In a previous blog, us talked around the impact that different waveforms have actually on RMS vs. Peak voltage. Now we’re going to look in ~ the implications of that when it concerns power consumption. For functions of discussion, we room going to use an old-fashioned irradiate bulb with a filament – something that’s simpler for many of us (especially me) to understand than the new-fangled light bulbs that present a tiny different snapshot (a subject for an additional blog). Girlfriend may have noticed that light bulbs have actually not just a wattage rating but a voltage rating as well. Us don’t think around the voltage rating together much due to the fact that we space usually talking just of a voltage rating the 120 volts. Therefore what walk a pear rating of “100 Watts” mean? precisely this – – when a specific voltage (120 volts because that a typical light bulb) is applied across the filament, the resistance of the filament will draw a current of .83 amps consuming power at the price of 100 Watts. Law a little an ext math, R(Resistance)=E(Voltage)/I(Amps) we can calculate that the resistance of the filament as soon as operating in ~ 100 watts is approximately 145 ohms. It should be noted that this is the resistance in ~ the operating temperature together the resistance does readjust (gets higher) as the filament heats increase but, for now, let’s say it is resolved at 145 ohms to store things simple. This is reasonable since the resistance that the filament counts on that temperature and the temperature go not boost or to decrease appreciably end a 1/60th the a second cycle.

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Note – A light pear rated in ~ 60 watts at a voltage that 120 volts has a different filament resistance to consume 60 watts instead of 100. The filament in a 60 watt bulb has a resistance that 240 ohms as compared to 145 ohms because that a 100 watt bulb.

The above assumes that the voltage resource is constant at 120 volts. But, once the source is a sine wave, the voltage is not continuous at 120 volts. An RMS voltage sine wave source varies in between minus 170 volts and plus 170 volts. A resistive fill produces the same current flow regardless of polarity. A positive voltage produces the exact same amount strength (amount the light) as the same an adverse voltage in the situation of a irradiate bulb. Thinking a tiny further though, since the voltage is varying and the resistance of the filament in the bulb remains the same, the power consumption varies in time with the median power consumption being 100 watts.


Current flow (amperage) varies with the voltage applied across the filament. As soon as there is no voltage (at the beginning of the cycle), over there is no power consumption. Basically the pear is off. As the voltage increases to 120 volts, the strength increases accordingly to 100 watts. As the voltage go even greater (the optimal of the sine wave) that reaches a top of roughly 170 volts. Due to the fact that the resistance that the filament doesn’t adjust (145 ohms) the current at 170 volts is 1.17 amps. The resultant power consumption at that immediate is just quick of 200 watts. Therefore the irradiate bulb, rated in ~ 100 watt at 120 volts is in reality consuming power at the price of 198.5 watt at the peak of the voltage cycle!


In researching for this blog (I carry out research this things) I discovered a video that illustrates the above. The filament never ever goes entirely dark in the video because that its thermal inertia however the pulsations in brilliance room evident.

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The “take away” right here is that a 100 watt light bulb is rated at 100 watt only when operated in ~ its to plan voltage (direct current or RMS). Decreased voltage results in less power while greater voltage outcomes in greater power uneven the resistance the the filament is changed. A 100 watt light bulb intended for operation at 120 volts is no much longer rated at 100 watt if the voltage changes!

Note – Yes, this is exactly why “long life” irradiate bulbs the the previous were rated 100 watt at 130 volts instead of 120. Once operated in ~ 120 volts, they were dimmer – no consuming a full 100 watts. This caused a longer operating life because the peak operating power was substantially reduced.