2n2222 for relay switching

Discussion in 'General Electronics Chat' started by Dritech, Jun 22, 2014.

  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Hi all,

    I am using a 2n2222 transistor for switching a 5V 185mA 27ohms relay. I calculated that the base resistor should be 270ohms. Why am I getting approximately 3.2V at the relay terminal and not a value close to 5V? 3.2V are not enough for switching my relay.
    Also, the 2n2222 transistor is getting quite hot.

    Any help or suggestion would be highly appreciated.
     
  2. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Got schematic?
     
  3. inwo

    Well-Known Member

    Nov 7, 2013
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    What is the base current or voltage ahead of the base 270ohm.
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    With the transistor saturated, you should have a voltage at the collector of ~0.3V, meaning that the relay should get 5V-0.3V= 4.7V.

    [​IMG]

    An logic-level NFET would be a better choice for a relay driver.
     
    Last edited: Jun 22, 2014
  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Is the transistor connected as an emitter follower or a saturated switch?

    ak
     
  6. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Hi, the 2n2222 have a base resistor of 270ohms which is connected to a micro. The collector is connected to the relay and the emitter is connected to ground.

    5V from a microcontroller.
     
  7. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    (5-.6)/270=.0163 A=16.3 mA

    Assuming Vcc=5 volts.
    (5-.4)/27=0.1704 A=170.4 mA

    Ok, looks about right, Ib*10 is about equal to Ic.
     
  8. Papabravo

    Expert

    Feb 24, 2006
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    Except the microprocessor cannot source 16 mA. It can probably only put the transistor into the linear range and that is why it is not turned on. What voltage is the processor putting out ahead of the 270 ohm base resistor?
     
  9. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Thanks for the replies. The PIC18f I/O can supply up to 25mA.
     
  10. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Note that when I changed the transistors to TIP122, the circuit worked (using same base resistor etc.). The problem is that I have a PCB design with the 2N2222 package so i cannot fit a TIP122.
     
  11. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    But not at 5V. When sourcing 25mA, the PIC pin wont even make it to 4V. Look at the data sheet.
     
    absf likes this.
  12. absf

    Senior Member

    Dec 29, 2010
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    TIP122 is an NPN darlington transistor. There are some darlington NPN in TO92 foot print eg. BC517. Just do a google search and see which one is easily available to you.

    Did you try MOSFET? I think that should be the right choice for your job.

    Allen
     
    Last edited: Jun 23, 2014
  13. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Guess again. While it may be rated to put out such a current without damage it does not mean the output will stay at 5V when doing so.

    Measure the PIC pin when doing this. Actually drawing the schematic and noting the voltage at every node is the best way to go.
     
  14. inwo

    Well-Known Member

    Nov 7, 2013
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    Measured?:confused:

    Then 2222 mis-connected or bad.
     
  15. Papabravo

    Expert

    Feb 24, 2006
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    It is easier for a pin to sink current than to source current. Are you sure it can source the required current and maintain the output at or near Vcc?
     
  16. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    We don't actually know what Vcc is. OP never said what it is.
     
  17. inwo

    Well-Known Member

    Nov 7, 2013
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    ..................................................................................................................
     
  18. MaxHeadRoom

    Expert

    Jul 18, 2013
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    +1
    After all the previous posts here on the subject, I am surprised why some still opt for a bi-polar for Pic or μp outputs?
    Max.
     
  19. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    My guess, based on my own education, is that we just follow our education. My curriculum spent a lot of time on BJT in lectures, homework and lab experiments. I had some instruction in FETs, but not anywhere as much as BJT and I don't remember doing any lab experiments with FETs.

    And inexperienced people like me don't really know the pitfalls of uC. We read on paper that it will do: "5 volts and 25 mA" and we take it for complete truth. Many times I look at datasheets and I know that I don't know what most of the stuff in the tables actually mean.
     
  20. MaxHeadRoom

    Expert

    Jul 18, 2013
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    It probably influenced mine ;).
    Having gone through the tube era, which is very close in concept to the FET I sort of had an affinity to it and the transconductance type of principle of a Mosfet.
    Max.
     
    Last edited: Jun 23, 2014
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