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  • How does a home appliance limit the amount of current that flows through it?
  • Are there some resistors set up in series in order to cut down the current flow before it actually reaches the device?

Assuming that there are $15 \;\text{A}$ under $240 \;\text{V}$ in each socket, most appliances would burn/get destroyed if there was no limitation to the current draw. But that would mean that huge amounts of energy were wasted in form of thermal energy and I assume that's not the case.

  • How does the electric circuit in our home "know" how much power to deliver to each socket?
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    $\begingroup$ How does your garden hose, which can push out maybe 2 GPM if you hold your sprayer wide open, "know" that you closed the handle on the sprayer and thus it delivers 0 GPM ????? $\endgroup$ – Kyle B Jun 13 at 19:17
  • $\begingroup$ How do you limit vehicle throughput on a highway? It's a combination of vehicle sizes, lanes available, and speed limit. E.g. at a 1mph limit, and if cars are 1/500th of a mile, each lane would be limited to an absolute maximum of 500 cars per hour (assuming no distance between cars etc, this is just a theoretical upper limit). How do you limit the throughput on your road, if the highway has a bigger throughput? Answer: lower the limits even further. $\endgroup$ – Flater Jun 14 at 12:48
  • $\begingroup$ Surely this idea can be simply disproved by observing a very low-powered device and seeing that it does not get excessively hot. $\endgroup$ – SiHa Jun 14 at 15:19
  • $\begingroup$ @pq89 You have it backwards. The idea behind electricity in simplified terms is that sources (outlet in your case) "push voltage" and sinks (any device connected to the outlet) "pulls the current". So the device only pull as much amps as it needs, thus it won't burn itself. $\endgroup$ – jnovacho Jun 15 at 12:38

14 Answers 14

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Your home circuit does not "know" how much current to deliver to each socket or appliance. The circuit supplies a constant voltage, and it is then up to each appliance to limit the current that it draws. Some simple appliances, such as lights with old incandescent bulbs or electric toasters or irons, are basically just a resistor (possible a variable resistor). Other appliances may contain a motor; the design of the motor will limit the current that it draws and this current will depend on the load on the motor.

Electronic circuits are often designed to run at a voltage that is lower than mains voltage, so electronic devices will typically have a circuit using transistors that supplies a very precise voltage to the rest of the device. Or they may work by charging an internal battery, which them provides current to the rest of the device - this then allows the devices to still be used when not directly connected to the mains supply.

Your home circuit (or each of the circuits, if your home has several) places an upper limit on the total current that can be drawn from it by using fuses. Individual appliances may also have fuses in their plugs or built into the appliances themselves. Some extension leads have their own fuses. But these fuses are intended to break the circuit in case of an accidental current surge (typically caused by a short-circuit somewhere) - they do not limit current in normal use.

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    $\begingroup$ Most electronics require a constant voltage, not a constant current - around your house it's pretty well just LEDs which require current regulation rather than voltage. Anything that runs off USB will be 5V +/- a bit. Likewise batteries are very much not constant current devices - to a first approximation they are constant voltage devices with a series resistor. $\endgroup$ – Pete Kirkham Jun 13 at 22:43
  • $\begingroup$ @PeteKirkham Thank you - I have edited my answer. $\endgroup$ – gandalf61 Jun 14 at 4:03
  • $\begingroup$ The voltage at the socket is not constant! $\endgroup$ – Volker Siegel Jun 15 at 9:30
  • $\begingroup$ @VolkerSiegel In the UK the mains RMS voltage is 230 V with a tolerance of +10 %\ -6%. For most practical purposes it can be treated as constant. $\endgroup$ – gandalf61 Jun 15 at 10:02
  • $\begingroup$ @gandalf61 I would not call it constant on this base, it even has a pretty large tolerance. But what I was thinking of is different, and directly relevant to this case: The voltage drops significantly if there is a high current. The maximal possible current appears when the sockets contacts are directly connected. In this situation, there is the minimal possible voltage. The extreme case is when the measurement contacts directly connect, in that case you have only 0 V. $\endgroup$ – Volker Siegel Jun 15 at 10:15
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There is already a good answer here, but I would like to add that a so-called "15A socket" is not so called because it "contains" 15A. The power grid and the wiring in your walls are capable of delivering hundreds or thousands of Amperes to any socket in your home. At least, it could do so for a brief interval before the smaller wires melt and start a fire within your walls.

