Measurement and analysis of automatic emergency lighting circuit

Figure 1 is the circuit I tested. To reduce the weight of the emergency light hanging on the wall, a capacitor step-down power supply is used. When there is a mains supply, the 4.5V DC voltage output from the rectifier bridge charges the rechargeable battery E through D5. At this time, the indicator lights LED1 (red) and LED2 (green) on the light box panel light up, indicating that there is AC. Q1 is turned on, so that the lower bias resistor R5 (also the load resistor of Q1) of the base of Q2 is connected to the "ground", but because the base of Q2 obtains a positive voltage of nearly 4V through D6, the potential is higher than the emitter, so Q2 is cut off, and Q3 is cut off because the base cannot obtain the bias voltage. Therefore, regardless of whether switches K1 and K2 are turned on or not, the lighting lamps ZMD1 and XMD2 will not light up.

Once the AC mains fails (or the key switch AJ used for "testing" is pressed to artificially create a power outage), the DC positive voltage provided by D6 to the base of Q2 is released. At this time, if K1 and K2 are pressed, the rechargeable battery E is connected to the lighting lamps ZMD1 and ZMD2. Since the voltage on the electrolytic capacitor C2 still exists at the moment of power failure, Q1 is still turned on. The voltage drop on R6 caused by the current flowing from E through D8 and D9 through R6 and R5 to the "ground" makes Q2 obtain the necessary conditions for conduction. Q2 is turned on, so Q3 is turned on, and the lighting lamps ZMD1 and ZMD2 powered by the rechargeable battery are lit, and will only go out when the current of the rechargeable battery E is exhausted and can no longer maintain the conduction of Q2.

In addition, in order to make the base potential of Q2 have a positive bias to maintain the on state, a path from R7 and D7 to the base of Q1 is designed to lock Q1 in the on state.

This circuit has a protective effect on overcharge and over discharge of the battery, and can prevent the battery from being damaged by a long power outage. Generally speaking, the lamp needs to be charged once every three months (about 22 hours), otherwise the battery life will be shortened.