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Making a safe and effective fly zapper

  1. Jun 20, 2018 #1
    I’m ‘prototyping’ a mains-powered bug zapper, based on a Cockcroft-Walton multiplier, and would be interested to hear opinions on how to make it safe and effective.

    From the start, I think the earth/ground reference should be removed with a small isolating transformer. Do commercial units have these? I can’t find a schematic. Surely a safety-rated device can’t rely totally on physically preventing the user from touching live parts.

    The multiplier is set up as below, with the output being taken between the left-hand node (ground symbol) and Vo. I’ve ensured that the neutral is connected at the bottom, but an isolating transformer would make that moot.
    70C671DA-50C1-43F9-AADC-4B868942AB61.png

    Vi is 240V ac; Cs are 1000V, 150 nF; Ds are 1N4007.

    I can’t measure the output voltage directly, but adding up that across each cap suggests 1440V. This chimes with the theoretical value of 2N(Vp), N being the number of stages (2 here) and Vp the peak input voltage of 330V or so.

    Short circuit current is around 10 mA.

    For the grid, I cut an oven shelf in half across the bars, then mounted and interleaved the two halves in wood blocks drilled for the purpose. A small, convenient fluorescent lamp (not UV) is lit behind the grid.

    My high voltage tester shows flashover with this set-up occurs at 3500V, with almost no leakage beforehand, so we’re well inside that.

    Questions:

    1. Voltage suitable?
    2. Current suitable?
    3. Safety recommendations? I will pot the multiplier and live connections, and enclose the grid, of course. A very small fuse will protect the device. My main worry is a failed-short component, especially a diode. Both caps and diodes are, I think, well rated. (Each cap, except C1, will be charged to 2Vp, or about 660V).

    4. What discharge resistor would be best to ensure the unit drains properly when unplugged? The total capacitance (1/C=1/C1+1/C2...) is 37.5 nF. With a 1.5M resistor, the RC constant will be 0.06 seconds. But at 1440 V, the leakage will be about 1 mA and the power dissipation 1.5 W. I think a higher value would be better.
     
  2. jcsd
  3. Jun 20, 2018 #2

    anorlunda

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    If those tennis racket bug zappers do it with a 9V battery, why can't you do it with a 9V wall wart? That would get rid of many of the safety concerns.
    20180513115430_470x.jpg

    There are numerous mains powered zappers on the market. I saw one for only $30. But the main thing it has that you don't is UL listing (what is the Euro equivalent of Underwriters Laboratory?). Here on PF, we can only discuss the hazards that you or we think of. UL does an inspection of the final device looking for hazards, and they are specialists on hazards.
     
  4. Jun 20, 2018 #3
    Fluorescent light.
     
  5. Jun 20, 2018 #4
    I think the CE mark is the Euro equivalent, or possibly BS xxxx, the British Standard.

    Of course I could buy one, but I’d rather make one.

    Surely a 9V adapter boosted to 1400 V is as safe/dangerous as an isolated mains supply, also boosted to 1400 V? This is not to be a hand-held device, but mounted high up on a wall somewhere.
     
  6. Jun 20, 2018 #5

    anorlunda

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    Ah, thanks.

    Nope, not true. It is not just voltage, but also the energy that can be delivered in a short circuit pulse. Have you ever gotten a static electricity shock from your car? You can generate as much as 30KV voltage with the seat of your pants against the car seat. But it is not injurious because there is very little energy at that voltage. It is the same with the zapper. It has enough energy to zap a fly, but not enough energy to kill you if you touch it. Compare it with a police Taser.
     
  7. Jun 20, 2018 #6
    Ah, yes. I meant if both the 9V adapter and mains versions were configured to deliver the same energy, which they would be if set up to achieve the same end (dead fly).

    For example, the CW multiplier I have made is current-limited to 10mA. If building a 9V-powered version, it would also have to deliver 10mA at the same open-circuit voltage (1400V).
     
  8. Jun 20, 2018 #7

    anorlunda

    Staff: Mentor

    But that ignores what I said about short circuit. To get a short circuit, something has to be wrong, or something fails, or something not wired right in the first place.

    You reminded me of a personal example. 52 years ago in EE lab, they told me to build a circuit for something simple. When I first turned it on, there was a flash and a bang, and it blew the 400 amp fuse for the entire lab. The problem was a component that shorted against the metal chassis. It was a type of fault that can not be deduced from the circuit schematic; but only by inspecting the finished product before it blew up. [By the way, they never let me make anything in EE lab again, and pushed me toward mathematical analysis of circuits. :smile:]

    Back to your project, there is nothing a mains supply can accomplish that a 9v version can't do also.
     
  9. Jun 20, 2018 #8

    berkeman

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    LOL. Yeah, that's a good way to get unpopular fast!
     
