Sizing Output Capacitance for Mains Dip Ride-Through
For an OEM controller that must survive a short mains interruption, output capacitance is an energy and interface requirement, not simply a larger capacitor value. Define the load that must remain alive, its minimum permissible voltage and the required duration; then qualify charging, reverse current, discharge safety and restart behavior with the power supply supplier.
The WEHO DR-30-12 illustrates the boundary. Its exact 12 V version is marked 2 A, and its supplied sheet lists 50 ms hold-up at 230 VAC and full load. That conditional figure is not a guarantee at every input voltage, nor permission to connect an arbitrary external capacitor bank. The retained sheet does not state a maximum approved external output capacitance.
Write the ride-through requirement before requesting a sample
Separate essential logic from dispensable loads. Keeping an event logger alive may need far less stored energy than maintaining every relay, lamp and actuator on the same rail. State whether the objective is uninterrupted control, completion of one transaction, or an orderly shutdown. These require different durations and may need different safety behavior.
| RFQ requirement | Why the supplier needs it |
|---|---|
| Essential load current or power during the dip | Defines energy demand; include any final write or communication burst |
| Highest starting voltage and lowest acceptable receiver voltage | Defines usable capacitor energy after distribution and isolation losses |
| Interruption duration and repeat interval | Defines storage and recharge requirements |
| Required restart and shutdown sequence | Prevents an unstable partly powered controller state |
| Temperature, space and service life target | Defines component derating and the sample validation conditions |
Estimate the storage, but do not order from the estimate alone
For an ideal constant-current load, the first estimate is C = I × t / (Vstart − Vmin). Suppose a hypothetical receiver needs 0.5 A for 0.2 s while its capacitor voltage falls from 12 V to 10.5 V. The ideal requirement is about 0.067 F, or 67,000 µF. This is a design example, not an approved DR-30-12 accessory value.
A constant-power load needs an energy calculation: C = 2 × P × t / (Vstart² − Vmin²) for an ideal direct supply path. At an assumed 5 W for 0.2 s over the same voltage window, the result is about 0.059 F. Real designs must account for conversion loss where applicable, capacitor tolerance, ESR-related voltage drop, leakage, temperature and aging. Use the actual voltage available after any blocking element, not the power supply’s nominal label alone.
The referenced discussion of hold-up energy storage explains the stored-energy relationship and why the storage voltage matters. It is general guidance, not a compatibility statement for the WEHO unit. Do not open or modify the supply’s internal high-voltage bus to increase hold-up.
Confirm charging and reverse-current behavior
A large empty capacitor looks like a demanding load during startup. Ask for the allowable external capacitance or an approved charging arrangement, including current limiting and the available recharge current while the normal load is operating. A design that survives one dip but cannot recharge before the next one does not meet a repetitive interruption requirement.
Also confirm whether stored energy can flow back into the supply or other branches after AC removal. Any isolation, discharge path and protection devices must be selected for the actual circuit. A capacitor remains an energy source after the mains disconnect opens; service instructions and verification points need to address that stored energy.
Compare a capacitor bank with a qualified buffer module
A discrete bank can be appropriate when the OEM controls the complete design and validation. A purpose-designed DC buffer may provide defined charging and power-path behavior, but it still needs the correct voltage, current, duration, temperature and interface evidence. Neither route becomes qualified merely because it can be mounted beside the DR-30-12 on a rail.
The DR-30-12 is a protected-indoor DIN-rail supply. Its source label states 100–240 VAC, 1.1 A, 50/60 Hz and +12 V, 2 A. Obtain the offered revision and matching terminal instructions before the assembly drawing is released; do not infer a buffer connection from a generic series diagram or reuse another voltage variant’s rating.
Make the sample test a purchasing acceptance gate
Agree the input condition, essential-load profile, dip waveform, repetition interval and pass/fail voltage at the receiver. Test cold start with discharged storage, interruption after full charge, repeated interruptions and recovery with the normal load connected. Record the exact supply and storage revisions, actual capacitance condition, fixture and temperatures. A photograph of an assembled cabinet is not a ride-through result.
If the sample fails, ask whether the limit is stored energy, charging current, voltage loss or receiver restart behavior before increasing capacitance. Retain the accepted configuration and change-notification requirements for repeat orders. Request quotations with sample and production quantities, destination and required documentation; delivery terms and availability must be confirmed for the order.
Buyer questions
Can the specified 50 ms simply be deducted from the required external storage time?
No. That figure applies at 230 VAC and full load under the supplied sheet’s conditions. The receiver voltage, actual input, load profile and power-path losses can change the usable interval. Ask the supplier to confirm the conditions and verify the complete arrangement before allocating any part of the requirement to the supply’s internal storage.
Is a larger output capacitor always a better purchasing choice?
No. More capacitance can increase charging demand, recharge time and stored energy during service. Request an approved capacitance or charging arrangement and compare the complete sample against the agreed dip and restart tests, rather than buying the largest value that fits.
Review WEHO DIN-rail power supplies و send the essential-load and interruption specification to WEHO. Ask explicitly for external-capacitance compatibility or an appropriate buffer architecture, rather than requesting “longer hold-up” without a voltage window.
