power output problem

Discussion in 'General Electronics Chat' started by dcd528, Jul 31, 2011.

  1. dcd528

    Thread Starter Active Member

    Jul 28, 2011
    31
    1
    I attached a sketch of the circuit. I have a 555 timer that is in single shot mode. When I trigger it the output biases a 2N3903 transistor. This transistor has +5V connected to the base and the emitter biases a TIP3055 transistor. There is a nichrome coil with a +9V connection and the other side of the coil is connected to the collector. The emitter is connect to ground through a 39 ohm resistor. The power to the coil cycles to match the timer output but the coil doesn't get enough power to heat up. If I remove the 39 ohm resistor the coil turns on when I trigger the timer and heats up but it never turns off.
    What I want to happen is the coil turns on and off in synch with the timer pulses and heats up. I'm stumped. Any help would be appreciated.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You have probably fried the 2N3903 transistor; it's likely shorted collector to emitter. You are using it as an emitter follower, so it dissipates a good deal of power.

    When the 39 Ohm resistor was in the path from the emitter of the 3055 to ground, the base and collector current of the 3903 was limited by it. When you removed that resistor, there was probably over an ampere of current flowing through the collector when the 555 output went high, and the 3903 was trying to dissipate several Watts of power as heat. It's only rated for 625mW maximum.

    The 555 timer is actually capable of more output current than the 3903 is. However, you still need to limit the maximum current output using a resistor.
     
  3. dcd528

    Thread Starter Active Member

    Jul 28, 2011
    31
    1
    When the timer goes hi the voltage through the 3903 goes to 4.4V and when the timer goes low the voltage through the 3903 goes to 0V (well a few mV). I am measuring the voltage from the 3903 emitter to ground. Doesn't this mean the 3903 isn't fried? What is the best way to get this working? Is my basic design right? What am I missing?
    thanks so much for replying.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Well, maybe it's not shorted then.

    Try placing a 20 Ohm resistor between the 3903 emitter and the 3055 base; and remove the 39 Ohm resistor.

    20 Ohms between the emitter and base will limit the 3903 current to a safe amount, and should turn on the 3055 reasonably well. If you're still having problems with the 3055 not turning off, then connect a resistor of about 100 Ohms from the base of the 3055 to it's emitter.
     
  5. dcd528

    Thread Starter Active Member

    Jul 28, 2011
    31
    1
    I added a 1K resistor between the 3903 and the base of the 3055 then a 100K from the base of the 3055 to ground. Now the coil heats.
     
Loading...