The "15A" means that the wiring and the socket have been deemed capable of safely delivering that much current, and somewhere in the circuit, there will be an over-current protection device (i.e., a fuse or a circuit breaker) that will open the circuit if the current consumed by the appliance (or a fault) exceeds that rating by a dangerous amount.

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Electric supplies to homes and industries are standardized in countries. This allows devices which are used in homes or in factories to be accordingly designed. For e.g. if the standard single phase RMS AC supply voltage is $240$ V, appliances would be designed such that they operate without electrical failure at that input voltage. What I mean by this is given $240$ V, the device would draw a maximum current. Usually for $240$ V, the sockets that appliances plug in support $5$A (for light devices) and $15$A for high power devices.

Are there some resistors set up in series in order to cut down the current flow before it actually reaches the device?

You mean the load of the device on the socket. Yes the load is so designed that at the standardized voltage, a certain maximum current would be drawn. The internal design of the device however doesn't just involve adjusting individual resistors and can be as complicated as that of microprocessors to as simple as that of a bulb. Overall though, the socket sees the Thevenin load.

Assuming that there are 15 Amps under 240V in each socket

A socket doesn't have any current inside of it. It is only a source of voltage. The plugged in appliance determines the current draw. The appliances that plugin into $15$ A sockets are designed to not draw more than $15$ A and expect the socket to be able to provide that much sustained current without burning and shorting.

How does the electric circuit in our home „know” how much power to deliver to each socket?

Electricians wire the sockets as per the maximum expected power consumption the the socket must support safely. For e.g. a $5A$, $240$V socket must be able to sustain a power draw of $5\times 240$ W. All circuits simply speaking, run back to the distribution panel of your house and plugin into the grid via fuses also designed to trip after a maximum power draw. So all in all, the maximum power that a household is expected to draw is known in advance and the house and the distribution of that supplied power to the different sockets in the house is done accordingly. So, the socket doesn't "know", it was designed so.

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    $\begingroup$ On the other side of things, beware of thinking that 15 amps is some physical maximum the outlet can possibly provide! It will quite happily provide far more current than that, until the fuse or breaker kicks in (and if the fuse or breaker is faulty or has the wrong current rating, well, that's how you get the worst kinds of house fires!) $\endgroup$ – Hearth Jun 13 at 16:48
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Voltage, current, and resistance are in a relationship: $V=IR$. This isn't to say that one "causes" the other. We don't always say that voltage "causes" resistance. It's just a relationship that's due to the laws of physics. If you change one variable in that equation, one or another must change.

Now, in the cases you are interested in, $R$ is typically seen as fixed. A motor does not change its resistance (handwaving away inductance, for now, which is a more complex topic). So, generally speaking, $V$ and $I$ will vary in tandem. If the voltage goes up, the current goes up. If the voltage goes down, the current goes down. And, of course, if the current goes up, voltage goes up (and if current goes down, voltage goes down).

The key to both house wiring and batteries is that your device is connected to something rather insistent. In the case of a battery, there is a chemical reaction which is rather insistent on $V$ being some fixed value. When you attach a load to it, such as a motor or a resistor with a given $R$, exactly $\frac{V}{R}$ current will flow through it, due to physics. It isn't a function of the battery, or the resistor, its a function of both. Given a 1.5V battery and a 100ohm resistor, $\frac{1.5V}{100ohm}=15mA$ will flow through the circuit.

