A lot of people don't pay much attention to power supplies until problems show up. We think you should always think about your power supply from day one - How are you going to power it? How long will the batteries last? Will it overheat? Can it get damaged by accidentally plugging in the wrong thing?
Here is the power supply that is used in many apple products:
All these power supplies have one thing in common - they take high voltage 120V or 220V AC power and regulate or convert it down to say 12V or 5V DC. This is important because the electronics inside of a computer, or cell phone, or video game console dont run at 120V and they don't run on AC power!
So, to generalize, here is what the power supplies for electronics do:
- They convert AC (alternating current) power to DC (direct current)
- They regulate the high voltage (120-220V) down to around 5V (the common voltages range from 3.3V to 15V)
- They may have fuses or other overcurrent/overheat protection
Hey, so if electronics can't run on AC, why doesn't wall power come in DC?
You may be wondering - "I have 20 wall adapters, this seems silly! Why not just have DC power come out of the wall at 5V?" Essentially, because modern electronics are very recent. for many many decades wall power was used to power light bulbs, big motors (like fridges, vacuum cleaners, washing machines, air conditioners), and heaters. All of these use AC power more efficiently than DC power. Also, different electronics need different voltages. So far its worked out better to have a custom power supply for each device although it is a little irritating sometimes!
|Power type in||Power type out||Technique||Pros||Cons||Commonly seen…|
|High Voltage AC (eg. 120V-220VAC)||Low voltage AC (eg. 12VAC)||Transformer||Really cheap, electrically isolated||Really big & heavy!||Small motors, in cheaper power supplies before the regulator|
|Low Voltage AC (eg. 20VAC)||High voltage AC (eg ~120VAC)||Transformer||Same as above, but the transformer is flipped around||Really big & heavy!||Some kinds of inverters, EL wire or flash bulb drivers|
|High Voltage AC (eg. 120V-220VAC)||High voltage DC (eg. 170VDC)||Half or full wave rectifier||Very inexpensive (just a diode or two)||Not isolated||We've seen these in tube amps|
|Low Voltage AC (eg. 20VAC)||Low voltage DC (eg 5VDC)||Half or full wave rectifier||Very inexpensive (just a diode or two)||Not isolated||Practically all consumer electronics that have transformer-based supplies|
|High Voltage AC (eg. 120V-220VAC)||Low voltage DC (eg 5VDC)||Transformer & rectifier
Combination of High→Low AC & Low AC→Low DC
|Fairly inexpensive||Kinda heavy, output is not precise, efficiency is so-so||Every chunky wall-wart contains this|
|High Voltage AC (eg. 120V-220VAC)||Low voltage DC (eg 5VDC)||Switching supply||Light-weight, output is often precise||Expensive!||Every slimmer wall-wart contains this|
Basically, to convert from AC to AC we tend to use a transformer. To convert from AC to DC we use a transformer + diodes (rectifier) or a switching supply. The former is inexpensive (but not very precise) and the later is expensive (but precise). Guess which one you're more likely to find in a cheaply-made device? :)
We left a few types out of this table because they're a little more esoteric or complex, such as the AC voltage doubler. These are still used but you're a little less likely to see them and they don't get used in power supplies you're likely to encounter.