  10. Jun 20, 2018 #9
    Do you mean if a part of the CW multiplier failed short, and we’re talking of the difference between a short across the mains, and a short downstream of a 9V switching supply (which presumably would shut down)?

    Currently, the protection is as follows, in order:
    1. 100A service fuse
    2. RCD (GFCI)
    3. 32A or 20A MCB, depending on where I plug in.
    4. 3A plug fuse
    5. The smallest fuse I have to protect the actual circuit is 315 mA F-type.

    Now, the 9V adapter would have 1,2 and 3, and possibly its own little fusible resistor. Also, some sort of short-circuit shut-down. Would you then feed this into some sort of single-transistor oscillator, step-up transformer, then a CW multiplier? We also have the fluorescent bulb to think of.

    My thinking is: the mains-powered version is very well protected, while the 9V version adds extra complexity, only to achieve the same result: 1400 V open circuit, 10mA short-circuit. Is this reasonable? Energetically, it doesn’t sit well either - we’re converting 240V ac to 9Vdc, then ‘inverting’ that through a transformer, then rectifying and boosting to 1400Vdc.

    Cf. 240V ac straight to 1400Vdc.
     
    Last edited: Jun 20, 2018
  11. Jun 20, 2018 #10
    I also built a home made bug zapper back when I was in high school.

    Attempt #1 involved hand winding a 10:1 transformer with 1000 turns primary and 10,000 turns secondary. I messed up the insulation, partially shorted the primary, and ended up with a 10,000 turn air core coil.

    Attempt #2 used a 800 volt TV power transformer with a series resistor to limit the current. It worked until the grid was completely covered with mosquitoes. It welded their little feet to the bars. I touched the grid with a finger tip, the result was a tingle.

    Attempt #3 removed the series resistor. It worked until the grid was completely covered with mosquitoes. It welded their little feet to the bars. No difference from attempt #2. I touched the grid with a finger tip, the result was two little arcs, one on each side of the finger. The dry abscess in my finger took a full year to work its way out. Then I gave up.

    Does anybody know the voltage of a commercial bug zapper? I know that 800 volts is not enough, and I question whether 1400 volts is enough.

    Safety requires isolation (a transformer) and current limited to a safe value.
     
  12. Jun 20, 2018 #11
    Yes, @jrmichler, as far as I can find out, 2kV is more like it for commercial units. I could add a stage or two more to the multiplier, but I fear the current would then be limited too much. A fly lobbed into the bars does die, but with two little arcs as you describe. I’m looking for the ‘pop’ you hear from a proper zapper.

    Having said that, those units are tremendously unhygienic. They blast insect entrails over a surprising area. Maybe something in between.
     
  13. Jun 27, 2018 #12

    sophiecentaur

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    Just make sure we don't read the newspaper headline "Flies 1 : Humans 0 "

    High volts are very risky - even when a very minor shock causes a horrific secondary accident.
    :))
     
  14. Jul 5, 2018 #13
    It seems to work pretty well with four stages (eight caps in total), achieving an estimated 2.6kV. Short circuit current is 4.5 mA.

    This gives the authentic ‘snap’ I’ve been looking for.
     
  15. Jul 5, 2018 #14

    CWatters

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    Personally I don't think we can easily give safety advice on a forum. Im retired now but have been involved in getting products CE and UL certified in a past life. Even though we had a lot of experience in house our first step was to take a prototype to a meeting at one of the test labs we were going to use for the formal test. It was quite common for them to pick up on something we had missed - such as the type of plastic used somewhere in the design or the dimensions of a ventilation slot.

    If you really must make it mains powered I would use an enclosed power supply with an IEC (aka kettle) socket on the back so all the mains wiring is done for you.
     
  16. Jul 5, 2018 #15

    anorlunda

    Staff: Mentor

    I had the option of working for UL upon graduation as an EE. Of course as a green student, one can't appreciate how rich the field of failure analysis can be, so I chose another path. Just about everyone admires inventors and designers more than those who support. Think of those who design Tesla cars compared to those who design the Tesla factory. But functions like those of CE or UL are vital to commerce and safety and they need their share of top notch engineers as much as any other industry.

    Students hear this! Just about any engineering specialty can be rich, rewarding, and intellectually challenging. Your choice of initial career path is much less important than your work ethic, intellectual curiosity, and the desire to help your employer succeed.
     
  17. Jul 6, 2018 #16

    CWatters

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    +1 Im reading this in a motel while on a trip to a university open day with my twins. Taking them to see a few of the engineering departments at Southampton so they get a taste for what's on offer.
     
  18. Jul 13, 2018 #17
    And here's the voltage of one of those handheld bug zappers that sort of resemble a ping pong paddle. Saw it at Walmart today. Looks like you found the correct voltage.
    20180713_145521A.jpg
     
  19. Jul 14, 2018 #18
    I haven’t managed ‘2750 volts of power’ yet, but I’ll keep trying!
     
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