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Parallel help

Discussion in 'General Electronics Discussion' started by PoliceDog, Aug 16, 2017.

  1. PoliceDog

    PoliceDog

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    Aug 16, 2017
    Hi, I'm new here and I have what I hope is a very simple question, but I'm not electronics genius.
    Anyway, I have a positive and negative terminal, both have two ports for each end of coils, so in a standard setup, each terminal will have two coils running in parallel, those coils have a resistance of 0.2 ohms therefore creating a total resistance of 0.1 ohms (4.2 volts, 176 watts).
    Suppose those two coils were now to touch each other, what happens to the resistance? Even further still if the coils were to interlock, therefore creating an 8 wrap spiral out of two 4 wrap coils, what now? Both coils still maintain their own identity and have their own legs in the terminal which provides the power to them, but the middle section is running rather more like a supercoil where all of the power is coming together, where do we stand?

    Thanks,
    Ben
     
  2. kellys_eye

    kellys_eye

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    If the coils were of insulated wire then nothing will happen.

    If they were bare wire it would depend on where they touched.

    You will only modify the magnetic field around the coils by changing their proximity to each other. Whilst magnetic flux can add/subtract (depends on the field polarity) they won't 'multiply' so perhaps 'supercoil' is a bit over ambitious.
     
  3. PoliceDog

    PoliceDog

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    Aug 16, 2017
    Supercoil perhaps being rather more of a poor term than an ambition. It's bare wire, small coils;- 4mm internal diameter over 4 wraps. If they were to be interlocked then they'd do so every other wrap, like a zip. Would this cause a drastic resistance change though? Either back up to 0.2 ohms or down lower to potentially unsafe levels. It's being fed from two battery cells in series.
     
  4. kellys_eye

    kellys_eye

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    Interweaving them - without touching - won't change the total circuit resistance (in your example 0.1Ω).

    Batteries? You are aware of the current such a circuit will draw...???
     
  5. hevans1944

    hevans1944 Hop - AC8NS

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    Is this another woo-woo science project? Supercoils indeed.
     
    davenn likes this.
  6. PoliceDog

    PoliceDog

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    Aug 16, 2017
    Indeed, though I am presuming they will touch in between each wrap as I have very little space at all to work with. Current indeed, 40A lithium batteries. It's pulse discharge not continuous. I'm not really aiming to create anything amazing or boost power or anything like that, I'm more concerned about accidentally dropping the resistance right down and causing the batteries to vent or go pop.

    So back to these two coils, they'll both be 0.2 ohm, but rather than sit beside each other they're interlocked, assume touching like a done up zip. Expected resistance? No change? Back to 0.2? or more like a single 8 wrap coil, thus maybe 0.4 ohms?
     
  7. PoliceDog

    PoliceDog

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    Aug 16, 2017
    Lol, not a science experiment I haven't done science since I left school a couple of decades ago.
     
  8. BobK

    BobK

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    If each turn shorts to a turn of the other coil, you have no coils and the resistance is essentially that of the leads.

    Bob
     
  9. PoliceDog

    PoliceDog

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    Aug 16, 2017
    Therefore 0.4 ohms?
     
  10. hevans1944

    hevans1944 Hop - AC8NS

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    Ah! Sounds like a coil for a vaping rig. About the right resistance and size. If the two coils are interlocking, I would expect that to yield a resistance substantially less than two of them connected in parallel at their ends, so less than 0.1 ohms, not 0.2 ohms, and definitely not 0.4 ohms. If the two coils are truly interlocked, it will appear as if they are a continuous chunk of metal, not coiled at all. See @BobK's post #8 above. This is probably NOT what you want to occur.

    I suggest you use a larger diameter wire (kanthal?) to make your coils, while keeping the same number of turns (4) and the same pitch as you used for the smaller diameter wire. This will allow you to substantially decrease the resistance of the coil, thereby increasing the power to it (E²/R), without changing the overall geometry. If you get too carried away on larger diameter wire, to the point where adjacent turns are touching, you could try slipping thin pieces of mica insulation between the turns. The mica should not be affected by the vaping liquid, nor add to its flavor.

