Look for better transistor by reading datasheet

Discussion in 'Analog & Mixed-Signal Design' started by Kelvin Lee, Nov 9, 2018.

  1. Kelvin Lee

    Thread Starter Member

    Oct 22, 2018
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    Dear Sir/Madam,

    I connect a PN2222A NPN transistor and a variable resistor at 1K, the motor on but the transistor is very hot. I think I need to replace another model transistor but I have no idea which model should be chosen when I reading the datasheet, which information I should focus? Can anyone suggest me the model of the transistor with an explanation by the figure from datasheet?

    Should I need to replace 1K resistor with other resistance if I change the transistor, how much I need? How do I calculate the resistance need?

    Best regards,

    Kelvin.
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    How much current does the motor draw?
    For good switching, the base current of the transistor should be about 1/10 of that current.

    Bertus
     
  3. Kelvin Lee

    Thread Starter Member

    Oct 22, 2018
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    0
    Dear Bertus,

    If 3V power directly connects to the motor, it requires 600mA to start and then around 290mA to run. There is no other load to the motor, later will add a fan blade.
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    At startup, the motor is hitting the absolute maximum current allowed for the PN2222:

    PN2222_ratings.png

    Better look for a transistor that can handle a bit more current.

    Bertus
     
  5. Kelvin Lee

    Thread Starter Member

    Oct 22, 2018
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    0
    Dear Bertus,

    Thanks for your reply. I tested with a fan blade installed to the motor, it requires around 800mA to start. According to your suggestion, I should look for a transistor with Ic more than 800mA, right? If 800mA is required, then the base current should be around 80mA, so the resistance should be around 39 Ohm, am I correct?

    May I know the base current is normally at 1/10 of collector current for every transistor? Is it the rule?

    Best regards,

    Kelvin.
     
  6. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    For hard switching the 1/10 is a rule of thumb.
    It will force the transistor in saturation, so there is less voltage accross the transistor to avoid heating.

    Also a kind of rule of thumb is that the switched current should be about half the maximum allowed for the transistor.

    Bertus
     
  7. Kelvin Lee

    Thread Starter Member

    Oct 22, 2018
    52
    0
    Dear Bertus,

    Many thanks for your patient to explain to my question, because of my unclear concept, do you mind quoting some example about half the maximum allowed for the transistor? At what situation I should consider 1/10 or half the maximum allowed?

    Best regards,

    Kelvin.
     
  8. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    When you want to switch 600 mA, the base current should be about 60 mA.
    The transistor should have a maximum current of at least 1200 mA.

    Bertus
     
  9. Kelvin Lee

    Thread Starter Member

    Oct 22, 2018
    52
    0
    Dear Bertus,

    Then how do I read the datasheet to know I find the correct transistor?

    Best regards,

    Kelvin.
     
  10. dl324

    AAC Fanatic!

    Mar 30, 2015
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    He already showed you. Look at the maximum ratings.
     
  11. Kelvin Lee

    Thread Starter Member

    Oct 22, 2018
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    Dear dl324,

    Sorry I don't understand the datasheet, do you mean 600mA is the maximum rating for PN2222A transistor, so the maximum current to the load is better less than 600mA but the transistor maximum allow current will be 1.2A but it will be very hot.

    What will happen if the load is over 600mA or 1.2A to the PN2222A?

    Best regards,

    Kelvin.
     
  12. dl324

    AAC Fanatic!

    Mar 30, 2015
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    It depends on duty cycle. At 100% duty cycle, the 2N2222A is rated for 600mA. Conservative designers would derate that to improve reliability. For hobby circuits, you can operate at 100% ratings if you want.

    The datasheet I have doesn't give data for peak currents above 600mA, so you need to do power dissipation calculations and insure that junction temperature doesn't exceed the maximum allowed. You also have to worry that the bond wires can carry the current you desire; if it's over the maximum continuous rating.
     
  13. dl324

    AAC Fanatic!

    Mar 30, 2015
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    This is curious. I checked another datasheet (FSC) and it gives a higher maximum current:
    upload_2018-11-9_11-7-6.png
    upload_2018-11-9_11-7-32.png

    EDIT: I see that FSC states that 1A is absolute max; which the device isn't guaranteed to survive. And OnSemi lists 600mA as a maximum rating.
    upload_2018-11-9_11-11-31.png
     
  14. crutschow

    Expert

    Mar 14, 2008
    20,065
    5,651
    The common 2N3055/TIP3055 power transistor would be more than adequate for your requirements.
    It has a 15A maximum current rating with maximum gain at around 0.3A.
    For the maximum startup current, a gain of 20 (base current = Icmax/20) should be sufficient.

    upload_2018-11-9_11-20-49.png
     
  15. Kelvin Lee

    Thread Starter Member

    Oct 22, 2018
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    0
    Dear dl324,

    Many thanks for your information, yes, sometimes we find like this, so it is very confused to our beginners to understand.

    Best regards,

    Kelvin
     
  16. Kelvin Lee

    Thread Starter Member

    Oct 22, 2018
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    0
    Dear crutschow,

    Very informative, may I know how I should interpret the statement DC Current Gain − hFE = 20−70 @ IC = 4 Adc from the datasheet of 2N3055?

    -hFE is 20 - 70, your equation let me understand how I can find how much of current I need to produce on base, at what situation we will do base current = Icmax/70 ?

    Best regards,

    Kelvin.
     
  17. Kelvin Lee

    Thread Starter Member

    Oct 22, 2018
    52
    0
    Dear dl324,

    What is the reference 1200mA from post https://forum.allaboutcircuits.com/...tor-by-reading-datasheet.153844/#post-1322662 ?

    Best regards,

    Kelvin
     
  18. Bordodynov

    Well-Known Member

    May 20, 2015
    1,923
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    See
    ZTX690B.png
     
    absf likes this.
  19. crutschow

    Expert

    Mar 14, 2008
    20,065
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    Those values of hFE are when you are using the transistor as a linear amplifier, not as a switch.
    When using it as a switch, an hFE value of 10-20 is typically used to insure the transistor is fully saturated.
    Notice that, for the saturation voltage measurement @4A, they use an hFE of 10.
    upload_2018-11-10_9-27-41.png

    I would use a value of about 20 for the maximum startup current, since that's well below 4A and its gain is significantly higher at lower currents.
    That will insure good saturation at the running current.
    Bordodynov, do you have an LTspice model for that transistor?

    Edit: The ZTX690B that Bordodynov posted looks like a good option.
    They use an hFE of 200 for the saturation tests.
    Bordodynov, do you have an LTspice model for that transistor?
     
    Last edited: Nov 10, 2018
  20. Kelvin Lee

    Thread Starter Member

    Oct 22, 2018
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    0
    Dear crutschow,

    I can now understand why most experts tell me to use 10 to divide the Ic and estimate the current need for the Ib, and then calculate the resistance value on Ib. It is because I use the transistor as a switch.

    Because of rule of thumb, that's why I cannot find the information from datasheet. Many thanks for your teaching.

    Best regards,

    Kelvin
     
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