A fluorescent lamp (LL) is a light source created by an electric discharge in a medium of mercury vapor and inert gas. In this case, an invisible ultraviolet glow arises, acting on the phosphor layer deposited on the inside of the glass flask. A typical scheme for switching on a fluorescent lamp is a ballast with electromagnetic ballast (EmPRA).
Device and description of LL
The bulb of most lamps has always had a cylindrical shape, but now it can be in the form of a complex figure. At the ends, electrodes are mounted in it that are structurally similar to some spirals of incandescent lamps made of tungsten. They are soldered to the external pins that are energized.
The gas conductive medium inside the LL has a negative resistance. It manifests itself in a decrease in voltage between opposite electrodes with an increase in current, which must be limited. The luminescent lamp switching circuit contains a ballast (inductor), the main purpose of which is to create a large voltage pulse for its ignition. In addition to it, the EMPA includes a starter - a glow discharge lamp with two electrodes placed inside it in an inert gas environment. One of them is made of a bimetallic plate. In the initial state, the electrodes are open.
The principle of operation of LL
The starter circuit for switching on fluorescent lamps operates as follows.
- Voltage is applied to the circuit, but first, no current flows through the LL due to the high resistance of the medium. Current flows through the cathode spirals and heats them. In addition, it also enters the starter, for which the applied voltage is sufficient to cause a glow discharge inside.
- When the starter contacts are heated from the passing current, the bimetallic plate closes. After this, the metal becomes a conductor, and the discharge ceases.
- The bimetal electrode cools and opens the contact. In this case, the inductor generates a high voltage pulse due to self-induction, and the LL is ignited.
- A current flows through the lamp, which then decreases by 2 times, since the voltage across the inductor drops. It is not enough to restart the starter, the contacts of which remain open during LL burning.
The circuit for switching on two fluorescent lamps installed in one lamp provides for the use of one common inductor for them. They are connected in series, but each lamp has one parallel starter.
The disadvantage of the lamp is to turn off the second lamp if one of them is out of order.
Important! With fluorescent lamps, special switches must be used. In budget devices, the starting currents are large, and the contacts can stick.
Throttleless inclusion of fluorescent lamps: schemes
Despite the low cost, electromagnetic ballasts have disadvantages. They were the reason for the creation of electronic ignition circuits (electronic ballasts).
How LL starts with electronic ballasts
The throttle-free switching of fluorescent lamps is done through an electronic unit, in which a successive change in voltage is formed when they are ignited.
Advantages of the electronic launch circuit:
- the ability to start with any time delay;
- Do not need a massive electromagnetic inductor and starter;
- lack of humming and blinking lamps;
- high light output;
- lightness and compactness of the device;
- longer service life.
Modern electronic ballasts have compact dimensions and low energy consumption. They are called drivers, placing in the base of a small lamp. The throttle-free inclusion of fluorescent lamps allows the use of conventional standard cartridges.
The electronic ballast system converts the 220 V AC mains voltage to high-frequency. First, the LL electrodes are heated, and then a high voltage is applied. At a high frequency, the efficiency increases and flicker is completely eliminated. The fluorescent lamp switching circuit can provide a cold start or with a smooth increase in brightness. In the first case, the life of the electrodes is significantly reduced.
The increased voltage in the electronic circuit is created through an oscillating circuit, leading to resonance and ignition of the lamp. Starting is much easier than in the classic circuit with an electromagnetic inductor. Then, the voltage is also reduced to the desired discharge holding value.
The voltage is rectified by a diode bridge, after which it is smoothed in parallel by a capacitor C 1 . After connecting to the network, capacitor C 4 is immediately charged and a dinistor breaks through. A half-bridge generator is launched on the transformer TR 1 and transistors T 1 and T 2 . When the frequency reaches 45-50 kHz, a resonance is created using a series circuit C 2 , C 3 , L 1 connected to the electrodes, and the lamp ignites. This circuit also has a choke, but with very small dimensions, allowing it to be placed in the lamp base.
Electronic ballasts have automatic adjustment for LL as characteristics change. After some time, a worn-out lamp requires an increase in voltage for ignition. In the EMPA circuit, it simply will not start, and the electronic ballast adapts to a change in characteristics and thereby allows the device to be operated in favorable conditions.
The advantages of modern electronic ballasts are as follows:
- smooth inclusion;
- profitability of work;
- electrode preservation;
- flicker exclusion;
- low temperature performance;
- compactness;
- durability.
The disadvantages are the higher cost and complex ignition scheme.
Application of voltage multipliers
The method makes it possible to turn on LL without electromagnetic ballast, but is used mainly to extend the life of the lamps. The inclusion scheme of burnt fluorescent lamps allows them to work out for some more time, if the power does not exceed 20-40 watts. In this case, the filaments can be either whole or burnt out. In both cases, the conclusions of each filament must be short-circuited.
After rectification, the voltage doubles, and the lamp lights up instantly. Capacitors C 1 , C 2 are selected for an operating voltage of 600 V. Their disadvantage is the large size. Capacitors C 3 , C 4 set mica to 1000 V.
LL is not intended for direct current supply. Over time, mercury accumulates near one of the electrodes, and the glow weakens. To restore it, reverse the polarity by turning the lamp over. You can set the switch so as not to remove it.
Starting circuit for fluorescent lamps
The starter circuit requires a long warm-up of the lamp. In addition, it sometimes has to be changed. In this regard, there is another circuit with heating the electrodes through the secondary windings of the transformer, which also performs the function of ballast.
When fluorescent lamps are switched on without a starter, they must bear the RS designation (quick start). A lamp with a starter start will not work here, because its electrodes warm up longer and the coils quickly burn out.
How to turn on a burned out lamp?
If the spirals are out of order, the LL can be ignited without a voltage multiplier using the usual EMPA circuit. The scheme for switching on a blown fluorescent lamp varies slightly compared to a conventional one. To do this, a capacitor is connected in series to the starter, and the pins of the electrodes are short-circuited. After such a small alteration, the lamp will work for some more time.
Conclusion
The design and scheme of switching on a fluorescent lamp is constantly being improved in the direction of profitability, reduction of size and increase of service life. It is important to operate it correctly, to understand the whole variety of manufactured types and to know effective connection methods.