Wirewound resistor TCR tells an engineer how much resistance may change with temperature, so an RFQ must state the resistance value, TCR band, temperature interval, power duty, mounting condition, and verification method together. A generic request for “stable resistance” is not enough. Vishay's CPW data, for example, separates ±30, ±50, and ±90 ppm/°C bands by resistance range, while Ohmite's RW series uses ±20, ±50, and ±90 ppm/°C bands. This guide turns those published limits into six source-backed RFQ checks for OEM buyers.
Key Takeaways
TCR and initial resistance tolerance are different specifications.
A ppm/°C limit is meaningful only with a defined temperature interval and measurement method.
Low resistance values often carry a wider TCR band than higher values in the same family.
Self-heating, mounting, and terminal temperature can dominate the real operating resistance shift.
Approve a production lot against a written test plan, not a catalog headline alone.
What does wirewound resistor TCR mean?
TCR converts temperature change into expected resistance change
Temperature coefficient of resistance, normally written as TCR, expresses the relative change in resistance for each degree Celsius or kelvin. The common engineering unit is ppm/°C. A nominal 100 Ω resistor with a maximum TCR magnitude of 50 ppm/°C has a first-order change of 0.25 Ω across a 50 °C temperature difference: 100 × 50 × 50 / 1,000,000. That calculation is a screening estimate, not a substitute for the manufacturer's specified curve, test interval, or tolerance limits.
Published TCR bands depend on construction and resistance value
One wirewound family can have several TCR limits. Vishay's CPW/CPWN datasheet specifies ±30 ppm/°C for 10 Ω and above, ±50 ppm/°C from 1.0 Ω to 9.9 Ω, and ±90 ppm/°C from 0.5 Ω to 0.99 Ω. Ohmite's RW series lists ±20 ppm for 10 Ω and above, ±50 ppm from 1 Ω to 10 Ω, and ±90 ppm from 0.1 Ω to 0.99 Ω. These are family-specific examples, not universal LORIDA specifications. They show why an RFQ must identify both the target resistance and the required TCR.
Which six checks belong in a wirewound resistor RFQ?
1. Separate resistance, tolerance, and TCR
Write the nominal resistance and tolerance first, then state the maximum positive and negative TCR. Do not assume that a tighter tolerance automatically delivers a tighter TCR. If the design needs only an absolute limit, write “|TCR| ≤ 50 ppm/°C” rather than using vague words such as low or stable. If a matched network is involved, specify whether absolute TCR, TCR tracking, or both are required.
2. Define the temperature interval and reference point
A TCR number without a measurement interval is incomplete. State the reference temperature, lower and upper test temperatures, stabilization time, and whether the requirement applies to ambient, body, or terminal temperature. Vishay's WSC/WSN surface-mount wirewound data explicitly measures TCR from -55 °C to +150 °C, illustrating that the interval is part of the specification. For an application with a narrower operating window, the buyer still needs a repeatable qualification interval.
3. Describe power and self-heating duty
TCR testing is usually performed at low measuring power to minimize self-heating, while the application may dissipate much more power. Ask for rated power, derating curve, expected continuous load, pulse load, duty cycle, and allowable surface or hotspot temperature. The Vishay CPW family spans 2 W to 20 W and states a short-time overload of five times rated power for five seconds. Those figures are family data, not a blanket recommendation; they demonstrate why power duty must be reviewed beside TCR.
4. Lock the mounting and cooling condition
Lead length, PCB copper, chassis attachment, airflow, spacing, and nearby heat sources affect resistor temperature. A part that meets the electrical requirement on an open laboratory fixture may run hotter inside an enclosure. Provide the intended mounting drawing and cooling condition, then ask the supplier to identify any required clearance, heat sink, or derating. For aluminum-housed or chassis-mounted wirewound parts, define the mounting surface and thermal interface rather than quoting only free-air power.
