Monday, February 14, 2011

High Volt LED Flasher


Here is a Flasher circuit that directly derives power from AC to give brilliant flashes at the rate of one flash per second. It uses a Diac as the main element to flash the LED through current pulses.

230 Volt AC is reduced to 50 volt DC by the dropping capacitor C1 and is rectified by the full wave bridge D1 through D4.Resistor R1 removes stored current from C1 when the circuit is unplugged and resistor R2 protect the circuit from inrush current.

The main element in the circuit is the Diac DB3.It is a semiconductor device that acts as a Voltage-Controlled switch. If a low voltage is applied to the Diac, it remains as an open switch passing little current. All diacs have a break down voltage VBO which is between 28 volts and 36 volts. If the applied voltage is above the minimum VBO, the Diac enters into the” Negative Resistance” region and heavy current passes through it. Diacs are commonly used in pulse generator circuits for driving SCRs and Triacs.

In the circuit, Diac forms a Relaxation Oscillator along with capacitor C2.When the capacitor C2 gets current, it charges slowly through R3.When the voltage in C2 increases above the VBO of Diac( 28 V), Diac conducts and current passes through the LED and it turns on. At the same time C2 discharges and the Diac becomes non conducting. Again C2 charges and the process repeat. This gives brilliant flashes at the rate of one per second.


Values of R3 and C2 determine the flash rate. With 100K resistor and 22 uF capacitor, the frequency will be around 1Hz. Value of the LED current limiter R4 is also important to determine the flash rate. Higher value above 220 Ohms will reduce the flash rate since the capacitor takes more time to discharge.If the current through LED is too high, increase the value of R4 to 1.5 K and adjust the flash rate by reducing the value of R3.

Caution: This circuit is extremely dangerous because there is no galvanic isolation from mains. Most nodes are at mains lethal potential and hence dangerous. Do not try to construct this circuit, if you have no experience in handling high voltage circuits.

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