Tips for Finding Current in a Circuit

March 5, 2020 Cadence PCB Solutions

 

Ohm’s law relates current with voltage and resistance

 

One of my previous trips abroad almost ended in a disaster. As a meticulous person, I’ve always kept my passport by my side. Before you wonder, no, I didn’t lose the passport but rather the arrival slip that I ought to have returned to immigration before boarding the flight. 

It took me 30 minutes of frantic searching before finding the slip in a heap of receipts. I’ve no idea how it ended among the receipts, nor how I’ve ended with spending almost all my money that results in that thick pile of receipts.

Thankfully, I didn’t have to go through the hassle of hours of interrogation by the immigration department. Similarly, you can get into various trouble if you forgot to calculate the current in your circuit, particularly when it’s crucial to the functionality of the design.  

Finding Current In A Circuit With Ohm’s Law

You can’t discuss calculating current without bringing up the most basic laws of electronics: Ohm’s Law. Ohm’s Law denotes the relation between voltage, current, and resistance. You ought to be familiar with the equation V = IR. 

From the equation, you can easily get the value of the current by dividing the voltage with the resistance or: 

I = V/R. 

That’s basic electronics that you ought to know before even drafting a circuit. The calculation looks simple when you have a simple circuit with a single voltage and resistor element. In a more complex circuit where you have a nest of series and parallel configurations, you’ll need to analyze the circuit between nodes. 

A voltage divider calculation will be handy in determining the voltage that falls between two nodes. It also helps to understand that electric current splits into different branches according to the resistance of the individual branch. 

Current Calculation For Capacitor And Inductor

As useful as it is, Ohm’s law only applies for resistive loads in the circuit. If you have a capacitor and inductor, you’ll need to approach the circuit analysis differently. In a DC circuit, where the voltage source is a DC signal, the following rules apply:

  • All inductors are short circuit.

  • All capacitors are open circuit.

However, the above assumptions are no longer true when AC signals are applied to capacitors and inductors. 

The current flowing through a capacitor from a sinusoidal voltage is stored as a charge and it’s given by the following formula:

I = C (dv/dt)

From the equation, it can be seen that the capacitor goes through a continuous series of charge and discharge when an AC voltage is applied.

 

Capacitors on top of a circuit

Capacitors behave differently in DC and AC analysis. 

 

As for the inductor, the calculation is slightly more complicated due to the characteristic of the component. Inductors produce an opposing force to the current that flows through it and the current is calculated by:

I = 1/L ∫vdt

When you plot the values of current against voltage for capacitors and inductors, you’ll notice that both parameters are 90 degrees out of phase with each other. 

Determining Current For Non-Linear Circuits

It’s a mistake to overlook the current calculation for non-linear components. Resistors, capacitors, and inductors are linear components, which means the resulting current is proportionate to the voltage applied. 

Non-linear components have an I-V curve that is not proportionate, in which you can’t determine the value of a current with a single formula. For example, a diode is a non-linear component as very little current passes through when the voltage across it is less than its forward voltage (Vf). As the voltage passes Vf, the forward current increases drastically and it’s only limited by the maximum limit of the component. 

 

A non-linear diode

Diodes are non-linear components where current is not proportionate to the voltage applied.

 

Transistors, varistors, and Zener diodes are some examples of non-linear components. When these components are placed in a circuit, you’ll need to consider how the non-linear I-V curve will affect the flow of the current to other components. 

It’s important to know the current that is passing through in a circuit if you’re working on a PCB. The amount of current will determine the thickness and width of the PCB trace. This is crucial when you’re dealing with high-power designs. 

Working with the suite of layout tools and analysis options from Cadence can make any of your current design challenges a breeze. However, manually calculating the current can be tedious, and even experienced designers do make mistakes. Therefore, it helps if you’re using a PCB software like OrCAD PCB Designer that’s equipped with a SPICE simulator. Verifying the calculation will minimize the errors. 

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

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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