I’ve been told electric (and electronic?) devices do not like AC current? Is that correct? Take an electric can opener. It needs AC rectified to DC? What about a PC? If this is true, then why is all that time spent studying L-R-C circuits?
This is false. Some devices need DC current, some use AC. I believe, although I’m not positive, that the majority of “small” devices convert the AC to DC before they do anything else.
AC has a distinct advantage over DC, though. Your home wiring is at about 120 Volts. But if the power company sent out 120 Volts over miles and miles of wire to your house, none of it would reach you. The solution is for them to send out electricity at 10,000 V or 100,000 V and step it down before piping it into your home. This is where AC has an advantage: transformers can be made (and easily, too) that have no moving parts to step voltage up and down. This is impossible with DC current.
And as to why we spend time studying LRC circuits: Other than lightbulbs, (which are just resistors [R] ) I can’t think of any practical application that doesn’t use inductors (L) and/or capacitors (C). They (along with other components) are what allows engineers to control electricity in devices. An example: in a disposable camera, a capacitor is charged by a battery, and that capacitor discharges quickly to flash the flashbulb. And they’re used for way more than that. Open up any electronic device you like, and I’ll almost guarantee you’ll see multiple capacitors and inductors.
And I know that “AC current” means “Alternating Current current.” Deal with it.
Most motor like AC. And heaters couldn’t care less, so you may as well run them from AC. (Converting AC to DC involves inefficiencies.) But most electronics want DC.
Another thing to remember: some clock circuits (e.g. some clock radios) “tap off” the 60 Hz AC for their time base reference.
Additionally:
The thread title refers to appliances. I just checked the circuit diagram on my roommate’s fridge, and it does NOT convert to DC. I don’t own an electric can opener, so I don’t know whether they use AC or DC. I would guess AC, since efficient AC motors are a dime a dozen and so converting to DC would be rather pointless and entail some loss of energy. (Converting AC to DC loses energy through heat. When the box along the power cord heats up, that’s why.)
True, though this is much less common now that really accurate quartz oscillators are cheap as dirt.
Suffice to say, it is appliance-dependent. Some appliances need AC, some appliances need DC, and some need both. It is easy to convert AC to DC, but it is not easy to convert DC to AC. This is just one of the reasons our power distribution systems use AC – it is good for all appliances. (The primary reason is because AC is more efficient to distribute vs. DC, as so stated by Mayo Speaks!.)
I was under the impression that the “L” of LRC could only work with AC current. For, isn’t the induction created by the alternation of the current (and thus the magnetic field)? Also, I was under the (false?) impression that DC circuits were much more simpler to analyze.
Blame the P-N junction. Actually, blame the vacuum tube diode and triode as well. All of these devices are polarity sensitive and need a clean supply of single polarity voltage. There aren’t too many things I can think of that will operate on alternating current. Incandescent light bulbs, induction motors and heating elements come to mind.
We study L/R/C networks because they do some interesting things and are the basic components of analog tuning circuits. R/L and R/C circuits act as simple high pass and low pass filters (depending on the arrangement of the components) and L/C circuits act as bandpass or bandstop (notch) filters at specific frequencies.
But even more importantly, we study the characteristics of resistance, capacitance and inductance because these properties are exibited by many ordinary components and their effects show up in unexpected ways. At high enough frequencies, the lead length of an electrical component can have significant inductance and interfere with the operation of the system. Miles of copper wire (which we normally think of as a conductor) can add up to hundreds of ohms of electrical resistance. Capacitance exists anywhere there are two conductive materials seperated by an insulating substance. This explains why rabbit ears were often so frustrating as we thought we had the perfect alignment only to have the TV picture go to hell as we back away from the set.
L/R/C networks are also the basic filtering components of AC to DC power supplies.
A can opener probably uses an AC motor, although I’ve never cracked one open to find out.
Couple things:
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Incandescent light bulbs are not “just resistors.” They are pretty complex, really. They have inductance, and their resistance is strongly dependent on current.
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Some appliances & circuits do not use discrete inductors or capacitors. Examples include certain industrial motor, lighting, and heating circuits.
