a high-voltage differential probe. This measurement arrangement is shown in figure 1.

The result of the measurement can been seen in the screen dump below. The power is calculated from the momentarily values of the voltage and current. The blue energy line represents the result of the integration of the momentaneous power.

Fig. 2: Start-up sequence.

During the startup of a TL the following distinct phases can been distinguished:

1. This is an inexplicable part. The voltage on the TL circuit is present, but the current and the power are very small.
2. During this phase, the gas in the starter warmed up. The current is low, as well as the power. This can been observed by the slight increase of the energy line.
3. The gas has heated up the bi-metal of the starter enough to close. The current flows now through the bi-metallic switch and the ballast by the filaments of the fluorescent tube. Because of the relatively high reactance of the ballast in comparison with the ohmic resistance of the filaments, the power is primarily reactive. Despite that the power amplitude is large, the energy use of this part is relatively low.
4. The bi-metal cool down and opens. A high voltage induction provides the ignition and the fluorescent tube light up. The current is determined by the induction of the ballast and fluorescent tube. The current and power amplitiude are now lower than during the ignition phase C, but because the circuit now has a more ohmic character, the TL use more energy. Note that te power curve now is asymetrical around the zero-line.

The power used by the fluorescent lighting is proportionate to the steepness of the blue energy line. The measurement shows clearly that during the normal working the energy line is the steepest and therefore the most energy is used. The startup requires a relatively low power.

MythBusters

The general prevailing idea is that a fluresent lightning during start-up requires more power than at a normal working stage. During a broadcast of MythBusters (episode 69) this assumption was examined and confirmed, unfortunately not correct. In the episode was a error committed which clearly shows where the myth is based on. Only the current during the start-up of the tube light was measured. Figure 2 shows that the amplitude of the current during the preheating, although c is greater than when the TL is light up d, but this says nothing about the energy used per unit time. For the true power at a certain time to determine the voltage must also be measured.
On the correct measure of power and energy see the article Theory en Definitions.

Unexplained part start-up sequence

So far this measurement. Remains only the incomprehensible part during a start-up. Repeated measurements of the start-up process tells that this current-less period only very rarely occurs. Because all components, ballast and the filaments of the fluorescent tube are conductive links, the starter is suspicious.

Measurement of the starter

Normaly a starter limits the voltage around 150 V. But even on the sinus peaks (325 V) and the fairly long duration (3 ms) of a voltage above 150 volts per half period, apparently the starter is not guaranteed to ignite. It was decided to investigate the reaction of the starter at different voltages. Therefor a step shape voltage is needed who rises quickly from zero to the desired voltage and remains until the starter ignites.

The test circuit

Fig. 3: Test circuit to determine the response of starters.

The heart of the circuit is a 1.8 mH coil. During conduction of the MOSFET, the voltage across the coil will charge the inductor. At the time the MOSFET goes off, the energy will transfer from the coil through the diode and 100 Ω resistor into the 10 nf capacitor. This increases the voltage in a very short time and drops afterwards very slowly.

By adjusting the voltage of the power supply or the time the MOSFET conducts, is the test voltage can be controlled.

The starter under test is connected in parallel across the 10 nF capacitor. The voltage on the starter is measured with a high voltage differential probe.

Test Measurement

Fig. 4: A impulse voltage across starter.

Figure 4 shows a typical course of the voltage across the starter. The voltage rises after the MOSFET stops conducting quickly to the desired voltage, 290 V here, and will remain until the starter ignite. The response during this measurement was 700 µs. After the igniting the capacitor discharge by the starter with a typical curve until a voltage of about 150 V remains. Hereinafter also the starter stops conducting and the decrease of the voltage across the capacitor determined only by leakage currents.

Multiple measurements shows that the reaction is very unstable but there is a clear depending on the test voltage. The higher the voltage the shorter the response time.

For this measurement the protective shell is stripped from the fluorescent starter and the internal capacitor is removed.

Spread in response time

Fig. 5: Spread in response time under daylight conditions.

Fig. 6: Spread in response time under dark conditions.

The ignition delay is anything but stable. The first measurements were carried out in the evening where the delay varies from almost 0 to many tens of milliseconds. When the next morning the same measurement was performed again showed that the reaction was much shorter than the night before, a very strange situation! After some experimentation showed that the cause was the ambient light. Apparently starters are very light sensitive. Possible the photons can speedup the ignition.

Figure 5 shows the spread in the ignition time in the situation where daylight entered the room. In case of the measurement where the starter is completely shielded from ambient light is shown in figure 6. The oscillosscope images are the result of many measurements. The difference between these two images shows clearly a light-sensitivity of the starters. The similarity between the measurements under light and darkened conditions is that in both cases a huge spread remains.

The variation in response, especially under dark conditions, could theoretically declare that a long delay can occur before the start sequence of a fluorescent tube light is initiated, not to be confused with the common repeated attempts to ignite fluorescent tubes.

The measurements in figure 6, in which photons are kept outside, shows that the ionization of the gas as almost immediately as after a very long time showed up. The suspicion exists that other energy-rich particles are responsible for the ionise of the gas in the starter. This should be further investigated:

External energy-rich particles

Fig. 7: Spread ignition delay of a TL starter with lead shielding plotted in a histogram.

To investigate whether external energy-rich particles play a role in ionise the gas is the TL-starter partially shielded with a lead role. The thickness of the vertical wall is 25 mm and the length of the role is about 50 mm. The starter is inside this role, and also shielded again for external light. Despite the starter to the sides is shielded and the ends are still open, still there must be a noticeable difference in ignition time to be observed.

There are two comparative response measurements done with and without lead shielding, in about 5000 sweeps. From both measurements the delay is plotted in a histogram as shown in figure 7. In these two measurements is no significant difference between whether or not lead shielding. It may be concluded that external energy-rich particles have not a great influence in ionise the gas in the TL-starter.

Radioactivity

If it does not come from outside it must come from the inside. Unfortunately, there is no possibility to screen the interior against itself to stop the possibility to assess it. A look around on the Internet shows that the light radioactive Krypton-85 is added to the starters to force a quickly ignition:

radionuclide: Kr-85
Activity criterion: 10⁴ Bq
Activity criterion: 10⁵ Bq/g
activity by product: 1,9.10⁴ Bq

Reverse polarity

Fig. 8: The ignition delay of the starter with reverse polarity.

A final mention is that fluorescent starters are polarity sensitive. During the same measurement conditions as in figure 7, the two connections on the starter are swapped. The ignition delay decreased a lot like figure 8 shows. A real explanation for this phenomenon is not immediately ascertainable.