Bad design example

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.

  1. 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!

  1. 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.

  2. 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).

Reply to
Pimpom
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I don't think so. In the olden days they'd use a transformer similar to a neon sign transformer but without the magnetic shunts so the short circuit current was much higher (to incinerate the bugs). Definitely kV.

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Best regards,  
Spehro Pefhany 
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Reply to
Spehro Pefhany

I think they get there by being cheap and no-name. So the basic sales strategy is to make trash that looks right on the outside and is so inexpensive to sell that people will both buy it and then not be motivated to return it (or you sell it so that it can't be returned).

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Tim Wescott 
Wescott Design Services 
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Reply to
Tim Wescott

It's also worth considering whether the bug killer, is killing the bugs you care about getting rid of.

I've read comments that indicate that the UV bug-killers do a good job of attracting moths... but they don't attract mosquitoes worth a darned. So, they *look* impressive - lots of moths flying around them, lots of crackling and sparking and flaming - but it's all "collateral damage". The biting-insect population isn't affected much at all.

The effective mosquito-killing machines seem to depend on the use of carbon dioxide as an attractant (and I think some of them also use other chemicals, similar to some emitted in human sweat).

Reply to
Dave Platt

"Spehro Pefhany" wrote in message news: snipped-for-privacy@4ax.com...

OK. I'll start with the doubler but be ready to change to a ladder.

Reply to
Pimpom

+1

I think a common "mosquito trap" just burns bottled propane to attract the buggers.

Getting rid of nearby sources of standing water probably is a more efficient remedy. I notice (when harvesting rainwater) that 3 or 4 days is all it takes before you can see larvae swimming around in it!

[Of course, if a neighbor has been negligent in keeping up with their pool chemistry, even active *fogging* won't control the beasts!]
Reply to
Don Y

Thanks for the input. Points noted.

My wife went through a severe bout of malaria in her teens. She hates mosquitoes and loves the battery-operated swatter rackets. I can get only Chinese products and they last an average of 3 mosquito seasons. But at $3-4 each (got a pair for $2.40 apiece last month), I can't complain.

Most of my state is in the hilly region and neither the heat nor the mosquitoes are as bad as in the plains of India. But we *are* in the tropics, squarely in the monsoon region, and the mosquito population is thick enough to get half a dozen or more bites while sitting on my porch in shorts and a T-shirt on a summer evening. By comparison, that would be closer to a hundred bites in places like Kolkata or Mumbai.

Reply to
Pimpom

which is a bit pointless. The high V bug killers are more about show than result in that one can kill them perfectly well with 240v. You just need closer electrode spacing, as 240 won't reach out the way HV does.

Optimistic - but as you've found, grossly exceeding rating can work.

The R should survive, the C probably won't. 0.22u would be closer. Don't forget a series R too, or the LED degrades and dies when C does.

Others do what you do, buy without finding out more

A candle for CO2, 240v current limited by a light bulb to a closely spaced grid for killing.

NT

Reply to
tabbypurr

Barbecued moths emit carbon dioxide.

But yeah, mosquito traps use carbon dioxide as a lure, either from an LPG flame (or from fermentation in some home-made ones). it seems that LPG is cheaper than dry ice.

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umop apisdn
Reply to
Jasen Betts

...................

I plead being in an uncommon situation. My location is so isolated that word-of-mouth about such things don't reach us and they are not available in local shops. Online reviews are not exactly abundant for such minor products.

Reply to
Pimpom

Don Y prodded the keyboard with:

A few drops of washing up liquid will cure that !

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Best Regards: 
                      Baron.
Reply to
Baron

Yes, not a lot one can do.

NT

Reply to
tabbypurr

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