A capacitor marked 225K 400V is usually a 2.2 µF capacitor with ±10% tolerance. It is commonly a film capacitor rated for 400 VDC, but the marking alone does not confirm its construction, AC rating, safety class or suitability for a particular circuit. Do not treat 400 VDC as equivalent to 400 VAC.
How to decode 225K 400V
225 means 2.2 µF
For many non-electrolytic capacitors, the three-digit code expresses capacitance in picofarads: the first two digits are significant figures and the third is the power-of-ten multiplier. So 225 means 22 × 105 pF, or 2,200,000 pF: 2.2 µF (also 2,200 nF). The last digit is not part of a 225 pF value. This is a common marking convention, not proof that every component bearing those characters is the same kind of capacitor. Electronics Notes explains the code convention.
K means ±10% tolerance
In the standard capacitor tolerance code, K means ±10%. For a nominal 2.2 µF part, that corresponds to about 1.98–2.42 µF under the manufacturer’s specified measurement conditions. Tolerance describes allowable capacitance variation; it is not a quality grade or voltage rating. Manufacturer references include TDK’s film-capacitor data and Vishay’s MKP 1845 datasheet.
400V is a voltage marking, not a complete application rating
On general-purpose film capacitors, a bare 400V marking commonly refers to a 400 VDC rating. Confirm the exact component’s datasheet: some parts state AC ratings separately, and the visible marking may be incomplete or part of a longer code. For example, one Cornell Dubilier part is listed as 2.2 µF, ±10%, 400 VDC, while Vishay documents distinguish DC and permissible AC ratings for its film-capacitor families (MKP 1845; MKT family).
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What kind of capacitor is it?
A 225K 400V part is often a non-polarized film capacitor, especially if it has a box-shaped or cylindrical body and two wire leads. Additional markings may identify the construction: MKT commonly indicates metallized polyester, while MKP or MPP commonly indicates metallized polypropylene. A manufacturer example explicitly identifies 225K 400V MKT as a film-capacitor marking (WEET marking and construction guide).
The code does not establish dielectric, temperature range, ripple-current or pulse capability, ESR, lead spacing, safety certification or whether the part is new and undamaged. Nor does it prove that the component is film: ceramic capacitors can also use 225K to indicate 2.2 µF and ±10%, as in this Kyocera AVX ceramic-part listing. Physical form and all markings matter.
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Ordinary film capacitors are generally non-polarized, so either lead can usually go in either direction in an appropriate circuit. Check the body and datasheet rather than assuming: a polarity stripe or +/− mark indicates an installation requirement. Capacitor markings may be split across package surfaces; TDK describes marking and ordering-code fields.
Why 400 VDC is not 400 VAC
AC voltage is commonly specified as RMS, while its peak voltage is higher: Vpeak = VRMS × √2. Thus, 230 VAC RMS reaches about 325 V peak, and 400 VAC RMS about 566 V peak. A 400 VDC rating does not establish suitability for 400 VAC; repetitive stress, transients and the capacitor’s actual AC rating also matter.
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- Lead Spacing (P): 10mm(0.394in). Lead Diameter ( ⌀d): 0.6mm ±0.1mm/0.02in. Dimensions (W×H×T): 11.5x9.5x5.2mm(0.45x0.37x0.20in). Operating Temperature Range: -40℃ to +105℃. Capacitance Tolerance: ±5%
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For mains connections, use a capacitor with the required safety classification. An X-class capacitor is intended for specified line-to-line positions; a Y-class capacitor is intended for specified line-to-earth positions. A general-purpose 400 VDC film capacitor is not a substitute for an X or Y safety capacitor merely because its voltage marking looks sufficient. See Vishay’s X/Y film-capacitor information and the safety-capacitor classification reference. Do not use a bare 225K 400V part in a mains position unless its exact datasheet and required approvals confirm suitability.
