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Greetings - Op amp problem...

Discussion in 'Electronic Basics' started by JohnB, Oct 12, 2003.

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  1. JohnB

    JohnB Guest

    Hello,

    I am building a Video detector circuit with a TL082. The goal is to
    detect a composit video signal and then turn on a relay. The circuit
    consists of two negative feeback amps in series which works great. IE
    the voltage goes to 11V when video is present and drops to zero when
    there is no video.

    My problem is as follows. I tie the output of the second amp into a
    2n2222 NPN transister intending to use it to drive the relay. The
    relay turns on but will not turn off when when the opamp tries to
    return to zero volts. the emiter is grounded , the base attached to
    the opamp output and the collector ties to the relay. The relay is
    then tied to +12V. Did I mension I am using +12/-12 powerrsupply...
    I want to keep things as simple as possiable.

    Ideas or suggestions?




    The circuit is two negative feedback amps
     
  2. Nigel

    Nigel Guest

    Where is the transmitter resitor tied (-12V or 0V)?

    Any resitors in your transistor circuit?

    What is the operating voltage/current of the relay coil?

    When you do get this working it is also advisable to put a reversed diode in
    parallel with the relay coil. This protects the transistor from damage from
    reverse EMF generated when the coil is switched on and off.

    My guess is that your have the emitter tied to ground. When you present 11V
    at the base, the transistor switches on because of the B-E voltage exceeds
    0.7V. If there is no base resistor, the circuit will try at set the emitter
    to 10.3V but it can't because it is tied to 0V.

    This means that the full 11V will be placed across the B-E junction which
    will certainly cause the transistor to switch on but may damage the
    transistor or have strange effects.

    I suggest you put a resistor between the op-amp and the base. This will
    carry the excess 10.3V and should be sized so that it provides sufficient
    base current which when multiplied by HFE of the transistor creates a
    collector current to meet the relay's requirements.

    e.g Rb = 10.3 / (HFE x Relay Operating Current)

    Make Rb a little smaller to allow for tolerances etc.

    If my assumption is incorrect please provide some more info on the
    transistor stage.

    Cheers,

    Nigel
     
  3. Gareth

    Gareth Guest

    Do you have a diode from the transistor end of the relay coil to the
    positive supply? A relay is inductive and when it is switched off
    quickly current will continue to flow in the coil in the same direction
    as before it was switched off, if you put a diode in, the current has
    somewhere to go (in this case back to the supply rail). If you don't
    provide a path for the current you could get a voltage spike which will
    damage the transistor.

    An alternative is to put a zener diode with zener voltage > the supply
    voltage but less than the maximum Vce for the transistor from the 2n2222
    collector to ground.

    +12V----+-------|
    ) -
    ( ^ diode
    relay ) |
    coil ( |
    --------+
    |
    |

    Does your op amp output ever go negative? - this could also damage the
    transistor.
     
  4. A resistor between the op amp output and the base is good.
     
  5. John Fortier

    John Fortier Guest

    Here's my suggested circuit:

    First, between the final TL082 output and the base of the 2N2222, place a
    diode to block negative going voltages. A 1N4248 is good here.

    Second, place the relay in the emitter circuit, with a 100 ohm resistor in
    series to ensure that the relay coil gets enough current to operate without
    any chance of burning out.

    Connect the collector to the positive supply and the resistor to earth.

    I've checked this in PSpice, and it should work

    May I suggest that a good move would be to go to the Cadence site,
    www.cadenmce.com, and download the, free, student version of
    PSpice/Schematics. You'll be able to try out all the various circuit
    variations and not build until you are pretty sure that your circuit will
    behave as you wish.

    good luck,

    John
     
  6. John Fields

    John Fields Guest

    ---
    Yeah. First, download the TL082 data sheet at

    http://focus.ti.com/lit/ds/symlink/tl082a.pdf

    and take a look at the gain-bandwidth of the opamp.


    Second, let us know what you mean by "composite video", and

    third, post a schematic of what you're talking about to a website
    somewhere or to alt.binaries.schematics.electronic.
     
  7. John Fortier

    John Fortier Guest

  8. JohnB

    JohnB Guest

    Thanks for your reply. I think the problem was that I usually use
    2n2222 as switches with TTL logic, IE much lower voltage. I do have
    the emiter tied to ground and I think the 11V accross the BE junction
    was too much. I replaced the 2n2222 with a TIP 122 and it works great
    now.


    I know. Actually Im using a 100uF cap because the relay was bouncing
    when being turned off. Is that a bad idea? Works nice.
    Definatly damaged.
    I will try this.
    Thanks!
     
  9. JohnB

    JohnB Guest

    Thanks for your reply.

    I have replaced the wimpy 2n2222 with a TIP122 and it works ok now. I
    think the 2n2222 might have worked if I had biased it correctly.

    Already have it.
    bandwith is not an issue. I rectify the ac with a small signal diode,
    resister and cap, before It gets to the first stage of the opamp.
    I believe the more technical term is NTSC. IE the type of video
    signal you would find on the back of a typical US VCR or DVD player.
    Thanks again.
     
  10. Nigel

    Nigel Guest

    Good to hear it is working now.

    The diode is the more traditional solution, smaller and cheaper than the
    capacitor, but if the you're happy with the capacitor and it works then I
    guess stick with it.

    I would still recommend a resistor between the op amp and the transistor
    base though.

    Cheers,

    Nigel
     
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