High Voltage Fly Killer

[Marvo]

I have a cheap and cheerful UV fly killer which is probably a couple of years old. I noticed the other day that the high voltage side of things is no longer working so I dismantled it. Inside, apart from the fluorescent ballast which is still working fine there's just a single transformer. Needless to say there's no fuses on the primary or secondary side and it's potted so it's not fixable.

I was thinking about just making a very simple multiplier out of loose components, I wasn't even going to etch a PBC, just twist and solder the leads and stick the whole thing inside some heat shrink when it's finished, maybe fill with resin if necessary. I'm not after winning any awards with this I just want something quick and easy that will work.

I know how to wire a mulitiplier with just caps and diodes but I have a couple of questions for the electronics guys.

1. If I'm using 230v AC input, how many stages would I need for a fly killer? I'm guessing the Tx was a few KV because it used to put quite a nice arc across the flys that got in there, I think it was a dragonfly however that caused its demise.

2. Would there be a need for primary and secondary side protection ie fuses or a limiting resistor perhaps?

3. Anyone any idea of actual component spec for the caps and diodes?

I'd obviously be much happier if this thing was non-lethal to people and not a fire hazard so any advice greatly appreciated.

This was the basic multiplier I was thinking of;

 

[Lucien Nunes]

Feeding a CW multiplier from 230V 50Hz would require rather large capacitors to kill a fly. The input voltage is multiplied by the number of stages off-load, but the reactances of the caps soon mount up to limit the current.

Top of head, if you wanted to dissipate 30W in the fly and you reckon you might need 3kV, you would be looking at 10 stages of 50µF, i.e. a shoe-box of motor-run caps. The problem is when the fly presents too low a resistance, the voltage will collapse because the source impedance is probably still too high. Typical applications of multipliers (e.g. CRT TV triplers) drive them with high frequency and/or higher input voltage per stage to make the capacitors practical.

I know nothing about fly killers but I wonder whether the original transformer was magnetically shunted like a welder, hence no need for fuses as it would self-protect to a degree against short-circuits?

 

[Marvo]

Thanks Lucien. I realise there a point of diminishing returns with the number of stages of a multiplier. The original Tx is a bit of a mystery, it's about 80 x 50 x 50 mm and it has a distinct pri and sec winding at opposite ends with no other visible components so it is quite plausible it's shunted because it does have that kind of layout.

I'm not sure about the technicalities of zapping flies like how many joules or watts are required, I was hoping it would be feasible with a small multiplier because the tennis racket type fly swatters must use something similar.

 

[Lucien Nunes]

I've not seen such a device but they probably run at a much higher frequency driven by a transistor oscillator, so the caps can be much smaller. Plus they won't be delivering as much power, presumably they are supposed to electrocute it rather than combust it and rely on the impact to obtain good contact.

 

[telectrix]

for what it's worth, i've measured the voltage output of a tennis racquet swatter and it's 1.4kV d.c.

 

[Marvo]

Lol, that wasn't with your tongue was it Tel? The output of the multiplier would also be DC. If around 1.4kV is the voltage they generally use then my second idea is out as well. I have a couple of recovered flyback transformers from old TV sets and I was going to see if I could rig one to run using a fluorescent ballast as a driver but I'd guess the output voltage could run at 30kV which might be a bit high in that case .

 

[guest119]

Electric insect zapper (killer) II.

 

[Lucien Nunes]

Interesting. That suggests much less power required than I had originally considered, based on Marvo's description of drawing a nice arc. I don't think you'll get much of an arc out of that or as much power as the size of his transformer suggests, but if it's sufficient to kill the fly, why not indeed.

 

[KFH]

From my experience the tennis racquet ones are hopeless other than as a fly swat and giving people a shock. I have a commercial zaper and that gives a very satisfying zap to the fly and can provide a zap for a long length of time if they get stuck, filling the room with the smell of burnt fly. I have tried some lower powered ones and they were pretty hopeless so go for the flyback transformer.

 

[Marvo]

I think his transformer is there just to ballast the lamp isn't it? Looks like it came out of some Cold War apparatus. The HV circuit is basically a straight 6 stage multiplier kinda similar to my drawing above from what I can see.