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Help identifying glass diode?

Discussion in 'Electronic Design' started by DaveC, Jul 6, 2004.

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  1. DaveC

    DaveC Guest

    Partial view here (it's V12, lower left in photo):

    Measured out-of-circuit, almost zero ohms, both ways. Printing (barely
    legible: "1(?)018P" (that first "1" could be any character or number with a
    vertical element).

    How do you tell if this is a simple signal diode or a zener? Do zeners come
    in this package?

    Any help identifying this and recommending a substitute would be greatly

  2. DaveC

    DaveC Guest

    Examining other similar diodes in the circuit, it seems to be a 1N4148.

  3. legg

    legg Guest

    This appears o be a diode used in a gate drive or current sensing

    To check the diode, you will have to desolder one end and remeasure
    the diode and the empty position to determine it's integrity and
    locate the real short, if the unpopulated location still looks low

    You must read and report lettering on the part to identify it's type.

    If used to prevent overvoltage on a current sensing position, it may
    have been a zener intended to short on sensor open circuit, to prevent
    damage to the more sensitive sections of the control circuit.

    If it is used in a gate drive circuit, it may have been a zener or
    diode that failed short after the failure of the power device being
    driven - in which case the driven power device and other components
    attached to the gate and source will measure incorrectly and need

    Failed zeners can usually be identified by the body marking.

  4. I've got some 3V9 zeners that look remarkably like that.
  5. DaveC

    DaveC Guest

    And I've got some that are just plain switching diodes.

    How do I determine what this diode is (was) without some markings?

  6. DaveC

    DaveC Guest

    I see some application notes for the SMPS controller IC this PS uses, and
    there is a zener diode at the FET's gate. The diode is "USD1120", but I can't
    find *any* data on the net about it.

    What diode is a USD1120?

  7. If it's across gate to source, it's a zener protection diode. Probably
    18 V, a 1N4746, looks like a fat glass diode.

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  8. legg

    legg Guest

    What is the lettering on the body of the part in question?

    Unless the app note is specific to this commercial device, it will
    only be usefull as general information.

  9. DaveC wrote...
    The USD1120 is a 20V 1A axial Schottky diode made by Unitrode,
    Solid State Devices, and Microsemi. I have a two-page Microsemi
    datasheet. It's not a zener diode.

    - Win

    (email: use hill_at_rowland-dot-org for now)
  10. Leeper

    Leeper Guest

    Based on the lettering alone, could it be a 1N918, which happens to be
    somewhat similar to a 1N4148?

    Possibly the P is a B or something like that?
  11. DaveC

    DaveC Guest

    Not *directly* across G-S; the current sense resistor is between ground and
    source. It looks like this:

    G |--+ D
    -+--| |<---+ IRFP340B
    | |----+ S
    | |
    | |
    - \
    ^ / 0.22 ohm 2w
    | \
    | /
    | |
    | |
    + --------+-

    The diode in question is not shown as a zener (as don't know if it is). Also,
    there is a protection diode internal to the FET between S and D, according to
    the data sheet.

    The glass diode (which looks very much like a 1N4148 nearby) has some partial
    markings which are: "C18P?" where the ? is an illegible character. Since the
    markings wrap around the diode, this character could be at the beginning or
    end of the number.

    The only hit I come up with when I search the net for this number is ComChip.
    They make SMD diodes, and one of the CZRM27 series is the CZRM27C18P. It is
    an 18v "voltage regulator diode", according to their data sheet.

    Other ideas? Where to find a replacement? Use just any 18v zener of 1w or so?

  12. DaveC

    DaveC Guest

    Turns out I actually have an 18v diode lying around. I'll use that.

    Thanks to all who piped-in.
  13. Clint Sharp

    Clint Sharp Guest

    Sounds OK, but I'd try and size match it, 1N4148 size devices tended not
    to be of the 1W rating when I was repairing this stuff though, more like

    Don't forget, if you don't push enough current through the zener, it

    The Zener is referenced in one of the UC384x datasheets (Not
    necessarily Unitrode's) and the reasons for it's existence are
    explained, IIRC you select the zener voltage to suit the FET but there
    seemed to be two 'standard' values for this part, 15v and 18v whichever
    PSU you opened.
  14. legg

    legg Guest

    So its a BZX55C18 (BZX79C18) or similar 18V 400mw(500mW) zener.

    You have not reported on impedance measurements of the part's vacant
    position, or the disonnected part.

  15. DaveC

    DaveC Guest

    Fairchild semi's data sheet now in hand.
    Diode is shorted in both polarities. Vacant position is relatively high
    impedance (10K ohms).

  16. DaveC

    DaveC Guest

    I'm no electronics professional, but mW rating is maximum power dissipation.
    I think I could use a 10W device here and I'd still get 18v out of it, no? I
    leave the "pushing enough current" to the circuit designers.
    The "18" on the diode defines it as an 18v device, to my satisfaction.
  17. GPG

    GPG Guest

  18. DaveC

    DaveC Guest

    Matching by eye (which I find generally reliable), the original zener diode
    matches up with the 500mW form factor.
  19. GPG wrote...
    Correction, my datasheet is from Unitrode.
    I suspect that's AIC's poorly drawn symbol for a Schottky diode.
    It's commonly used to insure that no improper substrate current
    flow can be induced by a fast-switching-induced negative gate-
    voltage spike. You can use any old 1n5817, etc., for the task.

    - Win

    (email: use hill_at_rowland-dot-org for now)
  20. DaveC

    DaveC Guest

    I've got 4 UC3843 data sheets, and none say anything about zeners (except an
    internal reference voltage for the IC), and they all use the USD1120 shottky
    diode at the gate of the FET.

    I'd like to see the explanation (although all I'm doing is replacing what the
    circuit designer put in there originally), out of curiosity. Which data sheet
    are you looking at?

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