Lt spice basics

Discussion in 'General Electronics Chat' started by whale, Dec 11, 2009.

  1. whale

    Thread Starter Active Member

    Dec 21, 2008
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    Can some one tell me from where can i get all basic transistor models for lt spice?

    Also,

    since iam new to ltspice , can i trust the circuits simulated in ltspice?

    Why iam asking it is because, while i simulated a amplifier circuit using bjt,i biased the bjt with collector -emitter voltage of 900v, but for that specific transistor the maximum collector -emitter voltage is 40v.
    When i run the simulation the output is normal with great amplitude about 850 v.the simulator haven't warned me about device failure due to high voltage at collector-emitter terminal.

    So , can i trust the simulator?

    Please make me clear.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    They are working on an upgrade to LTSpice.

    With that upgrade, if you exceed the current, power or voltage ratings of any device in the circuit, your computer will emit huge clouds of smoke before exploding in a fireball. :eek:

    But seriously, you have to view the results of a simulation realistically. If you knowingly subject a component to stresses beyond its' design limits, in real life it'll fail.
     
  3. whale

    Thread Starter Active Member

    Dec 21, 2008
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    iam asking how can i know the component is under stress by simulation?

    is there any way to find whether the current or voltage or power in the circuit is beyond the limit?
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    You have to look at the datasheet of the part in question. You can measure power dissipation in a component, but you still have to look at the datasheet to see what the part will handle.
    LTspice has a pretty good transistor library. You may already know this: After you place a generic transistor symbol on the schematic, right-click on it and select Pick New Transistor.
     
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    No Spice-based simulator makes any attempt to determine if voltage/currents limits are exceeded unless you explicitly model the breakdown conditions youself.

    Transistor models do not have parameters in the models with account for voltage/current limits. This is also a characteristic of all Spice-specific models, regardless of which simulator...
     
  6. Mike33

    AAC Fanatic!

    Feb 4, 2005
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    One thing that's helpful, when you are in operating point or AC analysis mode, is once you've run the simulation, move your cursor over the part in question. Press ALT while hovering, and the cursor becomes a thermometer. Click the part while still holding ALT, and the power thru the device will be graphed. This will let you see if you are exceeding power ratings!
    In conjunction w/the data sheet, you can then set the power rating for resistors (right click them, below the value is a box to display power rating). With a transistor, etc., you will see if the Max dissipation is being exceeded, and can choose a different device...
     
  7. kdillinger

    Active Member

    Jul 26, 2009
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    I do not know about LTSpice so you should visit the Yahoo user group to verify, but the Advanced Analysis tool set in PSpice from Cadence has Smoke Analysis.
    You can set the parameters of components and it will tell you if you are exceeding the stress limitations set by your smoke parameters.
     
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    It also works in Transient and DC Sweep analyses.
     
  9. Mike33

    AAC Fanatic!

    Feb 4, 2005
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    I don't want to hijack or anything, but does anyone know how to do the 'superimposed' graphs? Let's say I wanted to see what changing a capacitor value in a high-pass filter would look like, and desired having the results of 5 such changes displayed at once.
    Should I build 5 filters, with 5 sources, and give each C a different value? Or is there an easier way (with a .param command using steps or something?).

    I'm thinking of how the Valve Wizard gets those nice graphs showing the effects of different cathode bypass caps and tone controls ;)
    Thank you!

    Mike
     
  10. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Use the directive .step param Cx 10p 100p 10p, or one of the other .step options, such as .step param Cx list 10p 18p 27p. The value of the capacitor to be stepped has to be parameterized. Instead of entering a number in the value window, enter {Cx} or whatever name you want to give it.
    The help file can show you other forms of the directive. Search for .step.
     
  11. Mike33

    AAC Fanatic!

    Feb 4, 2005
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    Thanks, Ron, got it! Not too bad once you get the idea :eek:)
     
  12. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Excellent!:)
     
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