Help needed asap please! Basic Transistor problem

Discussion in 'The Projects Forum' started by tron, Apr 20, 2009.

  1. tron

    Thread Starter Member

    Apr 20, 2009

    I am working on a school project that needs me to make a transistor circuit.

    im using a tip102 npn transistor, here is a link for the spec sheet:

    when the base receives a 5V signal, the collector is hooked up to the positive end of a 12V car battery. this is connected to a solenoid that is then grounded.

    when i hook everything up there is no clicking from the solenoid (it isnt turning on obviously). what could this be? i looked at the datasheet of a tip102 and it seems fine.


    edit: the solenoid is 12V, 540mA, 6.5W
  2. markosillypig

    Active Member

    Jul 21, 2008
    can you tell me more about your project what it is foe and what does it have do do ?
    or have you a drawing or scamatic
  3. PRS

    Well-Known Member

    Aug 24, 2008
    Show us a drawing of your circuit. You'll get lots of help then. Otherwise we're in the dark.
  4. tron

    Thread Starter Member

    Apr 20, 2009
    sorry for not including a schematic. here is a crude drawing, i dont have multisim on my laptop so i did a quick mock up in paint

    the project is for an air shifting kit on a formula sae car. basically we have a microcontroller sending a 5V signal out and i want to turn the transistor on, complete the circuit for the solenoid, therefore activating a solenoid hooked onto a 3 way valve on an air tank. everything is all done except for this part.

    heres a quick drawing

  5. tron

    Thread Starter Member

    Apr 20, 2009
    i was thinking, would i need a pull up or pull down transistor? and on the spec sheet, it says the emitter base voltage is 5V. that means i need 5V to 'turn on' this transistor?
  6. Audioguru

    New Member

    Dec 20, 2007
    The max allowed emitter-base voltage is a reverse voltage and the darlington transistor is turned off. The max forward voltage to the base is about 2V and is like a dead short to the output of a microcontroller. The base current must be limited with a series resistor.
    A microcontroller "sends out" 5V when there is no load. Its max allowed output current is 25mA but when it is shorted it tries to output 60mA which causes it to melt or blow up.
  7. tron

    Thread Starter Member

    Apr 20, 2009
    Thanks for the reply. What would my solution be? Forgive me i am not a very advanced electronics student. Do you mean that according to the solenoid i have, i need to put a resistor on the base so it pulls the correct amount of amps? also i was thinking would i need a diode to prevent burning up the transistor?

    and finally, do you mean that i already blew my transistor up>?
  8. nucleargungus


    Apr 6, 2009
    I also searched for a solution to make TTL level logic turn on and off major loads.

    If you look at your data sheet you will prob see that Vce current is 500ma max or so. Try this circuit out. Buy a relay (shown below as a coil) that will support the current requirements of your solenoid. Use an ohm-meter. Divide 12volts by the resistance of your solenoids coil. Thats your current I=V/R. The diode in the circuit is REQUIRED! I know I've tried not using it. Not pretty.

    R1 = 1kohm (try different values)
    Diode = 1n4001 (suggestion, many will work)
    Transistor = 2n2222 (again a suggestion)
    Relay 12v coil, capable of withstanding your current requirements.
  9. SgtWookie


    Jul 17, 2007
    Robert's schematic should work for your circuit. For R1, you can increase it up to around 3k, but I suggest that you don't decrease it below 1k.

    Don't forget to use D1. Check the datasheet for your solenoid to make certain that it doesn't have one already built in. If it does, you will have to make certain that you connect the terminals properly.
  10. steinar96

    Active Member

    Apr 18, 2009
    The solenoid will attempt to maintain current trough it (inductive). So while the transistor is conducting the solenoid stores energy which it uses to try to maintain the current constant when it changes.
    What happens then when the transistor stops conducting is that the solenoid will induce a big voltage spike accross it's terminals in attempt to keep the current flowing and this voltage spike could destroy the transistor.

    The diode mentioned will prevent this from happening. The diode is a must :)
  11. tron

    Thread Starter Member

    Apr 20, 2009
    hello guys, im sorry i didnt update this project earlier but i managed to work out a solution! i looked at this article and it worked seamlessly the shift kit works perfectly and i couldnt be happier! i thought id post this in case someone else has the same problem i did and is searching for a solution.