Wikipedia:Reference desk/Archives/Science/2022 November 25

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November 25

If an incandescent and LED light bulb both are the same watts...?

Then they both use the same amount of amps, when plugged in the same voltage/Hz outlet? But the lumens would be different? 67.165.185.178 (talk) 05:23, 25 November 2022 (UTC).

Correct. Here is a comparison chart: https://www.voltlighting.com/learn/lumens-to-watts-conversion-led-bulb 41.23.55.195 (talk) 06:55, 25 November 2022 (UTC)

But this question is just as asking, are incandescent and LED light bulbs, are equally Ohmnic, or equally non-Ohmic, and the answer is still yes? 67.165.185.178 (talk) 16:25, 25 November 2022 (UTC),

The formula $P=VI$ relates the wattage $(\!$denoted by the letter $P)$ to the voltage $(V)$ and the current $(I).$ Ohm's law, $V=IR,$ relates the voltage to the current and resistance $(R).$ We can rewrite both equations by solving for $I.$ The first gives us $I=P/V,$ the second one $I=V/R.$ It follows that $P/V=V/R.$ Solving for $R$ establishes that $R=V^{2}\!/P.$ So the resistance of an element is determined by the combination of the voltage and its wattage.  --Lambiam 19:06, 25 November 2022 (UTC)

Ohm's law won't apply for something that's non-Ohmic, which was my question. 67.165.185.178 (talk) 19:53, 25 November 2022 (UTC).

Power=V.I If V is constant and power is constant then .... Greglocock (talk) 20:34, 25 November 2022 (UTC)

For Ohmic appliances yes. 67.165.185.178 (talk) 21:00, 25 November 2022 (UTC).

Okay, I just Google'd "are LED light bulbs ohmic?" and the 1st response was "LEDs are not Ohmic materials, meaning the relationship between current and voltage is not linear." So back to my older question on how do LED light bulbs work with the same voltage as incandescent bulbs. 67.165.185.178 (talk) 01:46, 26 November 2022 (UTC).

the efficiency of the LED is about 4-5 times that of the incandescent (light watts/electrical watts), and they undoubtedly have some electronics in the base of them, presumably some sort of rectifier and a voltage downconverter.. Greglocock (talk) 07:18, 26 November 2022 (UTC)

Modern lightbulbs - such as an LED bulb designed to fit in a standard socket - has an electrical ballast circuit.

These are diverse and complicated circuits; broadly, they convert and limit the power from the main supply (often using a switched-mode power supply).

Many vendors place a confusing "wattage equivalent" to help users compare brightness against an old-fashioned incandescent bulb. They often also print the actual power consumed, and often also specify the brightness in units of lumens.

In the USA, the Energy Labeling Rule requires light bulb manufacturers to give consumers key information in an easy-to-read format - so the bulb labeling is meant to provide useful information without requiring advanced knowledge of how the bulb actually works.

It turns out that a retail lightbulb is quite complicated, in this century; equally, we have more different types of lightbulb with diverse inside-bits.

One manufacturer that I like is Philips - specifically because they provide detailed engineering information about their bulbs - like these LED bulbs with datasheets. It just turns out that retail purchases across their product line-up can be difficult - you might have to order wholesale or through a specialty electrician!

Nimur (talk) 15:48, 26 November 2022 (UTC)

Neither incandescent nor LED lamps are "ohmic" if that means having constant resistance R so that Ohm's law quoted by Lambian exactly applies. An incandescent lamp has a positive temperature coefficient of resistance. The high inrush current to a cold lamp (e.g. R = 40 ohms) causes the tungsten filament to heat and the current to reduce in milliseconds to a stable illumination value e.g. I = 0.86 RMS amp, V = 120 RMS VAC, R = 140 ohms, P = 103 watt. A constant-voltage mains supply is suitable for an individual lamp. In equipment where many vacuum tube (valve) filaments are connected in series, direct connection to a mains supply can cause uneven overheating during switch-on so this is avoided by adding a negative-temperature-coefficient thermistor to the series circuit. LEDs have a steeply rising exponential current/voltage characteristic so LED lamps that contain them include a driver circuit that converts the alternating current from the mains to an effectively constant-current through the LED(s) - often this is achieved by switching the LED(s) on/off with variable duty cycle at an imperceptible high frequency. Philvoids (talk) 15:46, 26 November 2022 (UTC)

If you don't require R to be constant (whether as a function of temperature or as a function of voltage), Ohm's law applies just fine; simply define resistance by the formula R = I / V. Regardless, I = P / V, so two appliances with the same wattage for a given voltage draw equal amounts of current.  --Lambiam 11:19, 27 November 2022 (UTC)

That definition exercise seems to confirm the OP's opening words, that the currents in amps of the lamps are equal (while forgetting that the published "watt rating" of LED lamps is not objective). It is more constructive here to define luminous efficacies for tungsten incandescent bulbs 15 lumens/watt and for LED technology 75-110 lumens/watt. Explanation of the LED lamp driver technology can begin with the article Switched-mode power supply. Philvoids (talk) 18:06, 27 November 2022 (UTC)