Now there are limits to this, and these limits are where your question starts to take form. That chemical reaction in the battery is insistent, but it isn't unyielding. If you put a small enough resistance across the battery (such as when you short the battery with a very-low resistance piece of copper wire), the chemical reactions in the battery just can't keep up with the amount of electrons it has to push to keep up with that current. Now, we said voltage, current, and resistance are in a relationship. The load wire isn't change resistance (much... heating up does increase resistance for most wires). Thus, if the current isn't that high, the voltage can't be high either. So even if you have a "1.5V battery," if you short it with a wire, the voltage between the ends will plummet, to some value which corresponds to this maximum amperage. The maximum amperage of a AA battery is about 10 amps, and an 18ga wire 2-in long wire is about 1 milliohm. So, because these three variables go together, $V=10A\cdot1m\Omega=0.01V$.

The power coursing through your walls is backed by something even more insistent. Somewhere near your city, there's a steam powered generator that generates power for your whole city. The physics of this generator are designed to be very insistent: it will generate 120V (AC, but that doesn't matter for this discussion). If you try to draw enough power to overwhelm it like you did the battery, you're in for a surprise. This thing is designed to provide megawatts of power without flinching. And, indeed, if you do manage to start to overwhelm it (which could be done by a lot of industrial complexes colluding together), a technician there would observe the generator slowing down by a fraction of a Hertz, and would put more fuel in the fire, driving more steam through the generator. You will not win this battle.

If you do win this battle, you will have what is called a 'brown out.' It happens from time to time, often because something got interrupted in the power distribution.

So thus, in a residential setting, we like to say the voltage is "fixed" at 120V. Its hard to draw enough to beat the power grid. Likewise, the resistance is fixed, due to whatever component you are using. Thus, the current must match by $I=\frac{V}{R}$. It simply has to, due to physics. Questioning otherwise is like asking a fish why it swims, or air why it winds, or a giant meteor why it wipes out the dinosaurs. It's just how things work. (I lie a little here. Late you might learn about electric fields, and what happens when you really push systems to their limits. But for anything related to the wall outlets, this is good enough).

Now, you mention the socket "has" 15 amps. The reality is that the socket doesn't "have" amps. It simply supplies an amount of current corresponding to the voltage and resistance. This could be a bad thing. If you tried the experiment with shorting the AA battery above, you might have noticed something - it gets hot. Really hot. Hot enough to spot weld one of the wires to the battery after burning your fingers. (Don't try it... really. I've tried it enough for everyone. Nobody needs to get burned in the name of science!). Well, the battery wasn't that insistent. The power company is. It will provide you with enough current to correspond to 120V.

And that means heat. Lots of heat.

Frustratingly, it means heat in the wires inside the walls, not just your load. It means enough heat that it could actually catch your house on fire! So, to avoid this, we install circuit breakers (or fuses) to limit current. By that I mean, if the current required for that resistance and 120V gets too high, it will disconnect the circuit before the wires in the wall heat up enough to catch fire. Your "15 amp socket" is really a 15 amp set of wires, with a circuit breaker to enforce it.

Now, all that being said, there are devices which change their resistance. There are "active" components. Unlike resistors (which always obey $V=IR$), they are components which follow different rules. As an example, there's the BJT (bipolar junction transmitter) which is a "current multiplier" with three wires. The current from a point called the "collector" flowing towards the point called the "emitter" is always some multiple of the current flowing from the "base" to the "emitter." This multiplier lets us do things like amplifying signals.

In the case of a BJT if you do some clever circuitry with diodes to create a constant current from base to emitter, you will create a constant larger current from collector to emitter. If this collector/emitter pair is part of the "power supply" lines for your component, this system will create a different sort of "insistent" supply - one that tries to keep the current constant.

A BJT is an active component. It is not a resistor, so it is not bound to $V=IR$. However, we can handwave and pretend it is a resistor, as it has a voltage drop and a resistance. The physics of the BJT will adjust the voltage and/or the resistance to maintain the current. So, if you have a 120V input (forced by the power company), and say a 100mA current (chosen by how you hook up the circuitry on the "base" side of the BJT), you effectively have selected a "resistance." Were there to be a brown out, and the voltage drops, your BJT would apparently "adjust" its resistance to a lower value so that there's still 100mA flowing.

