Author Topic: This small circuit should never work, right?  (Read 823 times)

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Offline FriedMule

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This small circuit should never work, right?
« on: June 11, 2021, 12:34:47 am »
I am playing with falstad circuit simulator to get a better feeling for how things work. Just to test how accurate falstad is, did I try a circuit I do not mean should work but falstad seem to think it's just fantastic.

https://tinyurl.com/ydqte6em

May I please hear your opinion on what is wrong or right? :-)
Even if I appear online is it not necessary so, my computer is on 24/7 even if I am not on.
 

Offline T3sl4co1l

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Re: This small circuit should never work, right?
« Reply #1 on: June 11, 2021, 04:03:43 am »
Weird, it starts when viewing the link, but after reset and run it fails out as it should.

I have no clue what this is supposed to do but what the simulator is (or isn't) doing is trying to drive a zero-ohm voltage source, through a diode, into a zero-ohm voltage source (op-amp output).  This will not happen in the real world as op-amps do not deliver kiloamperes.

As far as accuracy of the simulator goes, it's pretty good for what it is, the above notwithstanding.  The only error I see here is a modeling error, or ignorance of what the models provided are.  The amps certainly shouldn't be labeled "TS912" when they behave wholly unlike any such part.

Tim
« Last Edit: June 11, 2021, 04:05:27 am by T3sl4co1l »
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Offline golden_labels

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Re: This small circuit should never work, right?
« Reply #2 on: June 11, 2021, 04:17:06 am »
Falstad’s circuit simulator is more of a educational tool than a proper simulator. Don’t get me wrong: it is amazing piece of work! But it has its limits. Since it’s only mode of operation is doing a blind step-by-step recalculation of the state, if you introduce some abrupt change by (dis)connecting wires, adding elements or changing paramters, it will continue from some garbage data. This is why resetting it solves the issue.

But every electronic simulator is merely a glorified calculator. Even the best ones. They take high-level description of a circuit, generate a model and calculate the results. Each model has its limitations and no tool of that kind can’t be used as if it presented reality.
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Offline james_s

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Re: This small circuit should never work, right?
« Reply #3 on: June 11, 2021, 04:22:28 am »
That Falstad simulator is pretty great, it isn't a professional tool by any means but it works remarkably well none the less and I've never found one that was easier to use. I've used it quite a few times to develop real circuits with good results, you just have to be aware of the limitations. It simulates rather ideal components, megavolt transistors and op amps are no problem in the simulator but don't exist in real life. It's a perfect tool for beginners to play around with.
 

Offline FriedMule

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Re: This small circuit should never work, right?
« Reply #4 on: June 11, 2021, 11:16:13 pm »
I have added a 100 Ohm resistor on top most line: https://tinyurl.com/yeo4c42x

The idea is an AC source but since falstad uses perfect components, have I added 50mA noise after the rectifier.
The center part is a circuit that shows max voltage (5V her) that have been through it.
The last box is an op amp used as a comparator that keeps voltage at max voltage (5V) without any deviation at all.
The three bulbs is fed the perfect 5V and needed current from input near the rectifier.

In short is the max voltage forcing a ripple free 5V and the bulbs is getting current from DC input.
The scope says the DC is 100% perfect, even that I have added noise.
Even if I appear online is it not necessary so, my computer is on 24/7 even if I am not on.
 

Offline golden_labels

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Re: This small circuit should never work, right?
« Reply #5 on: June 13, 2021, 07:35:37 pm »
In the model used there is no noise. The bulbs are powered from an ideal voltage source formed by the rightmost op-amp.

With the exception of the transient on startup, when the capacitor gets charged, the whole circuit is equivalent to something like this. Of course the value of the voltage surce depends on the rest of the circuit, but as soon as it settles it remains a constant 4.705V ideal voltage source with zero resistance and no dependency on the input noise (assuming it stays within the original range). During the startup period the voltage is not constant, but the op-amp still acts as an ideal voltage source with its value determined by the rest of the circuit: the voltage and current supplied to the bulbs do not depend on the current drawn through the top 100Ω resistor.
« Last Edit: June 13, 2021, 07:49:55 pm by golden_labels »
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Offline FriedMule

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Re: This small circuit should never work, right?
« Reply #6 on: June 15, 2021, 03:52:20 am »
Thanks, this is what irritates me a bit, the simulator assumes perfect components and perfect conditions.
I added the noise to add some ripple to the rectified signal, but it looks like no noise is going through.
My idea was (and I know it won't work) to make a 100% ripple free reference voltage to the last op-amp that then would do it's best to mach output voltage to the perfect input voltage and thereby make a 100% ripple free voltage:-)
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Offline golden_labels

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Re: This small circuit should never work, right?
« Reply #7 on: June 15, 2021, 09:13:20 am »
It’s not only this simulator. Any simulator of anything uses some model that approximates reality. In some situations it is similar enough, in some others — not. The user of the tool must understand its limitations. Simulators are not a laboratory. Those are mathematical equations and the visual part is just making it natural to assemble them. It’s up to you to decide, what and how to model.

In this case you probably wanted at least some small resistance on the output of the rightmost op-amp. However, the leftmost op-amp is also switching instantly from -15V to 15V back and forth, which will not be possible in realty. That may or may not be a problem. If it is switching fast, you may need to model reactance. The wires are not real wires: they are literal ‘=’ signs.

Falstad’s simulator is having a quite realistic LM741: see the element just under ideal op-amps in the menu. It includes the limited output current.
Worth watching: Calling Bullshit — protect your friends and yourself from bullshit!
 

Offline Terry Bites

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Re: This small circuit should never work, right?
« Reply #8 on: June 16, 2021, 01:26:24 pm »
Wow, what powers the opamps?
The only good simulator is copper!
MC12 is free, unlimmited and its sane. Installs in a flash!
https://www.spectrum-soft.com/download/download.shtm
 


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