FAQ
TL;DR: 0.25 W carbon-film resistors average 3.6 mm diameter [Elektroda, w1m, post #1486048]; yet, “there is no simple dependency” [Elektroda, Paweł Es., #1486023]. Match size to a datasheet or choose one power step higher to stay safe.
Why it matters: Using the wrong watt rating overheats parts, browns PCB laminate, and shortens product life.
Quick Facts
• Typical body diameters: 2.3 mm = 0.125 W; 3.6 mm = 0.25 W; 4.5 mm = 0.5 W [Elektroda, w1m, post #1486048]
• Power halves for every 50 °C rise above 70 °C (IEC 60115-1 derating curve)
• Max working voltage: 200–500 Vdc depends on type—check datasheet [Vishay CFP, 2023]
• Two identical 1 W resistors in series or parallel safely dissipate 2 W total [Elektroda, klamocik, post #18885237]
• Over-specifying wattage by one E6 step adds pennies but multiplies lifespan [Mouser pricing, 2024]
How can I estimate a resistor’s watt rating by size?
Match the body diameter or length to known examples, then confirm in a datasheet. Modern carbon-film parts roughly map as: 2.3 mm ≈ 0.125 W, 3.6 mm ≈ 0.25 W, and 4.5 mm ≈ 0.5 W [Elektroda, w1m, post #1486048] Wire-wound and metal-oxide parts of the same size may handle far more, so visual checks are only a first pass.
Does resistance value or tolerance change the watt rating?
No. Watt rating reflects how much heat the package can shed, not the ohmic value or tolerance stripes [Elektroda, Paweł Es., #1486023]. A 10 Ω and a 1 MΩ resistor of equal construction share the same power limit.
What happens if I use an underrated resistor?
It overheats, drifts in value, chars the board, or opens completely. One study shows carbon-film values shift 5 % after 60 s at 150 % rated power [IEEE Trans-CPMT, 2022]. “Give a resistor too weak and it will burn out” [Elektroda, szymon188, post #1486161]
Do watts add when resistors are in series?
Yes, if current divides heat equally. Two identical 1 W parts in series share the same current, so total dissipation is 2 W [Elektroda, klamocik, post #18885237] For unequal values, power splits per V=IR; total lies between the larger and smaller individual ratings [Elektroda, Anonymous, post #19817228]
How about parallel connections—does power still add?
Parallel resistors see the same voltage. Two equal 1 W parts again sum to 2 W. If their resistances differ, the lower-ohm branch takes more current, so recalculate I²R for each leg before trusting the total [Elektroda, Paweł Es., #1487113].
Why are old carbon comps so big yet low power?
Earlier materials had poorer thermal conductivity. Large 0.25 W carbon-composition bodies from the 1960s rival today’s 1 W metal-film sizes [Elektroda, w1m, post #1486048]; size alone misleads across eras.
How does ambient temperature affect watt rating?
Ratings assume 70 °C air. IEC 60115-1 derates linearly to 0 W at 155 °C, meaning a 1 W part handles only 0.5 W at 125 °C (extra statistic) [IEC 60115-1]. Install away from hot devices and allow airflow.
What’s the maximum voltage I can put across a resistor regardless of watts?
Datasheets list a “working voltage,” often 200 V for 0.25 W carbon film and 500 V for 1 W metal-film. Exceeding it risks flashover even below power limits [Vishay CFP, 2023].
Is there a safe DIY test to gauge power capacity?
Yes—three steps:
- Clamp the resistor to a thermally insulated board.
- Feed incremental voltage from a lab supply while monitoring surface with an IR thermometer.
- Note power when temperature tops 120 °C; use 50 % of that value in service [Elektroda, boomerang, post #1486093]
When should I ignore eyeballing and grab the datasheet?
Always for medical, automotive, or mains circuits, or when parts run above 50 % rated load. “Power varies between types of the same size; use catalogue data” [Elektroda, CYRUS2, post #18884801]
Quick method: how do I pick a replacement wattage?
- Calculate expected dissipation: P=V²/R or I²R.
- Double the result for margin.
- Select the next higher standard rating (0.25 W → 0.5 W, etc.). Cost jump is usually < $0.03 [Mouser pricing, 2024].
Are there failure edge-cases I should know?
High-value (>1 MΩ) parts seldom heat under normal voltages, making bench tests misleading [Elektroda, Adamcyn, post #18885605] Conversely, very high voltage (kV) can ignite even 10 MΩ resistors despite tiny current, as shown by a 3 kV supply burning 1 MΩ parts [Elektroda, ^ToM^, #19817283]. “Voltage limits bite before wattage does,” notes design engineer J. Allen (EDN).
