Does LT Spice show ripple voltage and other effects that one sees in a circuit?

Discussion in 'General Electronics Chat' started by thakid87, Jun 30, 2010.

  1. thakid87

    Thread Starter Active Member

    May 23, 2009
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    I don't have the parts required to build certain circuits and test them. To what extent does LT Spice simulate the circuits?

    For example if I create a power supply circuit, and I replace the filtering caps with different values many times, will it show the effects of said values?

    Basically, I want to know if I can sort of use it as an o-scope to learn how different valued components will affect a circuit.

    Thanks!
     
  2. retched

    AAC Fanatic!

    Dec 5, 2009
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    Yes. You can do quite a bit with LTSpice. It was made for powersupply circuits. You can inject transients, change frequencies, and edit every part of the circuit and scope the results.

    Have you signed up on the LTSpice Group at YahooGroups?

    They have hundreds of parts available there. And a great forum. I have had many questions, and I didnt even have to ask. I searched their group and found the answer every time. If you cant search and find it, ask and they will answer.
     
  3. thakid87

    Thread Starter Active Member

    May 23, 2009
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    I found it and signed up right after this post.

    THANKS!
     
  4. retched

    AAC Fanatic!

    Dec 5, 2009
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    Anytime, my friend.
     
  5. sceadwian

    New Member

    Jun 1, 2009
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    Yes but only in relation to the simulated power supply, NOT your power supply.
    It can NOT replace an o'scope, it can be used for general learning but only real world circuits and conditions will show you what you'll see in the real world. The real world and simulation will diverge at some point, if you have no way of determine where they do so (a scope) you can't.

    I would HIGHLY recommend real world experimentation especially with simple circuits and preferably a scope or a meter at the least.
     
  6. thakid87

    Thread Starter Active Member

    May 23, 2009
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    A meter I have access to, but I have very few components that I can actually mess around with. I understand that there is no replacement for the real thing, but this is what I can do for the time being.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Keep in mind that many components used in simulations are "ideal"; ie: no parasitic properties, and the tolerances (unless specified otherwise) are precisely what is specified; if you plug in 10uF for a capacitor, that's exactly what the simulation will use - whereas a real-world capacitor with a 10uF marking may be off by 20%.

    A real-world capacitor has more than capacitance; it has some inductance, some equivalent series resistance, and some "leakage" resistance between the plates. Fortunately, LTSpice includes a number of models for capacitors that more closely simulate real-world capacitors.

    Real-world inductors and resistors have parasitics, too.

    The wires used in LTSpice are also perfect. Zero resistance, no inductance or capacitance. If you wanted to get closer to the real circuit, you'd use small values of inductance, parasitic inductive coupling to traces that were going to be nearby, parasitic capacitive coupling to traces nearby, etc, etc. Just trying to build a really accurate model is a lot of work, and most of the time I don't bother.

    A SPICE simulation is a good starting point, if you keep the limitations in mind. However, once you get a circuit working in simulation, you will have a good starting point to begin testing with real-world components to see what you didn't model accurately.
     
  8. sceadwian

    New Member

    Jun 1, 2009
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    Then it's good enough, just remember to take simulator output with a grain of salt. I learned a lot from simulation.
     
  9. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    I gave up trying to get people to understand that. You wouldn't believe how many calls I got at National Semiconductor from idiots who never build a circuit up before going to production (just simulate).... then sue you when something doesn't work. In a way, we were as much a part of the problem as they were by giving them the models. But, it seems that these days schools focus on simulations and don't even teach reality anymore.
     
  10. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Some models allow you to insert specific parasitics (like ESR which is equivalent series resistance) but only in locations where they have defineable effects. No model can accurately reflect the effects of board layout or trace inductance.
     
  11. sceadwian

    New Member

    Jun 1, 2009
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    Those parasitics also don't fully characterize the truly complex nature of an IC. Most IC models for Spice aren't actual TRUE full spice models for the circuit, or they would take about a decade to fully simulate even power up oscillation; they're behavioral models. At best they are nothing more than an approximation if you don't know what you're doing, and the real possible crutches of the real world devices not just the models that simulate them well enough for general purpose use, specifics are a bitch.
     
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