This happens because we were just pretending the BJT with its base circuitry was a resistor. It isn't. But it turns out this is a useful fairy tale. When you get into more exotic systems like switching power supplies (which turn 120V AC into 12V and 5V DC for your computer), hand-waving all that fancy circuity turns out to be reasonable. We can view your computer as a "resistive" load with a "resistor" that is "tuned" just right to pull the right amount of power to feed the computer. Electrical engineers think this way all the time. However, if you wanted to get into the details of "why" the power supply acts like this, you'd want to remember that theses "resistances" were all just pretend, and there were really active components, and they followed different laws of physics.

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To very briefly summarize the other answers:

  • A power source is characterized by its voltage only.
  • A power consumer is characterized by its resistance only.
  • The combination of the two determines what the current will be (Ohm’s law: $I=\frac{U}{R}$).
  • If the resulting current is too much, bad things will happen. At best, an overcurrent protection device will trigger and cut off the power. At worst, something will overheat and cause a fire.

However:

Some devices do in fact exist that limit the current flow. They still obey the above principles, but they can manipulate their own resistance. For example, a device using USB Power Delivery to charge itself can ask the power source what the maximum allowable current is, calculate the appropriate resistance and then use some internal circuits to arrange that (the simplest way would be to only charge the full set of the battery cells if the source permits a high current, otherwise to disconnect some and charge them later). Another approach is for the load to switch itself on and off multiple times per second so the average resistance and average current are just right, and a capacitor can be used to smooth the current to avoid the short peaks—this is called pulse-width modulation.

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How does the electric circuit in our home „know” how much power to deliver to each socket?

It doesn't. It will supply as much as the appliance demands, up to a point.

How does a home appliance limit the amount of current that flows through it?

It depends on the appliance.

The simplest type is pure resistive, like a kettle or toaster. The resistance of the heating element is reasonably constant, as they tend to be made from a low tempco alloy and only reach (relatively) low temperatures (dull red is still relatively low in a toaster). It draws a current given by Ohm's Law.

Some types of appliance can draw a large inrush current, for instance filament lamps or transformer-input devices. The tungsten filament, being a pure metal rather than an alloy has a high tempco, and reaches a high temperature, so its hot (design) resistance is often 10x its cold resistance. The current taken at switch-on will therefore be 10x the running current. If a transformer is turned at voltage zero-crossing, it will attempt to reach twice its operating flux, and will usually saturate, drawing a very large current transient for the few mains cycles it takes it to settle down.

The wires supplying the socket can take this kind of intermittent over-current, absorbing the heat generated with little increase in temperature. Usually the protection device assigned to that circuit will also ride out a brief over-current event, whether it's a fuse or a circuit breaker.

If the socket is subject to a very heavy load (most are rated to be able to deliver many thousands of amps into a short circuit), then the fuse will blow or the circuit breaker will open before the wire temperature has risen high enough to damage the insulation.

Are there some resistors set up in series in order to cut down the current flow before it actually reaches the device?

Not in the house wiring, but some devices do actually include a small series resistor to limit current. The series capacitor + rectifier style of LED lamp uses a small resistor in series to limit the inrush current to a non-damaging level when it's first turned on. There's a compromise between a small resistor allowing a large inrush current, and a large one causing excessive heating during normal operation.

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  • $\begingroup$ If I'm not mistaken LEDs need to be protected against overcurrent at all times; their resistance is very low. $\endgroup$ – Peter - Reinstate Monica Jun 15 at 7:43
  • $\begingroup$ @Peter-ReinstateMonica What you say is true, but irrelevant. In the specific case I was referring to, a series capacitor style LED driver uses the AC impedance of the cap to provide current limiting. Inrush doesn't appear at the LED, that's shorted by a reservoir capacitor that I've not previously referred to, as it was irrelevant. The resistor is to protect the capacitor, rectifier and reservoir capacitor from the inrush current. Other, better, styles of LED driver manage LED current control in different ways. $\endgroup$ – Neil_UK Jun 15 at 9:06
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A toaster limits the amount of current flowing through it via its resistive elements which convert everything to heat.

