The circuit Incandescent Lamp Flasher in Figure is built around two power FETs, which are configured as a simple astable multivibrator to alternately switch the two lamps on and off. The R&C values given in the Parts List sets the flash rate to about 1⁄3 Hz. By varying either the resistor or capacitor values almost any flash rate can be achieved.
The power MOSFET can be paralleled without special current-sharing components to control larger load currents. This is an important feature when the device is used to turn on incandescent lamps, because the lamp’s cold resistance is much lower than the normal operating resistance.
A typical 12- to 14-volt lamp measures 6 ohms cold. When 12 volts is applied, the initial current drawn is 2 amps. The same lamp, when operating at 12 volts, requires only 200 mA. The hot resistance figures out to be ten times its cold resistance, or 60 ohms. This habit should be considered when picking any semiconductor device to control an incandescent lamp.
Lamp Flasher – Circuit Operation
The astable multi vibrator configuration alternately switches the two IRF510 MOSFETs on and off. The timing is controlled by the RC values – R1/C1 for one timing cycle and R2/C2 for the other. The flash rate is set to approximately 1⁄3 Hz with the specified component values.
Flash Rate Adjustment
The flash rate can be modified by changing either the resistor or capacitor values. Increasing the R or C values will slow down the flashing rate, while decreasing them will speed it up. This allows customization for different applications requiring specific flash rates.
Incandescent Lamp Considerations
When using incandescent lamps with semiconductor switches, consider the cold resistance characteristics. A 12-14 volt #1815 lamp has 6 ohms cold resistance but 60 ohms hot resistance – a 10:1 ratio. The initial current surge is 2 amps, while operating current is only 200 mA. This cold resistance behavior must be factored into semiconductor device selection.
This simple two-transistor astable multivibrator provides reliable alternating lamp Flasher control for automotive, marine, or signaling applications.