Racquet zapper adventures

[Don42]

Vicki loves, loves, loves her racquet zapper. She zaps spiders, flies, mosquitoes, any bug that has the misfortune to get her attention. She loves the smell of bugsmoke in the morning -- or afternoon--or evening.


Lately she noticed that her zapper makes a buzzing sound. Even I could hear it if I had my aids in. It ain't supposed to buzz. The elex that boosts 3 volts from a couple of D cells up to about 1500 volts operates at maybe 30 KHz, way higher than even dogs can hear. It isn't exactly a modern high-tech circuit running at a megahertz or more. Whaddya expect for $4.95 from Harbor Freight, right? It's not made using modern surface-mount technology, looks more like something from the 70s: a single-sided printed circuit board stuffed with thru-hole parts. One little transistor, no chips at all. But it works -- until it doesn't.

I took it apart in the kitchen at her house. Once apart (the zapper, not us) we saw sparking in the vicinity of one diode, a small cylindrical component not much bigger than a Lake Minnewaska mouse turd. Well, there ya go, mystery solved!

The circuit is obviously a three-diode Cockroft-Walton voltage tripler circuit. OK, I already knew that from previous bug-zapper investigations. I figured the diodes should be rated for 1000 volts each, not that the Chinese wouldn't use 600-volt diodes and hope for the best. For all I know, all 1N400x diodes (1N4000 thru 1N4007) come off the same production line and they just rate and label them differently to hit different price points in bazzilion piece quantites. Might a purchasing agent in the racquet zapper factory have a cousin in the diode factory down the road? It's an extremely common part, not to say vulgar like the president of one free-world country.

Sure enough, when I got home, took it apart again and pulled the sparky part, it turned out to be a 1N4007 which is rated for 1000 volts. I happened to have 20 of them in my goodie box, now down to 19. They sell for a dime apiece in oncesies, plus $8.95 for shipping of course.

I soldered in a new one, put the thing back together and pushed the button.

It buzzed.

WTF, over? I took it apart yet again. It's much nicer doing it at home where I have an electric screwdriver to spare my arthritic right hand. I discovered that it was sparking UNDER the diode, clear on the other side of the printed circuit board. Just for fun I tested the old diode I'd extracted. It tested good. I'd needlessly squandered a ten-cent part --3 cents at 1000 qty. Doncha hate when that happens?

The sparking was occurring between two circuit board traces that are about .040" apart so probably 1 mm. Chinese high voltage design. We can hope that they're making aircraft for the Russian Air Force.

I used a little battery-powered toy engraver that has a tiny burr to dig a little trench in the board between the two copper traces, then put some 5-minute epoxy on there to fill the trench and cover the traces. It's curing now. I knew I'd eventually find a use for that silly little engraver. I snapped it up when Menard's offered it on one of their ridiculous sales for maybe two bux. It runs -- more like walks -- on two AA cells. It does work if I don't push it too hard.

OK, test time, the epoxy has had 20 minutes or so to cure.

No buzzing. No sparks where sparks should not be -- but the zap on the grid of the zapper when I touch it with an insulated screwdriver is not it's former bug-burning energetic self. Rather than POP it's more like zzt. Maybe the epoxy needs to cure some more, else I may just have to admit defeat and blow $4.95 on a new one for Vicki. Some days ya can't pee a drop.

Chapter 2

I was sure that I had another racquet zapper circuit board, but I'd been unable to find it on the bench where I thought it should and would be. Oh well, right?
Today I was at the other bench in my lab, checking out the circuit board that had been erroniously sparking but maybe I fixed.

It wasn't fixed. It was no longer erroniously sparking where it had been, but it wasn't doing much else either. OK, that's about as far as curiosity took me since Vicki already has a brand new one to replace it. I bought it yesterday from Harbor Freight for $4.95.
What the hell, livin' large, right?

But then … but then … but thennnnn …ah ah… (Along came Jones, The Coasters)

… I found the other circuit board I remembered having. It was at the electronics test bench, not the general-purpose tinker bench.

It had some wires stuck to it so I guess I was at some time in the past investigating how the circuit works. Just for fun, I fired it up using a bench supply instead of messing with batteries, and brought an insulated screwdriver near the output wires.
POW!
Oh yes, oh my yes yes yes! Applied to a spider it's an arachnid annihilator; there'd be nothing left but leg bits, smoke and spider vapor. This one is WAY hotter than the one we have at the lake.

Unfortunately, I must have gotten the screwdriver a bit too close to a low-voltage part of the circuit because after that event the LED was still on but no sparks. Hm? Then I noticed that the ammeter on the bench supply said the circuit was drawing over 2 amps. Uh oh! I applied a scientific test to the transistor: touch it with a wet finger to see if it's spit-sizzle hot. Pssst. Yup, that transistor had gone to the geat circuit board in the sky.

You may not be amazed to learn that I just happened to have 9 of those exact transistors in my goodie box. It's an unusual transistor, but I have few of 'em -- perhaps because it's unusual and therefore interesting. I extracted the silicon corpse and installed a healthy little three-legged darlin'. Yup, that fixed it.

So now I think I'll remove all the test wires, install that circuit board in the body of her old racquet zapper, and maybe we'll keep that one at the lake for her arachidicidal spidy hunting adventures.