Fluorescent Lamps – Definition, Working & Application

What are Fluorescent lamps?

Fluorescent lamps are lamps in which light is produced as a result of flow of free electrons and ions inside a gas. A typical Fluorescent lamp consists of a glass tube coated with phosphor and containing a pair of electrodes at each end.  It is filled with an inert gas typically argon which acts as conductor and also consists of mercury liquid.

How does a Fluorescent lamp works?

As electricity is supplied to the tube through the electrodes, the current passes through the gas conductor, in form of free electrons and ions and vaporizes the mercury. As the electrons collide with the gaseous atoms of mercury, they give away free electrons which jump to higher levels and when they fall back to their original level, photons of light are emitted. This emitted light energy is in the form of ultraviolet light, which is not visible to humans. When this light strikes the phosphor coated on the tube, it excites the electrons of phosphor to higher level and as these electrons fall back to their original level, photons are emitted and this light energy is now in form of visible light.

Starting a Fluorescent lamp

In Fluorescent lamps the current flows through a gaseous conductor, instead of a solid state conductor where electrons simply flow from negative end to the positive end. There needs to be an abundance of free electrons and ions to allow flow of charge through the gas.  Normally there are very few free electrons and ions in the gas. For this reason a special starting mechanism is needed to introduce more free electrons in the gas.

Two starting mechanisms for a Fluorescent lamp

1. One of the methods is using a starter switch and a magnetic ballast to provide the flow of ac current to the lamp. The starter switch is required to preheat the lamp so that considerable less amount of voltage is required to trigger the production of electrons from the electrodes of the lamp. The ballast is used to limit the amount of current flowing through the lamp. Without a starter switch and ballast, a high amount of current would flow directly to the lamp, which would reduce the resistance of the lamp and eventually heat up the lamp and destroy it.

The starter switch used is a typical bulb consisting of two electrodes such that an electric arc is formed between them as current flows through the bulb. The ballast used is magnetic ballast which consists of a transformer coil. As AC current passes through the coil, magnetic field is produced. As current increases the magnetic field increases and this eventually opposes the flow of current. Thus the AC current is limited.

Initially for each half cycle of the AC signal, the current flows through the ballast (coil), developing a magnetic field around it. This current while passing through the filaments of the tube heat them slowly so as to cause production of free electrons. As the current passes through the filament to the electrodes of the bulb (used as a starter switch),  an electric arc is formed between the two electrodes of the bulb. As one of the electrode is a bimetallic strip, it bends as it gets heated and eventually the arc is completely eliminated and as no current flows through the starter it acts as open switch. This causes a collapse in the magnetic field across the coil and as a result a high voltage is produced which provides the required triggering to heat the lamp so as to produce the adequate amount of free electrons through the inert gas and eventually the lamp glows .

6 Reasons why Magnetic ballast is not considered convenient?

• The power consumption is quite high, about 55 Watt.
• They are large and heavy
• They cause flickering as they work at lower frequencies
• They do not last longer.
• Loss is about 13 to 15 Watts.

2. Using Electronic ballast to start the Fluorescent lamps

Electronic ballasts, unlike magnetic ballast provide the AC current to the lamp after increasing the line frequency from about 50 Hz to 20KHz.

A typical Electronic ballast circuit consists of an AC to DC converter comprising of bridges and capacitors which rectify the AC signal to DC and filter out AC ripples to produce DC power. This DC voltage is then converted to high frequency AC square wave voltage using a set of switches. This voltage drives a resonant LC tank circuit so as to produce a filtered sinusoidal AC signal which is applied to the lamp. As current passes through the lamp at high frequency, it acts as a resistor forming a parallel RC circuit with the tank circuit.  Initially the switching frequency of the switches is reduced using a control circuitry, causing the lamp to get preheated, leading to an increase in the voltage across the lamp. Eventually as the lamp voltage increases enough, it gets ignited and starts glowing. There is a current sensing arrangement which can sense the amount of current through the lamp and accordingly adjust the switching frequency.

6 Reasons why Electronic ballasts are preferred more

• They have low power consumption, lesser than 40W
• Loss is negligible
• Flicker is eliminated
• They are lighter and fit more into places
• They last longer

A Typical application involving a Fluorescent lamp – An Auto Switching Light

Here is a useful home circuit for you. This automatic lighting system can be installed in your home to light the premises using CFL or Fluorescent lamp. The lamp automatically turns on around 6 pm and turns off in the morning. So this switchless circuit is highly useful to light the premises of the house even if the inmates are not in home. Generally the LDR based automatic lights flicker when the light intensity changes at dawn or dusk. So CFL cannot be used in such circuits. In Triac controlled automatic lights, only the incandescent bulb is possible since the flickering may damage the circuit inside the CFL. This circuit overcomes all such drawbacks and instantly turns on/off when the preset light level changes.

How it works?

IC1 (NE555) is the popular timer IC which is used in the circuit as a Schmitt trigger to get a bistable action. The set and reset activities of the IC is used to switch on/off the lamp. Inside the IC there are two comparators. The upper threshold comparator trips at 2/3 Vcc while the lower trigger comparator trips at 1/3 Vcc. The inputs of these two comparators are tied together and connected at the junction of the LDR and VR1. Thus the voltage provided by the LDR to the inputs depends on the intensity of light.

LDR is a kind of variable resistor and its resistance varies depending on the intensity of light falling on it. In dark, LDR offers very high resistance as high as 10 Meg Ohm but it reduces to 100 Ohms or less in bright light. So LDR is an ideal light sensor for automatic lighting systems.

During day time, the LDR has less resistance and current flows through it to the threshold (Pin6) and the trigger (pin2) inputs of IC. As a result, the voltage at the threshold input goes above 2/3 Vcc which resets the internal Flip-Flop and the output remains low. At the same time, the trigger input gets more than 1/3Vcc. Both the conditions keep the output of IC1 low during day time. The relay driver transistor is connected to the output of IC1 so that, the Relay remains de energized during day time.

At sunset, the resistance of LDR increases and the amount of current flowing through it ceases. As a result of this , the voltage at the threshold comparator input (pin6) drops below 2/3Vcc and the voltage at the trigger comparator input (pin2) less than 1/3Vcc. Both these conditions cause the output of the comparators to go high which sets the Flip-Flop. This changes the output of IC1 to high state and T1 triggers. LED indicates the high output of IC1. When T1 conducts, relay energize and completes the lamp circuit through the Common (Comm) and the NO (Normally Open) contacts of the Relay. This state continues till morning and the IC resets when the LDR exposes to light again.

Capacitor C3 is added to the base of T1 for the clean switching of the relay. Diode D3 protects T1 from back e.m.f when T1 switches off.

How to set?

Assemble the circuit on a common PCB and enclose in a shock proof case. A plug in type adapter box is a good choice to enclose the transformer and the circuit. Place the unit where sunlight is available during day time preferably outside the home. Before connecting the relay, check the output using the LED indicator. Adjust VR1 to turn on the LED at a particular light level, say at 6 pm. If it is ok, then connect Relay and the AC connections. The phase and neutral can be tapped from the primary of the transformer. Take the phase and neutral wires and connect to a bulb holder. You can use any number of lamps depending on the current rating of the relay contacts. Light from the lamp should not fall on the LDR so position the lamp accordingly.

Caution: There is 230 Volts in the relay contacts when charged. So do not touch the circuit when it is connected to mains. Use good sleeving for the relay contacts to avoid shock.

Photo Credit:

• A Fluorescent lamp by wikimedia
• Starting Fluorescent lamp using a magnetic ballast and a starter switch by wikimedia