Voltage Reg. With Pass Transistor

Discussion in 'General Electronics Chat' started by mayhemsm, Mar 10, 2010.

  1. mayhemsm

    Thread Starter New Member

    Mar 25, 2009
    Hi I was wondering if anyone could look at this and tell me what I might be doing wrong, I seem to have killed the transistor.

    I was running it with a heat sink on both the LM317 and the transistor at about 12v / 4amps, and the transistor was hitting about 230 Deg. Fahrenheit in open air which is still inside the max temp for the transistor.


  2. SgtWookie


    Jul 17, 2007
    What was your input voltage to the 2N3055? 12v is the output, right?

    It has a thermal resistance from the junction to the case of 1.5°C per Watt, so the junction is considerably hotter than the case.
  3. mayhemsm

    Thread Starter New Member

    Mar 25, 2009
    Yeah 12v was the output, input was 15-16v.

    If I did over heat the junction would adding a fan likely be enough to keep it cool enough? Possibly adding a 2nd transistor?

    Also I've been looking at some other schematics, and I've seen several using a PNP type transistor like the schematic below rather than a NPN, is there any advantage to this?

  4. santonel

    New Member

    Feb 28, 2010
    Shouldn't 1 and 2 be your power in/out and 3 and 4 be ground? Or am I misreading the schematic?
  5. Audioguru

    New Member

    Dec 20, 2007
    Your first circuit used the 2N3055 transistor as an emitter-follower so the output voltage was poorly regulated.

    The next two circuits have the transistor as a current booster so the output voltage is properly regulated.
  6. Bychon


    Mar 12, 2010
    These circuits are right out of the text book. That means, "good". The only problem is in melting the transistors. Do you need help with the math for heat, temperature, and cooling?