Basic Circuit Laws

Engineers designing electronic circuitry for automotive use

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The basic laws of electrical circuits focus on a handful of basic circuit parameters, voltage, current, power, and resistance, and define how they are interrelated.

Unlike some of the more complex electronics relationships and formulas, these basics are used on a regular, if not daily, basis, by anyone that works with electronics. These laws were discovered by Georg Ohm and Gustav Kirchhoff, and are therefore known as Ohms law and Kirchhoff's laws.

Understanding these basic rules is critical for anyone designing a circuit, electronics, or an electrical system.

Ohms Law

Ohms law is the relationship between voltage, current, and resistance in a circuit and it is the most common (and most simple) formula used in electronics. Ohms law can be written in a number of ways, all of which are commonly used.

  • The current flowing through a resistance is equal to the voltage across the resistance divided by the resistance (I=V/R).
  • Voltage is equal to the current flowing through a resistor times its resistance (V=IR)
  • Resistance is equal to the voltage across a resistor divided by the current flowing through it (R=V/I).

Ohms law is also useful in determining the amount of power a circuit uses since the power draw of a circuit is equal to the current flowing through it times the voltage (P=IV). Ohms law can be used to determine the power draw of a circuit as long as two of variables in Ohms law are known for the circuit.

The Ohms law formula is a very powerful tool in electronics, especially since larger circuits can be simplified, but Ohms law is essential at all levels of circuit design and electronics. One of the most basic application of Ohms law and the power relationship is to determine how much power is dissipated as heat in a component. Knowing this is critical so that the right size component with the proper power rating is selected for the application.

For example when selecting a 50 ohm surface mount resistor that will see 5 volts during normal operation, knowing it will need to dissipate (P=IV=> P=(V/R)*V=> P=(5volts^2)/50ohms)=.5 watts) ½ a watt when it sees 5 volts means that a resistor with an even greater power rating than 0.5 watts should be used. Knowing the power usage of the components in a system lets you know if additional thermal issues or cooling may be required and dictates the size of the power supply for the system.

Kirchhoff's Circuit Laws

Tying Ohms law together into a complete system is Kirchhoff's circuit laws. Kirchhoff's Current Law follows the principle of conservation of energy and states that the total sum of all current flowing into a node (or point) on a circuit is equal to the sum of the current flowing out of the node.

A simple example of Kirchhoff's Current Law is a power supply and resistive circuit with several resistors in parallel. One of the nodes of the circuit is where all of the resistors connect to the power supply. At this node, the power supply is supplying current into the node and the current that is supplied is divided up among the resistors and flows out of that node and into the resistors.

Kirchhoff's Voltage Law also follows the principle of conservation of energy and states that the sum of all of the voltages in a complete loop of a circuit must equal zero. Extending the previous example of a power supply with several resistors in parallel between the power supply and ground, each individual loop of the power supply, a resistor, and ground sees the same voltage across the resistor since there is only one resistive element. If a loop had a set of resistors in series the voltage across each resistor would be divided according to the Ohms law relationship.