Connect with us

Transistor assisted Ignition

Discussion in 'Electronic Basics' started by Jack Jet Williams, Dec 4, 2003.

Scroll to continue with content
  1. Would someone PLEASE tell me how to wire a transistor to fire a
    ignition coil so the breaker points do not wear out so fast ? I have
    looked for circuit diagrams but have not found any.........Someone
    please help me.


    Jack Jet Williams
  2. The transistor is used as a switch, in the place of the contacts. The
    contacts would never have to be used, unless you decide to use them at low
    voltage to provide the sensing signal that tells the transistor when to fire.


    Chip Shults
    My robotics, space and CGI web page -
  3. Hi Chip,
    thanks - what I want to do is install this circuit on my 1974
    Honda 750 motorcycle - the points burn and pit pretty quickly - I know
    that a transistor could switch the the coils - then the current across
    the points would be in the milliamp range - and they would last for a
    long time - instead of the 8 amps. now across do you
    connect the transistor to the points ?

    Jack Jet Williams
  4. Would someone PLEASE tell me how to wire a transistor to fire afire. Cheers! Chip Shults
    Do you have the module? What type? Most newer ICM triggers a spark when
    grounding the input. Older ones when releasing (removing) ground. Use ICM
    (ignition control modules, aka igniter) as they range from tiny to large
    ones at the salvage yard. Unlike transistors, they limit current (lengthen
    coil's life) and produce sufficient current each cyle.

    The induced voltage is also determined by the rate of collapse, which is
    determined by the speed of the switching of the earth path. Another words,
    you will notice a bigger spark using the ICM.

    The bottom picture, figure 1, is a good replica block circuit of the
    reliable NEC for Honda ICM.- reluctor controlled. This one produce a spark
    when S1 is cut short of ground. S2 is left empty.

    Below is a picture, no internal circuit, of a 90's ICM computer triggerred.

    This one is a Toyota, computer triggered, block circuit.

  5. The points typically ground something (like the coil) when they close. This
    is the key- use a transistor that switches on when its base is taken low. A PNP
    bipolar power transistor would be a good solution.
    Connect the base of the transistor to two resistors- one is a fairly high
    value one, like 2.7 K and goes to the 12 volt supply. The other is perhaps 120
    ohms and goes to the contacts. What will happen is this. As the contacts are
    in the open state, the base is pulled to the 12 volt side and the transistor is
    turned off. When the contacts close, the base is drawn to the low side (ground)
    and the transistor switches on.
    Now, the emitter of the transistor will be connected to the coil to be
    driven, and the collector will be connected to ground. It is a good practice to
    place a series resistor (like 1 or 2 ohms) in series with the emitter to keep
    the current limited.
    The coil, incidentally, is an inductive device and will likely blow out a
    transistor the first time you fire it this way. The solution is to place a
    diode (sometimes referred to as a "snubber") in parallel with the transistor-
    across the emitter and collector.
    Use a fast rectifier for the diode and wire the cathode directly to the
    transistor's emitter. The anode of the diode will go to the collector.
    This will give you a circuit to play around with and see how it will work
    for your application.


    Chip Shults
    My robotics, space and CGI web page -
  6. Thanks Chip ,

    This 750 Honda is a 1974 model - it has no electronic module at all
    - the
    points are wired in series with the coil- and a capsaitor(did I spell
    that right?) is across the points - VERY simple - thanks for telling
    me how to do this with a transistor.

    Jack Jet Williams
  7. Pesovic Uros

    Pesovic Uros Guest

    Your braker points are wearing out because of capacitor.It.s maybe to small
    or it's not functioning.Place the new one(it should be 0.25uF).It's not
    smart to use PNP tanzistors.There are many NPN transistors made especialy
    for car ignition systems(with build-in protection against spikes made by
    coil).Some of them are:BU323, BU920, BU921, BU922, BUV37B (TOP3), BUW29,
    BUW81, BUX37, MJ10012, MJ10013, MJ10014, TIP662, TIP665, 2SD683, 2SD685.
    The transistorised ignition need to be made very good,need to be
    waterproof,sealed.If it's not build propery it can brought you many
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day