5. Check voltage, overload, and insulation constraints
The calculated power may be acceptable while working voltage or insulation is not. Vishay's CPW datasheet defines maximum working voltage by √(P × R) and lists 1000 VAC dielectric withstand voltage. Review those limits with creepage, clearance, terminal spacing, surge waveform, and fault energy. If the resistor is used for precharge, discharge, braking, or equalizing, provide the actual waveform and repetition rate; steady-state wattage alone cannot describe the stress.
6. Require a sample verification record
The RFQ should define sample quantity, instruments, four-wire or two-wire method, test temperatures, stabilization time, measuring current, calculation formula, acceptance limit, and report format. Record both resistance at each temperature and the calculated TCR. Keep lot identification and part marking in the report so the approval remains traceable. For production control, state whether the supplier must provide a certificate, periodic report, or only retain inspection data.
| RFQ field | Required entry | Why it matters |
|---|---|---|
| Resistance | Nominal Ω and tolerance | Separates initial error from temperature drift |
| TCR | Maximum ppm/°C and interval | Creates a testable drift limit |
| Thermal condition | Ambient, mounting, airflow, duty | Controls actual body temperature |
| Electrical stress | Power, voltage, pulse waveform | Prevents a TCR-only approval |
| Verification | Method, samples, report | Makes supplier results comparable |
How should an engineer calculate and verify TCR?
Use the same equation and controlled temperatures
A common two-point calculation is TCR = (R2 − R1) / [R1 × (T2 − T1)] × 1,000,000 ppm/°C. Use stabilized resistor temperatures, not only chamber air readings. Keep measuring power low enough that the measurement does not materially heat the element. For low-value resistors, use Kelvin connections where lead and contact resistance would otherwise affect the result. Record instrument model, calibration status, current, and dwell time.
Frequently asked questions
Is resistor tolerance the same as TCR?
No. Tolerance and TCR describe different errors. Resistance tolerance defines how far the measured resistance may be from nominal at the specified reference condition, while TCR defines how resistance changes as temperature changes. A part can have a tight initial tolerance but a wider temperature coefficient, or the reverse. In an RFQ, list nominal resistance, tolerance, TCR limit, reference temperature, and test interval as separate fields. This prevents a supplier from satisfying the tolerance requirement while overlooking the drift requirement that affects current, voltage division, timing, or measurement accuracy in operation.
Why do low-value wirewound resistors often have wider TCR?
Low-value constructions can show more influence from terminals, connections, and the chosen resistance alloy. Published families illustrate the pattern: Vishay CPW specifies ±30 ppm/°C at 10 Ω and above, but ±90 ppm/°C from 0.5 Ω to 0.99 Ω. Ohmite RW similarly lists ±20 ppm above 10 Ω and ±90 ppm from 0.1 Ω to 0.99 Ω. These examples do not define every product. They show that buyers should check the exact resistance band and ordered construction rather than applying the best headline TCR to the whole series.
Should TCR be checked at rated power?
Usually the coefficient is measured with low measuring power, while rated-power behavior is evaluated separately. A high test current can self-heat the resistor and blur the relationship between chamber temperature and element temperature. The RFQ should therefore define a low-power TCR measurement and a separate thermal or load test for the real duty. For the application test, document power, voltage, pulse waveform, mounting, airflow, ambient temperature, and steady-state body or terminal temperature. Reviewing both results prevents a laboratory TCR pass from masking excessive self-heating in the assembled equipment.
What information should I send for a wirewound resistor quotation?
Send the resistance, tolerance, TCR, temperature interval, power waveform, voltage, mounting, dimensions, quantity, and validation method. Add the intended function, such as braking, precharge, discharge, equalizing, current limiting, or measurement, because the same nominal wattage can experience very different stress. Include a drawing or available space, terminal style, cooling condition, and any pulse timing. If an existing part is being replaced, provide its verified datasheet and photographs without assuming that color or case size proves electrical equivalence. LORIDA can then review the inquiry against the relevant product family and identify missing inputs before sampling.
Sources and next step
Primary technical references
Last Updated: 2026-11-09
