Why is there ampere listed in DC power adapter?

Question

1.The current of a battery is decided by the resistance of electronics used.(Correct me if I am wrong).So there is no limit to current than can be taken from a battery (Except the time)

2.Is this also true for AC(No limit for current that can be taken out)
If both of the statements are true, then why there is ampere listed in the dc adapter (mobile adapter).

3.When I use the DC adapters for my project what should I take into consideration (Should I take the amperage listed into consideration)

Answer 1

The current of a battery is decided by the resistance of electronics used.(Correct me if I am wrong).

Correct mostly.

So there is no limit to current than can be taken from a battery (Except the time)

No longer correct. If you take too much current from a real battery, the output voltage will drop (due to internal resistance), and the battery will heat up. In extreme cases, the battery could catch fire, or ignite flammable materials nearby.

Is this also true for AC(No limit for current that can be taken out)

For the AC power supplied in your walls, the current is limited by a circuit breaker. If you attempt to draw too much power, the circuit breaker will trip and cut off the current.

The reason the circuit breaker is there is because if the current is too high, the wires and the outlet itself will heat up and could start a fire.

When I use the DC adapters for my project what should I take into consideration

You should use a DC adapter rated to provide more current than your project requires.

Answer 2

The maximum current that can be delivered by an AC power supply, DC-DC converter or other type of power supply is determined by the design of the power supply - how much current the components can safely handle, how much power can be dissipated by the components (and heatsinks) in the supply.

When selecting a power supply (AC adaptor) for your project, you must select a supply that will deliver the correct voltage, and AT LEAST the current required by your project. A higher current rating is fine (even desirable), as the load should only draw the current it requires.

Answer 3

I think you can figure out from the other answer that supply always exceeds demand unless you are thinking about coin cells trying to drive a relay. But there are some other facts you need may want to consider and read more about.

1. Every power source has a safe output limit and AC devices have breaker ratings for that device at max load. It is always capable of delivering more power for short duration, as the magnetic breakers have a time delay that has an inverse trip time with over current. Short circuits on the other hand, can be one thousand (1000x) the breaker current rating of 10A until tripped and they are designed for this possibility.

   • Overcurrent protection (OCP) is a must for every application where safety issues can occur. This is done with fuses, breakers and PTC thermal resettable types where applicable or with relays , crossbar switches etc.

2. Maximum power transfer occurs in practice ( and theory at half the no load voltage of an unregulated source when the load matches the supply. But this can cause thermal failures.

3. Normally unregulated power sources are designed to be within ~ +/-10% ( more or less) such as Car batteries when not being charged, AC grid to North American homes, LiPo cells from 3.7 to 3.3V/cell for most of its capacity. So unless you want to cause massive thermal issues with half the power lost in the power source or the wires to never draw even close to the unregulated output short circuit or maximum power limit of the source.

4. Closed Loop regulated Power Supplies are difference, because the negative feedback in the regulator lowers the effective output impedance and tightens the load regulation error. Although you can use 100% of the rated power output it is prudent for reliability reasons to leave sufficient margin for heat rise.

5. The short circuit impedance of AC distribution transformers of about 8% of the rated load meaning short circuit current is 1/8% of rated output or 12.5x rating of a typical 15kVA padmount. So a typical modern North American residence with 200A 240Vac split phase service could easily get 10kA short circuit current before the breakers trip. It can turn a huge screw driver tip into vaporized sputtered copper on plastic glass lens. ( personal experience )