Coils for relays and solenoids

What the coil has to do

A relay or solenoid coil converts current into a magnetic field that moves an armature. The coil itself is a stack of turns around a core or a bobbin, and its job is to produce enough magnetomotive force at the working gap to overcome the spring, the friction and the load.

The magnetomotive force is proportional to the ampere-turns, the product of current and turns. For a given geometry, the force at the armature rises with the square of the ampere-turns and falls as the air gap opens. That single relationship explains most of the difference between relay and solenoid design.

The coil also has to do its job without overheating, without breaking down and without changing its behaviour over life. Those are thermal, insulation and endurance questions, and they are often the ones that decide the wire and the bobbin.

Force against stroke in a solenoid

A solenoid starts with the armature away from the core and pulls it in. At the open position the air gap is large, the reluctance is high and the force for a given current is low. As the armature closes, the gap shrinks and the force rises steeply.

The design has to produce enough force at the worst point, which is usually the start of the stroke. That requirement sets the ampere-turns, and the ampere-turns set the wire and the window. A coil that is sized at the closed position will often fail to pull in.

The return spring adds force in the opposite direction. The coil must overcome the spring plus the load plus friction at every point in the stroke, with margin for low voltage and high temperature. A solenoid that just pulls in on the bench may stall in a warm cabinet.

Stroke also changes the inductance. An open gap gives lower inductance and a faster current rise; a closed gap gives higher inductance and a slower rise. A drive that ignores this may have a slow release or a high flyback spike.

Relay coils: a small gap and a defined threshold

A relay coil moves an armature across a very small gap. The force requirement is more stable over the stroke, and the design is usually judged by the must-operate and must-release values rather than by a force curve.

Must-operate is the voltage or current at which the relay is guaranteed to close. Must-release is the value at which it is guaranteed to open. Both are stated at a temperature and usually include a tolerance for the coil resistance, which rises as the coil heats.

Copper resistance rises with temperature. A winding at 100 °C has roughly 30 per cent more resistance than at 20 °C, so the current at a fixed voltage falls. A relay that operates cold may not operate hot, so the datasheet should state the temperature for the must-operate value.

Relay coils are often designed for continuous duty, so holding power is the limit. A latching relay changes that: the coil only pulses, so the thermal constraint is much lower. The drive and the coil design have to agree on the pulse width.

Heat and duty cycle

Copper loss is the main heat source. It is the current squared times the resistance, and the resistance rises with temperature. That feedback means a coil settles at a temperature well above the ambient, set by the thermal resistance of the assembly.

Continuous duty is the hardest case for a small coil, because the heat reaches a steady state. Intermittent duty allows a higher current for a short time if the average loss over the cycle stays within the limit.

Write the duty as a percentage and a cycle time, not just a word. A coil rated at 25 per cent with a 10 minute cycle is a different part from one rated at 25 per cent with a 1 minute cycle, because the thermal time constant decides how much of the peak the coil sees.

Ambient is part of the duty. A coil inside a sealed enclosure may see 20 K or more above the room, and one next to a power device may see more. Start the calculation from the air at the coil.

Pull-in, hold and the drive circuit

A solenoid needs more current to pull in than to hold. Many designs apply a pull-in current briefly, then reduce to a hold current with a resistor, a pulse width modulated supply or a current controller. That reduces heat and allows a smaller coil.

The drive has to match the coil. A coil designed for a 10 per cent duty pulse will overheat on a continuous supply. A coil designed for continuous duty will be larger and more expensive than it needs to be if the application only pulses.

The release behaviour matters as much as the pull-in. When the drive is removed, the magnetic field collapses and the current continues through the flyback path. A diode slows the release; a zener or a resistor speeds it up at the cost of a higher voltage spike. The coil insulation and the switch have to survive that spike.

For a relay, release time affects contact timing; for a solenoid, it affects the return stroke. Review the coil, the spring and the drive together, because a change in one moves the other two.

Wire, window and fill

The winding window sets the maximum copper. A larger wire lowers the resistance and the loss for a given current, but it reduces the number of turns for a given window. The designer trades turns against resistance until the ampere-turns and the thermal limit are both satisfied.

The fill factor decides how much of the window is copper. A layer wind gives a higher and more predictable fill than a random wind, which matters on a small coil where every turn counts. The bobbin wall and the insulation also take space that is not available to copper.

The wire grade should match the thermal class. A class F coil with a class B enamel is a weak point even if the bobbin and varnish are rated higher. The insulation class of the whole system is the limit.

Termination is part of the winding design. A fine wire coil with a long lead needs strain relief, or the first movement of the lead will break the wire at the bobbin edge. The joint and the lead route belong on the drawing.

Relay coil and solenoid coil priorities

The two products share a physics but not a specification. The table below shows where the design attention usually goes.

Proving the coil

A coil is proved by a combination of electrical, thermal and functional tests. Resistance at room temperature confirms the wire and the turns. Inductance confirms the magnetic circuit. A high potential test confirms the insulation.

The functional test should be done at the worst conditions: minimum voltage, maximum ambient and, for a solenoid, the worst stroke position. A pull-in test at nominal voltage on a cold coil proves very little.

A thermal test records the temperature rise to a steady state at the rated duty. The result should be compared with the class limit and the hot spot allowance, not just the average. A resistance measurement during the test gives the average winding temperature.

Life testing follows. A coil that passes first article may still fail after a few thousand cycles if the wire moves, the varnish cracks or the joint work hardens. Agree the cycle count and acceptance criteria before the test starts.