AKKN ElectronicsA 24 V DC industrial power supply starts to drop out under load, typically after the machine has been running for two or three full shifts. No-load output reads 24 V, but as soon as a 4 A motor starts, the supply cycles through an undervoltage restart. A visual check shows a slightly swollen electrolytic capacitor beside the input rectifier, with its safety vent deformed. The part carries the marking of a European capacitor brand that ceased production several years ago.
This failure signature is common in legacy European machinery. The original capacitor has reached the end of its working life, which is normal after years of thermal and ripple stress. The problem moves to a second stage when the replacement is selected by matching capacitance and voltage without checking the rest of the specification. The new capacitor fails within months because the original specification also included ESR at switching frequency, ripple current rating, and endurance. Cross-referencing an obsolete European part means reproducing those parameters, not just the printed values on the can.
Read the Failure Signature Before Desoldering
Start with the least destructive checks. Look for a bulged canister, electrolyte residue around the vent, lifted vent score lines, or a darkened PCB beneath the part. Measure the output ripple with an oscilloscope using a 10x probe and a short ground spring; a long ground clip adds inductance that distorts the measurement. On the failing supply, ripple at the output measured above 200 mVp-p while the design limit was 100 mVp-p. That amount of ripple points to a filter capacitor whose capacitance has dropped or whose ESR has risen.
Also record the case temperature under full load. A thermocouple on the canister of a 105 °C-rated part in a 40 °C ambient should stay below 75 °C. A noticeably hotter case indicates excessive ripple current or inadequate thermal design, and a replacement of the same size but a lower ripple rating will fail early.
Out-of-Circuit Checks and Acceptance Values
Discharge the capacitor through a 1 kΩ resistor and wait at least 30 seconds before handling it. Then remove the part from the board. Use an LCR meter with lead compensation and measure the following:
- Capacitance at 120 Hz for parts rated above 100 µF. The original tolerance was typically −20/+20%. If the measured value is below the nominal value minus 20%, the capacitor is aged; for a 470 µF part the boundary is 376 µF.
- ESR at the converter's switching frequency, usually 100 kHz. A common end-of-life criterion for aluminum electrolytic capacitors is ESR rising to twice the initial datasheet value. If the original datasheet value is unavailable, choose a candidate whose ESR is no higher than 1.2 times the measured value of a known-good part from the same design.
- Leakage current at rated voltage through a 1 kΩ limiting resistor, measured after five minutes. The acceptance limit for a new capacitor is typically 0.03 × C(µF) × U(V) in mA; for a 470 µF, 63 V part that gives 0.89 mA. An aged capacitor can show several times this value.
Cross-Reference Parameters for the Replacement
When the European brand is no longer available, use the original application notes to build a cross-reference table. The parameters below are the decision thresholds that matter in a switched-mode power supply environment.
| Parameter | How to verify | Acceptance threshold |
|---|---|---|
| Capacitance | LCR at 120 Hz or 1 kHz, as specified originally | Within ±20% of the original nominal value |
| Rated voltage | Compare to the DC bus voltage plus ripple peaks | Equal to or higher than the original; apply 80% derating for long service life |
| ESR at switching frequency | Measure at 100 kHz, or read from the datasheet | No more than 1.2 times the original ESR |
| Ripple current rating | Compare to the worst-case ripple current in the application | Replacement rating at least 1.2 times the computed ripple current |
| Endurance rating | Read from the datasheet at the upper category temperature | At least 5000 h at 105 °C for an industrial supply |
| Temperature range | Check the upper and lower category temperatures | −40 °C to +105 °C typical for an industrial aluminum electrolytic |
| Case size and mounting | Diameter, height, lead pitch, terminal style | Must fit the board keep-out; comparable can surface for heat dissipation |
Prevention and Sourcing Checklist
Once the legacy part has been mapped to a suitable cross-reference, prevent an early repeat failure with these checks:
- Replace both capacitors of the input filter stage from the same production lot, even if a single part shows visible damage. Aging imbalance and a higher ESR in the surviving part cause unequal ripple sharing.
- Derate the working voltage to 80% of rated voltage at temperatures above 85 °C.
- Keep at least 20% headroom between the calculated ripple current and the capacitor's maximum ripple rating.
- Apply the endurance rule of thumb: for an aluminum electrolytic, the useful life roughly doubles for each 10 °C reduction in core temperature below the rated value.
- If the application must start at a low ambient temperature, verify the capacitor's impedance ratio at −40 °C; high-Z values lengthen the charge time and can trigger undervoltage lockout.
- Request the distributor's documented cross-reference from the obsolete European brand to a current equivalent, and validate electrical parameters with samples before committing to a volume order.
An obsolete European capacitor does not leave the engineer without options. Provided the replacement matches the original on capacitance, ESR at the operating frequency, ripple current capability, and endurance, the supply can return to service with a predictable life. Measure before desoldering, compare datasheet parameters before buying, and the field symptom becomes a controlled repair instead of a recurring shutdown.
