On aircraft, it’s desirable to have lights in the wings that flash in alternating 1/2 to 1 second intervals, enhancing recognition by other aircraft. This circuit accomplishes such a flash operation in a somewhat unconventional manner.
The two lamps are wired in series (see the figure). Lamp L1 is brought to maximum brightness by a MOSFET (Q4) shunting the opposite lamp, placing all of the voltage across the ON lamp. To light the other OFF lamp (L2), the MOSFET Q4 is turned off, then MOSFET Q2 is turned on. However, a 30-ms delay is created during the switching when both MOSFETs are off. During this time, the OFF lamp is turned on through the ON lamp’s filament, limiting its inrush current to the ON lamp’s steady-state current.
The current-limiting delay produced by this simple circuit is as follows: With the timer output low, Q1 and Q4 are on, Q2 and Q3 are off, and L1 is on. When the timer output goes high, the gates of Q1 and Q3 will rise slowly. Once the gate of Q3 reaches 2 V, Q3 turns on, turning off Q4 and causing L2 to turn on through the filament of lamp L1. As the gates of Q1 and Q3 rise to the supply voltage, Q1 will turn off, turning on Q2. This turns off lamp L1 and sets lamp L2 to full brightness.
When the timer output goes low, the slowly falling gates of Q1 and Q3 will cause Q1 to turn on, turning off Q2. As a result, L1 turns on through the filament of L2. As the gates of Q1 and Q3 reach ground, Q3 turns off, turning on Q4. Once this occurs, L2 will turn off and set L1 to full brightness.
Because high lamp inrush current, which can be as much as 12 times the steady-state value, is the major source of lamp failure, this current-limited turn-on extends the service life significantly. In addition, the MOSFET that turns on only needs to supply the lamp’s steady-state current rather than high inrush current, minimizing its switching load. The MOSFET turning off isn’t subject to high di/dt transients because a cold OFF-lamp is present across the MOSFET to absorb any transients. If one of the lamps fails, its opposite lamp will still continue to flash.
This circuit is currently installed and operating on my Lancair 235 homebuilt airplane.