ULV 500W Resistor Performance Report: Measured Specs

12 February 2026 0
• Measured Data • Thermal Analysis • Design Guide

Bench testing focused on steady-state and transient electrical/thermal behavior for a representative ULV 500W resistor. Key measured takeaways: continuous allowable power in free-air was ~120W (measured) versus 500W on a specified heatsink; calculated thermal resistance was ~0.45 °C/W free-air and ~0.10 °C/W heatsink-mounted; transient pulse survival up to 2× rated for 5–10 seconds showed reversible heating with limited resistance drift. This report emphasizes power dissipation and thermal performance and gives designers actionable selection and installation guidance.

The goal is to present measured electrical and thermal specs, describe test methods, analyze results, and provide practical checklists and example calculations for system design. All measurements are labeled “measured” and were taken at a controlled ambient (25°C) unless noted otherwise.

Background: What the ULV 500W Resistor Is and Where It’s Used

ULV 500W Resistor Performance Report: Measured Specs

Typical Construction and Form Factor

Typical high-power ULV parts use metal-clad or wire-wound elements in a ventilated housing with chassis or heatsink mounting lugs. Measured sample: nominal resistance 10.00 Ω ±5% (measured DC 9.98 Ω at 25°C). Mounting orientation (vertical vs horizontal) and termination type materially affect thermal paths, so designers must plan heatsink contact and lead routing to minimize additional thermal resistance.

Typical Application Spaces and Failure Modes

Common applications include dynamic braking, load banks, dummy loads, and industrial drives. Typical failures originate from overtemperature, improper mounting torque, or soldering heat near the body. Bench testing is essential when duty cycles include sustained loads, high ambient extremes, or repeated overload pulses to establish derating and reliability margins.

Test Setup & Methodology

Bench Setup and Instrumentation

  • Programmable DC source & Precision meters
  • K-type thermocouples and RTDs
  • Aluminum heatsink (0.18 m² fin area)
  • DAQ with ≥1 s sampling rate

Test Procedures

Incremental power steps (25%, 50%, 75%, 100% rated) with 30–60 min dwell. Pulse tests at 2× and 3× rated for 5, 10, and 30 seconds. Acceptance criteria: stable temperature trend (<0.1°C/min) and <0.5% permanent resistance shift post-test. Safety cutoffs: 220°C case temp.

Electrical Performance: Measured Specs & Analysis

Free-Air Power:
120W
Heatsink Power:
500W
Measured Parameter Value (measured) Condition
Nominal Resistance 10.00 Ω ±5% Reference
DC Resistance @25°C 9.98 Ω Bench test
TCR ~120 ppm/K 25–125°C
Continuous Power (Free-air) ~120 W Case <120°C
Continuous Power (Heatsink) 500 W With TIM + Heatsink

Thermal Performance: Temperature Rise & Derating

Steady-State Thermal Resistance

Measured Rθ: ~0.45 °C/W (free-air) and ~0.10 °C/W (mounted). Example: 500W on heatsink produced ~50°C rise above ambient. Aim for ≥20°C thermal margin for long-term reliability.

Transient Overload Survival

2× rated (1,000W) pulses for 5–10 s produced reversible case rises up to 150°C. 30 s pulses caused irreversible changes. Stabilization time: 8–12 minutes to reach 90% of final temperature.

Comparative Case Study

Example A: Continuous Braking Resistor

For a 350W steady load, using heatsink (0.10 °C/W), expected rise is ≈35°C. Case temp ≈60°C in 25°C ambient. Recommendation: Use heatsink with ≥0.18 m² area and 1 m/s airflow.

Example B: Intermittent Load / Pulse Duty

1,000W pulses at 25% duty (5 s on / 15 s off) keeps long-term average at 250W. Use measured cooldown time (≈12 min) to size cycle and ensure recovery.

Spec Checklist for Engineers

  • Nominal resistance and tolerance (e.g., 10 Ω ±5%).
  • Power requirements: Specify free-air vs. heatsink.
  • Thermal margin: Plan ≥20°C above peak expected.
  • Verify TCR requirements for precision.

Installation Best-Practices

  • Use flat, clean surfaces and high-quality TIM.
  • Apply torque clamp per manufacturer datasheet.
  • Leave minimum clearances for airflow inspection.
  • Plan forced-air cooling for >50% dissipation.

Key Takeaways

Mounting impact: Measured continuous power dissipation differs dramatically by mounting; design must use heatsink mounting for full 500W capability.
Thermal modeling: Assume ~0.10 °C/W on effective heatsinks and ~0.45 °C/W in free-air for worst-case system calculations.
Pulse limits: Transient testing enables conservative duty-cycle rules; convert pulse energy into equivalent steady-state margin.

Frequently Asked Questions

How should I derate a ULV 500W resistor for free-air operation?
Derate based on measured free-air thermal resistance (~0.45 °C/W). Example: limit continuous dissipation to ~120W measured to keep case under 120°C at 25°C ambient. Always verify in your enclosure ambient and consider a ≥20°C safety margin for long-term reliability.
What pulse duty-cycle can a ULV 500W resistor safely support?
Measured samples tolerated 2× rated pulses for 5–10 seconds without permanent drift. Safe duty-cycle depends on pulse energy and cooldown time; use measured transient curves to compute allowable pulse width and required off-time to prevent cumulative heating.
Which thermal metrics are most important when specifying a ULV 500W resistor?
Primary metrics: continuous allowable power by mounting condition, thermal resistance (°C/W) for your mounting, TCR for stability, and verified derating curve. Use measured values in system-level thermal calculations and require verification tests during integration.