Working on a project box

Discussion in 'The Projects Forum' started by kiteh5088, Apr 24, 2014.

  1. kiteh5088

    Thread Starter New Member

    Apr 24, 2014
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    It's a personal vaporizer mechanical mod. Essentially two 18650 batteries in parallel firing into a resistor through solid copper wire. I had an OTTO P9 button in it, but exceeded the current limit and fried the button. The system runs on 4.2 volts maximum, but is running up to 60A of current. I am having trouble finding a button rated for that amount of current and was wondering if the current rating ascends and voltage descends as I think, and how they correlate
     
  2. wayneh

    Expert

    Sep 9, 2010
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    You need to find a switch rated to somewhat more than the max current you expect to switch, and be sure the specification for maximum voltage exceeds whatever you plan to use.

    I'm not sure what else you are asking?

    Some good suppliers can be recommended if you divulge your geographical location.

    Excuse my skepticism, but I doubt two cells can actually deliver 60A. Whatever the limit is, you should consider a fuse in your circuit.
     
  3. JohnInTX

    Moderator

    Jun 26, 2012
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    For DC switches there isn't usually a tradeoff between voltage and current ratings i.e. you can't run a switch a 1/2 its rated voltage and get twice the current rating.

    You need something from the big-amp world, RVs, boats etc. DelCity has lots of this kind of stuff.

    An alternative would be to use a large relay/contactor with the coil switched through the button.
     
  4. inwo

    Well-Known Member

    Nov 7, 2013
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    I would suggest an scr controlled capacitor discharge system.
     
  5. kiteh5088

    Thread Starter New Member

    Apr 24, 2014
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    sony vtc4 18650 cells are the ones I'm using, for Wayneh's skepticism.

    The box is fairly simple, 12 gauge solid copper wire, the two cells in parallel, to the resistor and back to negative. But with the OTTO P9 button, it was rated at 5 amps 28V DC, and didn't give out until I ran it over 27 amps. But I did run it at 21 amps and 12 amps with no issue at 4.2 volts.

    This was leading me to believe that with Current being power over voltage, the ratings for the button should scale down to some degree at lower voltages. Say a 10A at 28V DC button would be 18A at 14V, 36A at 7V, and 72A at 3.5V per say. Because the amount of power going through the button would be the same.
     
  6. kiteh5088

    Thread Starter New Member

    Apr 24, 2014
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    Another issue I'm running into is the size of the parts. I would like them to be fairly small, because the device is handheld.
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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    It is not completely the current factor, it is the plasma arc that often occurs when breaking a large DC current, the heat from the sustained arc will destroy/fry the contacts.
    Max.
     
  8. alfacliff

    Well-Known Member

    Dec 13, 2013
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    the curent a switch can handle is a function of the size of the contacts. the power rating, or voltage is more related to how far and fast they open to extnguish arcs. while a switch might be rated at 20 amps at 120 vlts, it will not carry 200 at 12 volts, there isnt enough metal to do that.
     
  9. wayneh

    Expert

    Sep 9, 2010
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    Thanks, I didn't realize such a thing existed. The 30A rating is the max spec for the battery, though, and I still doubt you are actually draining it at that rate. Hopefully your heater is the (current) limiting factor? If your system is designed so that the battery is the limiting factor, that might be a safety issue. In other words a fuse to protect against a short circuit might be a good idea.
     
  10. kiteh5088

    Thread Starter New Member

    Apr 24, 2014
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    The battery isn't, the resistor(atomizer) is. Essentially the batteries discharge through a coil that heats up, similar to a light bulb. The resistance of the coil is what regulates the amount of discharge the batteries give off. Now, 30A is max for the batteries continuous; 75 seconds or longer. These particular cells can deliver almost 160A of current in a momentary pulse. Not recommended, but the idea behind putting them in parallel is to eliminate battery strain with high power applications.

    You guy were saying something about the arc it makes, and that seems to be what happened with the other button. The solder actually seemed to melt out of it. But again, my curiosity with the stability of it at 21A is there. I have also seen other similar devices without the same problems, but people don't like to release button info.
     
  11. DC_Kid

    Distinguished Member

    Feb 25, 2008
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    power mosfet comes to mind, add a few of them in parallel??
     
  12. MaxHeadRoom

    Expert

    Jul 18, 2013
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    There are relays that switch high current DC have arc blow-out magnets by the contacts to extinguish the arc.
    I have seen arc on higher voltage contacts, once struck, sustain themselves and the current is enough to still provide to the load and will continue until the contacts are destroyed.
    If you have ever seen a plasma cutter in action, this is the same principle.
    Max.
     
  13. wayneh

    Expert

    Sep 9, 2010
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    Something like IRF6718 and a small switch for its gate might take up less room than a properly rated DC switch.
     
    Last edited: Apr 25, 2014
  14. DC_Kid

    Distinguished Member

    Feb 25, 2008
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    exactly, +1
     
  15. kiteh5088

    Thread Starter New Member

    Apr 24, 2014
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    Sorry for the late reply, busy weekend. What exactly is it? and how would wiring and such work?
     
  16. kiteh5088

    Thread Starter New Member

    Apr 24, 2014
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  17. wayneh

    Expert

    Sep 9, 2010
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    It's a transistor, specifically a high power N-channel MOSFET. Think of it as a switch on the ground wire. The MOSFET source pin goes to ground, and the drain pin goes to your load, the other side of which is connected to V+. The state of the "switch" - conducting or not - is controlled by the voltage on the gate pin of the MOSFET. The tiniest switch you can find could be used to apply v+ to the gate. This would turn on your load by allowing current to flow from drain to source across the MOSFET. A pull-down resistor (1-10kΩ, for instance) on the gate pin would pull it back to ground when the tiny switch is opened.
     
  18. kiteh5088

    Thread Starter New Member

    Apr 24, 2014
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    Okay, so in the context of this box; The + from battery would go in and it essentially has two outs. When not pushed it goes to ground,(wire grounded to aluminum box), and the other to the (resistor, atomizer, heating element) which would in turn follow back to battery -?

    And with this kind of application would there be any amount of internal resistance, reducing the voltage of the load?
     
  19. wayneh

    Expert

    Sep 9, 2010
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    This is what I tried to describe. The picture should do a better job.

    The pushbutton can be the smallest you can find since it doesn't need to carry much current.

    The MOSFET will have some internal resistance know as Rdson, the resistance between drain and source while the gate is driven "on" by the positive voltage applied to it. Data sheets provide this information. Since your power supply is ~4V, you will need what is called a "logic level" MOSFET to ensure the gate is driven fully on at that voltage.

    [​IMG]
     
  20. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    The one hurdle is that a 60 amp load on a 4 volt battery may drop the supply voltage - to a level below the saturation of the mosfet. The load resistance will be 70 mOhms so on-resistance of the Mosfet needs to be well below that to stay cool in this circuit (granted, it is the best circuit possible with a Mosfet). As someone said above, a parallel bank of MOSFETS will be best - to minimize resistance.
     
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