A few years ago, I bought a couple of bug zappers at ~US$3 for two, shipped. They had a beautiful violet glow to attract insects but one piece died in a few hours while the other one lasted all of 3 days.
Ever curious to find out what makes something tick (or fail to tick), I meant to take it apart and examine the circuit, but always either forgot or didn't have the time until now. This is what I found inside today:
The high-voltage generator is a classic 2-diode-2-capacitor half-wave voltage doubler. The LED current is set by a series capacitor followed by a bridge rectifier and a smoothing capacitor. The basic ideas seem sound enough but the choice of parts implies that the design was blindly copied from somewhere without understanding the factors involved.
- Voltage doubler: Both the 0.1uF input cap and the 1uF output cap are rated for 100V. This is for use with a 230V mains where the voltage across the output cap could exceed 600V and even the input cap would see >300V!
- LED drive: The series cap is a 0.82uF piece - too high for a single 5mm LED, and it's also rated for 100V. The bleeder resistor across the cap, which has practically the whole 230VAC across it, is a 0.25W type.
- The PCB is held simply by gluing the top of the sole electrocap (vent side) to the plastic frame holding the electrodes.
What surprises me is how they can keep these things in production and apparently sell them by the thousands year after year. I would've thought that, whatever the internal construction is, word-of-mouth about the extremely short life would be enough to reduce sales to the point where it's unprofitable to keep making them.
Anyway, I'm thinking of making use of the mechanical parts - box, electrode grill, LED diffuser, etc - to make a more reliable bug killer. (Wondering if doubling the mains voltage is enough).