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Why is my transistor overheating?

  1. Apr 9, 2016 #1
    Okay so I am having this problem with a transistor overheating in a motor control circuit and I am not exactly sure why it is overheating. There is a PWM signal being used to power the transistor from an MSP430 microcontroller. I am using a standard 1N004 kickback diode for the motor and an external power source that does not go above 5V or 3A for the purpose it's being used.

    This is the datasheet for the motor I am using go to the RE-380 model for specs.
    http://docs-europe.electrocomponents.com/webdocs/0032/0900766b800320a6.pdf

    The transistor that is overheating is a TIP3055 apparently rated to way more than what we should be needing, the datasheet is here too, we are using the NPN:
    http://docs-europe.electrocomponents.com/webdocs/0c65/0900766b80c65646.pdf

    And here is a basic picture of the circuit I am trying to implement:
    https://www.dropbox.com/s/dj5gnzo1aagnv66/20160409_213437.jpg?dl=0

    It shouldn't be this difficult, I have literally tried everything!
     
  2. jcsd
  3. Apr 9, 2016 #2
    The base current seems insufficient! I would use a power buffer with a much smaller resistor or a Darlington pair/transistor or a good MOSFET. The choice depends on the PWM frequency.
     
    Last edited: Apr 9, 2016
  4. Apr 9, 2016 #3
    Brilliant advice, thank you very much.
     
  5. Apr 9, 2016 #4

    tech99

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    Do you have a big heat sink on the transistor?
     
  6. Apr 9, 2016 #5
    I don't, I did think of doing this but I kept thinking to myself that the transistor really shouldn't be overheating anyway.
     
  7. Apr 9, 2016 #6

    tech99

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    Gold Member

    Well it is delivering 2 amps and will have a saturation voltage of perhaps 1 volt, so that is 2 watts dissipation for half the time, say 1 watt total. Maybe it is not being driven into saturation - that will make it inefficient and run hot - so try increasing drive first as mentioned previously.
     
  8. Apr 10, 2016 #7
    This datasheet includes some characteristic curves for the transistor:
    http://www.onsemi.com/pub_link/Collateral/2N3055-D.PDF

    Have a look at the curves for collector-emitter voltage vs base current. At just 1 A collector current and ~20 mA drive, it's hardly even on. At more collector current, the majority of the power delivered to your circuit is probably just being dissipated in the transistor.

    As already mentioned, you need a driver circuit or another transistor.
     
  9. Apr 10, 2016 #8

    rbelli1

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    You could also use a gate drive N-channel FET. That would give you even better performance than any bipolar you could find.

    BoB
     
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