A useful capacitor RFQ connects the part you want to the conditions it must handle. Give the supplier enough information to propose a configuration, and give purchasing enough detail to compare offers fairly. Start with the circuit role, electrical duty, thermal environment and physical interface, then add evidence and commercial requirements. This guide is for preparing that brief. Its general engineering references explain selection questions; they do not establish SMILER ratings, certifications, stock availability or performance in your equipment.
1. Identify the circuit role and the sourcing task
State whether the capacitor belongs on a DC bus, in a snubber circuit, in an AC filter, in a motor circuit or elsewhere. Include the equipment type and the component reference on the schematic. A converter power rating or a motor model may provide context, but it does not describe every condition at the capacitor. Show the relevant circuit position rather than expecting the supplier to infer it from the application name.
Separate replacement sourcing from new design work. For a replacement, attach the existing specification and complete marking, and identify which characteristics must remain unchanged. For a new design, identify required outcomes and adjustable constraints. A proposed alternative needs different scrutiny when it changes the design rather than reproduces an approved requirement.
2. Label voltage values and operating conditions precisely
Give the normal and maximum operating voltage, with AC or DC clearly identified. Where relevant, distinguish RMS, peak and transient values. Add the duration and recurrence of exceptional conditions. Do not place an equipment supply voltage and a capacitor terminal measurement in the same field without explaining the difference. Mark calculations as calculations and measurements as measurements, so the reviewer knows the basis of each value.
TDK's film capacitor technical overview distinguishes AC and DC ratings and explains that allowable voltage can depend on temperature. The practical RFQ consequence is to request a rating for the intended duty, not to assume that a DC label provides an interchangeable AC rating. Any derating rule must come from the selected component's documentation; this guide supplies no universal conversion factor.
3. Specify capacitance, tolerance and the component count
Write the required nominal capacitance and tolerance together. State whether the value belongs to one component or the complete capacitor bank. If the design contains several parts, show their arrangement and identify which item the supplier is being asked to quote. A request for a bank total without its configuration can produce offers that address different physical and electrical requirements.
If a range of values is acceptable, explain who will choose the final value and what decision remains open. Ask the engineer whether a proposed tolerance change affects the design before accepting a cheaper alternative. For a family of equipment, give each distinct electrical requirement a separate RFQ line rather than combining several variants under one series name.
4. Describe ripple, pulses and the demanding operating mode
Include relevant current waveforms, ripple frequencies, pulse repetition and operating-mode duration. If the available information is incomplete, say which measurements or calculations are still needed. For a snubber inquiry, ask the designer which pulse-related characteristics govern selection. For a motor starting inquiry, describe the starting sequence and restart conditions. For a filter inquiry, identify the circuit's relevant frequency content instead of providing only the mains frequency.
Ask for the proposed part's characteristics at stated conditions. A current value without a frequency or temperature reference can leave the comparison ambiguous. Keep continuous, RMS and peak requirements in separate fields where they are relevant. Have the engineering reviewer decide whether the supplied data answers the application question; do not compare unrelated catalogue numbers as if they were one performance score.
5. Explain where the temperature was measured
Describe the surrounding ambient temperature, cooling arrangement and nearby heat sources. If you have case-temperature measurements, identify the measurement location and the equipment operating mode. Do not label an enclosure-air measurement as a capacitor hotspot temperature. For a new assembly, identify the thermal assessment still required before component approval.
TDK explains that ripple and high-frequency current can produce self-heating in film capacitors. This is why the surrounding temperature alone does not fully describe the thermal duty. Include the electrical load and cooling conditions with a measured temperature. Ask the supplier which temperature definition and limits apply to the proposed configuration, and retain the conditions attached to any life statement you use in the design review.
6. Show the complete mechanical interface
Attach the permitted body envelope, mounting arrangement and connection geometry. For a board-mounted part, include the footprint and available height. For a terminal-connected part, show the mating interface and installation access. Distinguish fixed dimensions from preferences, including any room for an alternative outline. The space needed to make a connection should not disappear from the brief simply because the capacitor body itself fits.
Request a drawing for the proposed configuration. A family photograph is useful for orientation but does not define dimensional tolerances or every available variant. Ask the assembly team to review the outline before samples are ordered, especially when changing the component would also require a board, bracket or harness revision.
7. Connect approval requirements with circuit placement
List the required standards, classifications and supporting documents for the intended market. Ask for evidence covering the exact proposed part and ratings. Keep a material flammability description, a company management-system certificate and a component approval separate; they answer different questions. Your compliance reviewer should determine which evidence the equipment requires.
Vishay's safety capacitor application note explains the different placement of X capacitors across the AC mains and Y capacitors in line-to-ground filtering paths. That distinction makes the circuit position essential to a safety-capacitor inquiry. State the intended connection and required classification explicitly. A familiar model name is not evidence that a particular manufacturer's proposed part has the required approval scope, and the cited note does not certify any SMILER product.
8. Define what a sample must demonstrate
Identify the questions the sample evaluation should answer: physical fit, selected electrical characteristics, operation in the equipment, or a combination of these. Assign suitable methods and acceptance criteria through the responsible engineering team. A successful fit check does not resolve a missing approval document, and a component measurement does not necessarily complete the equipment qualification.
Connect the result to the proposed part identification and drawing revision. If the equipment changes during evaluation, identify which configuration the result represents. Keep the approval decision concise: accepted requirements, unresolved points and any agreed deviations. This gives purchasing a usable release basis without forcing it to interpret raw technical observations.
9. Make the commercial comparison like for like
Provide sample and production quantities, forecast assumptions, destination and requested timing. Ask for minimum order quantity, lead time, packaging, scope of supply and quotation validity to be confirmed. Separate a delivery requirement from a supplier commitment. If your schedule depends on sample approval, make that milestone visible rather than assuming production can begin before the technical decision.
Compare each offer against the same electrical and mechanical baseline. Put deviations beside the price, including documentary gaps, alternate connections and additional items. Ask whether an offer covers a component alone or a larger assembly. Resolve those differences before treating quotations as equivalent; otherwise the apparent price difference may simply reflect a different supply scope.
10. Send a brief that is complete enough to answer
Your first message can follow this order: application and circuit position; existing part or design reference; capacitance and voltage requirements; ripple, pulse and frequency conditions; temperature and cooling; outline and connections; required evidence; quantity and schedule. Attach the relevant drawing and mark unknown fields explicitly. A partial brief with clear questions is more useful than a complete-looking specification containing guessed values.
End with the decision you need next. That might be feasibility feedback, a proposed outline, a quotation against a fixed specification or a sample plan. Send your capacitor requirement to SMILER with that request so the discussion can address a concrete engineering and purchasing task. Final product properties and commercial terms require confirmation for the selected configuration.
Frequently asked questions
Can I request a quotation before every field is known?
Yes. Provide the verified information you have, mark the gaps and state whether you want feasibility feedback or a firm configuration proposal. Keep unresolved technical items visible so an initial commercial discussion is not mistaken for complete application approval.
Is a photograph enough for a replacement request?
It helps identify markings and the apparent arrangement, but it does not replace the electrical specification and outline. Add the equipment requirement and identify fixed interfaces. Ask for a comparison against that baseline before approving a proposed replacement.
Should the lowest price determine the selected capacitor?
Compare the technical and documentary scope first. Once offers address the same approved requirement, purchasing can assess price alongside delivery, packaging and other agreed terms. An unreviewed change in rating or configuration is an engineering decision, not just a price difference.
Sources & technical reading
External references explain general engineering principles. They do not establish SMILER product ratings or approvals.