TO-92 PNP recommendations for H-Bridge

Discussion in 'The Projects Forum' started by TerranDaniel, Oct 21, 2011.

  1. TerranDaniel

    Thread Starter New Member

    Oct 21, 2011
    2
    0
    Hello - first time poster here, with some basic experience in electronics but stronger on the computer side (CompTIA A+).

    The issue
    : I'm looking for PNP transistors capable of handling 300-500 mA @ 2.3 V DC, for building a small H-bridge to drive a DC motor.

    The project: I'm trying to design and build a simple, tiny robot car. It's another version of the common design where the car drives forward until it hits something, then reverses and turns, and then drives forward again. It would be easy, but the entire car is about the size of a dollar bill. It used to be a novelty wireless mouse, so size matters. That's why I'm avoiding MOSFETs.

    What I've done so far: Originally I had an H-bridge using only NPN transistors (attached), but the motor runs at different speeds depending on which side is active (Q1, Q4) or (Q2, Q3). So now I'm looking at the PocketMagic design. The control circuit will be a 555 monopulse sending a 1 or 2 second signal to the "reverse" side of the H-bridge.

    Circuit voltage: 3V
    Motor: 1.7v DC, 6Ω.

    Thanks in advance! I will reply, but it may take time depending on my schedule.
     
    Last edited: Oct 21, 2011
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    A 2N4403 is a 600mA PNP transistor in a TO-92 case. It is available all over North America.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Radio Shack sells two transistor assortments in 15-packs; one PNP and one NPN
    The NPN assortment generally contains:
    5 - 2N2222 transistors (rated 800mA, good for up to ~500mA)
    5 - 2N4401 transistors (rated 600mA, good for up to ~350mA)
    5 - 2N3904 transistors (rated 200mA, good for up to ~100mA)

    The PNP assortment generally contains
    5 - 2N2907 transistors (complementary to the 2N2222)
    5 - 2N4403 transistors (complementary to the 2N4401)
    5 - 2N3906 transistors (complementary to the 2N3904)

    These transistor assortments are probably one of the better values that Radio Shack carries, even though they're still pricey.

    I'm attaching a schematic that is similar to the PocketMagic design, but it uses CMOS TLC555 timers as Schmitt trigger inverters & transistor drivers. These CMOS 555 timers can operate with much lower voltage than the transistorized versions can. They have a weakness though, and that is the limited amount of current they can source; 10mA - they can sink up to 100mA. This design gets around the 10mA source limitation by avoiding using the timer output to source current at all; they only sink current.

    Another convenient thing for you is that it only requires a single logic input of 3v to run one direction, and 0v to run the other direction. This will work nicely with your one-shot 555 timer (you'll need to use a CMOS timer for that as well).

    The weakest spot in this design is the 1N914/1N4148 diodes. Radio Shack just doesn't carry much in the way of diodes. The 1N400x and 1N540x series that they DO carry are just terribly slow; they are OK for rectifying 60Hz AC, but lousy for things like this.

    You could double up on the diodes; use two for each position. Radio Shack sells 1N914/1N4148 diodes in a 10-pack. Either that, or order some Schottky diodes from someplace like Mouser or Digikey. 1N5817 thru 1N5819 would work very well.
     
  4. TerranDaniel

    Thread Starter New Member

    Oct 21, 2011
    2
    0
    Thank you, gentlemen! I'll try to post a video or pictures when the project is done.
     
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