A coil magnet is an electrical component formed by winding conductive wire into a helix that generates a concentrated magnetic field when current flows through it, converting electrical energy into mechanical force or storing energy in a magnetic field. In a real circuit, it introduces inductance that resists rapid current changes, while in an electromechanical installation, it changes electrical current into a physical linear or rotary pull to actuate valves, locks, or switches. Makers commonly confuse the coil (the copper winding itself) with the core or armature (the ferromagnetic material that moves or concentrates the flux), and they frequently mistake a solenoid's high holding force for its much weaker pull-in force at maximum stroke.
The Physics of Coil Magnets: Ampere-Turns and Force
The strength of a coil magnet is not determined by voltage alone, but by the Magnetomotive Force (MMF), measured in Ampere-Turns (AT). The MMF is the product of the current flowing through the wire and the number of loops in the winding. According to Georgia State University's HyperPhysics, the magnetic field inside an ideal solenoid is directly proportional to this current-turn product and the permeability of the core material.
Let's spec a standard 12V DC tubular solenoid with a coil resistance of 24Ω and 800 turns of 28 AWG magnet wire.
• Current (I): 12V / 24Ω = 0.5A
• Magnetomotive Force (MMF): 800 turns × 0.5A = 400 Ampere-Turns (AT)
• Pull-in Force at 5mm air gap: ~2.5 Newtons (approx. 250g)
• Holding Force at 0.5mm air gap: ~28 Newtons (approx. 2.8kg)
Note how the force drops exponentially as the air gap increases. A coil that can hold 3kg once closed might fail to pull a 300g load from 5mm away.
This inverse-square relationship with the air gap is why you must always size your coil magnet based on the pull-in force at maximum stroke, not the holding force listed on the spec sheet. If your mechanical linkage has slop or friction, the armature will stall mid-stroke, the air gap will remain large, and the coil will draw continuous stall current until the insulation melts.
Where You Meet Coil Magnets in Practice
You will encounter coil magnets across almost every domain of electrical and electronics work. Understanding the specific flavor of coil you are dealing with dictates how you drive and protect it.
- Relays and Contactors: The coil here is purely an actuator. A small 12V DC coil current (usually 30mA to 150mA) generates enough flux to pull a steel armature, closing heavy-duty contacts that switch AC mains or high-current DC loads. AC contactor coils feature a copper 'shading ring' embedded in the iron core to prevent the armature from vibrating and buzzing at 120Hz during the AC zero-crossing.
- Solenoid Valves: Used in irrigation, pneumatics, and brewing setups. The coil pulls a steel plunger against a spring to open or close an orifice. These are often rated for continuous duty but run hot.
- Magnetic Locks (Maglocks): Used in access control. These are essentially massive, open-frame coil magnets with no moving parts. The armature plate is the door itself. They are fail-safe (unlock when power is cut) and require massive holding current.
- Inductors and Chokes: In power supplies (like buck converters), the coil magnet is used strictly for energy storage, not mechanical movement. The core is usually ferrite or powdered iron to minimize eddy current losses at high switching frequencies.
Decision Tree: Sizing and Selecting Your Coil
Choosing the wrong coil magnet leads to weak actuation, thermal failure, or blown driver transistors. Use this decision matrix to lock in the right component for your automation build.
| Application Requirement | Coil Type Needed | Concrete Part Recommendation |
|---|---|---|
| Linear push/pull, <10mm stroke, low voltage DC | Tubular DC Solenoid | JF-0630B (12VDC, ~5N pull-in, 8mm stroke) |
| High force, short stroke, AC Mains switching | Laminated AC Solenoid | Ledex 4B (120VAC, intermittent duty) |
| Hold a door/gate closed securely | Magnetic Lock (Maglock) | Seco-Larm E-941SA (12VDC, 600lbs hold) |
| Switch high current loads (motors, heaters) | Contactor / Power Relay | Omron G7J-4A-B (24VDC coil, 25A contacts) |
Drive Circuit Realities: Flyback Diodes and Inrush
Driving a coil magnet is not as simple as connecting it to a battery. Because it is an inductor, it stores energy in its magnetic field. When you interrupt the current (turn off the MOSFET or transistor), the magnetic field collapses rapidly. According to Faraday's law of induction, this rapid change in flux induces a massive reverse voltage spike—often hundreds of volts—which will instantly destroy your driving transistor or microcontroller GPIO.
As detailed in Electronics Tutorials, you must provide a safe path for this stored energy to dissipate. This is done using a flyback diode (also called a freewheeling diode) wired in reverse bias across the coil terminals.
A standard 1N4007 rectifier diode is cheap and handles the voltage spike fine, but it keeps the current circulating longer, causing the solenoid to release sluggishly. If you need a fast drop-out time (e.g., for a high-speed sorting gate), use a Schottky diode like the 1N5819, or place a Zener diode (e.g., 15V) in series with your flyback diode to clamp the voltage higher and burn off the energy faster.
The Thermal Trap: PWM Holding Current
Solenoids require high current to pull the armature in across the air gap, but require very little current to hold it once closed. If you leave a 12V tubular solenoid energized at 100% duty cycle, the $I^2R$ heating will eventually bake the 28 AWG magnet wire, melting the thin enamel insulation and causing an internal short. For any application where the coil must remain energized for more than 30 seconds, use a microcontroller to drive the MOSFET with a PWM signal. Hit it with 100% duty cycle for 150ms to pull it in, then drop the PWM to 25% to hold it. This reduces thermal dissipation by 75% and vastly extends the coil's lifespan.
Frequently Asked Questions
Can I run a 12V DC solenoid on a 24V power supply?
Yes, but only for very short, intermittent pulses. Doubling the voltage doubles the current and quadruples the heat ($I^2R$). You can use 24V to get a faster, harder pull-in strike, but you must use a PWM driver to drop the effective voltage back down to 12V (or lower) immediately after the armature seats, or the coil will burn out in under a minute.
Why is my AC contactor coil buzzing loudly?
AC contactor cores have a copper 'shading ring' (a single shorted turn of copper) embedded in the face of the laminated iron. This ring creates a secondary magnetic field that is slightly out of phase with the main field, preventing the armature from losing its magnetic grip during the 120 times per second that the AC sine wave crosses zero. If the core face is rusty, dirty, or if that shading ring is cracked, the armature will physically vibrate at 120Hz, creating a loud, destructive buzz. Clean the mating faces with electrical contact cleaner and a lint-free cloth; never use oil or grease.
How do I measure the inductance of an unknown coil magnet?
You cannot measure inductance with a standard multimeter (which only reads DC resistance). You need an LCR meter. Set the meter to measure inductance (L) at 1kHz. Be aware that if the coil has a ferromagnetic core that is currently extended (open air gap), the inductance will read much lower than when the armature is fully seated (closed magnetic circuit). Always measure inductance with the armature mechanically held in the fully closed position to get the operational value.