A transformer connected to AC power mains limits the amount of current flowing through its primary windings via inductance: inductance is a form of opposition to the flow of AC. Even though the windings consist of copper wire that has low DC resistance, we don't cause a short circuit when we plug a transformer "wall-wart" into a socket. This is because a coil generates its own AC voltage which cancels most of the applied AC voltage. This is known as counter EMF.

A device with a motor, such as a vacuum cleaner, limits the amount of current flowing through it via inductance also, since its motor contains coils. Additionally a spinning motor generates a voltage which balances the applied one, which is called "back EMF", discussed in the same above article. Back EMF isn't confined to alternating current: a DC motor that is freely spinning without doing any work (other than overcoming a tiny amount of friction) draws very little current, because it is acting as a generator, producing an almost equal back EMF voltage.

A voltage source limits current. If we apply, say, 25.00V to a load which consists of a 24.95V voltage source of equal polarity, with a 1Ω resistor in series with it, what we actually have is a equivalent to a circuit which applies a 0.05V across the resistor, and that produce only 50 mA of current:

schematic

The schematic on the left is the same as the one on the right; I just drew it differently; as you can see the voltage sources add together and cancel, leaving 0.05V. Thus the 24.95V source in series with the 1Ω is very effective at limiting the current. Here, the 25.00V could be a power source such as a power supply, and the 24.95V voltage source might represent the back EMF generated by a motor.

If we apply a workload to the motor, it will slow down and generate less voltage, causing more current to flow.

When you plug the hose of a vacuum cleaner, the motor speeds up. Why is that? Intuitively, it may seem like it's trying to work harder against the blockage. But in fact, it's speeding up because it's not moving air, and therefore doing less work. Less work means it spins faster, closer to the synchronous frequency at which it would theoretically be doing zero work. Because of this, the appliance then draws less current.

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I've +1'd @gandalf61's answer for using the term "constant voltage" which is an important electrical concept, and the rest of that answer is spot on too.

I would add that two different concepts are often discussed using the same word, "limited".

In normal operation, the current that an appliance will allow to flow is limited by its resistance, or in the case of motors or transformers, a property called "reactance", both of which are measured in ohms and which for simple purposes we can consider very much the same. For example, in a 120-Volt circuit, a lightbulb with a resistance of 144 ohms will allow about 0.8 amps to flow, resulting in power usage of 100 watts. A heater that puts out, say, 1200 watts will allow 10 amps to flow, and ditto for a motor that's rated at 1200 watts at a given speed and load.

Often you'll hear that a fuse or circuit breaker "limits" the current in a circuit. I think this is misleading, or at least we need to understand it works in a very different way. A fuse or breaker has no effect at all so long as the current is below its limit. When the current exceeds its limit, the breaker or fuse "opens", which is to say, breaks the circuit, so that no current flows at all. Say we plugged two of those 1200-watt heaters into the same circuit, so that the combined current draw was 20 amps. (Or, imagine that an appliance failed in a way that the current could bypass the resistance, so that it could flow with no limit at all.) If the circuit was protected by a fuse rated at 15 amps (common for North American homes), the fuse would blow and the circuit would be dead. So it's "limited" the current, but in a very different way.

In technical contexts fuses and breakers are called "overcurrent protection devices"; if you see documentation referring to "current limiters" then they are probably talking about very different devices. For example there are variable-resistance components that increase their own resistance as more current flows through them. This has the effect of "dropping" more voltage at this device, reducing the voltage available elsewhere in the circuit (assuming a constant-voltage source) and thus the power available to those other components. You don't find these in home wiring projects; I believe they are used in things like power tool battery packs, to make sure the tool doesn't try to pull more current than is safe for the batteries.

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Your house knows nothing. ((Jon Snow))
But the appliances builders know something:

  • They know the voltage of your outlet
  • They know how to build a transformer
  • They remember well Ohm's law: $I=V/R$

Since they know the voltage of your outlet they can equip the appliance with the right transformer, to get the voltage output of their choice; and of course they build the damn appliance, so they also know its resistance; and with Ohm's law they now get the current flowing into the device. And that's game, they know everything there is to know.

