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Triac Circuit Not working right ?

Discussion in 'Electronic Basics' started by Stacy, Nov 24, 2004.

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  1. Stacy

    Stacy Guest

    I have built a circuit based on. It uses a Triac and a Optoisolator 3031 to
    turn power on. Solid State Relay
    by Tony Van Roon

    I hope that someone can answer a few questions ( If you could be simple on
    your explanation as anything involved with get me lost)

    1. The circuit I thought is suppose to be a Latching circuit. That is, from
    my understanding, you apply the 5vDC for a second and the 120 light will
    turn on. When you remove the 5v DC the 120 Volt light stays on. This does
    not work for me. The 120 Volt light stays on only when I keep the 5 v DC
    applied. When I remove it the light goes out.

    2. The circuit does not work when I have the .01uf capacitor connected as
    shown in the schematic. That is, the light simply comes on and stays on.
    This is regardless of the 5 v DC.

    Can anyone help me to understand. Or perhaps if the circuit is wired up as
    described its suppose to exibit these symptoms ?

    It frustrates me as Im trying to learn that I think I have done all the
    right things, but then it doesnt work.

  2. John Fields

    John Fields Guest

    That's the way it's supposed to work. When you remove the 5V you are
    no longer supplying current to the LED in the opto, which disconnects
    the TRIAC's gate from its trigger source, the mains.
    Make sure you have the capacitor connected to the junction of R3 and
    R4 _only_, and that the gate of the TRIAC is _only_ connected to R5
    and IC1-4. The "hump" in the wire connecting C1 to R3 and R4 means
    that it jumps the wire it's crossing, not that it's connected to it.
  3. Stacy

    Stacy Guest

    Thanks John, I'm glad it works sorta the way it is suppose to. I think I
    have read about 5 or 10 of similar circuits and some of them they call them
    latching. I must have had that confused.

    Yes the "hump" is NOT connected. -thanks.

    When you say
    I do have the Cap between R3 and R4. From there the cap is connected to a
    common line that has the R5 / MT1 and load connected up to it. (bread

    ..01u R5 MT1
  4. John Fields

    John Fields Guest

    For your circuit, if the load is an incandescent lamp there's no need
    for the 2.2k or the 10k resistors or the capacitor, since what you
    have is what's called a "static switch" and you have no need for phase

    I think van Roon was confused and wanted to use the 2.2k and the 0.1µF
    as a snubber. Take a look at

    for some circuits that work.
  5. Look at the datasheet for the MOC3010. There is a sample circuit which
    could be better for powering a lamp.


    See Figure 6.

    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
  6. Stacy

    Stacy Guest

    Thanks I will do some experimenting with them.

    Ive done a little reading and found this ..

    To prevent sensitive gate devices from false triggering due to high rates of
    rise of off state voltage, 1 kW resistor in parallel with a 10nF

    capacitor may be fitted between gate and cathode (gate and terminal 1 for a
    triac). This approach is less effective for standard gate

    devices. In this case, the preferred option is to fit an RC snubber between
    anode and cathode (T2 and T1 for a triac) to reduce the dVD/dt below

    the critical value.

    I did read some other documents about snubbers before and wondered if this
    was the problem.

    So the questions I have is, When would the circuit that I started out with
    work exactly as drawn. Is the schematic wrong ? Is it for electric motors
    etc. That is, a non resistive load ?

    I suppose the begging question which you can't answer is why would an
    intelligent electronics person put a schematic out there when it does not
    work ? Or at best is not really going to work on an ordinary Incandescent
    Light Bulb which it describes as the load?

  7. There are *lots* of circuits that don't work on various web pages. There
    are also lots of circuits that work, but only marginally, with parts
    that may have different specs than your part.

    I'm guessing that Tony Van Roon tried the circuit out, and for the triac
    he was using (which is probably different than the one you are using) it
    worked fine. The amount of current required by his triac was possibly
    less that that required by yours.

    Another flaw with the 'van roon' circuit is that the circuit is powered
    at line voltage when the lamp is disconnected. If you touch it after
    you've unscrewed the bulb, you'll get shocked. Many circuits put the
    load on the high side (the Line side, black wire or small hole on the
    plug) for that reason. The center tap of the socket (which is hardest to
    touch) should be at Line, and the screw part should be at Neutral. The
    fuse should also be on the Line side, preferably before the lamp itself.

    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
  8. Stacy

    Stacy Guest

    Thanks for that. I did actually do some other circuits like the one you
    indicated. Although I used a 3031 (as I recall) The reading I did seemed to
    indicate that the 3031 does the Zero thing which minimized RF if there are
    TV's/ Radio etc around. At least that seem to be what I understood. I kinda
    wish that people would actually tell us the parts they used and then give us
    the theory as to how to arrive at the right part. This would help me anyway.
    If I can see exactly what they used. This way I can figure out their math
    and test mine and hopefully come to the same conclusions. When I finish a
    little project I write the my observations, and what chips I used that
    worked so Next time I dont have to expriment all over again.

  9. rayjking

    rayjking Guest


    You are seeing what happens when a circuit is built with trial and error or
    leaving out the most important parameter such as gate current sensitivity
    range for the triac. Also there are many thousands of folks that dabble in
    electronics that are successful in building one circuit that works. It is
    quite another answer for a circuit that is described in enough detail to be
    mass produced or copied with the expectation of working.
    The above issues are the difference between the digital designs and analog

  10. John Fields

    John Fields Guest

  11. rayjking

    rayjking Guest


    You are seeing that the circuit does not work as expected or suggested when
    the circuit was offered. There was not enough information to hint at the
    potential problem of gate sensitivity or special selection of components.
  12. John Fields

    John Fields Guest

    Most of us here reply to newsgroup posts by bottom-posting, so if you
    don't mind please follow that convention.

    You misunderstood my post. What I was questioning was your statement

    "The above issues are the difference between the digital designs and
    analog designs."

    Which makes no sense to me. What were you trying to say?
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