    If I am completely wrong on what you are trying to do... well, nevermind!
     
  11. PoliceDog

    PoliceDog

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    Aug 16, 2017
    To put more to it, it's going into a vaping device. Those familiar with them will know the typical setup. The coils themselves sit tight so there are no gaps between the wraps (or turns) (so why doesn't the current just take the shortest route across the top??). This proposal would have opened those gaps up between the wraps / turns into which the second, normally parallel coil would now interlock, allowing for a single wick to be threaded between them as if it were one coil not two, giving more space in the 24mm build deck. However I wanted to be sure I wouldn't end up with a 0.05 ohm monster that sent the current rocketing and blow up the batteries.
     
  12. PoliceDog

    PoliceDog

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    Aug 16, 2017
    Correct, you posted as I too was writing. So you think that in doing this I would probably drop the resistance somewhat. It's stainless steel "juggernaut" wire, which has a resistance of around 2.97ohms/meter
     
  13. kellys_eye

    kellys_eye

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    The coils are probably insulated although you might not be able to see it.
     
  14. PoliceDog

    PoliceDog

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    Aug 16, 2017
    Potentially disastrous then. Good job I posted here first. I tried to make a big coil with 350 micron wire and I simply could not power the 6ft of cable needed for a tri-core braid, so I am exploring other options. It's not just power here, it's about space. The deck is not designed to be used with just one coil, so locking two together seemed to free that space up as they are both going from, and go the same terminal.
     
    Last edited: Aug 16, 2017
  15. hevans1944

    hevans1944 Hop - AC8NS

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    The purpose of the coils is to create surface area for heating the liquid, as well as increasing the effective resistance in a given volume. You do not want adjacent turns to short together! Stainless steel wire, IIRC, is used with a temperature-controlled box mod and runs at a higher temperature than kanthal. The temperature is derived by "measuring" the resistance of the wire, using the same microprocessor that is controlling the current. The 2.97 ohms per meter for "juggernaut" stainless steel wire is valid only at room temperature. It increases as the wire gets hot, which allows the microprocessor to infer its temperature by "measuring" its resistance: R = E/I.

    Why do adjacent, tightly wound, turns not simply "short out" and produce an absurdly low resistance? There are many factors affecting the actual end-to-end terminal resistance, including surface oxidation, contact geometry, and how much pressure there is between adjacent turns. In any event, it would be most prudent to actually measure the coil resistance after winding the coil. This is easily accomplished by momentarily passing a known current through the coil and noting the voltage drop across the terminal ends. For low-resistance coils, this "test current" should be several amperes to allow for a reasonably measurable voltage. For example, ten amps through a tenth ohm coil would produce one tenth volt or 100 millivolts, easily measured by most digital multimeters. If you leave the current on long enough to heat the coil, you should see the effect of temperature on the coil resistance.

    Yes, there are a few vapers here who build (or try to build) their own rigs. I didn't, preferring to use off-the-shelf commercial rigs purchased as a name brand (Eleaf iStick Pico) from a reputable vape shop in Dayton. I started vaping in 2011, on the recommendation of a hospital doctor, after I experienced congestive heart failure and was hospitalized. Unfortunately, I have had to give up vaping since moving to Florida, after experiencing a heart attack that required open-heart coronary artery bypass graft surgery. Not saying smoking or vaping had anything to do with my heart problems, but nicotine does constrict the arteries so I finally gave it up this year. My heart needs all the blood flow it can get!

    You lost me there... why would you need to power six feet of cable? And what is tri-core braid? Is this drip vaping with the goal being to produce prodigious quantities of "smoke" vapor? When you get to this level of vaping, is nicotine still a component of the vaping liquid, or is it all about "taste" and "back of the throat" feel without the need to satisfy nicotine cravings? Just curious. Seems there is a vape shop in every strip shopping plaza here in Venice. Not sure who the customers are though, because I don't visit vape shops anymore.

