Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Circuits Question -> Laptops

  1. Nov 15, 2005 #1

    I'm a chemical engineering major who's taking a product design class. Hating all the different classroom desk/chair arrangements in my building, my group chose to redesign classroom desks/chairs (it has nothing to do w/ ChemE, I know). Being a ChemE, I only know about circuits from my Phys II course that I took 2 years ago.
    Here's the deal: my groups wants to run circuits through the desks to allow people to plug in their laptops. There will either be one long desk or blocks of desks that can be electrically connected together. Here's what I know (or so I hope):

    1) The volage from the classroom's outlet is 120 V
    2) The current is about 20 A
    3) The maximum current a laptop uses is about 4 A

    Of course, the circuits will be connected in parallel to equally share current.

    Therefore, my group concluded that the maximum number of computers plugged into the desk outlet before the fuse is blown is 5. Does this seem reasonable?

    Also, does anyone know anything about the plug of a laptop? Will it convert the 120 V to the amount that the laptop uses ~ 22 V? Like I said, I know very little about circuits.

  2. jcsd
  3. Nov 15, 2005 #2


    User Avatar
    Gold Member

    Yea it would.That plug you may be referring to is called the adapter. It steps down the voltage.

    Let me ask you a quick question, how do you know the current is 20A. The current would only increase when you add a load (in this case a laptop). Are you referring to the maximum current flow the wire can handle before it starts to get hot an poses a threat
    ? Well in that case, yes. This fuse has to break the circuit to prevent a fire for example.
    Last edited: Nov 15, 2005
  4. Nov 15, 2005 #3
    What's wrong here is the laptop current figure. 20A for the 120V mains seems reasonable for the fuse, but a laptop will not draw ~500w. That is completely crazy. It might draw 5A from the power supply at 12V, the power supply is not drawing 5A from 120V to get that. Probably more like <1A, when the efficiency is accounted for.

    Thus, the circuit will be fine with even 20 laptops plugged in, assuming you don't pass 20A through a skinny wire at some point.
  5. Nov 16, 2005 #4


    User Avatar
    Science Advisor
    Gold Member

    What I would do is measure the current on the 120 VAC source when one laptop is attached. Then figure how many you can hook up.

    If you don't have a AC current meter, then look at the power (watts) the laptop consumes then find the current. i.e. I = P/V
  6. Nov 16, 2005 #5
    Total power avialable from the outlet is 20A * 120 V = 2400 W, my laptop uses 65 W, so if you divide 2400/65 = 37 laptops
  7. Nov 16, 2005 #6


    User Avatar

    Staff: Mentor

    Most laptop power supplys (the transformer block in the middle of the wire) say what their input amperage is. High-end laptops could pull up to about 1.5A, but most are less than 1. The output tends to be pretty high - 19V or so, so that's where the figure of 5A comes in, giving a maximum of about 95W.
  8. Nov 16, 2005 #7
    Hey guys,

    Thanks for responding to my concerns. I have one more question about circuits. Is there only one fuse that I can blow for a classroom?

    For instance, if I have 3 outlets in the classroom, is the maximum number of computers that I can have connected to the desk, going by waht's calculations, 37 or 3x37=111?


    By the way, if you have any comments about features you hate or love about classroom desks/chairs, please let me know :)
  9. Nov 16, 2005 #8


    User Avatar
    Gold Member

    You have to determine which breakers control which outlets. If there's any decency in the world, the breakers will be labelled as to what loads are assigned to them.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook