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Efficient LED power supply has battery backup
Release time: 2020-09-02 17:46:06  Hits: 3

LEDs find wide use in emergency lighting because of their high efficiency and control simplicity. The circuit in Figure 1 provides a highly efficient and reliable design for emergency LED lighting at 3 to 6W. The circuit's input is 12V ac, which the full-wave bridge rectifies and one or two capacitors filter into dc. The battery (not shown) is a 12V lead-acid type. IC1 compares the battery voltage to the supply voltage. When the rectified voltage drops below the battery voltage, the battery takes over to provide LED power.

IC1 , a Linear Technology LTC4412, controls two external PFETs that create a near-ideal diode function for switching between ac and battery output. The PFETs' voltage drop is only about 20 mV compared with a normal 0.7V diode-voltage drop. Pin 5 is low when ac power is off, so you can use this pin to turn on a warning LED through another PFET. IC2 has an internal current limit of 1.5A. Resistor R1 limits IC2 's input; when the current reaches a certain level, Q4 turns off the charging circuit. This IC does not require reverse-diode protection.

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