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    $\begingroup$ My phone charger knows nothing about Ohm's law, quite to the contrary: At 230V it draws 0.1A, at 115 it draws 0.2A! $\endgroup$ – Peter - Reinstate Monica Jun 15 at 7:30
  • $\begingroup$ @Peter-ReinstateMonica correct me if I'm wrong, but doesn't your phone charger charge your phone at the same rate regardless of what mains voltage you plug it into (120V vs 240V)? I bet it probably does. Since the voltage is lower (120V < 240V) but the power is the same, that means it needs more amps for the same power. That means that the average resistance of your charger goes down to allow for that larger current. So, in fact, it still knows about Ohm's law. $\endgroup$ – user1969903 Jun 17 at 5:31
  • $\begingroup$ @user1969903 I'd call it Mho's law: The current is inversely proportional to the voltage. $\endgroup$ – Peter - Reinstate Monica Jun 17 at 7:34
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It can all be explained by the resistance of the involved parts.

  • How does a home appliance limit the amount of current that flows through it?

    • It limits the current by behaving like a resistor
  • Are there some resistors set up in series in order to cut down the current flow before it actually reaches the device?

    • The relevant resistor is the device itself, but there is a resistor in series that reduces the current flow a little. It is the wire itself. It only has a small resistance, compared to the device, so it has only a small effect.
  • How does the electric circuit in our home „know” how much power to deliver to each socket?

    • It knows it from the resistance of the device that is connected to the socket.
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I am sure all the other answers are excellent, providing varying degrees of details of all the physics involved. If anyone wants a more naive but perhaps more intuitive explanation, here's my two cents.

First off, let's not think of an electrical device as a collection of discrete parts (ICs, capacitors, inductors, resistors, etc.), but instead, as a black box. How does the electric circuit in our home "know" how much power to deliver to the box?

The answer is that it doesn't. All the electrical circuit in the home knows, is that it supplies whatever box plugs into it with the same amount of volts, that is to say, the same force (electromotive force, to be pedantic). In other words, it just knows that it should try to shove electrons though that box with a certain force (typically 120V for the US).

With this in mind, it's the box that knows it will be plugged into something that will try to shove electrons with 120V of "force" through it. As such, it is designed to cope with this. The box can be bigger or smaller. The bigger the box is, the more electrons can be shoved through it with the same force (120V), i.e. the more electrical current (amps) can flow through it. By this, I'm not refering to the pysical size of the box / device, but the overall electrical resistance of the device.

So how does it know how much current it needs? This depends on what the box does and how efficient it can do it. Let's consider a toaster. All it needs to do is get hot enough to brown some bread, and do it fast enough that you don't feel like throwing it out the window. This means it needs to use a certain amount of electrical power, whatever that figure may be. Therefore, the box, as it were, is designed so that its resistence allows enough current through it to consume that ammount of power.

The thing is, no matter how big the box is, it will always resist electrons being shoved through it (unless you cool it to near absolute 0, but that's out of scope for this answer). This means that the box will heat up as those electrons are flowing. Because of this, the box is made out of materials that can handle that heat by dissipating it to the environment, preferably before the box catches on fire or melts.

In the case of the toaster, the heating element uses a certain length of nichrome wire. This wire is designed to have just the right electrical resistence that enough current can pass through it so that it can get red hot and toast your buns. It is dissipating the heat to the bread.

In the case of a fuse, the wire inside the fuse is designed with a resistance that allows it to safely handle the current it's supposed to allow through. If something were to happen, such as a voltage surge or a decrease in resistence, which means more electrons are now able to be pushed through, the wire in the fuse heats up faster than it can dissipate the heat and consequently melts, breaking the circuit and preventing whatever device it is protecting from catching on fire and burning down your house.

How does a home appliance limit the amount of current that flows through it?

By limiting its electrical resistance so that the current draw (at mains voltage), as well as the heat it needs to dissipate, are within spec.

Are there some resistors set up in series in order to cut down the current flow before it actually reaches the device?

