A magnetic field unit quantifies the strength and direction of the magnetic force exerted over a specific area or volume, dictating how strongly a magnetic field will interact with moving charges or magnetic materials. When you are designing a motor controller, selecting an inductor core, or troubleshooting EMI on a PCB, the specific unit you use—Tesla, Gauss, Oersted, or Ampere/meter—determines whether your components will operate efficiently or saturate and fail.
The Direct Answer: Magnetic Field Units Defined
In modern electronics and electrical engineering, the two dominant units you will encounter for magnetic flux density are the Tesla (T) and the Gauss (G). Both measure the B-field, which is the actual magnetic flux passing through a given area.
The Golden Conversion: 1 Tesla (T) = 10,000 Gauss (G) = 1,000,000 microtesla (µT)
Because a single Tesla is a massive amount of magnetic force (roughly the field of an MRI machine), component datasheets and bench measurements almost always use millitesla (mT) or Gauss. A standard N42 neodymium magnet produces a surface field of about 450 mT, or 4,500 G.
What does this unit actually change in a real circuit? The flux density directly dictates the physical limits of your magnetic components. If you push a ferrite transformer core past its saturation flux density (usually around 350 to 400 mT at room temperature), its inductance collapses to near zero. The resulting current spike will instantly destroy your switching MOSFETs. Knowing your exact mT or Gauss limits is the difference between a functioning power supply and a smoking workbench.
The Core Confusion: Flux Density (B) vs. Field Strength (H)
The most common mistake hobbyists and junior engineers make is confusing magnetic flux density (B-field) with magnetic field strength (H-field). They are related, but they measure entirely different physical phenomena.
- B-Field (Flux Density): Measured in Tesla or Gauss. This is the resulting magnetic field you actually measure in the air gap or inside a core. It is what a Hall effect sensor reads.
- H-Field (Magnetic Field Strength): Measured in Amperes per meter (A/m) or Oersteds (Oe). This is the magnetizing force applied by a coil of wire, dependent only on the current and the number of turns, regardless of the core material.
Think of the H-field as the water pressure you apply to a sponge, and the B-field as the amount of water the sponge actually holds. The relationship between them is the material's permeability. If you are calculating the pull-in force of a relay or the torque of a motor, you care about the B-field (Tesla/Gauss). If you are calculating the required current to magnetize a core, you calculate the H-field (A/m).
Worked Example: Sizing a Linear Hall Sensor for a BLDC Motor
Let's apply these units to a real workbench scenario. You are building a custom commutation circuit for a brushless DC (BLDC) motor. The rotor uses N42 neodymium magnets, which have a surface flux density of roughly 450 mT (4,500 Gauss).
If you place a standard digital Hall switch (like the ubiquitous Allegro A3144, which triggers around 30 mT) directly against the rotor, the sensor will permanently latch on, and your microcontroller will read a continuous HIGH on the GPIO pin. You need a linear Hall sensor that outputs an analog voltage proportional to the mT field.
We select the TI DRV5055A1. Here is the bench math:
- VCC: 3.3V (to match our ESP32 ADC)
- Quiescent Output (0 mT): VCC / 2 = 1.65V
- Sensitivity: 25 mV/mT
- Linear Range: ±66 mT
If we mount the sensor directly on the stator, the 450 mT field will saturate the ±66 mT range. We must introduce an air gap. By mounting the sensor 5 mm away from the rotor, the flux density drops to a manageable 50 mT.
Calculating the ADC Voltage:
Vout = Quiescent Voltage + (B-field × Sensitivity)
Vout = 1.65V + (50 mT × 0.025 V/mT)
Vout = 1.65V + 1.25V = 2.90V
A reading of 2.90V sits perfectly within the ESP32's 0-3.3V ADC window, giving you a clean, high-resolution sine wave for precise motor commutation.
Where You Meet Magnetic Field Units in Practice
Beyond motor commutation, you will run into Tesla and Gauss specifications in three critical areas of electrical design:
- Inductor and Transformer Core Saturation: As noted by TDK Electronics, standard manganese-zinc ferrite materials (like PC95) saturate at roughly 390 mT at 25°C, but this drops to about 320 mT at 100°C. If your flyback converter design calculates a peak flux density of 350 mT, it will work on the bench but fail under full load when the core heats up.
- Current Sensing: When using a Hall effect current sensor (like the ACS712), the internal concentrator translates the magnetic field generated by the conductor's current into a localized mT field across the silicon die. A 30A trace might generate a localized field of 50 mT inside the sensor package.
- EMI and PCB Routing: High dI/dt switching nodes generate intense, localized H-fields. If you route a sensitive analog trace (like an I2C or SPI bus) too close to a switching inductor, the changing flux density (measured in µT) will induce noise voltages via Faraday's law of induction.
Decision Tree: Which Sensor and Unit Scale to Pick
Selecting the right magnetic sensor requires matching the expected flux density (in mT or Gauss) to the sensor's linear range or trip point. Use this decision matrix to pick your component.
| Application | Expected Field Strength | Sensor Type Required | Concrete Part Pick |
|---|---|---|---|
| Compass / Earth's Field | 25 to 65 µT (0.25 to 0.65 G) | Magnetometer (I2C) | QMC5883L |
| Fridge Magnet / Proximity Switch | 5 to 20 mT (50 to 200 G) | Digital Hall Switch | TI DRV5012 |
| BLDC Commutation / Joystick | 20 to 100 mT (200 to 1000 G) | Linear Hall (Analog) | TI DRV5055A1 |
| High-Current Busbar / Welding | > 200 mT (> 2000 G) | Fluxgate or Isolated Hall | LEM HTFS 200-P |
The Default Pick: For 90% of workbench motor commutation, custom proximity, and linear position projects, default to the TI DRV5055A1. It covers the highly useful ±66 mT range, operates from a 2.5V to 5.5V VCC, and interfaces directly with a 3.3V microcontroller ADC without requiring an external op-amp or level shifter.
Frequently Asked Questions
Can I convert Oersted (Oe) to Tesla (T) directly?
No. Oersted measures the H-field (magnetizing force in a vacuum or air), while Tesla measures the B-field (resulting flux density). In a vacuum, 1 Oersted roughly equals 1 Gauss (0.1 mT). However, inside a ferromagnetic core, the permeability multiplies the effect. You must know the exact permeability of the core material to convert the applied Oersteds into the resulting Teslas.
Why do modern datasheets use mT instead of Gauss?
The Tesla (and by extension, the millitesla) is the official SI unit for magnetic flux density, as defined by the National Institute of Standards and Technology (NIST). Gauss is part of the older CGS (centimeter-gram-second) system. While legacy American magnetics companies and older hobbyist tutorials still heavily use Gauss, all modern silicon manufacturers (TI, Allegro, Infineon) specify sensitivity in mV/mT.
My multimeter has a 'mT' setting, but it reads 0 near my motor. Why?
Standard multimeters with magnetic attachments only measure static (DC) magnetic fields. If you are measuring the stator of a running AC or BLDC motor, the field is alternating rapidly. The multimeter's sampling rate is too slow to capture the RMS equivalent of the moving flux. You need an oscilloscope connected to a linear Hall sensor to view the dynamic mT waveform.
