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Assuming all connections are DC (avoiding any RMS, phase, and impedance calculations), if the power of an appliance is constant, and I change my country from a 240 V to 120 V one, my appliance will draw more current in the 120V country.

For that would it change its resistance? If yes, how does it do that? Do all appliances have a variable resistance? Can I use a 60 W light bulb cross country?

ocrdu
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awhitesong
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1 Answers1

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You start your question with a false assumption: that something like a 60 W light bulb has a constant power. Instead, it has a constant resistance, and is rated for 60 W wherever it is intended to be used.

A resistive load (like an incandescent light bulb) rated for 60W is assigned that rating based on the intended supply voltage. Using Ohm's law we can determine some other properties. We know the intended voltage (120 V) and its power rating (60 W), so we determine its resistance:

\$R = E^2/P\$

\$120^2 / 60 = 240 Ω\$

Now that we know the load (bulb) resistance, we can apply the higher voltage (240 V) to see what happens.

\$P = E^2/R\$

\$240^2 / 240Ω = 240 W\$

Since the filament in the bulb is not designed for 240 watts of power dissipation, it will burn itself out quickly.

Other loads, like computer power supplies, can sometimes operate at different supply voltages. (Sometimes it is necessary to toggle a switch, and sometimes the supply can auto-compensate.) In switching power supplies like that, they can often accommodate a wide variety of supply voltages. The device depending on that power supply can then consume a more-or-less constant power from one region to another.

A device that can handle different voltages doesn't alter its resistance to maintain a given power output, and therefore is not said to be a "resistive" type of load. Instead, it could be characterized as an inductive and/or capacitive load.

JYelton
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  • Got it. So some capacitive/inductive loads must be changing their impedance if not resistance to auto compensate because in the end it all comes down to it for drawing a different current, doesn't it? – awhitesong Jan 14 '22 at 00:45
  • Simplified, a switching power supply will use pulses of current through an inductor to achieve some other, regulated, voltage. The duty cycle and frequency which it does this can be variable, and so would then behave differently with different input voltages. For example, a SMPS might have a higher duty cycle with a lower input voltage. If you treat the device as a black box then you could say it is changing its characteristics depending on the supply. (It basically is a complex load that consists of resistive, inductive, and capacitive elements.) – JYelton Jan 14 '22 at 00:50
  • Got it. That's all I wanted to clear for a basic understanding. This comment is basically what I wanted to know. You can edit it in the answer if you want to. Now that I have a direction, will read more on this later. Thanks a lot. – awhitesong Jan 14 '22 at 00:53
  • @awhitesong No problem. You may also want to read more about the different types of loads here: https://electronics.stackexchange.com/q/91975/2028 (The takeaway being that most devices are a combination of the types.) – JYelton Jan 14 '22 at 00:57
  • That's a great resource, thanks! – awhitesong Jan 14 '22 at 01:06
  • An incandescent light bulb is a very poor example of something with a constant resistance. – Hearth Jan 14 '22 at 02:01
  • @Hearth Yes, it changes with temperature, but for the purposes of the scope of the question, that detail seems unnecessary. – JYelton Jan 14 '22 at 07:02
  • Re, "false assumption," That's not _entirely_ true. Electronic devices that have "smart" (a.k.a., "agile") power supplies often _do_ use constant power. Such devices are relatively new in the world. A few decades ago, they didn't exist. Usually such devices can be identified by markings on the device near its power inlet, or on its external power brick if it has one, that say something like, "INPUT: 100-250V 50/60 Hz" – Solomon Slow Jan 14 '22 at 14:08
  • @SolomonSlow I characterized the false assumption by specifically citing an incandescent bulb, which OP used as an example. I also believe I covered these types of devices where I stated sometimes these supplies can auto-compensate for voltage differences. Based on the question scope, I purposely minimized overly technical details. – JYelton Jan 14 '22 at 16:28