Circuits and Passives
Model class: Standard approximation
Hold-up capacitor designer
Estimate ideal hold-up capacitance for a constant-current load between a starting and minimum voltage.
Interactive engine
Start with the stated conditions.
Values stay in this browser. Choose a representative scenario, then calculate deliberately.
Example ready
Calculate to inspect the result.
The result will identify the direct answer, assumptions, and any warning that changes the next decision.
How much does each input matter?
Each row holds every other input fixed at its current value and moves one input by ±10%, using the same model as the main result. It identifies which input is the binding lever, not a full tolerance stack.
Assumptions to check
- The entered hold-up-capacitor values represent the stated operating condition.
- This standard approximation is evaluated in the declared lumped or first-pass model.
- A hold-up-capacitor calculation is not a component qualification or safety approval.
What this hold-up-capacitor calculation establishes
Estimate ideal hold-up capacitance for a constant-current load between a starting and minimum voltage. The useful result is the stated electrical quantity and the decision it supports, not an unstated claim about a finished product. This engine keeps the governing relationship visible so an input, unit, condition, or model boundary can be reviewed before a value becomes a component or layout choice.
The constant-current estimate uses C = I times time divided by the allowed voltage drop. Treat the number as a first-pass result for the declared operating point. When a source, load, temperature, frequency, waveform, component tolerance, or measurement condition changes, repeat the calculation at the relevant corner rather than assuming the nominal answer persists.
Worked decision context
A 100 mA load held from 5.0 V to 4.5 V for 10 ms requires 2.0 mF before margin and ESR. That example verifies the equation and illustrates the scale of the result, but it does not select a part by itself. Compare the result with available values, ratings, tolerance bands, and the receiving circuit or physical environment before implementation.
Use the primary output to identify the binding constraint. If it leaves little margin, document which input dominates and use selected-part data, a higher-fidelity model, simulation, or measurement. This is especially important when a small numerical difference changes a thermal, timing, noise, or reliability decision.
Limits and validation handoff
It omits capacitor tolerance, ESR step, source dynamics, load conversion efficiency, DC bias, and recharge. The calculation does not silently include omitted parasitics, installation conditions, manufacturing variation, or product policy. Those conditions can be decisive even when the arithmetic is exact for the selected model.
Record inputs, units, model assumptions, and the intended decision with the result. Verify the leading risk against the selected component data sheet and a representative measurement when the circuit has consequential energy, high voltage, safety, compliance, or reliability requirements.
Common mistakes
- Treating the hold-up-capacitor result as a guaranteed operating limit rather than a first-pass standard approximation estimate.
- Mixing a data-sheet value measured under one condition with this hold-up-capacitor calculation performed at another.
- Selecting a component before checking the boundary this calculation names: it omits capacitor tolerance, ESR step, source dynamics, load conversion efficiency, DC bias, and recharge.
Model limit and handoff
Keep the entered hold-up-capacitor conditions with the calculation, then validate the binding limit using the selected component, physical implementation, and representative operating corner.
FAQs
Is this hold-up-capacitor result sufficient to approve a design?
No. It applies standard approximation reasoning to the entered hold-up-capacitor values and names the checks that still need selected-part data, a higher-fidelity model, or measurement. It omits capacitor tolerance, ESR step, source dynamics, load conversion efficiency, DC bias, and recharge.
What does this hold-up-capacitor calculator assume that could make the result wrong?
The constant-current estimate uses C = I times time divided by the allowed voltage drop. If the entered values do not match the real operating condition, the result no longer describes the actual circuit.
Where should this hold-up-capacitor result go next?
Compare this hold-up-capacitor result with Battery and stored-energy estimate limits, then use the stated next decision below the calculator to move from this first-pass number toward an implementation.