Need help controlling 14 volts 10A with transitors.

Discussion in 'The Projects Forum' started by atokatim, Jan 13, 2008.

  1. atokatim

    Thread Starter New Member

    Jan 13, 2008
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    Hey everyone. I have been dabbling with electronics again and need help controlling 14vdc, 10A with a transistor or some kind of switch. I am using a micro controller to send out a positive output, but the highest i can get the output of the chip is 5vdc. I need to run a large 10 amp motor based on when the chip sends out the signal to turn it on. I do not want to use a relay because of the size of my box it is going in. And help would be greatly appreciated!

    Thanks!
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    If it's just an on/off control, a transistor can switch voltage onto a FET gate that can handle the current. It's late, but I'll post up a circuit if nobody beats me to it by tomorrow.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    Can you switch the ground to the motor on and off instead of having to switch the positive side? It's OK if you can't; it's just more efficient if you can.

    Will you need to reverse the direction of the motor, or just turn it on and off?
     
  4. atokatim

    Thread Starter New Member

    Jan 13, 2008
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    well, the motor is already grounded to the case so I have to have only positive going to the motor. I cannot remove the case either. It would be simpler to be able to just run positive to the motor instead of changing the way the power connected. In order to test the circuit before I mount everything, I need to just test it on a 12v light and see if reacts correctly. The motor just needs to turn on or off.

    I thought FET transistors lock once turned on. It has been awhile so I am really rusty on alot of this. Thanks for helping me!
     
  5. atokatim

    Thread Starter New Member

    Jan 13, 2008
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    oh yeah... one more thing. the voltage coming in from the power supply to control the circuit and the motor has to be right at or a little under the original voltage on the output to go to the motor.
     
  6. nanovate

    Distinguished Member

    May 7, 2007
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    You are thinking of thyristors.


    Here is an example of what you could do.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    No, SCRs stay "on" until the current drops to zero, regardless of control input. JFETS and MOSFETS can be switched on and off pretty easily. They do have a fair amount of capacitance on the gate to overcome, but that's hardly even worth considering for a strictly on/off situation.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Yes, have a look at the schematic that nanovate posted.

    If your MPU is TTL, use a 2.7K resistor for R2; if it's CMOS, use 10K.
    The NPN transistor can be a 2n2222, a 2n3904, basically almost any switching NPN transistor.
    Use a 1K resistor for R1.
    The MOSFET is a P-channel. I'll suggest using an IRF4905; it has a nice low on resistance of around 0.02 Ohms, can handle up to -55V, is rated for up to -74A and comes in a TO220AB package; easy to work with. You should use a heat sink on it anyway, just to keep it's resistance as low as possible.
     
  9. hgmjr

    Moderator

    Jan 28, 2005
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    Nanovate,

    The drain and the source of the mosfet appear to be reversed.

    hgmjr
     
  10. atokatim

    Thread Starter New Member

    Jan 13, 2008
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    so my best bet would be to find an FET transistor that can handle 10A constant?
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    I already recommended one in my prior post, an IRF4905. It's a P-channel power MOSFET. It's somewhat overkill for what you're doing, but you can buy one for a couple of dollars - and it's not likely to break, either.

    Note hgmjr's comment; the source and drain are indeed reversed on the schematic. It won't work very well like that.
     
  12. nanovate

    Distinguished Member

    May 7, 2007
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    That's what I get for staying up late ;)
     
  13. atokatim

    Thread Starter New Member

    Jan 13, 2008
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    Ok, i figured out that i could use a smaller switching NPN transistor to control a larger PNP transistor rated at 40V 10A, but now I ran into the problem that There is not enough current to run the motors. I attached an image with part of my schematic. the output of the chip is around 5vdc.
     
  14. Audioguru

    New Member

    Dec 20, 2007
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    Your NPN transistor is completely wrong.
     
  15. atokatim

    Thread Starter New Member

    Jan 13, 2008
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    what is wrong with it?
     
  16. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
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    The emitter of your NPN should be tied to -. The collector and collector resistor should be tied to the base of your power PNP. The other end of the NPN collector resistor should be tied to +.

    Check here for more information: http://www.allaboutcircuits.com/vol_3/chpt_4/5.html
     
  17. Dookie

    New Member

    Jan 18, 2008
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    You can also use a good old relay . No loss at all , but put a "spark-eating" diode across the contacts . Use a transistor to separate the MCU from the relay coil . ;)
     
  18. hgmjr

    Moderator

    Jan 28, 2005
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    Though a relay is one way to go, the original poster specifically indicated that they did not want to use a relay.

    hgmjr
     
  19. Dookie

    New Member

    Jan 18, 2008
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    Sorry! :rolleyes:
     
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