FAQ
TL;DR: Need a 0.5 W resistor? Using two 0.25 W parts in series or parallel gives the same 0.5 W rating—a 100 % increase—because, as one engineer states, "power is added"[Elektroda, CMS, post #3741568]
Why it matters: This lets you solve parts shortages without changing circuit behaviour.
Quick Facts
• Two identical resistors (series or parallel) double the wattage; four quarters equal 1 W[Elektroda, kankan, post #3741584]
• Series chains split voltage; parallel nets split current[Elektroda, szod, post #3741914]
• Typical film resistors derate 50 % at 70 °C[Ohmite Datasheet, 2022].
• 3-resistor strings routinely handle ≥1 kV to cut cost[Elektroda, jony, post #3742173]
• Two 230 V bulbs in series draw roughly 140-150 W, not 200 W[Elektroda, żarówka rtęciowa, #3742158].
Can I swap one 0.5 W resistor for two 0.25 W in series?
Yes. Each half sees half the voltage, so each dissipates 0.25 W; together they handle 0.5 W[Elektroda, CMS, post #3741568]
Does the same wattage addition work for parallel wiring?
Yes. Parallel parts share current, not voltage. Two 0.25 W parts still dissipate 0.5 W total[Elektroda, żarówka rtęciowa, #3833104].
How do voltage and current split between identical resistors?
Series: voltage divides equally, current stays equal. Parallel: current splits equally, voltage stays equal[Elektroda, szod, post #3741914]
What if the resistors are not identical?
The lowest-watt part limits chain power. One 0.1 W in series with 10 W still limits to 0.1 W[Elektroda, zybex, post #4083072]
How can I build a 2.2 kΩ 1 W resistor from four 2.2 kΩ 0.25 W parts?
- Make two parallel pairs (each 2.2 kΩ || 2.2 kΩ = 1.1 kΩ, 0.5 W).
- Connect the two pairs in series (1.1 kΩ + 1.1 kΩ = 2.2 kΩ).
- Combined rating: 0.5 W + 0.5 W = 1 W[Elektroda, Aleksander_01, post #3833048]
Is power always simply added, whatever the connection?
For equal-value parts, yes; total allowable dissipation equals the sum. Unequal parts need individual checks[Elektroda, emka371, post #3833110]
What happens during an overload edge case?
Exceeding the smallest resistor’s rating causes it to overheat first, often within seconds, leading to smoke or open-circuit[Elektroda, zybex, post #4083072]
How do I derate resistor power for ambient temperature?
Most carbon-film parts drop to 50 % power at 70 °C and 0 % at 155 °C[Ohmite Datasheet, 2022].
Is there a quick rule for N identical resistors?
Total wattage = N × single-wattage. Total resistance: N×R (series) or R/N (parallel).
Do these rules change for capacitors or light bulbs?
Yes. Capacitors follow inverse rules; incandescent bulbs are non-linear and drop to ~70 % rated power when halved voltage is applied[Elektroda, żarówka rtęciowa, #3742158].
Which tools help calculate combinations?
Free calculators like LottoCAD’s resistor tool speed up series/parallel math[Elektroda, Pegelmesser, post #3833182]
How do I estimate power dissipation fast?
Use P = V²/R or P = I²R. Measure voltage across, or current through, the resistor with a DMM; plug into formula[Horowitz & Hill, 2015].
