Series-connected aluminum electrolytic capacitors need voltage balancing because leakage differences can force unequal voltage across otherwise identical cans. Nichicon's example uses two 400 V, 470 uF capacitors at 60 deg C and calculates about 100 kohm per balance resistor. This is an application example, not a universal LORIDA value. The checks below help OEM buyers specify and validate a real capacitor string.
Series connection raises voltage capability, but it does not guarantee equal voltage sharing.
Set balancing current from documented leakage behavior, resistor tolerance and temperature, then check resistor power loss.
Measure every capacitor voltage after stabilization and during charge, discharge and restart events.
Use matched parts, equal cooling and the same date code where practical.
Why Does Voltage Become Unequal in a Series String?
Steady-state sharing follows leakage behavior
In a series string, each aluminum electrolytic capacitor has its own leakage-current-versus-voltage characteristic. TDK notes that the unavoidable spread can be extensive. The total bus can look correct while one unit is closer to its rated limit than expected.
Dynamic sharing also depends on capacitance
During charging and fast regeneration, individual voltages depend on capacitance and circuit impedance before leakage dominates. Nippon Chemi-Con warns that frequent charge-discharge operation can make voltage balance difficult even when resistors are installed. Therefore, the RFQ must identify whether the string sees a steady DC bus, repeated regeneration, pulsed charging or hot restarts.
What Do Manufacturer Guidelines Actually Require?
Use a resistor across each capacitor
Nichicon states that when more than two aluminum electrolytic capacitors are used in series, applied voltage must remain below each capacitor's rating and voltage must be applied equally by a balancing resistor connected in parallel with each capacitor. Nippon Chemi-Con gives the same practical direction for series input-filter banks. Sources: Nichicon Application Guidelines; Nippon Chemi-Con Technical Note.
Keep stress and aging conditions comparable
TDK recommends equal voltage, current, ambient temperature and cooling conditions across a bank, plus the same date code. These controls reduce avoidable drift caused by airflow, mounting or production-age differences.
How Should the Balancing Resistor Be Chosen?
Start with leakage-current spread, not a copied resistor value
TDK cites a rule of thumb in which balancing-resistor current is about 20 times expected leakage current. Nichicon presents an equation based on allowed voltage and leakage-current differences. Both methods require resistor current to dominate uncertain leakage spread.
Verify loss, voltage rating and tolerance
Lower resistance improves passive equalization but increases continuous dissipation. Check loss from the actual resistor voltage, plus power, working-voltage and tolerance limits. Include resistor temperature rise inside the real enclosure, not only a room-temperature bench calculation.
| Design input | Why it matters | RFQ evidence |
|---|---|---|
| Maximum DC bus and transient | Defines the worst voltage the complete string must withstand | Waveform, tolerance and protection behavior |
| Capacitor count and topology | Determines voltage-sharing and failure propagation | Series/parallel schematic |
| Leakage-current limit | Sets the balancing-current basis | Selected series datasheet and temperature |
| Balancing resistor | Controls static sharing and continuous heat | Resistance, tolerance, voltage and power ratings |
| Charge/discharge profile | Reveals dynamic imbalance not seen at steady state | Startup, regeneration and restart waveforms |
| Cooling and mounting | Temperature changes leakage and lifetime | Airflow, spacing, clamp and terminal drawing |
Which Exact Data Points Are Useful for Review?
Nichicon's worked example is a calculation reference
Nichicon's technical note demonstrates two 400 V, 470 uF capacitors at 60 deg C. It uses a 10% voltage-balance target, a temperature coefficient of 2.0 and a leakage-variation coefficient of 1.4, producing 364 uA of calculated leakage variation and approximately 109 kohm, rounded to 100 kohm. Those five numbers belong to that worked example and must not be copied into another design without verifying all assumptions.
Temperature can multiply leakage spread
The same Nichicon note states that leakage current, referenced to 20 deg C as 1, is generally 2 to 3 times at 65 deg C and 3 to 5 times at 85 deg C. Therefore, use stabilized hot measurements and worst-case data rather than one room-temperature incoming value.
