Wednesday, March 21, 2018

Bug zapper power supply

I have a $4 bug zapper that uses two AA batteries for a voltage source. I've measured the output voltage before to about 1.4kV and today I decided to measure the power supply operating frequency and ripple amplitude. Here's the bug zapper with an improvised high-pass filter and my trusty pocket scope standing by:

Hey, this actually works! Notice the hands-free high-tech solution for keeping the bug zapper on while I fiddled with scope settings:

And a close up of the scope:

So the operating frequency is a disappointingly low 18.8kHz, but the ripple amplitude is an impressively low ±30mV riding on 1.4kV DC. A modified bug-zapper booster design for neon glow lamps definitely seem worth revisiting down the road, but for now I'll stick to refining the voltage-multiplier version.

Voltage-multiplier boost converter for neon glow lamp

And now back to the regularly scheduled programming! I had second thoughts about pursuing the inductor-based boost converter design to power a neon glow lamp with a standard 9V battery. Non-ideal effects are significant for both the inductor and the transistor switch and it wasn't obvious that I'd be able to get the booster to operate above a megahertz. I might return to this design later and hope to ultimately replace the inductor with a flyback transformer, but for now I'm going with a more straightforward voltage-multiplier boost converter.

For the first iteration, the square-wave oscillator is unchanged except that it's input voltage is doubled to 18V by using two 9V batteries in series. I wasn't sure how many stages the voltage multiplier would need, but with 18V input I got away with using a 4-stage Greinacher (a.k.a. Cockcroft-Walton) multiplier. Here it is on two mini breadboards (voltage multiplier to the left and NE555 square-wave oscillator in the back to the right:

The components were what I had available. The diodes were bargain "germanium" diodes I bought a couple of years ago. I'm pretty sure they're just generic silicon signal diodes, but they seem to work well enough for this application. The voltage rating of the caps was a bit marginal and I used the lowest capacitance values a SPICE simulation told me I could get away with without reducing the output voltage. I did this over two months ago and I don't remember the oscillator frequency, but it must have been less than 200kHz. Anyway, this first crude attempt was encouraging enough. Here's a cheap multimeter showing an output voltage of 80.8V, more than enough to light a neon glow lamp: