Help with a simple on-pcb circuit

Discussion in 'The Projects Forum' started by sstbrg, Sep 28, 2009.

  1. sstbrg

    Thread Starter Active Member

    Nov 29, 2008
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    Well, I have this small project on a PCB and I'm using SMD components. I'm concerned about a relatively-high voltage part of it.
    My question is whether this digital switching circuit will work or not.
    I need to switch a bright 28v DC lamp, which I had for quite some time and decided to put for a good use, using low voltage. I tested this circuit with larger components on a matrix board which worked great, but I'm concerned about the SMD versions.

    Datasheets:
    http://www.fairchildsemi.com/ds/PN/PN2222A.pdf (MMBT2222A)
    http://www.st.com/stonline/books/pdf/docs/2156.pdf (LM124D)

    Circuit
    http://i34.tinypic.com/307onqq.png




    My other choice for a transistor is a BS170F MOSFET. Also SOT-23 package.
     
    Last edited: Sep 30, 2009
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I don't know why you're using the opamp like you are.

    The output will always be in saturation, trying to go to one rail or the other.

    If the output is high, it will put about 20mA through the base of the transistor.

    I don't know how much current your lamp draws. It's shown as an LED, but I don't know of any LEDs that are rated for 28v.

    What is the 3.3v input supposed to be, a signal from a computer or microcontroller?
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    896
    The circuit shown will have an output of about 23V, not 28V.
    If a Mosfet is used as an output source-follower instead of as the transistor emitter-follower then the output will be only about 20V.
     
  4. sstbrg

    Thread Starter Active Member

    Nov 29, 2008
    54
    0
    First of all, thanks for the replies!
    Looks like my first post wasn't written clearly...

    "The output will always be in saturation, trying to go to one rail or the other."
    The input can be either 3.3v or 0v. It's from an FPGA.

    "I don't know how much current your lamp draws. It's shown as an LED, but I don't know of any LEDs that are rated for 28v."
    As far as I know, 28v is the maximum rating - didn't try going above it since it's my only lamp (or LED). It shouldn't draw above 100ma of current according to my matrix circuit.

    "The circuit shown will have an output of about 23V, not 28V.
    If a Mosfet is used as an output source-follower instead of as the transistor emitter-follower then the output will be only about 20V."
    Both voltages are fine, I'm more concerned about whether the transistor can handle the power in the real world, practically.
     
    Last edited: Sep 30, 2009
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Power in a transistor causes it to heat up. Power is the volts across it times the current in it. It is simple to calculate the power dissipation of a transistor then look on its datasheet to see if it will melt.
     
  6. sstbrg

    Thread Starter Active Member

    Nov 29, 2008
    54
    0
    Yeah, well that's obvious, but the question is how will this transistor behave in the real world? What will be its Vce (or Vds) not on paper? Plus that lamp or LED that I hoped somebody here would recognize.
    Found a picture online, mine looks similar but it's red: http://i33.tinypic.com/2nsc7cj.jpg

    Thanks anyway, I should finish soldering it sometime next week or in two weeks and I'll see if it blows up :D
     
  7. rickmartin

    New Member

    Sep 26, 2009
    27
    1
    Your lamp is manufactured by Chicago Miniature and is called a "cartridge lamp". You can see it here. It is an incandescent lamp and not an LED. The maximum voltage across the MMBT2222A will be at maximum when the lamp is turned off, and minimum when it is on, but will be well below the absolute max spec of 40 volts for the transistor. The ON current is also below the 1A max for the transistor. The power dissipation is your only foe, then, and is helped by the fact that your op amp will be driving the transistor into full saturation (on and off only states will cause the transistor to dissipate less power than, say, 50% of full power for the lamp). The ON resistance of the transistor, then, is what will determine how much voltage is dropped across it when the lamp is on. If the ON current through the transistor multiplied by the ON voltage drop across it does not exceed 350mw, then you will be okay. Sorry, I can't give you more specifics.
     
  8. sstbrg

    Thread Starter Active Member

    Nov 29, 2008
    54
    0
    Thanks for the detailed post, especially for the details about the lamp! :)
    I guess I'll have to find out experimentally how much voltage will fall on the transistor. On paper it should be OK, because how much voltage can fall on it in saturation? Can't be too much, but I still don't have a real number. That's what I wanted to find out in this thread, from others' experiences with these SMD components.
     
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