High Frequency Emergency Fluorescent Light
The objective here is to construct an emergency fluorescent light circuit that requires neither a choke nor a starter unlike conventional mains operable fluorescent light. Such emergency lighting units having different lighting power
capabilities are available in abundance in the market. The circuit given here is simple and highly| efficient. It operates from a 6V rechargeable battery (Lead-Acid type). The battery is trickle charged when the mains is present. The battery with a capacity of 6Ah (this is the battery used in this project) is capable of providing uninterrupted lighting for four hours in the absence of mains. The switch over is fully automatic i.e. When the mains goes OFF, the battery comes into the lighting circuit automatically and there is no requirement of manually switching on the light.
The emergency fluorescent lights are based on the principle of high frequency lighting. Typically, the frequencies the applied pulse train is around 30 to 35kHz and the initial peak amplitude (when the light is not yet on) of the pulses is around 750 to 800 Volts. When the tube lights up, the peak amplitude falls to a value depending upon the output power delivery capability of the inverter circuit supplying high frequency pulse train to the tube. The circuit operation can be described as follows: In the presence of AC mains power, the AC/DC power supply arrangement consisting of a step down transformer with center tapped secondary (T-1), the two diode conventional full wave rectifier constituted by diodes (D1) and (D2), the filter capacitor (C1) and the three terminal regulator (VR-1) generate regulated even which charges the battery through diodes (D3) and (D4). Since the emitter potential of transistor (Q) is less than its base potential by two diode drops, (Q1) is surely in cut-oft As a result, no voltage appear? at the collector of (Q1) and hence the drive (circuit for the DC/AC inverter portion remains without its DC supply.The fluorescent tube (a 6Watt, 9 inch tube in this case) stays OFF. To sum up uphill now, when ever AC power is ON, the battery is getting charted but is not supplying any powered the DC/AC inveter circuit feeding the fluorescent tube. LED-1 is ON and it indicates that mains is present. LED-2 is OFF and it tells that transistor (Q1) is in cut-off. When the mains power is OFF, diodes (D3) and (D4) are reverse biased, tsansistor (Q1) conducts and the DC voltage is now available at the collector terminal of (Q1) and hence for the drive circuit of DC/AC inverter. Case of transistor (Q2) gets drive pulses and it is switched ON and OFF alternately Basically, transformer (T-2), transistor (Q2) and the drive circuit constitute an externally driven DC/AC inverter of the fly back type. During every conduction time (when the output of 555 timer is LOW) of the transistor (Q2), energy is stored in the primary of the inverter transformer and during every OFF time (when the output of 555 timer is HIGH), the energy stored in the previous cycle is transferred to the secondary circuit. The inverter transformer has been so designed here that it produces a train of pulses with a frequency of about 30kHz and a peak amplitude of 750 volts. When such a pulse train appears across the tube it lights up and the peak pulse amplitude drops to about 150 volts.
Capacitor (C8) limits the tube current. The transformer and the drive circuit have been designed to feed a 6 Watt, 9 inch tube.
The PCB layout the components layout are respectively shown in Figs. 20.2 and 20.3 respectively. Transformer (Tl), Transformer(T2), transistor (Q2) and the battery are not the part of the PCB. These are mounted separately. Transformer (T2) (the inverter transformer) is wound on a good quality ferrite rod (10mm diameter and room long) usually seen in transistor sets. These are easily available in the market. The dots shown in the transformer windings indicate start of windings assuming that both primary and secondary are wound in same direction. Other details such as number of primary and secondary turns, the gauge of the wire to be used etc. are given in the parts list.
Resistors and Capacitors
R1 : 470Ω, 1/4W
R2 : 3.3K, 1/4W
R3 : 1 .5K, 1/4W
R4 : 100 Ω, 1/4W
R5 : 10 Ω, 5W (Wire wound)
R6 : 4.7 Ω,2W
R7, R8 : 4.7K, 1/4W
C1 : 470µF, 16V (Electrolytic)
C2 : 0.01µF(polyester)
C3 : 0.01µF (Ceramic disc)
C4, C6 : 0.01µF (Ceramic disc)
C5 : 10µF, 16V (Electrolytic)
C7 : 2.2µF, 16V (Electrolytic)
C8 : 0.01µF, 1KV (Polyester)
Semiconductor Device: and ICs
D1 to D5 : 1N4001 or Equivalent
LED-1, LED-2 : Preferably two different colour LEDS
Q1 : SK100 or equivalent
Q2 : 2N3055
Q3 : 2N2222
VR-1 : Three terminal regulator type 7809
IC-1 : Timer 555
1. S1 : Mains power ON/OFF switch
2. T-1 : Mains transformer, Primary : 230VAC,
Secondary 9-0-9, 500mA 3. T-2 Inverter transformer, primal:
Core : 7.5cm long ferrite rod (Fig. 20.4)
6 Watt, 9 inch fluorescent tube with appropriate holder, 8-pin IC base, LED holders, multistrand wires, solder metal, mains power cord, suitable mounting cabinet etc.Note: The photograph offering rods shown in Fig.20.4 is only a representative photograph to give you an idea of how this component typically may look like.