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SMT is pretty easy

Discussion in 'Electronic Design' started by Tim Williams, Aug 21, 2010.

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  1. Tim Williams

    Tim Williams Guest

  2. Tim Williams

    Tim Williams Guest

    Err, typo... Multisim doesn't have an acceptable LM393 model, so I found a
    random dual comparator with the same pinout. Actual device is LM393, and
    saturation is noticably sharp in both directions (~150mV).

  3. legg

    legg Guest

    Turn-off, enforced by Q3, will be limited by the 100R mos output
    impedance of the comparator.

    Not so the other side, but then the comparator output is clamped at
    two diode drops above gound. Not much turn-off voltage for Q3, and
    nearly the entire supply across the comparator's 100R mos driver -

    Maybe the comparator's output transistor is open circuited.

    Not breadboarded as drawn, anyways.

    Nice handiwork though. Pitty.

  4. Tim Williams

    Tim Williams Guest

    LM393 doesn't exist, but there is TLC393.

    Last time I tried a comparator, it wasn't a comparator, it was an op-amp.

    And for that matter, half the op-amp models suck -- one circuit I tried
    with an LM358 on +12/0V rails blissfully generated outputs in the 30V
    range. Others get the bandwidth or slew rate wrong.

    Multisim 10, BTW.

    TL494 / TL598 don't come with it either, and I haven't seen them available
    (at least in any obvious fashion; they didn't seem interested enough to
    add an automatic "find models here" feature, but rather a painful database
    entry process). I've done more than a few schematics with them and just
    placed DIP16s as placeholders. At least the footprint is right then.
    Good thing I can simulate these chips faster on paper, or test on the
    breadboard, than any simulator anyway.

    And then there's the mess of "untestable" circuits I've come up with,
    which I can only imagine come from terrible models or some bizarre set of
    required simulator conditions (none of which is specified, if it even
    could be). But that's more of a SPICE thing than Multisim's fault

  5. legg

    legg Guest

    What's wrong with Multisim's LM393 model?

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