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Thread: A wiring question

  1. #11
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    Quote Originally Posted by Midlife View Post
    You added the electrons bit while i was typing. I had been trying to think about how this was explained but it was 30 plus years ago so there is no hope of me trying explain it now.
    Yeah, same boat but even longer for me
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  2. #12
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    Generally speaking, yes either way would be fine. The reason they put it on the positive side is because it's isolated and if it decides to short out to earth then the fuse should catch it. If you're overly OCD about it though, you can run two fuses.

    From an high voltage electrical standpoint (in guitar amps when I've done them) it's in line with the positive flow.
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  3. #13
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    Cheers mate


    In the diagram, the "load switch" ie the mosfet/s switch the negative side so as a matter of convenience, I was thinking of connecting the battery positive to the atomiser case (what is usually the negative) with a metal strip. Now while almost all mech's are not fused, I wanted to include some and thought it'd be convenient to place them on the negative side of the battery (and after the feedback and reading the article I linked), it'd be prudent to place them (2 fuses) between the atomiser "positive" and the mosfet/s.
    Last edited by gtadmin; 31-07-17 at 09:59 PM.
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  4. #14
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    I'm late to the party but anyway...


    Quote Originally Posted by gtadmin View Post
    This is for those with an electrical bent ... does it make any difference whether the fuses are in the positive side of the battery (as per the enclosed diagram), or in the negative side (between the mosfet's source {S} and negative terminal of the battery/s)?
    No it wouldn't make any difference in this case, however standard convention means that engineers will always put the fuse on the +ive, and will often put the -ive to chassis ground. Putting the fuse on the -ive could have some unexpected results, for example if someone modified the ground on a more complex circuit, like adding some shielding or some other component taking it straight back to -ive. As far as functionality is concerned though, the amps will be the same either way and the fuse will still work (FUBARs not withstanding)


    Or even between the atomiser negative and the drain {D} of the mosfet?
    Definitely don't do that. In the admittedly unlikely case of the voltmeter (or the wires to it) shorting, it would completely circumvent the fuse but the button might get a bit hot - exactly the kind thing a fuse on the +ive is meant to prevent.
    Last edited by fabricator4; 31-07-17 at 11:09 PM.
    Chris: Tobacco free since 17:00 15th March 2013.


  5. #15
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    Better late than never

    The voltmeter will not be part of the circuit as previous experience shows that it'll be unnecessary (you know when the battery drops below 3.8V), I just didn't want to redraw the circuit
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  6. #16
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    Quote Originally Posted by gtadmin View Post
    Better late than never

    The voltmeter will not be part of the circuit as previous experience shows that it'll be unnecessary (you know when the battery drops below 3.8V), I just didn't want to redraw the circuit
    Fair enough, but I still wouldn't do it. A failure of the MOSFET that produced a short between gate and source would have the same effect. I don't know how likely that is, but in my experience semis can have two catastrophic failure modes - either a dead short, or completely dead. Possibly the first followed by the second - that's usually the black crispy ones.

    Definitely put the fuse on either wire directly from the battery - it's the only way you can guarantee it will isolate the battery if something goes wrong.
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  7. #17
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    I've experienced a mosfet failure where it hard shorted, battery kept discharging with no way to turn it off It was a 650mAh eGo ... chucked it outside onto the back "lawn" until it fully discharged, then disposed of it. A 1.4Ω coil caused all that mayhem ...
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  8. #18
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    Everything I've read here makes sense for the most part, nothing to add.

    But the only thing I could say is that make sure the fuse is the first thing the battery current sees, and the closer the better. Make sure any wiring between the battery and the fuses is doubly safe (double insulated!?) and whatever else you do in the circuit makes zero difference. Me, I'd run 14AWG high grade wire to the fuses, covered with additional insulation and wires hot glued into place so there's almost no possible way for the unfused wiring to become loose. As you've got two supply wires you need to make sure all that wire is secure and safe. If one battery wire comes loose, it's 'hot' through the other wire coming around the fuse.

    Fuses are intended to protect the wiring in a mains circuit, and any additional fuses in equipment are intended to protect the equipment from becoming dangerous. Fuses in the shown location do both, so don't change it.

    General convention is to use -ve as earth too, and all electronics is based on this convention so go against it with possible adverse consequences. As in the battery casings are -ve, which pretty much dictates you've got to use a -ve ground too.

    (And yes, electrons flow from -ve to +ve. Electrons are negatively charged otherwise they aren't electrons...)

    Stu.

    (Card carrying electrician.)
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