Connect with us

I'm not biased, and neither is my transistor

Discussion in 'Electronic Basics' started by Phideaux, Jan 11, 2004.

Scroll to continue with content
  1. Phideaux

    Phideaux Guest

    It's amazing how the simple things seem to elude me. I've set up this
    circuit to drive a relay from a CMOS gate output.

    When the gate goes high, it puts 10.8V on the 1K resistor to base.
    The transistor just sits there. No current flow from collector to
    emitter. Yes, they do use the same power supply, i.e. they share the
    same ground and source. No glow from the LED. Nadda. FYI, I can't
    seem to grasp the art of oscillators either. Maybe a torch is needed
    to bring some warm to this transistor? What have I done wrong?
  2. Tim

    Tim Guest

    Have you tried driving the base of the transistor from the power supply
    rather than through the CMOS gate? Does it work that way? Most CMOS gate
    outputs only drive small current devices. Could that be the challenge?
  3. Your diagram appears to be using a transistor correctly. So I suspect
    that your circuit is not actually connected the same as your diagram.
    Measure the voltage on the base, emitter and collector pins with
    respect to ground during operation and get back to us. A digital
    photo of the circuit posted to the web site might also be helpful. I
    made made silly errors on circuits and absolutely could not see them
    until someone else pointed them out.
  4. John Larkin

    John Larkin Guest

    The CMOS source may not be able to actually put 10 volts into a 2K
    load, and the assumption of b=200 is way optimistic.

    Put a voltmeter across the base and collector resistors and calculate
    the actual currents to see what's going on.

    Figure this out - quantitatively - before you tackle oscillators.

  5. Ian Bell

    Ian Bell Guest

    Indeed, most people forget beta depends on vce and at saturation, which is
    where this circuit should end up, beta is probably little more than 20.

  6. So many things....

    2N1104 is a ANTIQUE!
    It is INCAPABLE of Hfe over 150.

    You need to graduate to power FETs.
    Even the simplest Power FET will do this job with room to spare.
    For example, IRF210.
    No gate (Base) resistor needed, and CMOS will drive it just fine.

    | __O Thomas C. Sefranek
    |_-\<,_ Amateur Radio Operator: WA1RHP
    (*)/ (*) Bicycle mobile on 145.41, 448.625 MHz
  7. Phideaux

    Phideaux Guest


    Does it work that way?


    Most CMOS gate outputs only drive small current devices.

    True, sad to say, but true none the less.

    Could that be the challenge?

    It could prove to be one, but I don't think so in this case.

    All good points Tim. I quess it's quite often the simple things that
    we overlook. In another circuit, I miswired one lead of an IC and the
    circuit that had been working quit. It took 30 minutes of going over
    it for me to find it. In this case, I think there's something wrong
    in my head!
  8. Joe

    Joe Guest

    Hi Phideaux,

    Are you sure you have the part number of the transistor correct? I did a
    google search and checked a couple of catalogs. No sign of a 2N1104. The
    closest I found was a 2N1100 germanium pnp transistor.

    Assuming it really is a npn transistor, why the 120 ohm resistor in the
    collector line? Also, what type of cmos gate is driving it?

  9. Tim

    Tim Guest

    I could not find a pinout of the 2N1104, so I got a cross-reference from
    NTE. Is this the pinout of the transistor?

    Are you certain you have it connected correctly? If so, have you tried using
    the 12v power supply to activate the base to light the LED by itself?
    (remove the 120ohm resistor/relay)

    If that does not work, have you tried jumping the 2N1104 collector and
    emitter with a jumper wire to insure the LED and relay work when hardwired?

    After that, it will be voltage checking with an o-scope or voltmeter. I
    would recommend getting the LED/transistor circuit working first. Then add
    the relay and get it to work, then the CMOS gate.

    One step at a time...
  10. John Larkin

    John Larkin Guest

    It's a TO-5 silicon NPN, actually not a bad part for its age.

  11. JeffM

    JeffM Guest
    I'm with TCS on this one.
    Microamps beat milliamps and there's fewer parts to buy/fail.
    BTW, what's with that 120 ohm shunt?
    Pull that sucker out first thing.
  12. Phideaux

    Phideaux Guest

    You are correct about the 2N1104 being an antique. Looking at your
    call sign, WA1RHP, you may be old enough to appreciate MARS program
    refuse. Gee, I must have 1,000 or so lying about, so they are handy,
    plentiful, and most importantly FREE. Perhaps you have made the
    aquaintence of an old friend of mine, Paul Childress, WA5JIY? I have
    him to thank for the abundance of antiquated surplus! Anyone in need
    of a fan cooled 500W dummy load, in a water tight metal case? (Gives
    new meaning to the phrase Old Navy!) Or silver plated variable air
    gap capacitors? The list goes on and on. I still would like to
    master the art of biasing transistors. Old Paul tried to teach me,
    but I was young and not wanting to think much about serious matters
    back then. Youth, it is indeed wasted on young people.

    But alas, I think I'll take your lead about the Power FET. It'll get
    the job done, I just hope Radio Shack has them at the store. And
    thanks, the trip down memory lane has been good.
  13. Phideaux

    Phideaux Guest

    Joe, it is quite old, and it is an NPN silcon. The "7" digit output
    from a CD4017B decade counter is driving it.
  14. Yea, Free is often tempting...
    Sorry, out of the 650,000 hams here, I know a lot of them, but not Paul.
    You will get many requests for this, I lust after these kinds of loads.
    I have some of these.
    Send me your questions via E-Mail.
    | __O Thomas C. Sefranek
    |_-\<,_ Amateur Radio Operator: WA1RHP
    (*)/ (*) Bicycle mobile on 145.41, 448.625 MHz
  15. Michael

    Michael Guest

    How come nobody has told you to put a diode accross the relay coil to
    prevent back EMF blowing up the transistor?

    Connect a 1amp diode, with cathode (stripe) to the + side of the coil.

    If your circuit never worked then this probaly isn't your initial
    problem, but is still required.
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