Avian’s Blog

A couple of months ago I noticed that the local DIY store sells "flashing lightbulbs" at some discount price (around 4€ if I recall correctly). Since it looked like they could contain a xenon flash lamp I bought one just so that I could play a bit with this interesting component. Recently I got some time to actually take it apart and have some fun with it.

What they are selling as a lightbulb is actually a small electronic circuit that drives a lamp from AC line voltage from a standard E27 socket (similar to common compact fluorescent lights). Here's the circuit from this particular lamp:

A xenon flash lamp is a gas-discharge lamp that can make intense pulses of visible light. This achieved by sending a large pulse of current through the xenon gas. The current, which flows between two main electrodes (A and C on the diagram above) excites xenon atoms which then emit visible light as they return to their ground state. The power source for the burst is usually a charged high-voltage capacitor, which also helps defining the energy of each flash of light.

It would be hard to charge the capacitor to the breakdown voltage, where the lamp would start conducting on its own. So the gas is initially ionized by applying a high voltage spike on a third electrode (B) that is outside the isolating glass bulb. Although this doesn't send any current through the gas itself it creates an electric field around the main electrodes that is strong enough to strip xenon atoms of some of its electrons. This initial ionization then provides a starting point for the main discharge.

So how does this particular circuit achieve that? The main capacitor C2 is charged directly from the mains voltage through a rectifier D1. Together with it a smaller starter capacitor C3 is also charged. After the voltage on it reaches a threshold voltage or around 70V, a small neon lamp NL ignites. This sends current into thyristor Q1's gate, which switches it on. A current pulse travels through the primary winding of a small transformer T1, which produces a voltage spike on the open-circuit secondary. This spike ignites the main lamp, through which the main capacitor discharges, making the flash of light.

After the cycle both capacitors start to charge again and the whole cycle repeats approximately once per second. How this period is calculated is left as an exercise for the reader.

That's it for now. Next time: how to modify this circuit to do something more interesting.