Newbie with a 2N6277 transistor problem

Discussion in 'The Projects Forum' started by neville s, Jun 16, 2015.

  1. neville s

    Thread Starter New Member

    Jun 16, 2015

    I am new to electronics and have run into a problem with one of my first projects and am hoping someone can offer advice.

    I have built a simple voltage regulator using a LM317 and a variable pot. The output of the LM317 is connector to the base of the 2N6277 power transistor. I have connected a LED to the emitter of the transistor to check everything is working. So far all works great. I have also checked with a multimeter and I get behaviour as I expect. Led switches on and off and voltages are what they should be.

    Now comes the tricky bit. This is all powered by a 12V 100aH lead acid battery (Overkill, I know but I wanted lots of juice). When i replace the LED with a DC motor (which requires lots of juice) and connected in the same way as the LED I cannot get the motor to turn over. The multimeter shows correct voltage changes when i change the variable pot but nothing else happens.

    The motor is 9.6 volt RS540SH motor. When connected directly to the battery (i.e. at 12 volts) it runs without a problem. As soon as I step the voltage down to 9.6 volts with the LM317, nothing happens. Turning the variable pot up to full (i.e. giving about 11 volts) also does nothing.

    The motor needs about 9.6 volts and about 8 amps, the transistor should be able to supply this easily and the battery by itself has no problem at all.

    What am I missing?


  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    You have connected the power transistor as an emitter-follower, which is not the preferred method of how to boost the current from a LM317. Look at the applications section of the LM317 data sheets (check TI and others; I cant remember which one it is on) for the proper method of boosting the current. This is typical of the correct method, but it requires a PNP pass transistor. If you want to use an NPN transistor, you will need this compound connection.

    Be advised that the transistor will have to be on a huge heatsink. It will be dissipating P = i*e = 8* (12.6-9.6) = 24W, more if you intend to speed control the motor, which will require about a square foot or so of aluminum mounted vertically, or lots of fins... Without a heatsink, you likely vaporized your transistor in about the first 1/4sec.
    Last edited: Jun 16, 2015
  3. DickCappels


    Aug 21, 2008
    You are also missing a schematic :)
    Seriously, we have to make several assumptions in order to try and help you based on the functional block type description you provided. Please post a schematic.
  4. sheldons

    Well-Known Member

    Oct 26, 2011
    Unfortunately the crystal ball I hire out to our other members has been sent off for calibration and polishing. your schematics. ....