Kirchhoff's Laws and Their Implication in PCB Design

Are laws meant to be broken? Well, it seems so when parenting is concerned. As a child, mom’s words are the law, and I can’t remember how many times I’ve broken them. Now, I find myself on the receiving end as I’m blessed with a son, and I’m about to give up demanding obedience. 

But laws are there for a reason. It helps keep things proper and ensure conflicts are resolved amicably. When you’re designing a PCB, there are a few fundamental laws that you need to keep in mind, as they directly influence your approach in the design itself. One such law is the Kirchhoff's law, which every electronics graduate should be familiar with. 

What is Kirchoff's Law? 

Let’s take a trip down memory lane and rediscover what Kirchhoff’s law is all about. You may vividly recall how your lecturer states that the Kirchhoff's law dealt with the values of voltage and current in a closed circuit. 

To be specific, there are two parts of Kirchoff’s Law, which are the Kirchoff’s Current Law and Kirchhoff's Voltage law. 

Kirchhoff's Current Law specifies that the total of all current entering and exiting a single node must equal zero. In other words, the law indicates how conservation of charges is applied in a closed circuit.

Meanwhile, Kirchhoff's Voltage Law declares that the summation of voltages in a closed circuit will always equate zero. 

Both Kirchoff’s law formed the fundamentals of circuit designs and determined the best practices that ensure reliability and functionality of a PCB.

Kirchhoff’s Law and the 4-20mA current loop. 

In most commercial and consumer electronics, you’ll be accustomed to digital signalings. But when you’re designing for industrial applications, you’ll find that current loop signalings, such as the 4-20 mA, is the preferred method. 

Transducers usually use the 4-20 mA current loop. Parameters like air-flow, pressure, and velocity are transferred by current instead of voltage. Values between 4 mA and 20 mA indicate a corresponding value to the actual readings. These sensors are usually placed far from the receiving controller, and the input values are critical to the operation of the controller.

4-20mA industrial sensors are based on Kirchhoff’s Current Law

The question of why 4-20 mA current loop is preferred to digital signaling can be answered by Kirchhoff's law. Kirchhoff's Voltage law implies that voltage drop is to be expected over the length of cable. Besides, electrical noises may be coupled and affect the integrity of the reading. 

The 4-20 mA current loop takes advantage of the Kirchoff’s Current Law, which states that the total current entering and exiting a node must be equal. This means that the current generated by the sensor will not suffer any loss as it travels along a distance of the cable. Theoretically, the value that the receiver picks up is equal to what the sensor generates.  

Kirchhoff’s Law and current densities.

It’s easy to reflect and be guided by Kirchoff’s law when it’s a straightforward application like the 4-20 mA current loop. But there are aspects in the PCB design where the Kirchoff’s Law holds true but often overlooked. 

And we’re talking about dealing with current densities. 

It’s common knowledge that PCB traces can only hold an amount of current before heating up. Excessive heat will wear off the copper traces or in some cases, result in broken traces. The PCB trace current-carrying capacity is determined by the width and thickness of the copper. 

Most PCB designers have no issues in getting the PCB traces directly connected to the high-current component right. But problems usually surfaced at the traces where more than one high-current traces combined.

There’s a limit of how much current a PCB trace could take.

The Kirchoff’s Current Law states that the total sum of current entering and entering a node must equal zero. This law means that the PCB trace that carries the combined current must have a sufficiently larger width to prevent heat built-up. 

For example, the return path that is subsequently leading to GND must have the right physical dimension to hold the peak current. Failing to spot these current densities hot spot will result in potential issues when they are deployed. 

To prevent that, you’ll want to hold Kirchoff’s Law in mind and utilize advanced features in the PCB design software you’re using to spot high current densities traces. You’ll want something like the OrCAD's PSpice Simulator to model current parameters accurately. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.