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Homework Help: Powering electromagnets question

  1. Mar 10, 2014 #1
    So i'm trying to build some electromagnets and i have some questions regarding how to power them.

    Imagine that i have a battery of 6 Volts and a coil with x gauge with y meters of wire that will equal 1 Ohm of resistance. So if i want to run it with 1 Amp of current i just put one 5 Ohm's resistor in series and i get the current i want. So far so good right?

    Here's my confusion.

    If instead of a coil i wanted to power leds that had foward voltage of lets say 2. I could only power 3 of them. No matter the current im putting in (right?).

    So in the coil case, instead of the 5 Ohm resistor what if i put 5 coils of 1 Ohm? i would still be running 1 Amp but instead i would be powering 6 coils instead of just 1 (?!) Isn't there something that limits the number coils i can use just like the LEDS case?

    i think i'm missing something very basic here

    thank you for your time, i'm still very new to electronics.
  2. jcsd
  3. Mar 10, 2014 #2


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    Staff: Mentor

    Hi electrosiri. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

    The limit on the number of coils you can power in a series connection is limited by the wire resistance, as you point out. :smile:
    Last edited by a moderator: May 6, 2017
  4. Mar 10, 2014 #3
    So does that mean that i will save energy if i put them in series with no resistor, rather than putting them in parallel with a resistor?

    I know there's probably some advantages of putting them in parallel. But just in terms of energy, seems like putting them all in series should save alot of energy.

    Ty for the quick answer.
    Last edited by a moderator: May 6, 2017
  5. Mar 10, 2014 #4


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    Staff: Mentor

    It sure does. That's assumng you are making use of those extra coils! If you are including them just for their resistance, then you'd be no better off. Whereever you have a resistor, or a resistance, it is turnng some of your electrical energy into heat. That's a loss in efficiency considerations.
  6. Mar 10, 2014 #5


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    Gold Member

    Much the same applies to LEDs. If you look at LED strip lights they typically use groups of three LEDs in series with one resistor. They could use one resistor per LED but that would waste more power in the form of heat and would need more components.

    Aside: electrosiri - In your example you proposed 3 x 2V LEDS operating from 6V. That wouldn't be recommended because the current wouldn't be well controlled. For example suppose when the LEDs were made they had a voltage of 1.9V instead of 2V. Three would have an operating voltage of 5.7V. To limit the current to 1A you would need a resistor with a value of R = (6-5.7)/1 = 0.3 Ohms. OK so far but what if the next batch of LEDs had a voltage of 1.8V? The current would be (6-5.4)/0.3 = 2A. eg a small variation in the LED forward voltage would change the current dramatically. For this reason you would be advised to use only 2 LEDs in series (4V) allowing 2V across a bigger resistor. There is a trade off to be made.... On the one hand you don't want to waste power in the resistor but on the other hand you want the current and brightness to be well controlled so they all look the same. You might also like to consider what happens if the 6V power supply isn't exactly 6V? What if the battery was a bit flat and only delivering 5.5V?
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