Decoding the "102" Inductor Value: 1mH or 1µH?
If you are squinting at a tiny component with "102" printed on the top and wondering what the actual inductance is, you have hit one of the most common traps in passive component identification. Unlike resistors and capacitors, which universally use picofarads or ohms as their base unit for the standard 3-digit EIA code, inductors split into two distinct camps based on their application.
The direct answer: The "102" marking means 10 × 10² (1,000) of the base unit.
• On standard power inductors, chokes, and through-hole parts, the base unit is microhenries (µH). Therefore, 102 = 1,000 µH = 1 mH.
• On tiny RF/microwave SMD chip inductors (like 0603 or 0402 packages), the base unit is nanohenries (nH). Therefore, 102 = 1,000 nH = 1 µH.
How do you know which one you are holding without an LCR meter? Look at the physical volume. A 1 mH inductor requires hundreds of turns of fine enameled copper wire wrapped around a high-permeability ferrite core. It is physically impossible to fit 1 mH of inductance into a 0603 (1.6mm x 0.8mm) ceramic chip. If your "102" component is larger than 3mm (e.g., a 1210 package, a 7x7mm shielded can, or a radial leaded choke), it is 1 mH. If it is a microscopic surface-mount chip, it is 1 µH.
Reading Physical Markings and Package Clues
Manufacturers use several methods to stamp the 102 inductor value onto physical parts. Understanding these visual cues saves you from desoldering a perfectly good component just to measure it.
- Printed 3-Digit Code (SMD Power): Common on shielded and unshielded ferrite drum inductors (e.g., 5x5mm or larger). "102" is printed in white or laser-etched on the top shield. Base unit is µH (1mH).
- Color Bands (Radial Leaded): Older through-hole power chokes use resistor-style color coding. Brown (1), Black (0), Red (×100) = 1,000 µH (1mH). The final band is usually silver (±10%) or gold (±5%) tolerance.
- Alphanumeric Codes (RF SMD): High-frequency inductors from brands like Coilcraft or Murata often abandon the 3-digit system to avoid this exact confusion. You will more commonly see "10N" (10nH) or "1R0" (1.0µH). However, if a generic RF chip does use "102", the base is nH (1µH).
- No Marking (Molded Ferrite): Many small multilayer ceramic inductors are unmarked. If you suspect an unmarked 0805 chip is a 1µH inductor, you must pull it from the board and test it with an LCR meter set to 1 MHz.
Inductor Construction Types and Selection Matrix
Knowing the value is only half the battle; the internal construction dictates where the part can survive. Substituting a 1mH multilayer ceramic chip into a DC-DC power rail will result in instant saturation and a short circuit. Use this matrix to match the construction to the job.
| Construction Type | Core Material | Typical Tolerance | Tempco (ppm/°C) | Best Application |
|---|---|---|---|---|
| Shielded Ferrite Power | Ferrite drum + powder shield | ±20% | N/A (Non-linear) | DC-DC buck/boost converters, high-current filtering. Handles high Isat. |
| Unshielded Ferrite Power | Exposed ferrite drum | ±10% to ±20% | N/A (Non-linear) | Cost-sensitive power rails, low-profile designs. Prone to radiating EMI. |
| Multilayer Ceramic (RF) | Low-temp co-fired ceramic (LTCC) | ±2% to ±10% | +100 to +250 | RF matching, high-frequency filtering (>100 MHz). Low current, low DCR. |
| Wirewound RF | Alumina or air core | ±1% to ±5% | -20 to +50 | Critical RF oscillators, narrow-band filters. Highest Q factor available. |
Failure Modes: What a Bad 102 Inductor Looks Like
Inductors are generally robust, but they fail in specific, identifiable ways when pushed beyond their datasheet limits. Before you blame your switching regulator IC, inspect the inductor for these symptoms.
1. Thermal Overload and Core Saturation
Visual Symptom: The epoxy coating on a radial inductor is melted, cracked, or discolored brown. On SMD shielded types, the plastic overmold may bulge, or the PCB pads beneath the part show heat-tinting (darkened FR4).
The Physics: The circuit exceeded the inductor's saturation current (Isat). The ferrite core lost its permeability, causing the inductance to drop to near-zero. The component effectively became a low-resistance wire, drawing massive current and overheating.
