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Repurposing switcher for HB-LED flashing beacon

11 Aug 2014  | Babu Ta

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This flasher/beacon circuit can be utilised as a distress signal on highways, a direction pointer for car parks, hospitals, and hotels, etc. The circuit uses a power LED, and provides more light than a typical incandescent lamp flasher. Use of a 6V or 12V SLA lantern battery makes the circuit portable.

The heart of the circuit is an MC34063 monolithic switching regulator sub-system, originally intended for use in DC-DC converters. This device contains a voltage reference, comparator, controlled duty cycle oscillator with an active peak current limit circuit, driver, and a high current output switch, all in an 8-pin DIP.



The circuit briefly flashes a 1W power LED from a 6V to 12V DC supply – at about 5% duty-cycle. Current limiting to the LED is accomplished by monitoring the voltage drop across R1, a 1Ω sense resistor placed between VCC and the output switch, pin 1.

The maximum current capability of a 1W white LED is about 350mA. At the beginning of a cycle, C1 starts to charge, and LED current rises rapidly, along with the drop across R1 which is monitored by the Ipk sense pin, IC1-7.



When this voltage becomes greater than 330mV with respect to pin 6 (i.e., 330mA), the current limit block in the IC provides additional current to charge the timing capacitor C1. This causes it to rapidly reach the upper oscillator threshold, at which point the output switch turns off and C1 discharges. Flashing rate can be altered by changing the value of C1. 100µF gives approximately 4Hz.

Since The LED flashes on very briefly, thermal issues are minimal and a star MCPCB is sufficient to cool the LED.

This application note further describes IC1's operation.

To download the PDF version of this article, click here.




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