How Should a First Article Be Tested?
Measure every capacitor, not only the string
Use probes and equipment rated for the common-mode voltage. Record each capacitor during startup, steady load, maximum ambient, shutdown and hot restart. Confirm the measurement method does not change the sharing circuit, and log stabilization time.
Repeat after thermal stabilization
Measure capacitor-case and balancing-resistor temperatures after thermal equilibrium. Compare positions near fans, busbars and heat sources because an open-bench result may drift inside a cabinet. Link voltage, temperature and operating state on one timeline.
What Failure Modes Must the Architecture Consider?
A shorted capacitor can overvoltage remaining units
Nichicon warns that if one side of a series arrangement is shorted, the other may see overvoltage. TDK distinguishes series-parallel and parallel-series banks because a short can propagate differently. Review fusing, discharge, detection and safe shutdown as well as normal equalization.
Stored voltage can return after discharge
Nichicon notes that a potential can reappear between terminals even after a capacitor is discharged and gives a 1 kohm resistor as a discharge-treatment example in its handling guideline. Service procedures must follow the selected capacitor and equipment documentation, verify voltage with a suitable instrument, and never assume a balancing resistor alone creates an immediately safe touch condition.
What Should an OEM RFQ Include?
Send the complete electrical duty
Provide maximum and nominal bus voltage, transient waveform, series and parallel count, ripple spectrum, startup time, regeneration profile, restart interval and required discharge time. Include the proposed balancing-resistor network and the allowed per-capacitor voltage spread. Do not specify only total capacitance and voltage.
Send the mechanical and validation package
Include case size, terminals, polarity controls, mounting, airflow, nearby heat sources and measurement points. Buyers can review LORIDA's screw-terminal capacitor range, a representative product page, and the related VFD DC-bus selection guide.
Frequently Asked Questions
Can two 400 V capacitors automatically replace one 800 V capacitor?
No, two 400 V capacitors do not automatically create a safe 800 V component. The total voltage can divide unevenly because leakage current, capacitance, temperature and dynamic operating conditions differ between cans. Each capacitor must remain below its own rating under steady state, startup, regeneration, restart and fault conditions. Use manufacturer-approved series-connection guidance, a balancing network, appropriate voltage margin and per-capacitor measurement. Also compare the required lifetime, ripple current, mechanical layout and fault response with a suitable single high-voltage capacitor option. The choice is a system design decision, not simple arithmetic.
How much current should flow through each balancing resistor?
The balancing current should be based on worst-case leakage-current spread and the permitted voltage imbalance. TDK cites a common rule of thumb of about 20 times expected leakage current, while Nichicon provides an equation-based method. Neither is a substitute for the selected series datasheet and system validation. Higher current can improve static sharing but raises continuous resistor loss, temperature and standby consumption. Verify resistor tolerance, working voltage, power rating, enclosure temperature and the effect of parallel groups. Record the design basis in the RFQ so supplier and OEM calculations can be compared.
Are balancing resistors enough for regenerative drives?
Not always; passive resistors may not control dynamic imbalance during frequent charge and discharge events. Nippon Chemi-Con explains that individual charging voltages depend on capacitance variation and that maintaining balance can be difficult during intensive cycling even with resistors. For regenerative drives, servo amplifiers and pulsed systems, provide the waveform, repetition rate, restart interval and discharge path. Measure every capacitor during the transient, not only after stabilization. The final architecture may require a product intended for that duty, a different topology, active balancing or additional protection.
What is the most important production acceptance test?
The most important acceptance test is a repeatable per-capacitor voltage and temperature measurement under the real worst-case operating profile. Confirm total bus voltage, each capacitor voltage, resistor temperature, case temperature and stabilization time during startup, steady load, high ambient, shutdown and hot restart. Use the same probe positions and operating sequence across samples. Add incoming checks for part identity, date code, capacitance, leakage and mechanical fit, but do not treat a room-temperature component check as proof of system-level sharing. Retest after changes to airflow, enclosure, busbar or resistor selection.
Last Updated: September 28, 2026
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