Cell phone chargers need a much smaller transformer, usually ones that sit on the wall. Lamps cords don't need any transformer. Why do I need a giant brick in the middle of my laptop charger?
2 Answers
Mobile phones do not need as much power as a laptop in order to charge. You'll notice that a phone battery may last a day or two running on battery, whereas a laptop might struggle to last 4-5 hours.
The power equation states that P=IV, where I is the current and V is the voltage. A typical phone charger outputs 5 volts at 1 ampere, giving a power of 5 watts. Laptop chargers vary, but taking my own charger as an example, it outputs 19.5V at 10.3A - giving a power of just over 200W.
The extra current requires thicker wires and more space to stop the generated heat from melting the casing or setting fire to the surface it rests on. Indeed, most laptop transformers become quite warm after only 15-20 minutes of use.
To answer your question about lamp cords: a lamp does not require a transformer, as it is designed for local mains power (which may be 240V or 120V, depending on where you are). Phones and laptops are designed with a certain voltage in mind, and use tranformers to 'bring the voltage down' to their level.
Your cell phone charger supplies 5-10 watts or so, your laptop charger supplies 65-150 watts. E.g the label on one cell charger on my desk it says "OUTPUT 5.0V = 2.0A", and on one laptop charger it says "OUTPUT 19V = 4.74A". Power is voltage times current, so the cell charger supplies 10W, and the laptop supplies 90W.
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1I learned it once, but I don't remember. You don't explain why more power means you need a larger magnetic core in the transformer ?– mverooneNov 20, 2013 at 8:37