    Anyway, if you really are intent on interlocking two identical four-turn coils, try my suggestion of using pieces of mica insulation between the turns to keep them from shorting out to each other. There will be hardly any voltage between the turns, so very thin pieces of mica, which are easily flaked with a single-edge razor blade from a thicker piece, should work fine. Perhaps you can slightly expand the coils to make room to slip the mica between the turns, and then compress the coils back together with a small vice or pliers. And be sure to measure the resistance of the final result. That's how you prevent disastrous things from happening.

    Have you paid any mind to how you plan to control the discharge current from your re-chargeable lithium ion battery? This is NOT a trivial problem and usually involves a MOSFET and pulse width modulation with some sort of current, voltage, or combination feedback, usually mediated with a microprocessor.
     
    davenn likes this.
  16. davenn

    davenn Moderator

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    just buy the proper coils, they are readily available online
     
  17. hevans1944

    hevans1944 Hop - AC8NS

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    Yep, that's what I did when I was vaping. But I noticed when I started vaping that there is a sub-culture of vapers who want to DIY the whole vaping rig... except maybe for the glass atomizer. I could sort of understand the allure of winding your own coils... some vapers really push the limit on current in attempting to generate huge clouds of vapor. This obviously shortens the life of the coil, so if you can wind your own... well, wire is cheap and only short lengths are required. You do need a steady hand and good eye though. Most coils are only slightly larger than SMD components!

    As it turned out, vaping nicotine-laced "juice" as a substitute for cigarettes does not require a lot of power. I never broke a single coil in the five plus years that I was vaping. I did drop the vape and broke the atomizer, requiring the purchase of new rig. And then my wife decided to give up cigarettes and switch to vaping to get her nicotine "fix" so a second rig... then a third for increased atomizer capacity... then a fourth for its top-filling capability (no need to take the atomizer apart to add liquid). Beginners like me usually start with "stick" vapes that are about the diameter of a small cigar, only longer. These are "okay" but have limited battery capacity and need frequent charging and frequent re-filling of the atomizer chamber.

    When I dropped my first vape and broke the atomizer, I decided to "move up" to a so-called "box mod" which has a larger rechargeable lithium-ion battery and (depending on price) several neat features such as an oled alpha-numeric display and different operating modes for different wire materials. These usually come with an atomizer tank that holds upwards of 20 ml of vaping liquid. The last one I purchased (before quitting vaping) had an atomizer with a rotating valve at the top, sealed with some sort of silicone gasket, that allowed a slot to be exposed through which liquid could be poured to fill the atomizer tank.

    Vaping does appear to be fully mainstream now, with lots of people trading their tobacco sticks for vape sticks for whatever reasons... cost, less health risk, no odor or second-hand smoke, and very little regulation at this time. That is destined to change. Follow the money! In the USA, the FDA is likely to get involved by declaring vaping a "drug delivery system," especially if medical and recreational marijuana use is involved. The tobacco companies stand to lose billions in sales and governments billions in tax revenues if vaping isn't regulated.

    So, vapers, enjoy it while you can. And be safe with your "mods" and lithium-ion rechargeable batteries: these things are potential un-thrown hand grenades if not charged and discharged properly. There is probably a good reason they are not allowed to be shipped on domestic air carriers..
     
  18. PoliceDog

    PoliceDog

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    Aug 16, 2017
    Indeed, it can get quite serious. I have seen some quad coil rigs running 0.03 ohms on an 8.4volt series parallel configure pushing out the best part of 800watts. I'm not quite that crazy. Still, before I got to even try any of the proposed above, the mod went into spontaneous meltdown this morning and carbonised everything inside the atomiser. I'm not sure if it's a good job I was nowhere near it at the time or not.
     
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