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Diac's Again

Discussion in 'Electronic Basics' started by Trudeau, Jan 20, 2005.

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  1. Trudeau

    Trudeau Guest

    I have several general questions on Diac's.

    It seems that some electronic places dont sell Diac's . I went to one place
    that I buy parts from and they said they dont sell them.
    Another place didnt have them out on the shelf for me to look at but I had
    to have a part number. The sales person was somewhat unfamiler with them.
    Are diac's little used ?
    It also seems that there are a relatively small number of Diacs NTE I think
    has about 4 that I can see.
    I'm just wondering if anyone knows why the Diac seems to be so maginalized.

    Some web sites list them as "Bidirectional Diode Thyristors". At least I
    asume thats a Diac. eg. It is in the Thyristor family, it is two diodes
    oposite each other and it is bidirectional.

  2. Chris

    Chris Guest

    There aren't as many manufacturers. Try Teccor, which makes DIACs in
    the DO-35 (small signal axial diode package) as well as DO-214 (surface
    mount). One distributor is Digi-Key; another is Mouser (they only
    stock the thru-hole parts).

    DIACs were made primarily for analog phase control of AC loads using an
    RC charging circuit. That's nowhere near as amazing as it was in the
    '60s, when these parts were developed. The simple phase control
    circuit (view in fixed font or M$ Notepad):

    o -|___|--o--------------.
    Load | |
    | |
    ..-. |
    | |<----. |
    | | | | Q4010L
    '-' | _|_
    120VAC | | |<| V_A
    o------o-|\|-/ |
    | |>| |
    --- HT-35 |
    --- |
    | |
    | |
    created by Andy?s ASCII-Circuit v1.24.140803 Beta

    has a lot of limitations. Non-resistive loads don't fire well, the R-C
    charging constant is dependent on line voltage, and these circuits all
    have "snap-on hysteresis", which means you don't have good control of
    the phase at the low end. The circuit tends to generate EMI/RFI, which
    is a no-no. Also, these triggers have asymmetry in their bidirectional
    trigger voltages, which leads to a DC component applied to the load
    (can be very bad for inductive loads, lets the smoke out).

    These problems led to some manufacturers dropping out of this market in
    the 80s and 90s. Also, some of the ICs which did this job far better
    and more elegantly than a standard DIAC were single-source, and were
    dropped due to lack of demand and yield issues (I still like the old GE
    ST-4, and have a treasured few tucked away -- but I'm not sure exactly
    why anymore. I guess it's like that when you get older ;-) ). After
    the 2N4991/2/3 (Motorola) and the ST-4 (GE) left the picture, the stuff
    that was left mostly DIACs triggered at higher voltages (30V and up),
    which again reduced the market (you can't turn on the triac until the
    AC gets up to 30v). This made the whole concept even less popular.

    These days a lot of phase control of AC loads is done digitally,
    especially with small microcontrollers. All you need to do is read the
    AC line zero crossing, determine if you're at 50Hz or 60Hz, and measure
    out a time until triggering the triac. You can get full 180 degree
    phase control, and can get easy integration with any sensors attached
    to the stuff you're controlling, as well as a plethora of added
    features which jack up the price. Sure beats one plain Jane dial for
    control, I guess.

    By the way, a DIAC is more complicated than just two avalanche/zener
    diodes back-to-back. Once the voltage across the DIAC reaches the
    trigger voltage, it turns on and remains on until the voltage reaches
    the breakback voltage, typically 7V or so. This gob of current being
    discharged from the cap into the gate of the triac is what turns it on.
    Try reading a few appnotes from the Littelfuse/Teccor knowledgebase to
    get up to speed:

    particularly AN1003 - Phase Control Using Teccor Thyristors

    as well as the DIAC datasheet:
    Good luck
  3. Hi,

    Diacs are mostly used with Thyristors or Triacs to control
    a lamp (so called Dimmer).

    The better Technology is to always switch power at zero crossing
    of mains voltage and to feed the power while whole periods to the lamp.
    For this purpose there are IC's to control the Thyristor/Triac.

    Because Diacs are used in a such special application, the market
    is small and not so many manufacturer and types.

  4. Trudeau

    Trudeau Guest

    Thanks Chris,

    I have read some tutorials and lessons on line regarding Diac's. And will
    read the little fuse faq. I just sort of thought that Diacs would be a
    usefull thing. in reducing stray low voltages. Admintedly the circuit has to
    withstand the dam bursing when it suddely conducts but it just seemed like
    usefull tool and simple. I wanted one that had a breakover voltage of about
    20volts but virtually they all start at about 28. I probably wount buy it
    because I'm not going to mail order one part for 2 bucks.

    Thanks again for the usefull info.
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