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Need Some Help Finding the Right MOSFET

Discussion in 'General Electronics Discussion' started by Supercap2F, Apr 11, 2015.

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  1. Supercap2F

    Supercap2F

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    Mar 22, 2014
    Hello Everyone!! :)

    Well I'm working on a project that uses a single Li-poly battery for it's power. But unfortunately, I could only find a Li-poly cell that has no protection circuity on board that would fit my needs (Here).

    So, I need to design an under voltage protection circuit. I don't need any over voltage protection, as I'm just going to charge them up with something like this. First, I thought that I could just find a chip to do the protection, but I couldn't find any that would just do under voltage protection. I also need the protection circuit to drop hardly any voltage at all, because of 3.3 volts power supply requirements of the rest of my circuit.

    After doing a whole lot of thinking, I decided to try something like this:
    REG.JPG
    So the way I would think this would work would be: power comes into the LDO regulator from the Li-poly battery at Vcc. Then the MOSFET pulls the Enable pin high, which then turns on the regulator. But then when the battery drops to about 3.45V the MOSFET stops conducting and R14 pulls the Enable pin low, turning off the regulator and the rest of the circuit (at 3.45 volts the regulator is almost to the point where it won't work anymore due to it's 0.13V drop out voltage).

    Now the problem is, I have no idea how to select the MOSFET, or even if it's around the right way! :confused::p I wonder if someone could point me in the right direction as to selecting it? Or even if this design is plausible!

    Thanks very much Everyone!!! :)
    Dan
     
  2. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Hi Dan
    Why could you only find a battery without protection? Batteries with protection have been qualified to be safe under most conditions. Please reconsider. What battery spec do you need.
    Cheers
    Adam
     
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  3. Supercap2F

    Supercap2F

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    Mar 22, 2014
    Hey Adam!
    Well I looked just about everywhere that I trusted. The whole project actually uses two batteries, one for some motors and one for the controller circuit. The motors will draw about 3A max, and they don't go through this part of the circuit. The controller circuity that will be going through the regulator will draw about 2A max. I would like to have at least 1000mAh capacity too.

    Those 1000mAh batteries I found have a 20C rating, which means they can have about 20A drawn out. They're just the perfect size too...
    Thanks Adam! :)
    Dan
     
  4. Arouse1973

    Arouse1973 Adam

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    What voltage Dan?
     
  5. Supercap2F

    Supercap2F

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    Mar 22, 2014
    For the controller circuit: 3.3V
    For the input of the regulator: 9V or less
    Dan
     
  6. Arouse1973

    Arouse1973 Adam

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    Can you use one battery for all?
    Adam
     
  7. Supercap2F

    Supercap2F

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    Mar 22, 2014
    Well I would like to have separate batteries since the motors will cause a whole lot of noise on the power lines.
    Dan
     
  8. Arouse1973

    Arouse1973 Adam

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    So for Lipos you got a few choices as you you know. 4.2, 8.4, 11.6 volts. You need to choose one.
    Adam
     
  9. Supercap2F

    Supercap2F

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    Mar 22, 2014
    Well the problem with the 8.4V ones now that I think about it, would be increased weight. So the 4.2V ones would be the best for this project.

    Thanks again for your time Adam!
    Dan
     
  10. Supercap2F

    Supercap2F

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    Mar 22, 2014
    Hey Adam! Where you able to come up with anything?
    Thanks
    Dan
     
  11. Arouse1973

    Arouse1973 Adam

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    Hi Dan
    So you need a 4.2 V lipo rated at 1000 mA/h?
    Adam
     
  12. Supercap2F

    Supercap2F

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    Mar 22, 2014
    Yeah pretty much. :) Less then 8x5cm in size would be about right too.
     
  13. Arouse1973

    Arouse1973 Adam

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  14. Supercap2F

    Supercap2F

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    Mar 22, 2014
    Well that one only has a max discharge of 2 Amps, I need at least 3A for the motors...

    Edit: I take it back! I actually need one the can discharge at at least 3.2A, turns out the motors have a 1.6A stall current. :oops: Sorry about that!
    Dan
     
  15. Arouse1973

    Arouse1973 Adam

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    Ok so you might need a different battery. Have you looked at model RC batteries?
    Adam
     
  16. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
  17. Supercap2F

    Supercap2F

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    Mar 22, 2014
  18. Arouse1973

    Arouse1973 Adam

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    Yes it will have inside the pack. Any batteries that have a set of wires and or connector when sold must be qualified. It's the law over here.
    Adam
     
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  19. Supercap2F

    Supercap2F

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    Mar 22, 2014
    Hmm. Okay, that one looks like it might work then. But I don't like how its coming from Hong Kong (sending payments over their kind of creeps me out :D)... I'll see if I can find one like that from somewhere local.

    Thanks again Adam!!! :)
    Dan
     
  20. Arouse1973

    Arouse1973 Adam

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    Yeah no probs it was an example to show you what's out their. Check with your local hobby shop.
    Adam
     
    Supercap2F likes this.
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