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Need help with surface part ID and finding a schematic

Discussion in 'General Electronics' started by Phil S, Feb 16, 2005.

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  1. SteveB

    SteveB Guest

    I fix surface mount power supplies for a living and still I can't recognise
    some of these parts. Even having a rough idea doesn't help a lot, for
    instance the pink non-surface mount 15/16 items are diodes but could be any
    type and rating, but they look a bit like the bidirectional avalanche ones
    as used in the gates of triac ac dimmer circuits as I can't see a cathode
    marking. Measure them on a DVM diode range to see if you get an approx 0.7v
    drop across them, if so then they are ordinary diodes or perhaps zeners, if
    not they may be the bidirectional ones I mentioned.
    Item 17 is a capacitor, probably about 100nF 50v, but again that's pure
    guesswork as it could be 10nF 500v.
    Other guesses:-
    1,2,3,4 are power diodes, very likely all the same type.
    5,19 (presuming there are 3 connections) are power transistors, FETs ,
    triacs or thyristors. My guess is triacs because of those pink things). I
    would expect them to both be of the same type.
    11 is a small value capacitor, perhaps 1nF 50v
    12,13 are probably transistors or less likely small FETs but there are
    diodes and zeners the same shape with 3 connections where one goes nowhere.
    7,8,9,10 no idea but as there are 4 of them and seem to be only two
    connections I suspect medium power diodes.

    I wish you luck as it is almost impossible to identify unmarked surface
    mount components accurately. I would measure everything with a DVM, if
    anything measures short then remove it and check it's still short off the
    pcb for starters. Check all identical looking components actually measure
    the same on a DVM diode range both ways round. These sort of semiconductors
    tend to go short or very low resistance (<100 ohm) in both directions when
    they blow up. Unfortunately they also sometimes go short first then blow
    open circuit if lots of current then flows as a result of the short. If you
    get 0.7v readings across 2 or 3 contacts of the 2 or 3 available then that
    component is likely OK (this is NOT foolproof!). The 5,19 components are
    the most likely to have failed and if they have will be difficult to
    identify. If only one has gone short then use the other to find out if it's
    a triac or whatever. If you don't know how to identify basic semiconductor
    types with a meter ask here.
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