# AoE - Thevinins Theory Question

#### bwilliams60

Joined Nov 18, 2012
1,197
So I have been on here for a while and can find my way around automotive stuff fairly well, both electrically and inside ECMs, ECUs etc but now I am finding myself being very challenged. I received the Art of Electronics for Christmas and dove into it head first andmade it 16 pages in before I remembered why I don't like this type of electronics. Like everyone else it gets deep in math and I am more about understanding how things work and not the physics of it. So I am going to take it on the chin and try to grasp the whole package now but I will need some help.
What is the real purpose of Thevinins theorum and how does it apply to everyday use? How do I apply this to a circuit problem?
I think if people can relay this in laymans terms to me, it will make the journey through this book a lot easier. There will be a lot more questions to come. Looking forward to the journey. Merry Christmas to all who have made it to the end of this post

#### BobTPH

Joined Jun 5, 2013
2,051
The simplest useful case would be a voltage divider. Thevenin's theorem will give you the output voltage and series resistance that it is equivalent to, which will help you determine what happens when you load the output.

Bob

#### bwilliams60

Joined Nov 18, 2012
1,197
So far that is what I have gotten out of it. Is there more to it than that? Is this a useful application for diagnosing electronic problems? Where would I use it? Or would I use it?

#### drc_567

Joined Dec 29, 2008
735
... 'maximum power transfer' ... It is different for DC and AC circuits.
DC ... R_load = Rth
AC ... R_load = Rth* ... (where Rth*=Rth-conjugate)

#### AnalogKid

Joined Aug 1, 2013
8,143
Is there more to it than that?
Yes, but not at your level.

Note - that is *not* a snarky answer. Everything about electronics IS math. Rules of thumb come from somewhere. Ohm's Law starts with the charge and spin of the electron and some frightening math from the 1860's, cleaned up in the 1890's. In round numbers, everything else is Oliver Heavyside's complex vector analysis way way way watered down. The vast majority of electronic circuit designers never use any of that stuff, but every now and then having a long unused dim recollection of understanding it comes in handy.

Here is a useful application. Suppose you have a 2-resistor voltage divider as one input to a comparator, with a noise filter capacitor across the shunt resistor. When the circuit first powers up, it will take some time for the divided voltage to wander up to its operating value because the resistors and capacitor form an R-C ramp circuit. Calculating the R-C noise filter corner frequency and the ramp properties (such as how long the user has to wait before the output of the comparator can be trusted) are much easier if you can reduce the two resistors to one equivalent resistor. Thevenin does that for you.

Will the circuit work less well because you can't spout Thevenin's derivation from memory? Nope.

Will you probably apply Thevenin incorrectly in the future? Yup.

Should you any lose sleep over it? Nope.

ak