Solenoid Duty Cycle and Coil Heating, Explained

Duty cycle = on-time ÷ (on-time + off-time) × 100%. A solenoid energized for 2 seconds and then off for 6 seconds runs at a 25% duty cycle. One that stays energized indefinitely runs at 100%, also called continuous duty. European datasheets often give the same figure as ED, from the German Einschaltdauer, meaning on-duration.

Duty cycle alone is not enough. The longest continuous on-time matters too, because a coil heats up during a long pulse even when the average is low. Datasheets pair an intermittent rating with a maximum on-time, and a specification should do the same.

Coil power is current squared times resistance, and nearly all of it becomes heat. Solenoid force rises with coil current, so more force takes more power, and more power means a hotter coil. The limit is the temperature the coil insulation and surrounding materials can tolerate. Magnet wire and coil systems are rated by thermal class, such as Class B at 130 °C, Class F at 155 °C, and Class H at 180 °C, and the hottest point in the coil must stay below that rating at the highest ambient temperature.

Heat also feeds back on performance. Copper's resistance rises about 0.4% per °C, so as the coil warms it draws less current from a fixed voltage, and force falls. A solenoid has to meet its force requirement hot, at the lowest supply voltage, not only when it is cold.

Because the coil cools during the off-time, a solenoid run intermittently can accept more power than one held on continuously. As a first approximation, when the cycle is short compared with the coil's thermal time constant, the allowable power scales inversely with duty cycle: a solenoid that can dissipate a given power continuously can take roughly four times that power at a 25% duty cycle. More power means more force from the same size, which is why intermittent-duty solenoids are smaller for a given force.

The gain has limits. Once the magnetic circuit saturates, extra current adds heat without much extra force, and a long on-time can overheat the coil even at a low average duty cycle.

• Peak-and-hold drive: apply full current to pull in, then drop to a lower holding current once the air gap is closed. Holding a closed gap takes far less current, so heat falls sharply.
• PWM current control: regulate coil current electronically, so the holding power stays constant as the supply voltage and coil temperature change.
• A latching (bistable) design: a permanent magnet holds the plunger with no power at all, which removes the duty-cycle limit on holding.
• Better heat paths: mounting to a metal structure, thermally conductive potting, and a coil designed for the real thermal environment.
• A larger solenoid: more copper and surface area dissipate more heat, at a cost in size and weight.

Give the on-time, the off-time, the longest continuous on-time, and the highest ambient temperature, and say whether the solenoid will be mounted to a metal structure or thermally isolated. With those, a solenoid can be designed to hold for as long as the product needs without running hot.

We model coil temperature rise for your duty cycle and ambient, choose the wire and insulation class to suit, and design the drive, often peak-and-hold or PWM, together with the coil. Prototypes are built in-house in two weeks and characterized on the bench for temperature rise and for force when hot, not only at room temperature.