INTRODUCTION TO ELECTRONICS: Thevenin’s Theorem and Equivalent Circuits

In the diagram below, a circuit is placed within a box. Next, the circuit is opened on the opposite end of the voltage source inside so that two output terminals are visible. The current ( I ) and potential difference ( V ) in relation to the newly created output terminals is then measured:

The ammeter and voltmeter have been diagrammed in a way that reflects their functionality: an ammeter has current pass through it, so it must be connected in series with a circuit wire to operate. Conversely, since voltages in parallel are the same, the voltmeter is connected in parallel to measure voltage drops that occur across resistors.

The key to understanding Thevenin’s Theorem is the ability to conceptualize how things may be observed from different vantage points. In regard to electrical points-of-view, what is “ seen “ by an observer is determined by provable rules of physics. For the time-being, however, let’s entertain ourselves with relatively simple examples of observations from alternative vantage points.

First, consider a mirror image. Looking into a mirror, we are able to see a standing image of ourselves. If we begin to wave our left hand, the leftmost hand of the image will begin to wave as well. Let’s now assume that an observer is viewing an individual from inside the mirror. The observer within the mirror will see the aforementioned moving hand as being located towards their right-hand side. Within the field of organic chemistry, mirror images of molecules are referred to as enantiomers. The function of two enantiomers may be different within an enzyme or biochemical process, but spatial arrangement of their single-atom constituents are identical. Let’s now look at an example of different vantage point using numbers:

( THREE / TWO ) = SIXTEEN = 16

From an opposite viewpoint, the above equation appears as follows:

( EERHT / OWT ) = NEETXIS = 16

The rearranged letter sequence above is analogous to a circuit being viewed conceptually from another angle. The output of the second equation, however, must be equal to that of the first.