What is Lenz Law and How Does it Affect PCB Design?

November 1, 2018 Altium Designer

Inductive coil

Team meetings can be frustrating if you have a team member who is constantly opposing your ideas just for the sake of doing so. Dealing with opposing current is much like dealing with such an opponent. Simply following the norms and coming up with sound arguments isn’t enough; when dealing with opposition, in team settings as well as PCB design, you need to develop an alternative approach.

What is Lenz Law?

In case you need a refresher on your physics fundamentals, Lenz Law states that any induced electromagnetic field (back EMF) will result in current and magnetic fields that are opposed to the change. This theory can be simplified by the Lenz Law equation:

The negative sign on the equation signifies the opposing change that occurs as the magnetic flux opposes the change in the induced back EMF.

Lenz Law can also be expressed from another perspective where induced current flows in the opposite direction to the change causing it. This statement, in fact, brings you closer to the effect of Lenz Law in PCB design.

Lenz Law, Back-EMF, and Inductor Coil

Energized inductor

Figure 1: Inductor is energized

The simplest demonstration of Lenz Law can be arranged with a simple connection of a DC battery, switch, and an inductor coil, as displayed in the above figure (Figure 1). When the switch is closed, forming a complete circuit, the current flows in a counterclockwise manner. According to the Lenz Law, an electromagnetic field on the inductor will be induced in a direction opposed with the current flow caused by the battery.

De-energized inductor

Figure 2: Inductor is de-energized

As the back EMF builds up at the inductor, the switch opens and the circuit breaks. As Lenz Law emphasizes, induced current always opposes the factors that change it. As a result, the magnetic field at the inductor changes direction and polarity as it attempts to continue the flow of the current. The opposing electromagnetic field that occurs when the circuit is broken is called back EMF.

Back EMF is fundamental to the operation of electric motors as it creates an opposing magnetic field that turns the rotor. The back EMF in a motor always assumes almost the same value of the voltage supply.

Disruptive Effect of Back EMF And Prevention

While back EMF can be the driving force of DC motors, it can also be the menace that causes multiple problems on a PCB. One of the most inductive elements on a PCB design is a mechanical relay. Mechanical relays consist of inductive coils that become electromagnetic when energized.

Energizing a mechanical relay is generally harmless, but when the relay is released, the back EMF generated can affect hardware stability. For instance, microcontrollers may experience a hard reset each time a relay is released, or the back EMF may introduce large enough current in reverse polarity to damage immediate components.

The following schematic (Figure 3) shows a mechanical relay that has been de-energized. The back EMF induced at the inductive coil of the relay attempts to maintain the flow of the current when the relay is energized. As the transistor is now in an ‘off’ state, an increasing positive voltage may cause damage if it exceeds the breakdown voltage of the junction.

De-energized relay with back EMF

Figure 3: Relay is de-energized, producing back EMF

Back EMF can also cause arcing on the relay if you’re connecting a DC motor to open contacts of the relay. As DC motors are made of inductive coils, the same theory of Lenz Law applies when it is disconnected. As the back EMF attempts to maintain the decreasing current, a high reverse potential may cause arcing over the gap of the relay contact. This phenomenon may induce electromagnetic interference (EMI) that compromises hardware stability.

The easiest way to mitigate the effect of back EMF is using a flyback diode. This is done by placing a diode across the inductive coil in reverse polarity when the coil is energized. When the coil is de-energized, the diode becomes forward biased, providing a path for safely discharging the back EMF without affecting other nearby components.

With reliable PCB design software like Altium  Designer®, you can leverage an expansive to prevent unwanted effects of Lenz Law and keep your PCB layout organized. If you’re dealing with issues related to EMF or want to learn more about the effects of Lenz Law on your PCB design, talk to an Altium expert.

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Altium Designer

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