Reverse Transistor?

Discussion in 'General Electronics Chat' started by nar, Dec 1, 2008.

  1. nar

    Thread Starter New Member

    Dec 1, 2008
    Hi I was wondering if there is a transistor comparable to the TIP42C (PNP Power transistor) that would work in opposite as a switch? (On with no base voltage)

    thanks & any replies are appreciated,


    Last edited by a moderator: Dec 1, 2008
  2. bertus


    Apr 5, 2008

    When you have two TIPS and drive them with opposite signals , they wil take over from one to the other.

  3. beenthere

    Retired Moderator

    Apr 20, 2004
    You can find an NPN compliment to the TIP42, but transistors are active devices. They have to have a voltage present to turn them on.
  4. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    Offhand, I don't think there is anything that will work without voltage. However, a PNP can be used to provide a voltage to a load when the input is low or GND, relative to a positive supply voltage. To turn off the xstr, the input must essentially be equal to the positive supply voltage.

    See attached for an example.
  5. SgtWookie


    Jul 17, 2007
    A TIP41C is the NPN complement to the TIP42C.

    Both the TIP41C and TIP42C require current flow through the base in order to provide amplified current through the collector.

    I suggest you remove your E-mail address from your post, as otherwise "spam-bots" will pick it up and your in-box will be loaded with garbage.
  6. nar

    Thread Starter New Member

    Dec 1, 2008
    Thanks for all the replies & they helped. I'm trying to make a continuous spark ignition relay circuit for use with model gas engines with small contact points. The problem was with the small contact area of the points, the amperage has to be low to keep from burning them up.

    Looking on the internet & with your answers I was able to get a circuit to work with the engine but there is stil bugs with it. The circuit I'd like to have work would be the one below. It would run very little amperage through the points & has a transitor to step up the power to a 12 volt coil relay. The TIP41 transistor is suppose to turn on when the relay circuit is broken, the TIP41 opens all the battery power into ignition coil.

    I've spent hours playing around with this & searching the internet for info but am still struggling. Any suggestions? I'd like to keep it simple as possible for me :)

    thanks & any replies are appreciated,

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  7. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    If the TIP42 is a PNP, and the 300 ohm is the base, then there is nothing to turn it off. Place a 10K resistor from the base to the +BAT (or to what I assume is the emitter on the right).

    This may not be the only issue, but it is one.
  8. nar

    Thread Starter New Member

    Dec 1, 2008
    Nomurphy thanks for the info & sorry about the drawing :) I'm still learning the sybols for everything. The base is to the left, collector middle & emmiter right. I see what you're saying about nothing to turn the TIP42 off but it seems like any drawing for model ignition you find on the internet doesn't have a bleed resistor. Do they neutralize themselves internally to off when the base is disconnected? I can see with adding a resistor to the base it would speed up the off time.

    thanks again for the input & hope to play with the circuit tonight.

  9. mik3

    Senior Member

    Feb 4, 2008
    There is a transistor which conducts current with its gate (like base for BJTs) at 0V. Its the FET transistor and to switch it off you need to apply a negative voltage with respect to its source (like the emitter on a BJT).
    As I see from your diagram you are using the transistors as switches. Why don't you use MOSFETs? Their major purpose is for switching applications.
  10. kvshetye

    New Member

    Nov 2, 2012
    Replying to old thread for anyone searching for the similar function.

    Both can be NPN

    - Kailas Shetye