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Alternistor = trade name for traic?

Discussion in 'Electronic Repair' started by N_Cook, May 23, 2013.

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  1. N_Cook

    N_Cook Guest

    No other difference? alternistor = triac ?
    its just that a number of apparently unrelated companies use the term
    alternistor for what otherwise seems to be traic
  2. Phil Allison

    Phil Allison Guest

    ** Nope.

    ** The term refers to a special type of triac that has a very high dv/dt
    rating - so high, that in most applications there is no need for a RC

    With ordinary triacs, a fast rising voltage wave or a high frequency wave
    will cause the device to trigger itself on.

    Not so with an alternistor.

    ..... Phil
  3. Lee

    Lee Guest

    They seem to be a special case version though:

    Also, Wiki says: "A TRIAC which can only operate in quadrants I through
    III, and cannot be triggered in quadrant IV, has improved turn-off
    (commutation) characteristics.
    These devices are made specifically for improved commutation when
    controlling a highly-inductive load, such as a motor or solenoid, an
    application where normal TRIACs have problems due to high
    voltage/current angles"
  4. N_Cook

    N_Cook Guest

    I'd managed to miss that distinction along the way. It seems odd that RS,
    probably the largest industrial electronic components supplier in the UK ,
    makes no mention of them alongside or within their triac listings.
  5. N_Cook

    N_Cook Guest

    alternistor "600 volt"
    throws up no UK suppliers for 600V alternistors
    luckily no one ever uses a snubber in this use, usually just a triac, so
    replaced with an ordinary 600V triac and turns on and off ok, presumably
    maker had a stock of alternistors , so used them instead of a triac there
  6. Lee

    Lee Guest

    Depends whether you count Digikey as a UK supplier (they have UK pricing
    and supply to the UK) - they list them anyway.
  7. N_Cook

    N_Cook Guest

    Digikey seemed to list alternistors but not 600V ones.
    The failed one has no T1-gate resistance in the 10s to low hundreds of ohms,
    is that a (sometimes) characteristic of alternistors ? otherwise no gas
    venting through the epoxy fill or other signs of distress
  8. Phil Allison

    Phil Allison Guest


    ** The wiki quote is a bit misleading - cos there are two specs for dV/dt.

    1. The regular spec referring to the "withstanding" condition where the
    triac is in the off state.

    2. The other called " Commutating dV/dt" - referring to the condition where
    the triac has just stopped conduction because the load current has fallen
    below the holding threshold, about 15mA to 50mA for most devices. At this
    critical point in time, the device will retrigger if the voltage rises too
    fast. The back emf from an inductor can easily do so.

    The commutating figure is generally much lower that the regular one, up to
    100 times lower - eg the triac ( or SBS ) used in the MOC series of
    opto-couplers has a regular dV/dt of 10V/uS, falling to a mere 0.1V/uS while
    commutating off.

    A simple RC snubber across the main triac normally solves the problem.

    BTW: If properly designed, the triac/SBS in a MOC goes off as soon as the
    driven triac begins to conduct so the commutating dV/dt issue never arises.

    .... Phil
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