AC/DC Power Adapters: Input & Output Specifications

In summary, the conversation discussed the workings of power adapters, specifically the brick-type ones that come with laptops and external hard drives. The question was raised about how the output current can be greater than the input current, using an example of a laptop power brick with an input of 100-240V~1.7A and an output of 18.5V == 3.5A 65W. The explanation provided was that there is a transformer inside the adapter that converts the high voltage AC to a lower voltage AC, which is then converted to the final DC voltage. This allows for the output current to be higher than the input current, while still conserving power.
  • #1
habitat
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in regards to power adapters, mainly the brick-type ones that come with laptops and external hard drives, how is the output current greater than the input current?

for example, i see a laptop power brick with the following:
input 100-240V~1.7A 50-60Hz
output 18.5V == 3.5A 65W


how exactly does this work with a branch current method of analysis if the branch going into the power brick is 1.7A? i see that you can pull current from voltages, but if only 1.7A is going into the network, where are the other carriers coming from to bump up the output to 3.5A?

i've been trying to think this out but i can't arrive at anything. what i left off on is that 1 ampere = 1 coulomb per second. if the input is 1.7A = 1.7 coulomb per second operating at 50-60Hz, perhaps it actually collects enough current to drive 3.5A?

i'm a bit lost.


thanks!
 
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  • #2
yes but its 1.7 A at the mains voltage 100 - 240V. But you then drop the voltage and you can increase the current. I don't have the formulae for showing the workings. Some one else is likely to be able to do that.

Same thing happens around the other way. I have a PSU that can supply for example... 12V at 5A. If I double the voltage, I'm going to 1/2 the current capability
That is. The PSU has a particular max Wattage rating V x I (current) = Watts if you change V then I is going to change to keep that max Wattage

Thats a basic way of looking at the problem :) I'm sure others could explain it in more detail :)

cheers
Dave
 
  • #3
Welcome to PhysicsForums!

There's a transformer inside that converts the high voltage AC to lower voltage AC (which is fairly close to the output DC voltage). This lower AC voltage is then converted to the final DC voltage.

The formula for power, given voltage and current, is:
[itex]P=I*V[/itex]

Since power is conserved (more or less, some of the energy is lost as heat during the transforming), you can find the input and output voltages:
[itex]P=I_{in}*V_{in}=I_{out}*V_{out}[/itex]

More on this topic at:
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/transf.html
 
  • #4
Great! thanks a lot!
 

FAQ: AC/DC Power Adapters: Input & Output Specifications

1. What is the difference between input and output specifications for AC/DC power adapters?

The input specification refers to the amount of voltage and current that the power adapter can accept from a power source. This is usually listed in a range, such as 100-240V AC. The output specification, on the other hand, refers to the amount of voltage and current that the adapter will provide to the device it is powering. This is usually listed as a single value, such as 12V DC.

2. Can I use an AC/DC power adapter with a different input or output specification than what is listed on my device?

No, it is important to match the input and output specifications of your power adapter to those listed on your device. Using an adapter with a different specification can potentially damage your device or cause it to malfunction.

3. What does the polarity specification mean for an AC/DC power adapter?

The polarity specification refers to the direction of the electrical current flow in the adapter. This is important to know because using an adapter with the wrong polarity can also damage your device. The polarity is typically indicated by a symbol, such as a plus and minus sign, on both the adapter and the device.

4. How do I know if an AC/DC power adapter is compatible with my device?

You can check the input and output specifications of both the adapter and your device to ensure they match. You can also look for compatibility information provided by the manufacturer of your device or the power adapter.

5. Can I use an AC/DC power adapter with a lower output specification than what is listed on my device?

No, it is not recommended to use an adapter with a lower output specification than what is listed on your device. This can cause your device to not function properly or not receive enough power to operate at its full capacity.

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