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circuit calculation

Discussion in 'General Electronics Discussion' started by natklopper, Jan 19, 2014.

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  1. natklopper

    natklopper

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    Jan 19, 2014
    Hi, I'm trying to run a lamp (which draws 0.6A at 3.6v) and an led array (Which draws 0.01A 5.4v) Similtaniously however I want to be able to turn them off/on separately and have one power supply in of 9v. How can I get this to work with resistors because I can't get my head around it and I'm sure someone on here can.

    Array: 0.01A 5.4v

    Lamp: 0.6A 3.6v

    Many thanks, Nathaniel.
     
  2. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    You could do that, but is it not optimal. You will waste a lot of power in the resistor, particularly for the lamp.

    But the calculations go as follows:

    R = (Vb - Vl) / I

    Where R is the resistor, Vb is the battery voltage, Vl is the lamp (or LED) voltage and I is the lamp or LED current. Calculate a separate resistor for the lamp and the LED and put it in series with the light.

    For example, for the lamp:

    R = (9 - 3.6) / 0.6 = 9 Ohms.

    The power wasted in the resistor is I^2 R, in this case 0.6^2 * 9 = 3.24 Watts. This is a lot of power. You would need a 5W resistor and it will get hot.

    See if you can do the same two calculations for the LED.

    A better solution would be to use a buck converter to step down the 9V to 3.6V for the lamp. The resistor should be fine for the LED because of the lower current. The buck converter can step down the voltage while wasting only about 15 to 20% of the power, whereas the resistor in this case is wasting 1.5 times the power used by the lamp.


    Bob
     
    Last edited: Jan 19, 2014
  3. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Turn on and off, what about a switch?
     
  4. natklopper

    natklopper

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    Jan 19, 2014
    I feared this would be the case, cheers bob, do you know what other method I could use?
     
  5. natklopper

    natklopper

    3
    0
    Jan 19, 2014
    Yeah, I will use a switch to turn them off and on, it was just getting the voltages right for the units.
     
  6. BobK

    BobK

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    1,688
    Jan 5, 2010
    I mentioned it in the last paragraph of my post. A buck converter. You can get these on Ebay for a couple of bucks.

    Bob
     
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