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Looking to Cross ref a Diac & Triac set

Discussion in 'Electronic Components' started by Tim, Apr 17, 2007.

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  1. Tim

    Tim Guest

    Hello again, I have a circuit for an electronic Variac that uses a Diac
    and Triac to control the output. It looks like a glorified dimmer
    circuit. Anyways it uses part # MAC2-4 for the triac and MPT-28 for the
    trigger diode (diac). I haven't been able to cross reference these
    puppies on the net. I hav checked the usual sources, but came up empty.

    Any help would be appreciated.


    - Tim -
  2. What load and operating voltage?

    Generally a 10 amp triac and any 'normal' diac will do. This isn't precision
  3. Ray King

    Ray King Guest

    The triac is not critical almost any will work but the diac is one that
    triggers at 28 volts + or - 4 volts.
  4. Eeyore

    Eeyore Guest

    They're ancient, that'll be the problem !

  5. Tim

    Tim Guest

    Hmmm, this circuit was drawn in 2005, August 18th to be exact. It's
    strange that the guy used old parts in the design. It also appears that
    he has spec'd and drawn a triac, but the pins are marked cathode, anode
    and gate, like an SCR. I think it should have MT1, MT2 and gate for a

    The schematic is here;

    - Tim -
  6. Tim

    Tim Guest

    Yes, I figured I'd use the parts I have kicking around the shop, and I
    thought mebbe someone would reconize the parts and tell me the ratings
    on 'em.

    The schematic is here if you are interested in the project;
  7. Tim

    Tim Guest

    So is that what the -28 is for? Kinda makes sense. I am planning on
    using some TO-220 15A triacs I have here. I'll be sure to get a 28v diac

    The schematic is here if you are curious as to the application;

    Thanks for the info,

    - Tim -
  8. Ray King

    Ray King Guest


    Is this a diode on the ac input of of this circuit? Also is the load a
    transformer? If either is true then the circuit will not work. You can not
    control the primary of a transformer with a triac or two scr's at least over
    the full range. you can go from zero to aproximately 75% depending on the
    inductance vs the load and wheather or not the load is resistive or
    rectified with a cap filter.

  9. Tim

    Tim Guest

    I had intended to use this as a variac to lower the operating voltage to
    an RCA TV, to allow me to reset it's eeprom. The set uses a switching
    power supply inside. It's a trick that some guru came up with to solve
    the eeprom programming issue without a proper chip programmer.
    Apparently these sets had a problem with this chip getting scrambled due
    to bad grounds, bad soldering, and power flucuations.

    I don't have access to a real variac, nor the funds to buy one, even if
    one was available locally.

    - Tim -
  10. You can with the right design. Fan controllers do it.
  11. Ray King

    Ray King Guest


    Fan motors are high slip and less inductive and more resistive. This similar
    to a transformer with a full resistive load ( pure resistive). The fan motor
    does not have as much mutual/leakage reactance that looks inductive to the
    primary as the transformer with a pure resistive load.
  12. Franc Zabkar

    Franc Zabkar Guest

    MPT-28 is a 28V diac:

    I don't see a MAC2-4 in either my 1984 Motorola catalogue or my NTE
    1990-91 catalogue, but the "-4" appears to refer to a 200V device.
    FWIW, the MAC20-4 is a 200V, 15A triac.

    - Franc Zabkar
  13. Ray King

    Ray King Guest

    There is a very good circuit for controlling the primary of any inductive
    load. It consists of a switch in series with the ac input to the transformer
    that pwm's the load. During the off time a second switch turns on in
    parallel with the load. Each switch consists of fets or IGBT's either in
    anti series or across the output ( + and - ) of a 4 diode bridge( fast
    diodes ) . The fets/igbts are transformer driven ( both switches are driven
    with the same transformer ) This will prevent the two switches from
    conducting at the same time ( shoot through ). The output can be filtered so
    the output looks like the output of a variac. The use of scr/triacs can not
    be filtered very well and reek havoc with the switchers with cap input
  14. Ray King

    Ray King Guest

    I have a 1979 mot catalog that does not list the MAC2-4 ----I have looked in
    a 1981 D.A.T.A. BOOK with no results. The lowest numbers for the MAC is
  15. Eeyore

    Eeyore Guest

    It mnay simply be what he had in his box of bits !

    It should.

    They are non-critical as previously mentioned by others. Choose a triac to suit
    your load. It's basically a triac based dimemr. Check any other similar designs
    online for comparison.

    Did you spot the deliberate mistake btw ?

  16. Eeyore

    Eeyore Guest

    That may not work very well actually.

  17. Tim

    Tim Guest

    Deliberate mistake? In the drawing? I can't see anything obvious...

    - Tim -
  18. ehsjr

    ehsjr Guest

    I don't think it's a deliberate mistake. I suspect
    it is just poor drawing of what is supposed to be
    the output connector.

  19. Tim

    Tim Guest

    I got the TV set re-programmed without needing the variac thing. I
    pulled the chip and re-programmed it in my eprom programmer using data
    from a fellow tech on the net.

    I did not need to build the circuit after all, but I appreciate
    everyone's input on the project.

    - Tim -
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