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hi all i want to know the the limit of my circuit,upto which it can withstand,so that i can select or change the components accordingly.When i tried simulation in NI MltiSIM, it works fine,but the issue is when you apply 50A directly to an LED, it'll still work fine,I am wondered how is that possible??I would like to know which other simulators can show "when my circuit will get fried?"

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  • \$\begingroup\$ this is the first time i've seen(on stack exchange community sites) no response even after an hour... \$\endgroup\$ – test Dec 21 '11 at 4:24
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    \$\begingroup\$ NI Multisim is pretty niche. Give it time. \$\endgroup\$ – Connor Wolf Dec 21 '11 at 8:28
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    \$\begingroup\$ Also, circuit simulators are pretty universally mediocre at best. They generally only produce results as good as your understanding of the circuit. \$\endgroup\$ – Connor Wolf Dec 21 '11 at 8:29
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The problem is that you are expecting a model to have more functionality than it has. Most models are simply parameters in an equation. They are designed to approximate the actual device behavior within specific operating conditions. For example, a diode is modeled using the following equation:

\$\Large I_f = I_S(e^{\frac{Vf}{NV_t}} - 1)\$

Where Vf is the applied voltage and If is the forward current. Notice that there's nothing stopping me from putting 50V as the applied forward voltage, and I will definitely get an answer from the simulator. It will be completely nonsense, but the model is assuming that the behavior of the diode is always described by that equation.

I have seen device models with voltage breakdowns and melting currents, but typically those are in "high end" models used for IC design. It is specifically coded into the models. I'm using Spectre and HSPICE, but I don't think that's going to help you much. If you MUST use a simulator to identify if a part is going to fail, then you need to understand when the part is going to fail and watch the simulation results for those conditions.

Relying on a simulator to guarantee whether or not a part works is asking for disaster. Simulators model everything as "ideal" - and that is making a LOT of assumptions. Keep in mind that a simulator considers everything perfect unless told otherwise. Doing conceptual design with a simulator is dangerous.

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Some simulators will issue a warning when an excessive current is detected in a circuit. These warnings are typically triggered when a current exceeds 100 or 1000 Amps, for example. The purpose of the warning is not to tell you when the failure limit of a part is reached but only to let you know you might have set up a circuit incorrectly or with unrealistic parameters.

In fact, the device models themselves don't even include the self heating behavior that, in the extreme, leads to damage. Even if they did, your circuit drawing would then have to include information about heat-sinking and air-flow in order to produce accurate results.

What you have to do is run the circuit simulation to find out what currents will be produced if the device continues to operate nominally, and then compare that result to the datasheet limits for each part.

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  • \$\begingroup\$ so we can not simulate for the same purpose?is actual implementation the only way out? \$\endgroup\$ – test Dec 21 '11 at 6:52
  • \$\begingroup\$ You can simulate to see if you exceed the parts limits, to a certain extent. But you will have to check the currents against the datasheet yourself -- the simulator won't do it for you. And you will need to leave some design margin because the simulator won't include self-heating effects. Also, you'll need to consider that your simulator model for a given component represents a "typical" part -- real parts vary from the nominal, and you also need to consider whether a worst-case part (say highest or lowest gain, highest or lowest resistance, etc) will function in the design. \$\endgroup\$ – The Photon Dec 21 '11 at 16:34
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I accept with The Photon. It may not possible to detect accurately without giving too much parameters like heat-sinking, air-flow and such.

And I haven't seen any simulator to do the job of warning you before or at the frying point.

But I would like to say that you should not trust the SPICE. That is because

  1. Analog guru Bob Pease says so :)
  2. It is not 100% true all the time, it may lead to production mistakes that cost you much.
  3. If you base your circuit design on SPICE with trial and error, then you will not improve your circuit design skills and practice your theory.
  4. If you do not practice your circuits in real world, you will never get the experience of the real world and will not be able to design circuits for real world, because you won't know much about the real world. ( That is lots of real world! )

These are the things I've learned and still learning.

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