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Scr firing

Discussion in 'Electronic Basics' started by Robotnik, Mar 2, 2007.

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  1. Robotnik

    Robotnik Guest

    Hello all,

    We have been trying to build a prototype of a single-phase power
    controller.The amount of power to be output is controlled by a PID
    algorithm. The PID algorithm resides in the external flash associated
    with a processor. The output from PWM module of the processor controls
    the firing of the SCR. Let me add that the input to the PID algorithm
    are the current temperature of the heat lamps which are controlled by
    the SCR. The temperature is measured using thermocouple and acquired
    using SADC.The part till the output from PWM is complete.
    I am not sure how to proceed from there.We need an
    optoisolator for electrical isolation.And then there is the SCR
    firing circuit with a couple of SCRs connected in anti-parellel
    fashion. What else shall I need to consider the SCR firing circuit.
    Or do I need to change my plans and go in for Mosfet based induction
    heating.Could you please shed some light on the above problem.

    Thanks in advance,
    Robotnik
     
  2. Chris

    Chris Guest

    Hi, R. The easiest, most straightforward way to go is to purchase a
    solid state relay (SSR) that will handle your problem. SSRs have the
    optocoupler and thyristor built in. Heating lamps generally provide a
    more or less resistive power factor, so I'm not sure why you seem to
    be focused on inverse-parallel SCRs. For most loads, a triac-based
    SSR should do well.

    A couple of caveats here. First, to be safe, think 2 watts power
    dissipation in the SSR per amp of load current. Purchase a heat sink
    accordingly, use thermal heat sink compound to mate the SSR to the
    surface of the heat sink, and use forced air where appropriate. Make
    sure to purchase a SSR which is "random turn-on". *DO NOT* get one
    which is "zero crossing", unless your control algorithm takes that
    into account.

    Apart from that, you might get better advice if you described your
    problem a little better. For instance, what kind of heating lamps do
    you have, are you shooting for phase control, integral half cycle
    control, or proportioning on/off control of your load, and of course,
    what's the load power, current, and voltage?

    Note also that, if this is a school project, a SSR probably won't cut
    it -- you'll have to "show your work" rather than just buying a
    module.

    Feel free to post again.

    Cheers
    Chris
     
  3. Why do you want to do this? What would define a successful project? Why not
    an off the shelf solution?
     
  4. For SCR controlled AC, you need to sense zero crossing, and add a delay for
    phase modulation. For slow response like a heater, you can use PWM
    techniques, but only if the PWM frequency is much lower than the AC line.
    In other words, you just turn on the power for a few seconds, and then turn
    it off for a few seconds. You probably don't want to use the PWM module for
    this.

    Induction heating is something else altogether, using RF coils to heat a
    metal object. Heat lamps are more properly called radiant heating, using
    infrared to heat the object.

    For antiparallel SCRs, you need two isolated gate drive circuits. Pulse
    transformers are the simplest and cheapest, but may not be as reliable as a
    well defined gate pulse circuit, which uses a narrow high current pulse
    followed by a "back porch" of gate current just over the minimum turn-on
    spec. You also should be careful to avoid gate current during the quadrant
    where the SCR has reverse voltage applied to it.

    You can also use a single SCR connected to the + and - terminals of a full
    wave bridge, so you can use a single gate circuit. However, the diodes
    waste power and the SCR works twice as hard.

    For any SCR gate drive, be sure the isolation is sufficient. An
    optoisolator like 4N25 has isolation voltage of over 4000 volts, which is
    just enough for a 480 VAC circuit. If you build gate drive power supplies,
    make sure the transformers are rated for the mains voltage you are
    controlling. If you use DC/DC converters, isolation voltages are often only
    500V, which is just barely enough for 120 VAC. The isolation barrier must
    be designed to withstand high voltage transients on the power line without
    degradation. A breakdown will allow mains power to connect to your logic
    circuitry, with dangerous and catastrophic results.

    These concerns are another reason to use a commercially available solid
    state relay.

    Paul
     
  5. Your two main choices,m ere are phase controlled power (fast
    smooth and electrically noisy) or burst fire integral half
    cycles (slower, simpler, lumpier and electrically quieter)
    For quantity one, you best bet is an appropriate solid state
    relay. The line voltage isolation is included, both for the
    control signal and for the heat sink.

    There are instantaneous turn on types, if you want to use a
    small transformer to provide a line frequency zero crossing
    reference for the micro, so you can delay the firing pulse
    with software, for phase control. There are zero crossing
    turn on types for burst pulse control, where you need only a
    slow PWM digital output to control the average power.

    There are also units available that convert a variable DC
    control voltage or current into phase or burst pulse power
    control, if you have a DC control output available. The
    last kind are most expensive, but you will definitely spend
    more designing, building and debugging your own, if you are
    going to need only 1 or a few.
     
  6. Marra

    Marra Guest

    I have done PID loads of times over 25 years for motors, but its a
    similar principle for controlling heating.
    In all cases I used the mains as a reference and the output from the
    PID was scaled to provide a pulse of between 0 and 9ms. A triac was
    used in every project.
    I have to say that PID is not always to convert into a real project
    especially for a beginner.
    I found that often a bit of fine tuinin gwas required to get the
    critical damping just right.
    If its not right you will get hunting of the speed or the motor will
    grumble badly (not a problem with heat lamps)

    www.ckp-railways.talktalk.net/pcbcad21.htm
     
  7. Try also looking at:

    http://focus.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=slaa043a&fileType=pdf

    Might give some ideas.

    Jon
     
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