I was bitten by this little engineering method when I moved from the U.S. to Australia and brought along my digital alarm clock. I plugged it into my transformer, which nicely stepped the 240V down to the 120V the alarm clock needed. Unfortunately, the 50Hz (vice 60Hz) frequency of the current remained. I set my clock to noon, and an hour later it read 12:50. I scratched my head and reset it for 1pm and an hour later it read 1:50pm. Once I realized that in one hour it was only getting 5/6 of the number of “beats” from the power company, I knew why I needed to purchase an Australian alarm clock.
-Tofer
Inductance is defined as the opposition to a changing current. Capacitance is the opposition to a changing voltage. That means inductors act as a choke at high frequencies and a good conductor at lower frequencies.
That’s because DC circuits don’t do a whole lot by themselves. When you start adding semiconductors, then DC circuits become more useful.
There’s really no such thing as a pure DC circuit. All circuits must turn on and turn off, so there is always a transient response. And most circuits are susceptible to EMI. So what the non-engineering world calls “DC” we call “steady-state DC response.” 
A lot of interesting things can happen with DC circuits such as:
- Voltage drop
- Voltage division
- Leakage current
- Seebeck effect
- Contact resistance
- Peltier effect
- Triboelectric effect
- Electrochemical effects
- Hall Effect
Band Names!
A general rule of thumb I’ve noticed is that if something with a motor uses batteries, it’s DC driven. If it doesn’t, it’s usually AC driven. Every can opener and blender I’ve ever taken apart has had an AC motor in it. Vacuum cleaners, refridgerator motors, and older electric shaver motors are all AC. Newer shavers tend to be battery powered, and DC. Toasters and electric heaters use AC. Electric grills and hotplates use AC. Clothes washers and dryers use AC, but may have a DC circuit for the control (also may have AC).
TVs, microwaves, stereos, and computers all use DC, and often have multiple voltages present.
Inductors aren’t used much in the real world (compared to capacitors and resistors, at least) because inductors are more expensive, and can usually be replaced with a less expensive circuit containing a capacitor. However, that doesn’t mean that learning how inductors work is a waste of time. It’s important to know how they work, just to understand the basics of how electricity in general works.
Also, in the real world, there’s no such thing as a resistor, a capacitor, or an inductor. As Crafter_man pointed out, a light bulb isn’t a simple resistor. A resistor isn’t really even a resistor. The typical el-cheapo wire wound resistor that you buy at radio shack looks more like an inductor than a resistor at high frequencies. Real coils of wire aren’t inductors. They also have resistance and a bit of capacitance between the wires. Heck, just a section of wires like the wire in your house will have inductance, resistance, and capacitance. If you are designing circuit boards, there is resistance, capacitance, and inductance all in the little traces of copper on the board. You aren’t intentionally using inductors, but you still have to deal with inductance.
If you want to slow down a relay, one cheap and easy way to do it is use a capacitor. Relay coils are inductive. Now you’ve got a classic RLC circuit that you need to analyze, straight out of a first year EE text book. A relay coil is an inductor that stores charge. A capacitor stores charge. Pick exactly the right values, and the relay is going to “chatter” every time you turn it off because the energy is just going to bounce back and forth between the coil and the cap. How do you know if it’s going to chatter? That’s what that section of your EE book called “transient response of an RLC circuit” is for.
Power systems tend to be a tad bit on the inductive side due mostly to motors. The power company compensates for this using capacitance to counteract the inductance.
At higher frequencies, inductors do actually become quite commonly used. At RF frequencies, the bit about inductors being easily replaced by cheaper capacitive circuits no longer holds true.
Inductance is everywhere. You have to understand it if you want to understand electricity.
Minor nitpick. DC to DC transformation is possible with devices that have no moving parts, and is done all the time using semiconductors. Some DC to DC “transformers” also work by using an oscillator to convert to AC, then a transformer, then a rectifier to convert back to DC. Technically, this is kinda cheating since the actual voltage transformation is done via AC, but this type of DC transformer has no moving parts. You can also use diode and capacitor “charge pump” circuits to change voltages. The typical PC power supply will rectify the AC, then use DC to DC transformers to generate the various voltages the computer needs. This allows them to make smaller and more efficient power supplies than by simply tapping off various AC voltages and rectifying them.
That said, you do have a valid point. An AC transformer is just a couple of wires and a hunk of iron. You don’t get much simpler than that.
Edison wanted DC. Westinghouse wanted AC. There’s a good reason Edison lost.
Well that’s news to me. Thanks for fighting my ignorance.
Maybe this is why I only have a C in Intro to EE…