The pulse of power: Mastering the PWM relay

Imagine a component that combines the heavy-duty muscle of a power relay with the surgical precision of a digital signal. That is the essence of a pulse width modulation (PWM) relay. While traditional switches are often strictly binary, the integration of pulse width modulation allows engineers to go beyond simple “on-off” control, enabling significant power savings and reduced heat signatures.

The “PWM relay” myth

While high-speed switching is often associated with the solid-state relay (SSR), the real magic happens when applying these pulses to a standard electromechanical relay (EMR). By modulating the “hold current” of an EMR coil, you can prevent overheating and drastically extend the life of your hardware. Whether you are managing automotive solenoids or optimizing industrial control panels, understanding the synergy between PWM and EMR is the key to transforming a basic mechanical switch into a sophisticated, energy-efficient power management tool.

However, if you head to an electronics distributor, looking for a “PWM relay,” you will likely hit a dead end. You cannot easily buy a dedicated PWM-enabled or PWM-driven EMR off the shelf because PWM is not a physical feature of the relay itself; it’s a control strategy applied by the external circuit.

To achieve this, you typically need a devoted relay driver or a microcontroller to manage the signal. By sending a high-frequency pulse to a standard, inexpensive EMR, you effectively turn a “dumb” mechanical switch into a “smart” energy-saver. While an SSR is natively capable of high-speed switching for load modulation, using PWM with a traditional EMR is specifically about optimizing the coil’s efficiency, allowing you to reap the benefits of mechanical isolation without the drawback of a roasting-hot solenoid.

The “holding current” tweak

Nowadays electromechanical relays are widely used across automation systems because they enable a low-power signal to control a high-power circuit. Yet, the conventional method of relay operation is relatively energy-intensive, often producing excess heat and demanding a sizeable power supply. In practice, energizing a relay requires more power than simply holding it in the active state.

This opens the door to efficiency gains: by applying pulse width modulation to the coil’s holding current, we can reduce the duty cycle and thereby lower the average current. The result is decreased power consumption, less heat generation, and improved thermal management—particularly valuable in applications that employ banks of relays.

As a quick design example, begin by switching the relay driver MOSFET fully on to apply voltage to the coil for at least 100 ms. During this initial energizing phase, set the duty cycle to 100% to ensure the MOSFET is fully on, and the relay pulls in reliably.

Once the relay is engaged, transition to PWM control with a reduced duty cycle—say 50%—to sustain the relay state while cutting power consumption. This approach maintains functionality while significantly lowering average current draw, reducing heat, and improving overall efficiency.

Figure 1 Basic schematic illustrates PWM control for lowering relay coil holding voltage. Source: Author

As an aside, while current is the physical mechanism at play, “holding voltage” is a very common industry term because engineers often think in terms of the voltage applied to the circuit.

Practical switching: EMRs and PWM

On the workbench, additional considerations arise when using PWM to drive EMRs.

In conventional relay designs, the nominal coil voltage must be continuously applied to keep the relay energized, which reduces overall energy efficiency. By contrast, PWM-driven relays can operate with reduced effective coil voltage, significantly lowering power consumption, an advantage in energy-conscious applications.

PWM drivers regulate the effective voltage by adjusting the duty cycle of a DC signal at a fixed frequency. A quick note: Duty cycle is usually given as a percentage, while duty ratio is the same concept expressed as a fraction. Relay coils, being inductive, respond to duty-cycle transitions with current fluctuations. The resulting ripple depends on coil inductance, suppression circuitry, PWM frequency, voltage level, and duty cycle.

Best practice is to begin with a 100% duty cycle until the relay pulls in and stabilizes. The required time varies with relay type and excess voltage but typically falls between 100–500 milliseconds. Afterward, the duty cycle can be reduced to maintain holding current.

Higher PWM frequencies reduce ripple, allowing lower effective coil voltages while keeping other parameters constant. Frequencies in the 20–100 kHz range are generally recommended. Since effective coil voltage equals the product of supply voltage and duty cycle, tight regulation is essential. Even small supply variations demand rapid duty-cycle adjustment—within a few milliseconds—to prevent the effective voltage from dropping below the relay’s minimum requirement.

For reliable performance, coil current must always exceed the holding current plus a margin for shock and vibration. If current falls below this threshold, the armature may release, causing repeated pull-in cycles. Such instability can lead to humming noise, unintended contact opening under load, or even contact welding.

Notably, an increasing range of EMRs now support PWM-regulated holding currents to improve thermal management and efficiency. By modulating the duty cycle once the armature is seated, these relays minimize steady-state power dissipation. The Omron G2RL-1A-E-PW1 exemplifies this trend, featuring a coil architecture optimized for PWM and reduced-voltage holding.

Figure 2 The G2RL-1A-E-PW1 relay utilizes PWM control to minimize coil power consumption and heat. Source: Omron

What is more, dedicated PWM current controllers like DRV110 and DRV120 are specifically engineered to optimize relay and solenoid operation through precise waveform regulation. These ICs rapidly ramp the current to a peak level to ensure the plunger or contactor fully seats.

Once actuation is confirmed, they transition to a significantly lower hold current, which maintains the magnetic field while drastically reducing power dissipation. By managing this peak-to-hold transition automatically, these controllers prevent thermal overhead and extend the operating life of the inductive load.

Figure 3 A prewired DRV120 module empowers makers and experimenters to slash relay power consumption by automatically transitioning from pull-in to hold current. Source: tindie

Clever pulses never stop

Where does this leave us? Whether through basic RC mechanisms, dedicated integrated solutions, or the efficiency gains of PWM applied to electromechanical relays, engineers have a wide range of proven strategies to reduce relay energy consumption.

This is more significant nowadays in the era of EVs and e-mobility, where every watt saved translates into extended range and smarter system design. Yet beyond the established lies the experiment, where unproven methods await bold exploration.

Energy efficiency is not just about saving power; it’s about sparking possibilities, and the next breakthrough may come from your own trial and error. If you have worked with PWM-driven electromechanical relays or discovered alternative approaches, share your insights in the comments and help expand the collective knowledge base for engineers everywhere.

T. K. Hareendran is a self-taught electronics enthusiast with a strong passion for innovative circuit design and hands-on technology. He develops both experimental and practical electronic projects, documenting and sharing his work to support fellow tinkerers and learners. Beyond the workbench, he dedicates time to technical writing and hardware evaluations to contribute meaningfully to the maker community.

Related Content

The post The pulse of power: Mastering the PWM relay appeared first on EDN.

Become a member

Become a member

Become a subscriber

Become a sponsor