An electronic magnet is the common layman's term for an electromagnet—a coil of wire wrapped around a ferromagnetic core that generates a magnetic field only when electrical current flows through it. While the name suggests solid-state electronics, it is fundamentally an inductive electromechanical component. What it changes in a real circuit is the introduction of high inductance; it stores energy in a magnetic field and will violently release that energy as a reverse voltage spike (back-EMF) the moment you cut the power. Beginners frequently confuse the electronic magnet (which creates a magnetic field to do physical work) with a Hall effect sensor (which detects a magnetic field), or mistakenly assume it behaves like a simple resistive load like a heater.
The Physics and the Inductive Kickback Problem
When DC current flows through the coiled wire of an electronic magnet, it generates a magnetic flux that concentrates through the iron or steel core, creating a powerful pulling force. The core material and the number of wire turns dictate the strength. However, that same coil acts as an inductor, resisting changes in current.
Inductive kickback is the electrical equivalent of water hammer in plumbing. When you suddenly close a valve on a fast-moving pipe of water, the physical momentum of the water creates a massive pressure spike that can burst the pipe. Similarly, when you open a transistor switch on a current-carrying coil, the momentum of the electrons collapsing the magnetic field creates a voltage spike that bursts through your silicon.
A Worked Numeric Example: The 1.75 Million Volt Spike
Let us look at a standard 12V DC access-control maglock (a heavy-duty electronic magnet rated for 600 lbs of holding force, typically costing around $55 to $80). It typically has a coil resistance of 24 ohms, drawing 500mA in steady state. The inductance ($L$) of a large iron-core coil like this is roughly 3.5 Henrys.
If you use an ESP32 and a logic-level MOSFET (like the IRLZ44N) to switch this off, the transistor turns off in about $1 \mu s$ ($10^{-6}$ seconds). The formula for inductive voltage is $V = L \times (di/dt)$.
- V = 3.5 H $\times$ (0.5 A / 0.000001 s)
- V = 1,750,000 Volts
Obviously, the voltage does not actually reach 1.75 million volts in the real world. Instead, it spikes instantly until it finds a path to discharge. It will avalanche through your MOSFET's drain-source junction (which is usually rated for only 55V), permanently shorting the silicon. Worse, the transient can couple back into your microcontroller's 3.3V rail, bricking the ESP32.
Sizing and Driving an Electronic Magnet from a Microcontroller
You cannot drive an electronic magnet directly from a microcontroller GPIO pin. An ESP32 DevKit v1 GPIO pin is limited to roughly 40mA absolute maximum, while even a tiny 5V relay coil draws 70mA to 120mA. You must use a driver transistor.
For DC loads under 2A, an N-channel logic-level MOSFET like the IRLZ44N or a Darlington array like the ULN2003 are the bench standards. Below is the exact pin mapping and component list for driving a 12V maglock with an ESP32.
| Component | Connection / Value | Purpose |
|---|---|---|
| ESP32 GPIO 26 | 100$\Omega$ Resistor $\rightarrow$ MOSFET Gate | Limits inrush current into the gate capacitance to protect the ESP32 pin. |
| MOSFET Gate to GND | 10k$\Omega$ Resistor | Pull-down resistor; ensures the magnet stays OFF while the ESP32 is booting. |
| MOSFET Drain | Electronic Magnet Negative Terminal | Switches the ground path for the coil. |
| MOSFET Source | System Ground (12V PSU GND) | Common ground reference (must be shared with ESP32 GND). |
| Flyback Diode (1N4007) | Across Magnet Terminals (Cathode to +12V) | Clamps the reverse voltage spike to ~0.7V above the supply rail. |
When writing your firmware, remember that turning the GPIO HIGH energizes the magnet. If you are building an access-control system, you must decide whether your installation requires a fail-safe configuration (magnet loses power and door unlocks during a power outage, standard for fire exits) or fail-secure (door remains locked, used for server rooms or vaults). Most commercial electronic maglocks are fail-safe by design.
Where You Meet This In Practice
While the underlying physics is identical, electronic magnets are packaged differently depending on the mechanical work they need to perform. Understanding these variations helps you select the right part and power supply for your project.
| Application | Typical Specs | Core / Armature Type | Primary Function |
|---|---|---|---|
| Maglock (Access Control) | 12V/24V DC, 300-500mA | Flat mating steel plate | Secures doors; high holding force, zero moving parts. |
| Linear Solenoid | 12V/24V DC, 1A-3A peak | Plunger inside a tube | Creates linear pushing/pulling motion (e.g., door latches, valves). |
| Relay / Contactor Coil | 5V to 240V AC/DC, 20-100mA | Hinged armature | Closes high-current electrical contacts; isolates control from load. |
| Magnetic Chuck | 110V DC, variable current | Interlocking pole grid | Holds ferrous workpieces flat on CNC mills and surface grinders. |
For hobbyists and DIY smart-home builders, the 12V linear solenoid (often sold as 'electric door strikes' or 'cabinet locks' for $8 to $15) and the relay coil are the most common encounters. Both require the exact same flyback diode protection outlined above. For deeper reading on inductive load suppression, the All About Circuits DC Textbook chapter on Inductors provides excellent foundational math, while the Adafruit Motor Selection Guide offers practical wiring advice for makers.
Frequently Asked Questions
Can I power an electronic magnet directly from an Arduino GPIO pin?
No. An Arduino Uno (ATmega328P) GPIO pin has an absolute maximum rating of 40mA, and the recommended continuous limit is 20mA. Even the smallest 5V reed relay coils draw roughly 70mA to 120mA. Driving a magnet directly from the pin will overheat the microcontroller's internal trace, permanently damaging the I/O pin or destroying the entire chip. Always use a transistor (like a 2N2222 BJT or an AO3400 MOSFET) as a switch.
Why does my ESP32 reset when my magnetic lock turns off?
This is a classic symptom of inductive kickback causing a brownout. When the MOSFET switches off the magnet, the collapsing magnetic field generates a high-voltage spike. If you are missing a flyback diode, or if your breadboard wiring has long, untwisted jumper wires acting as antennas, this electromagnetic interference (EMI) couples directly into the ESP32's 3.3V voltage regulator or the EN (enable) pin. The voltage dip triggers the internal brownout detector (BOD), forcing a reboot. Install a 1N4007 diode directly across the magnet terminals and add a 10$\mu$F decoupling capacitor across the ESP32's 3.3V and GND pins.
What is the difference between an electronic magnet and a Hall effect sensor?
They are opposites in the signal chain. An electronic magnet (electromagnet) is an actuator; it consumes electrical power to generate a physical magnetic field to move or hold metal. A Hall effect sensor (like the common A3144 or SS49E) is a sensor; it contains a semiconductor element that outputs a voltage or digital signal when it is exposed to an external magnetic field. You use a Hall sensor to detect if a permanent magnet on a door is closed, and you use an electronic magnet to lock that door.
Do AC electronic magnets need a flyback diode?
No, standard flyback diodes do not work on AC circuits because AC current alternates direction, meaning a diode would either conduct continuously (shorting the AC supply) or block only half the cycle. AC contactors and AC solenoids generate inductive spikes just like DC ones, but they are suppressed using an RC snubber network (a resistor and capacitor in series, typically 100$\Omega$ and 0.1$\mu$F) or a MOV (Metal Oxide Varistor) placed across the coil terminals to clamp the voltage symmetrically.