Where a 2.2 µF film capacitor may be used
Depending on its specific design, a 2.2 µF film capacitor may serve for DC blocking or coupling, filtering, timing, energy storage, a snubber, a DC link, or a ballast or switching circuit. These are possibilities, not guarantees based on the value code. High-current, resonant, PFC and pulse applications can demand particular ripple-current, dv/dt, temperature and lifetime ratings. Manufacturers maintain separate families for these stresses; see TDK’s snubber, resonant and PFC film-capacitor families and Vishay’s application and rating data.
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Identify the exact component before replacing it
- Record every marking. Photograph all sides and note the logo, full code, voltage notation, safety symbols and date or lot codes. Small parts may have markings split between surfaces.
- Identify its physical construction. Note whether it is a radial or axial film part, a disc or surface-mount ceramic, an electrolytic can, or another form.
- Check for decisive markings. Look for MKT, MKP, MPP, X1/X2, Y1/Y2, an explicit VAC or VDC rating, and polarity signs.
- Measure the body and lead spacing. A nominally equivalent part may not fit the board or preserve required clearances.
- Use the circuit position as a clue, not a substitute for a datasheet. A part across line and neutral may be an X capacitor; one from line to earth may be a Y capacitor. A part across a switch or winding may face snubber or pulse stresses.
- Find the manufacturer’s datasheet or verified part listing. A distributor photo by itself cannot establish the full electrical specification.
Choose a replacement by more than capacitance and voltage
For a direct replacement, verify the original circuit requirements and compare the following:
- Capacitance and tolerance: typically 2.2 µF and ±10% for this marking; use the circuit’s specified tolerance if different.
- Voltage type and rating: confirm DC versus AC, and suitable margin for the circuit’s operating voltage and transients.
- Construction and dielectric: match the specified film type or other technology where its electrical behavior matters.
- Safety approval: retain the required X/Y class and approvals in mains-suppression positions.
- Ripple and pulse performance: check current, dv/dt and pulse-energy ratings for power, snubber and resonant circuits.
- Temperature, lifetime and physical fit: verify operating conditions, lead spacing, size, mounting and insulation clearances.
A 450 V or 630 V rated part can sometimes replace a 400 VDC part if capacitance, construction, pulse and ripple behavior, temperature and physical fit are appropriate. A larger printed voltage rating alone does not make it a suitable replacement. Likewise, a 2.2 µF electrolytic is usually not a drop-in substitute for a film capacitor: polarity, ESR, ripple behavior and suitability for bipolar or high-frequency signals can differ.
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For example, DigiKey lists the Cornell Dubilier 225MPW400K as a 2.2 µF, ±10%, 400 VDC axial polypropylene part, and the 225PPB400K as a radial part with the same nominal capacitance and DC voltage. These examples show why package and datasheet checks matter; neither is automatically right for a particular board. A part number containing 225K is not a universal replacement specification.
Test it carefully—and treat it as charged
Visual inspection may reveal cracks, bulging, melted or discolored encapsulation, damaged leads or signs of overheating. A clean appearance does not prove that a capacitor is sound; it can fail open or short, leak, or drift.
A capacitance meter can give a basic value check, but it does not test high-voltage insulation or leakage, AC-current capability, pulse endurance or safety certification. If you are qualified to service the equipment and have its service procedure and properly rated instruments:
- Disconnect power and follow the equipment’s service procedure.
- Use an appropriate, properly rated discharge method; do not short the capacitor directly with a screwdriver.
- Verify the voltage is discharged with a meter before touching the circuit.
- Isolate at least one lead from the circuit before measuring capacitance, then compare the reading with the specified tolerance.
Do not choose a universal resistor value or discharge time: the safe method depends on circuit voltage, stored energy, the resistor’s pulse rating and the service procedure. A 2.2 µF capacitor charged to 400 V stores about 0.176 J, using E = ½CV²; equipment may contain other capacitors or charge it above that nominal figure. It can remain charged after unplugging, so leave mains and high-voltage repairs to qualified personnel if you lack the equipment or experience.
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