Thursday, April 16, 2015

The VIR Triangle

The VIR triangle, also known as the triangle of electronic truth, is mostly used to illustrate the concept of Ohm’s Law.  Ohm’s Law simply states that the current flowing through a resistive element is proportional to the voltage drop measured across the element.

Otherwise known as:  I=V/R


Figure 1. VIR triangle with expressions

The relationship of a potential drop across and current through a resistive element is displayed in Figure 2.

Figure 2. Illustration of I=V/R across resistor

The concept of infinite and zero resistance.

What is the first thing that crosses your mind when I say “absolute zero”? Maybe of a temperature so low that it suspends all molecular motion, otherwise known as zero kelvin (0K).  Others may think of superconductors, material that is cooled below a critical temperature to achieve zero resistance.  I usually think of a superconductive loop of wire, a looped wire carrying a once injected current perpetually and indefinitely.  There really is no wrong answer as the question is open ended, ‘absolute zero’ is just such a cool phrase that I had to use it.

Under normal circumstances conductors will always display some resistance, despite how small that resistance may be.

 The resistance of any material can be defined as:

Where ρ is the resistivity of the conductor in question, L is the length of the conductor, and A is the cross-section area of the conductor.  From the expression above it is easy to see the relationship of L and A in the resistance of a material -- decreasing the Length and increasing the area of the conductor will lower the resistance.

Even though wires do in fact have some small resistance, we will consider it negligible in our circuits.  Therefore wires in our future circuits will be considered as having a value of 0 ohms.  This is displayed in Figure 3.
Figure 3. Illustration zero resistance as a short

On the opposite end of the spectrum is infinite resistance.  We can easily deduce from the expression above that increasing the length or decreasing the area of a conductor will increase the resistance, but how can we truly reach infinite resistance? Well, to be infinitely resistive no current should be able to flow through the conductor.  The only way to accomplish zero current is to have the signal path completely open, removing any possible connection from point A to point B, as shown in Figure 4.

Figure 4. Illustration infinite resistance as an open

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