Test: Measure DC Resistance (DCR) with a multimeter. It will often read near 0Ω (short) or slightly lower than the datasheet spec due to copper annealing.
2. Mechanical Shock (Cracked Ferrite)
Visual Symptom: A hairline crack running through the ferrite shield or drum core, often invisible without a 10x loupe. Common in boards subjected to drop testing or ultrasonic cleaning.
The Physics: Ferrite is essentially brittle ceramic. A crack introduces an unintended air gap into the magnetic circuit, drastically dropping the inductance and causing the part to radiate electromagnetic interference (EMI).
Test: Requires an LCR meter. DCR will measure perfectly normal, but inductance (L) will read 20-50% below the 1mH nominal value.
3. Internal Wire Break (Open Circuit)
Visual Symptom: The component looks pristine. No burn marks, no cracks.
The Physics: Thermal cycling or a mechanical flex event snapped the fine enameled copper wire inside the winding, usually right at the termination point where it solders to the end cap.
Test: Multimeter reads "OL" (infinite resistance). Note: Always measure out-of-circuit; parallel PCB traces can mask an open inductor by providing an alternate DC path.
The Substitution Decision Tree: Picking the Right Replacement
When your exact OEM inductor is on a 40-week backorder, you need a systematic way to pick a replacement that will not blow up your prototype. Follow this decision path to terminate on a concrete part number.
| Circuit Function | Critical Parameter to Match | Secondary Parameter | Concrete Default Pick (1mH / 102) |
|---|---|---|---|
| DC-DC Power Rail Filter | Saturation Current (Isat) ≥ Peak Switch Current | DCR ≤ Original (to prevent voltage drop) | Bourns SRP1265A-102M (1mH, 0.75A Isat, shielded) |
| Input EMI / RFI Choke | Impedance (Z) at target noise frequency | Rated RMS Current (Irms) ≥ Max Load | Wurth 744774210 (1mH, 0.5A, high impedance) |
| RF Matching (0603 size) | Inductance (1µH) + Self-Resonant Freq (SRF) | Q-Factor > 30 at operating frequency | Coilcraft 0603CS-102XJRB (1µH, wirewound, ±5%) |
| Low-Freq Signal Filtering | Inductance tolerance (±10% or better) | Physical footprint / height limit | Taiyo Yuden CB2518T102M (1mH, unshielded, low profile) |
Safe Substitution Rules When the Exact Part is Missing
If you cannot source the exact Bourns or Wurth part listed above, you can safely substitute a generic inductor provided you strictly adhere to these four engineering rules. For a deeper dive into magnetics selection, the Coilcraft Inductor Basics knowledge center provides excellent manufacturer-level guidance on these parameters.
- Inductance (L) Must Match Within Tolerance: For power supply filtering, a 1mH ±20% part is acceptable. You can substitute a 1.2mH or 800µH part if the regulator's compensation loop is stable. For RF matching networks, you must hold ±2% to ±5% tolerance; a 20% variance will shift your antenna resonance and kill your link budget.
- Saturation Current (Isat) is Non-Negotiable: Isat is the current at which inductance drops by 30%. If your original part had an Isat of 1.5A, your replacement must be ≥ 1.5A. Substituting a part with a lower Isat will cause the inductor to saturate during load transients, resulting in massive current spikes that will destroy your switching MOSFET.
- DC Resistance (DCR) Should Be Lower or Equal: DCR causes I²R heating and voltage drop. If your original 1mH inductor had a DCR of 0.5Ω, do not substitute one with 1.2Ω. The higher resistance will cause the part to run hotter and drop the output voltage of your power rail. You can find detailed thermal models for power inductors using tools like the Wurth Elektronik REDEXPERT simulator.
- Self-Resonant Frequency (SRF) Must Stay High: Every inductor has parasitic parallel capacitance, creating an LC tank circuit. Above the SRF, the inductor stops acting like an inductor and becomes a capacitor. Ensure your replacement's SRF is at least one decade (10x) higher than the switching frequency of your circuit. For a 500kHz buck converter, an SRF of 5MHz or higher is mandatory.
By combining the physical size heuristic with the EIA base-unit rules, you can instantly decode whether that "102" marking means 1mH or 1µH. Keep your LCR meter on the bench, verify the Isat and DCR before soldering, and your substitution will hold up to production-level stress testing.
