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epicly easy question

Discussion in 'General Electronics Discussion' started by abkarch, Nov 17, 2011.

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  1. abkarch


    Nov 17, 2011
    Im completely new to all this. So im just making sure i have everything straight.
    Lets say I was hooking a EL wire up to USB.
    The maximum current of USB is 500 mA. Therefore, the EL wire must have a current input less than or equal to 500 mA.

    USB has a max voltage of about 5V. Therefore, the EL wire was have also less than or equal to 5V. is this correct?

    Also, lets say i wanted to hook 2 fans up to one USB port. Is it right to say that both combined can have no more than 500 mA and 5V? so if each fan was 235mA and 2V, than it would be fine? (these are random numbers, pulled out of my head)

    And im not exactly sure how resistors come into all of this, could someone explain it simply?

    Oh, and if A 10V fan was connected to a 5VUSB, would the fan just spin slower?
    Last edited: Nov 17, 2011
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010

    No, it should be exactly 5V.

    The voltage must match (you can have a little leeway in somethings)

    Yes, if they are both 5V fans then they need to be connected in parallel and in total draw no more than 500mA

    No. In general you can't put motors in series (so two 2.5V motors in series may sound fine, but won't work) although *some* things can be put in series.

    The total voltage needs to add up to (in this case) 5V, and the current required from both needs to be identical.

    A resistor is a simple device that will drop a certain voltage if a certain current is passed through it (determined by the resistance and current). It also works the other way around, if a voltage is placed across it, a certain current (determined by the resistance and voltage) will flow.

    Ohms law is the equation which gives the relationship between a resistor's value (in Ohms), the voltage across it (in Volts) and the current through it (in Amps).


    There is also a chance that it won't spin, or won't spin until you give it a bit of a nudge.

    There is also a small probability (unlikely with a fan, but possible with other things) that it could draw more current than it does at 10V, possibly overloading your power supply or damaging the device.

    Or, (again highly unlikely with a fan) it might just work normally.
    Last edited: Nov 18, 2011
  3. abkarch


    Nov 17, 2011
    Thanks, that helps ALOT. is this diagram a series or parellel circuit? i THOUGHT it was parellel, but not entirely sure.

  4. BobK


    Jan 5, 2010
    That would be parallel.

    A series circuit would go like this:

    +lead from USB to + lead of first fan
    - lead from first fan to + lead of second fan
    - lead from second fan to - lead on USB.

  5. abkarch


    Nov 17, 2011
    thanks for the help!
    So just to be clear, in a parellel circuit, the voltage of both fans would be exactly 5V EACH?
    And in a series circuit (which cant be done with motors, as you say), each electric device attached would have to add up to 5 volt?
    Last edited: Nov 17, 2011
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010

    The reason it won't work with motors is that they don't represent a simple load. Their current requirements go up and down as the motor turns. This is not normally an issue, but if you connect 2 in series each of them sees voltages that vary as the demand for each motor changes. Since they're unlikely to be exactly synchronised, things get pretty chaotic.
  7. abkarch


    Nov 17, 2011
    thanks for all the help!
    one more thing...
    i used this:
    with these numbers:
    Source voltage: 12
    diode forward voltage: 3.3
    diode forward current (mA) 20
    number of LEDs in your array 24

    And it gave me a 3x8 array. Could i do a 4x6 array with no problems? (Sorry to ask so many questions but i have like 5 things i want to do at this point, and they seem basic enough for me to do)
    Last edited: Nov 18, 2011
  8. Resqueline


    Jul 31, 2009
    No, you can't connect 4 white (or blue) LED's in series and run them from 12V (without adding "fancy" electronics).
    3 * 3.3V = 9.9V, leaving 2.1V for the resistor.
    4 * 3.3V = 13.2V, lacking 1.2V for a resistor.

    As for fans, in practice you can connect them in series (even quite unequal ones). On average they'll share the voltage quite fairly between them.
    Of course, with one small & one big 12V fan in series on 24V - the small fan can get quite overtaxed.
  9. abkarch


    Nov 17, 2011
    ohh ok thanks.

    Also...I understand how AC and DC work, but i wanted to get a switch. its says max of 250VAC.
    If i got a DC wall adapter...would it work fine with a switch?
    I guess a proper question is i know how they work, but how to do they work differently with circuits?

    also, are there small boards that have a DC plug jack in them? so i plug it into the jack and solder the wires onto the board?
  10. davenn

    davenn Moderator

    Sep 5, 2009
    Yes that switch would be ok to use on DC :)
    Often you find they have AC and DC ratings written on them but more commonly you may see 2 different AC ratings eg. 240VAC 1Amp and 120VAC 3A

    If a breadboard already has a DC socket on it, most likely it will already be wired to the + and - lines of the breadboard probably via a switch. That would totally depend on the manufacturer.
    the common breadboard are those white strip ones and you have to do all wiring to the board

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