Different Voltage Components in a series circuit

Discussion in 'General Electronics Chat' started by aguywithfeet, Jul 12, 2014.

  1. aguywithfeet

    Thread Starter New Member

    Jun 6, 2014
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    I have been reading through the lessons and trying to build a simple circuit while im at it. I am having trouble figuring out my nest step.

    I have 5 LEDs in series with a 100 ohm resistor. They are fed by an 18vdc supply. I have 4 rows of these setups in parallel with each other. Each led drops 3.2v and draws 20 miliamps. This part works.
    I was thinking of adding a dc fan which is 12v 150miliamps. I thought if I added this in parallel I could make it work, but now I am thinking that this would cause all the leds to receive 150 ma also. would I need to change my resistors for this to work or is this even possible with it being 12 v?

    I am trying to figure it out on paper but I just cant grasp this. Any help would be appreciated
     
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    No, parallel circuits take their own current, so putting your fan across the 18v supply won't alter the leds current, the fan would just use more current as its runing on 6v more.
     
  3. paulktreg

    Distinguished Member

    Jun 2, 2008
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    You may damage the 12V fan however so use a 7812 or something similar to provide 12V for the fan.
     
  4. bwilliams60

    Active Member

    Nov 18, 2012
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    In a series circuit, the current will change as you add each load in the circuit as long as the voltage stays the same. So in each one of your branches of the parallel circuit, as long as you dont add any loads in series with the existing loads in each branch, your current will remain the same. When you add on the fan in parallel, it will draw its own current without affecting the other branches, however it will increase the overall current taken from the source.
     
  5. wayneh

    Expert

    Sep 9, 2010
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    Picture the lamps in your home. They're connected in parallel. They don't "care" whether other lamps are on or not. It only matters when the load makes a serious dent in the total available power. This is not the case in you home (within reason) but may be an issue with anything powered by battery. In other words your LEDs may dim a little when the fan is added.
     
  6. aguywithfeet

    Thread Starter New Member

    Jun 6, 2014
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    ok I am having trouble putting my mind around the fact that current stays the same.
    I don't understand why if i feed 18v to 5 leds, each one will drop the voltage in that branch 3.2 volts, about. this would leave 2 volts which i assume goes back to the negative or return. So in my mind the fan would take its 12 volts and 150 ma and the rest would go back to negative.
    I have been an electrician for 10 years. I don't know why this doesn't click. Is it a characteristic of dc that current is constant and voltage changes?

    sorry for the dumb questions
     
  7. NorthGuy

    Active Member

    Jun 28, 2014
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    Same as with water. Imagine water running through a pipe or a hose. No matter what you do with the pipe, the amount of water running through the pipe is the same everywhere.

    Same with electrical things connected in series, except electrons flow through, not water.

    Voltage is like water pressure.

    Resistor is like a thin section of pipe. It limits the flow through the whole pipe.

    LED is like a check valve with a strong spring. You need a certain pressure to push water through.
     
  8. bwilliams60

    Active Member

    Nov 18, 2012
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    Okay lets start fresh. In any given DC circuit, you will usually have five things. Power source, conductors, circuit protection, control device and a load. In a perfect world, the conductors, circuit protection and control device will have very low resistance and will use very little voltage. Source voltage is used push electrons through the load and change electrical energy into heat, light, movement. In doing so, the bulk of the source voltage gets used up so that there really is almost no pressure pushing the electrons back to the source. Voltage drop is very hard to understand for most people.
    Series circuits as said previously, have only one path for current to flow, so no matter where you look in the circuit, current will be the same. The only thing that can change that is a change in voltage or a change in resistance.
    In a simple parallel circuit, you will add another load across the first load as you well know but lets look at it this way. The original series circuit is still there with the same components and the current in that portion of the circuit has not changed. By adding a second circuit, if you wanted to, you could isolate it the same way and its characteristics would only be dictated by the load in the circuit. By adding a parallel circuit, you have now increased the overall current being used, but each circuit in the parallel circuit is fed the same source voltage and the only thing that changes is the load or resistance of the branch. That is why they work so well with headlights. One headlight dies out and the other headlight is totally unaffected and the amperage running through it remains the same as when both headlights were on. The overall current flow has decreased by 50% if both lamps had the same resistance.
    Just think of each branch having the same power source and it is it's own little circuit. Same five components and nothing changed but the load values.
    In your LED circuit, I will assume you have a resistor dropping the voltage down to light the LED. So guessing, I would say that roughly 15-16VDC is dropped on the resistor, leaving 2-3VDC to power the LED. That is the circuit. Each of those branches will be the same. On the fan, you will drop 18VDC and if not rated for it, you might get a good smoke show after a few minutes.
    Hope this helps. If you want the math, we can help out with that as well, maybe that is the way to go.
     
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  9. aguywithfeet

    Thread Starter New Member

    Jun 6, 2014
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    I appreciate the effort. Let me make up a random circuit and try to list values, then perhaps you can correct me.
    Bear with me though I work nights.
    To be clear though, current being constant is a characteristic of the circuit being series or of being dc?
     
  10. MrChips

    Moderator

    Oct 2, 2009
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    Be careful with your choice of words.

    "Constant" means that it doesn't change.

    In a series circuit, the current is not necessarily constant. It is the same value through each element of the circuit. If you change the resistance of any element, the current will become a different value. But the current remains the same for all elements in the circuit.

    This can hold true for AC circuits as well as DC circuits.
    In an AC circuit, we can consider the average AC current (or if you wish, the RMS current) through each resistive element and the argument remains the same as for DC circuits, i.e. the current is the same in each element.
     
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  11. aguywithfeet

    Thread Starter New Member

    Jun 6, 2014
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    Perhaps semantics is muddying it for me.
    In any section of a circuit that is series, current will be equal for each component. Which requires resistance or voltage to change per component. Resistance is set by the component construction, so the only thing left to change in ohms law is voltage, correct. So in basic low level terms. Any components which are in series will have equal currents. Any components in parallel will have the same voltage, until the series portion begins.Once the series section connects to the return path, the series section, is now in parallel with the energy source, so for all intents and purposes, there is no "loss" of voltage when you measure it from positive to common.
    Now to be sure, if I placed a meter lead in between a resistor and an led being fed with 6v, assuming 3v drop on led, and placed the other lead on common, I would read 3 volts correct?
    I think I got this now, even if that didn't make sense
     
  12. MrChips

    Moderator

    Oct 2, 2009
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    Bingo!.....
     
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  13. aguywithfeet

    Thread Starter New Member

    Jun 6, 2014
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    Awesome. Now let's talk harmonics.......just kidding
     
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