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How to get 12 volts from electrical outlet?

  1. Feb 6, 2012 #1
    Hi everyone,

    I need to power up some small LEDs in series and I need about 12 V total. I can use an A23 12 V energizer battery to achieve this but I want to know if I can use an electrical outlet to do it. So I would need to go from about 120 V (standard North American output) down to 12 V. What is the best way of safely doing this?
    Last edited: Feb 6, 2012
  2. jcsd
  3. Feb 6, 2012 #2


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    Staff: Mentor

    Welcome to the PF.

    Just get a 12Vdc "wall wart" power supply. It already has the safety approvals, and they are relatively cheap.

  4. Feb 6, 2012 #3
    Hello ylzxu and welocme to Physics Forums.

    Do you realise that your batteries give 12V DC whilst the mains gives AC power?

    Your LEDs will work from either but the voltages will be slightly different. You need 9V AC rather than 12 for the LEDs and to reduce the series resistor by 1.4 (divide by 1.4) to restore the current drawn to the same level and maintain the brightness.

    Just to correct an oversight by Berkeman.

    His text talks of a DC adapter, but the rating on the picture clearly states output : 12V AC.

    You should check with your shop which one you are going to buy.

    The 12V (DC) or 9V (AC) adapter is necessary to provide safety isolation between the mains and your circuit.

    go well
  5. Feb 6, 2012 #4
    Thanks for the replies guys.

    Sorry for the stupid question, but where would I connect the negative terminal of the series circuit? I know the positive terminal connects to the adapter.

    Also, with regards to what you (Studiot) said, could you possibly explain what you mean by "restore the current drawn to the same level."
    Last edited: Feb 6, 2012
  6. Feb 6, 2012 #5
    The connector on the output lead is usually ether the type shown in Berkeman's picture or the laptop/calculator type with a hollow barrel and metal on the inside and outside.

    Either way the polarity of the connection should be marked on the plate of the adapter in as in the symbols on my (sorry it's so scruffy) sketch.

    AC adapters don't have +ve and -negative and can be connected either way round so there is no polarity symbol on Berkeman's pic.

    go well

    Attached Files:

  7. Feb 6, 2012 #6


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    Staff: Mentor

    Thanks for the catch, Studiot! I neglected to look closely at the picture I snagged from Google Images. Glad you were paying attention!
  8. Feb 6, 2012 #7
    If you put too much voltage across a diode you can damage or destroy it. The peak voltage AC is 1.4 times the stated voltage. So the peak voltage of 9volts AC is 9*1.4 = 12.6 volts.

    However with AC the voltage is only there for a short period per cycle, whilst the DC voltage is there all the time.

    The brightness of an LED depends upon the current (not voltage).
    The LED will actually pulse on and off 60 times a second with your AC.
    You need to increase the current during the on time to maintain the brightness.
    The current is controlled by the series resistor. That is its purpose.
    To increase the current you need to decrease the resistor by the factor of 1.4.

    Some LEDs have a built in resistor that you cannot alter.
  9. Feb 6, 2012 #8
    Too much pumping iron if you ask me.

  10. Feb 6, 2012 #9
    Thank you very much guys, I very much appreciate it.
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