The device itself is the thing that provides the resistance which determines the current. As others have pointed out, depending on what the device is, it can have a mostly constant resistence (e.g.: toaster heating elements) or it can be something more dynamic (e.g.: switched power supplies found in phone chargers). Also as others have said, resistors can be used to protect discrete component inside of the device, and while they do contribute to the overall resistence of the device, these are there to protect the actual components (LEDs, transistor junctions, etc.), not the device as a whole, and I'd say are negligible for the purpose of this answer.

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  • How does a home appliance limit the amount of current that flows through it?

Strictly speaking, the way an appliance limits the current flowing through it is usually with a fuse, either a replaceable one or a part of it's circuit board which will melt if too much current passes through it. Electronic devices and appliances are designed to draw only a certain current and if more than that is flowing through it then something has gone wrong.

In normal operation the voltage supplied is more-or-less constant, and so the amount of current drawn for simple devices is based on the resistance of the device. A heater might have two elements, each of which is basically a resistor, if you have one switched on it draws 4A and if you switch both on each draws 4A and so the heater draws 8A. For motors, they have a resistance in their windings and also create a voltage as they spin so the effective voltage is less the faster they go, so the current is highest when they are starting, but the useful power is the current times their generated voltage, so the current is limited either by the load applied to the motor shaft or, if the load is too much for the motor to turn, the resistance in the windings.

  • Are there some resistors set up in series in order to cut down the current flow before it actually reaches the device?

Sometimes, but usually the device is created in a way that the work done by the current (power is current times voltage drop) does something useful. For example, a USB power supply will typically be a switched mode power supply, the mains voltage being rectified and then switched to create a high frequency AC which drives a small transformer, and there will be a chip which senses the voltage drawn and regulates how the switch operates to ensure it stays within limits. So if you plug a USB lamp into that power supply, the LEDs in the lamp will draw more current from it, which will cause the voltage to drop slightly, the chip will sense that and increase the drive to the transformer, and because more magnetic flux in the transformer means more current then more current will be drawn from the mains. This can be 90% efficient, but is obviously more complicated than a heater or simple motor.

Two exceptions are some LED indicators on some devices, which do sometimes just use a resistor, and some capacitive dropper LED drivers add a series resistor to prevent large currents if they are switched on at a peak in the AC waveform. In the indicator case it's typically done on high power devices and in comparison is such a small current that it doesn't matter to the overall efficiency of the device - although the indicator itself is only about 1% efficient, if it's indicating whether a the element on an iron is switched on then it is only wasting 0.1% of the total power of the appliance. In the capacitive dropper case the resistor only needs to be high compared to the internal resistance of the capacitor to prevent the capacitor heating up and exploding when you switch the lights on, and once the capacitor is operating normally most of the voltage is dropped across the capacitor and so the power wasted by the resistor is small.

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The appliances uses the electrical energy to get some useful effect. Sometimes it means high currents, as for an electric shower. For a voltage of 127 V, it can work with a current of 40 A, which requires thick wires between the fuses and the shower.

There are industrial devices that needs much higher currents (big motors, electric arc furnaces for example), the point is: they can not be used inside a home, because the wiring are not designed for them.

It is not that the appliances have some resistance to limit the current, but that if they need high currents, they are not suitable for home usage.

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Perhaps it is best explained on a simple example:

Say you want to manufacture a simple electric radiator:

  1. The tension (voltage) of the network is a given constant,say $Vo = 220 V$. (of course it is AC, but this value is just an average)

  2. You know that for heating a given room you will need say $1kW$ ($1000$ Watts) of power.

  3. What resistance will provide you with that amount of power?

$$ W = VI = RI^2 = V^2 / R$$

Given $W$, you can determine $R:$

The current used by the above construct is:

$$ I = V / R $$

  1. Step 1: Check how to choose the components of your system needed to get the job done.

  2. Step 2: Check how much current said chosen setup will drain from the power grid (and if your fuses are powerful enough to let such a current through).

Example:

You want to build an electrical radiator of 1000 W (1kW) and the average network tension is 220 V.

The formula above tells you that you will need to use a resistance of 40 Ohms, which will drain a current of 5 Amperes from the network.

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