AKKN ElectronicsA 24 V industrial power supply in a packaging line starts resetting sporadically. On the oscilloscope, the +24 V rail shows a 250 mV pk-pk ripple that was not present during the previous preventive maintenance. The switching transformer hisses, and the largest electrolytic capacitor, a 400 V / 47 µF part, looks normal from the outside. Replacing all three electrolytics with parts of the same capacitance and voltage seems to solve the problem, but the ripple returns after a few hundred hours.
This pattern is typical when an obsolete European capacitor brand is replaced with a generic equivalent. The failure is not caused by capacitance loss; it is caused by ESR and ripple current ratings that do not match the original series. Before ordering a substitute, check the parts under stress and measure what matters.
Start with the Parts That See Electrical and Thermal Stress
Open the unit and check the primary-side bulk capacitor first, then the DC link capacitors after the rectifier. A capacitor at end of life may show a bulged vent on the can, a discolored PCB under the component, electrolyte residue around the leads, or a lifted solder joint caused by repeated heating. Absence of these signs does not clear the part: many dried-out electrolytics keep their original geometry.
Record the markings before you remove anything. European manufacturers often indicate the series on the sleeve or can, using a code that identifies the ESR class and lifetime grade. Two capacitors with identical "47 µF / 400 V" printing can have different ripple ratings and different end-of-life behavior.
Measure ESR, Capacitance, and Leakage Before You Cross-Reference
The table below gives field acceptance thresholds for aluminum electrolytic capacitors used in power supplies. The values assume a measurement temperature of 20 °C and a meter that is accurate at the stated frequency.
| Parameter | Measurement condition | Acceptance threshold |
|---|---|---|
| Capacitance | 120 Hz | -20 % / +50 % of rated value; a reading near the lower edge is acceptable after 10+ years of service |
| Dissipation factor | 120 Hz | ≤ 0.2; above 0.3 the electrolyte has dried out |
| ESR | 100 kHz | ≤ 1.5 × datasheet maximum; if the datasheet is gone, no more than double the ESR measured on a known-good part |
| Leakage current | Rated voltage, 5 min | ≤ 0.01 × C × V (µA), with a minimum of 3 µA |
Use an ESR meter at 100 kHz. A capacitance-only meter at 120 Hz will often show the part within tolerance long after the electrolyte has dried out. For the ripple current, measure the AC voltage across the terminals with a 20 MHz bandwidth-limited oscilloscope and estimate the RMS current using I_rms = V_ripple_pk-pk / (2√2 × ESR). If the resulting current is more than 80 % of the replacement part's rated ripple current at the actual case temperature, the substitute will not survive the same load profile.
Why a Same-Value Replacement Can Fail
The original European part was usually a 105 °C low-ESR series with a rated ripple current in the hundreds of milliamperes at 100 kHz. A general-purpose substitute from a stock bin may offer the same capacitance and voltage but only a fraction of the ripple rating. At the same switching ripple, the substitute dissipates more heat because its ESR is higher, and its rated lifetime (for example 2,000 h at 105 °C) is far shorter than the original series. The result is a capacitor that measures fine on a multimeter but fails thermally after a short run. As a rule of thumb, electrolytic lifetime doubles for every 10 °C drop in core temperature; that works in reverse when the part runs hot.
What to Check Before You Order a Substitute
- Photograph the original marking before removal: capacitance, voltage, series code, date code, temperature rating, case diameter and height, pin spacing, and mounting style.
- Verify the ESR class from the series code. European brands used distinct designations for low-impedance, long-life, and standard types; a generic cap with the same value is rarely a valid replacement.
- Compare three parameters besides capacitance and voltage: maximum ESR at 100 kHz, rated ripple current at 10 kHz or 100 kHz, and load life at rated temperature.
- If the original datasheet is no longer published, have the distributor provide a comparison of these parameters for the candidate substitute.
- At ambient temperatures above 85 °C, use a 105 °C rated part; do not downgrade a 105 °C original to an 85 °C substitute.
- Select a part whose ripple current rating is at least 20 % above the measured ripple current at the capacitor location.
- Confirm that the DC link voltage rating is equal to or higher than the original, and that the PCB pad spacing matches; forcing a different can size can create mechanical stress on the leads.
A failed capacitor in an old European power supply is an obvious symptom of a thermal problem, not the root cause. The repair is complete when the replacement's ESR, ripple current rating, and endurance are compatible with the original series — not when the capacitance values happen to match.
