Why go through the effort of coverting AC current into DC current? I know that AC current is needed to transfer electricity through long lines. Why shouldnt the AC be stepped down to 110v or 220v, and then go right into the appliance? Why convert it into DC?
Because many of the appliances you use have semiconductor diodes which are biased.
They need current flowing one direction.
Yeah, you can’t make a microprocessor run on 120V AC. The electrons get confused.
Pretty much all electronics things like amplifiers, digital circuits need DC power to operate.
Even before we started using semiconductor diodes vacuum tube amplifiers needed DC power.
It’s obvious to anyone who has worked with electronics, but the question is not so simple to answer.
First of all, semiconductor devices (diodes, transistors, FETs, and all their myriad variations) have a preferred direction for current flow. This means that if they are operated on AC, one of two things will happen - they will either be “off” 50% or the time when you expect them to be “on,” or they will be “out of control” 50% of the time - you won’t be able to turn them off. Also, digital devices use a reference voltage to determine whether an output or input is a “1” or a “0.” If you were using AC, it would make things way more complicated, since the voltage is aways changing.
Oh, I forgot about the vacuum tube diode and triode.
Y’all must have some fancy new appliances, because with the exception of my toaster oven which has a nifty digital display I don’t have a single appliance that has any need for DC.
And AC isn’t required for long distance power transmission, high voltage is. It’s just much easier to convert AC voltages with simple transformers.
For the sake of clarity, let’s all acknowledge that “Most Appliances” is not “All Appliances”. In fact, it may not even really be “Most”.
When I picture an appliance, I usually see something that uses electricity for either mechanical power (motor) or heat (heating elements). These basic functions can be directly powered by full-voltage AC electricity.
Down-voltage DC is only for electronic (“solid-state”) control circuits or low-voltage IF (“intermediate frequency”) audio or video circuitry.
This latter type of circuits use either signals of specific frequencies and very low voltages operating in circuits made with low-voltage semiconductors (transistors, chips, etc.).
Older appliances with electromechanical controls may not have stepdown transformers and DC rectifiers at all.
Nowadays, however, I guess “solid state” is the default for everything except mechanical and thermal power, so transformer/rectifier stuff is much more prevalent.
assuming you’re talking about things like laptops and such which have power bricks or “wall warts,” it’s mostly out of a desire to move the DC power supply outside of the appliance for any number of reasons (space, heat, cost.) Even if a laptop were to plug directly into the AC mains, it would still have to have a DC power supply inside it.
Core_dump, your TV, computer and its peripherals, radios, CD player, DVD or Blue Ray player, and digital video recorder are a few of the things you may own that use DC.
Sure, you plug them into the wall, but that AC is rectified into DC for the appliance to work. Sometimes this is done in a wall wart adapter and other times the circuitry is inside the appliance.
I don’t own a TV/DVD/etc and I guess I didn’t consider the computer to be an “appliance”. At any rate, I forgot about the microwave so I’ve been busted.
So you are posting from your toaster, are you? Cool. 
You might ask the question in reverse: Why can some appliances run on AC with no conversion?
Answer: Some things, like light bulbs and heaters, work fine on whatever you give them. Sure, the light bulb goes on and off 60 times a second in the US, but our eyes can’t see it and the bulb doesn’t care. Same with heaters.
And there are AC motors that run at 120 or 240 volts, so no conversion is needed there (refrigerator, washer & dryer motors).
It’s only electronics, designed to work on DC at low voltages, that must be handled differently.
Actually, the filament in incandescent lamps has enough thermal mass that the output doesn’t change very much over one cycle. Line-powered LEDs are a different story - they can have noticeable (and annoying) flicker.
no you’ve been rectified.
We could make a couple of lists:
Uses AC directly (;)):
oven/stove
toaster
toaster oven
dishwasher
washer
dryer
electric water heater
refrigerator
coffee maker
blender
vacuum cleaner
Requires conversion to DC:
microwave oven
computer
television
telephone
radio/stereo
DVD/VCR
iPod/MP3 players
digital displays for any of the AC appliances
The AC list is a little longer, but I’m sure I’ve missed some things in both lists. Also, it depends on what you consider to be an “appliance”.
good list.
to add completeness the digital displays for any of the AC appliances could include control functions for the appliances.
The just made up gazpacho rule of appliances “something for the house that uses power which needs to be delivered on a truck”. So culling your list and adding my comments for my house.
oven/stove Electronic control needs DC
dishwasher Electronic control needs DC
washer Electronic control needs DC
dryer Electronic control needs DC
electric water heater Probably AC but mine is gas so it has no plug at all.
refrigerator Electronic control needs DC
TV Needs DC
Only a fraction of the electricity in appliance with motors gets converted to DC. The motors themselves are run on AC.