Fire safety evacuation emergency signs

Fire safety evacuation signs are widely used in supermarkets, shopping malls, stations, cinemas and other public places as a sign to indicate the safe exit passage when power outages or fires occur. This Aohui brand safety sign is reliable, has a long emergency time (more than 2.5 hours), and a low failure rate. In a supermarket, more than 20 of these emergency exit signs have been used continuously for more than 2 years without any damage. The circuit and working principle are described as follows based on the actual object.

The circuit diagram is shown in the attached figure. It mainly consists of four parts: power indication POWER circuit, charging indication CHARGE circuit, fault TROUBLE indication circuit, power failure emergency inverter conversion circuit. Emergency inverter high frequency voltage lights up 8W fluorescent lamp for safety marking and general lighting.

1. The power supply circuit and the power indication circuit are stepped down by transformer T1 and rectified by rectifier diodes VD1~4. After filtering by C1, a 7V DC voltage is output to supply power to the entire circuit. The power indication POWER lights up the light-emitting diode LED2 through the current-limiting resistors R4 and R6.

2. The charging circuit and indication circuit are to charge the nominal 6W1800mAh nickel-cadmium battery pack through the power supply through the current limiting R1 and the isolation diode VD5. The battery pack is packed with 5 batteries, and the actual object is 4 nickel-cadmium batteries with 1 empty shell, so the actual nominal voltage should be 4.8V/1800mAh. 7V voltage. It is also limited by resistors R4 and R5 to light up the CHAGE charging indicator LED1. As the battery is gradually filled, the charging current gradually decreases, and the voltage drop across R1 also gradually decreases. The voltage at point A will gradually increase, causing the bias voltage of VT1 to rise accordingly. When the bias voltage gradually rises to the conduction voltage of VT1, VT1 changes from cut-off to conduction, the charging indicator LED1 goes out, charging is completed, and the battery enters the floating charge state.

3. The fault TROUBLE indication circuit is composed of VT2 and VT8. When the voltage of the rechargeable battery gradually rises to the breakdown voltage of the voltage-stabilizing diode VD8, the base bias circuit of VT2 also rises accordingly, causing VT2 to enter the conduction state from the cut-off state, and the fault indicator LED3 lights up. Therefore, from the principle analysis, this circuit should actually be a rechargeable battery saturation indication circuit.

4. The power failure emergency inverter conversion lighting indication circuit is mainly composed of VT3, VT4, VT5, VT6, pulse transformer T2 and SW lamp. When the mains power is normally supplied, the output DC voltage is added to the base of VT4 through R4, R12 and VD9. Since VT4 is an NPN tube, it is forward biased and saturated and turned on, and the other DC voltage is added to point B through VD6 (measured 6.8V), and the voltage of the positive pole of VD7 is the output voltage of the nickel-cadmium battery and is added to point C through R9 (measured 5.8V). Since the negative pole voltage of VD7 is higher than the positive pole, VD7 is reverse biased and cut off. Since VT3 is a PNP tube, it is also cut off, and VT5 and VT6 have no bias voltage and stop oscillating. When the city power is off or the test button S1 is pressed, the output DC voltage slowly drops through capacitor c1. When the voltage at point B drops below the voltage at point C, the negative voltage of VD7 is lower than the positive voltage, and VD7 changes from reverse bias cutoff to forward bias conduction. The base of VT3 is biased and turned on through R9, VD7, R8, and VT4. The nickel-cadmium battery voltage is added to the base of VT5 or VT6 through VT3, R15 or R16 to make it saturated and turned on, and the feedback coil L of T2 is used to form a push-pull oscillation. The high-frequency voltage output by it lights up the 8W fluorescent tube through C5 to enter the emergency indication, and the voltage output by VT3 is added to the base of VT4 through VD10 and R14 to maintain the saturation conduction of VT4. When the switch S2 is pressed, the base of VT4 is short-circuited to the ground, causing the bias voltage to drop to 0 and cut off, and the base of VT3 also changes from forward bias to reverse bias and cut off. VT5 and VT6 stop oscillating, and the 8W lamp goes out.

After deep discharge, the charging current of this sign is relatively large at the initial stage of recharging, and the current limiting resistor R1 has a power value of 1W, which is too small and easily causes heat and discoloration. To prevent R1 from burning out when recharging after deep discharge of the nickel-cadmium battery, the current limiting resistor R1 can be replaced with a 16Ω/3W resistor to further improve the working reliability of the sign.