Thévenin's theorem

[CrackBadger]

So Thévenin's theorem for linear electrical networks states that any combination of voltage sources, current sources and resistors with two terminals is electrically equivalent to a single voltage source V and a single series resistor R.

**broken link removed**

I have no idea what this means. Are they saying the second circuit will do exactly the same thing the first one does without the need for having all those extra power sources and resistors?

Is this theorem important to know? I'm reading the art of electronics at the moment and they're explaining alot of weird concepts that I don't understand because they don't really explain what the point of these circuits are.

Do I have to learn all these concepts before I can gain a basic understanding of electronics or is there a quicker way of going about it?

 

[Mendoza]

Thevenin's theorem does not get rid of the "extra power sources and resistors" it simplifies what can be a complex circuit into one that is far easier to understand. The Rth and Vth will be a combination of the sources and resistors in your original more complex circuit...

For example with the Thevenin equivalent above if you were to add a load on to the output ports A & B, e.g a resistor. You could calculate the voltage across the resistor using the simple voltage divider rule...

Unfortunately, yes you will need to learn these concepts, however it may look crazy right now (and it may still may be so for many) but over time it will start becoming easier. I always believe, understand the fundamentals and you'll go far!
Good luck

 

[ljcox]

Thevenin's theorm is very useful for simplifying calculations.

I also use McMillman's theorm.

 

[DennisFerron]

Some people think you have to know every bit of theory before you can do any practical applications. That's b.s. The truth is you will not really understand or be ready for the theory until you have tried some practical circuits and see why the theories are useful.

You can build simple circuits with little more understanding than Ohm's law - in fact I would say that Ohm's law is the only one you absolutely must learn.

Maybe it's because I'm self-taught in electronics, but I have a very low opinion of people who think they know electronics, but have only read about it in classes and books, and not done it with real circuits. I believe experience is the only truly correct teacher of electronics, and the real world is the best classroom for it. If you end up blowing up a few components along the way because you tried wiring up a circuit you didn't fully understand, great! That's part of experience, and the lesson will stay with you longer than anything you study for a college course or exam.

Definitely do learn the theory too along the way; the theory is what makes it a science and not alchemy. I'm just saying that I think studying the theory should follow the experience, not precede it, and that you should absorb it one piece at a time while using it.

Edit: P.S. In 20 years I've never needed Thevenin's theorem to design a circuit.

Edit 2: The trouble with college courses on engineering subjects is that there is a difference between an academic, and a practicing engineer. An academic will value the theory for it's own sake, and expect you to learn all the theory whether you need it or not. A practicing engineer views the theories and formulas as tools, just like any other tool like a wrench or a screwdriver. The tools are there when you need them; you grab the tool you need for the job and use it when you need it. A mechanic doesn't study tools, he studies engines; similarly I don't study formulas, they're my tools. I use formulas and theorems, what I study is circuits.

 

[RadioRon]

Interesting question from the op. I've just yesterday run across a lesson in how/when to limit the use of Thevenin's theorem. The lesson I learned is that the theorem helps you understand how the black box with terminals A and B will behave FROM THE LOAD'S POINT OF VIEW, but it doesn't necessarily give you insight into the workings inside the box. Take for example, the situation where you have an antenna inside the box, one that is an ideal antenna perfectly matched to a 50 ohm port. The thevenin equivalent impedance looking from the load back into the box is 50 ohms alright, but you can't say that the 50 ohms, which is your radiation resistance, means that half the power from the voltage source is re-radiated. Admittedly this is advanced stuff so the point may be lost on some, but it struck me as quite interesting.

Oh, and on the op's final question, I must agree that the most effective way to learn the theory is to interleave the reading of theory with the practical building of circuits. In fact, I learn best when I'm stuck on a circuit and have to go back to the theory to understand what's going on. At least then I'm motivated to read those often dry and boring textbooks more carefully. The key is to know in which textbook and what chapter you should be looking first.

 

[CrackBadger]

Some people think you have to know every bit of theory before you can do any practical applications. That's b.s. The truth is you will not really understand or be ready for the theory until you have tried some practical circuits and see why the theories are useful.

Thanks for clearing that up for me. I was driving myself insane trying to learn all this theory without ever having put any of it into practice. The most complex circuit I've built is a power source, an switch, a pot and an LED.

I realize the importance of Ohms law and use it to calculate voltages, currents and resistances etc. but the more complex concepts I haven't been able to fully understand and put to use yet.

 

[ljcox]

Here is an example of how Thevenin's theorm makes the maths easier.

Using Kirchoff laws leads to 3 equations and 3 unknowns.

Using Thevenin leads to 2 equations and 2 unknowns.

 

[eblc1388]

Just be careful that the Thévenin equivalent refers to what user do at terminals A & B and said nothing about what inside the black box.

As far as any user measurement or loading is carried out at terminals A & B, the blackboxed circuit and the Thévenin equivalent circuit will appear to perform the same and it can not be distinguished. However, in real life the circuits are different.

For example, the blackboxed circuit could be heating up 100 times faster than its Thévenin equivalent.