Elementary Circuit question

Discussion in 'General Electronics Chat' started by Res Earch, Jul 4, 2016.

  1. Res Earch

    Thread Starter New Member

    Jul 4, 2016
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    Hi

    I know next to nothing about electronic theory.
    i am going to ask a very very elementary question which i have been searching the web for answer but to no avail, hope everyone has patience with me please.

    What i know :
    1) power is relative to voltage and current. that is, to get same power, either increase voltage or current. If voltage is to remain the same, if you increase current, power will increase.

    2) If 2 x1.5V, 15A battaries are connected in
    a) parellel, the voltage remains the same but current is x2.
    b) series, voltage x2 but current remains the same.

    hence power from parallel or series batteries connection are the same.

    3) bulb will be equally bright if you connect 2x AA bat in parallel or series.

    4) 2 x AA will be make a bulb brighter than 1 x AA.

    what i don't understand is :

    1) when someone designs a bulb, what are the considerations to determine what voltage or current to adopt when power (Watt) does not determine brightness.

    similarly, for design of any simple DC electronics product, what do you use to decide if you require parallel or series battery to give same power
    when one can vary either voltage or current. That is, why use eg 1.5V when 1.0V can be used by increasing the current ?

    2) What goes into producing a bulb that will lead to an increase voltage/current requirement ?

    Thank you
    Jocey.
     
  2. crutschow

    Expert

    Mar 14, 2008
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    Your premise in 3) is incorrect. If you increase the voltage on the bulb, the current and power will also increase, since the battery current rating doesn't determine the bulb current, the battery voltage and bulb resistance does.

    A bulb is designed with a resistance that will give the desired wattage for a given applied voltage.
    The bulb wattage does indeed determine the brightness.
    Why do you think it doesn't?

    Incandescent bulb resistance is controlled by the diameter and length of the filament, which in turn, controls the bulb current and power.

    Electronic circuits are designed to draw the required current for a given voltage range.
    For voltages outside that range the circuit may not work properly or burn out.
    Thus the number of batteries required in series or parallel depends upon what voltage and current the circuit requires.
    Circuit designed to operate exclusively from batteries are often optimized for the voltage that common battery types generate (or multiples of that voltage).

    I have a small Sony transistor radio which runs from one AA 1.5V alkaline or 1.2V NiMH battery and will operate down to near 1V while still delivering good, clear earphone volume (which is rather amazing to me).
    It was designed that way because one AA battery battery will last longer than two AAA batteries which occupy about the same volume but are more expensive and have less total energy than one AA battery.
     
  3. Res Earch

    Thread Starter New Member

    Jul 4, 2016
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    please correct me.

    Re : 3) bulb will be equally bright if you connect 2x AA bat in parallel or series.

    I read from the internet that P = V*I across a conductor.
    So, if 2 AA is connected in parallel, the current (I) is still that of 1 AA.so P = 2 V*I
    if 2 AA is now connected in series, the Voltage (V) remains the same but current is now 2I
    and P = Vx2I = 2 V* I too

    Hence, if the bulb remains unchange, the brightness will remain the same in theory.
    Is my understand correct ?
     
  4. Res Earch

    Thread Starter New Member

    Jul 4, 2016
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    I mean, am i correct to say :
    1) Brightness of the same bulb B1 in a cuicuit with 2AA bat connected in parallel is the same as another circuit where the 2AA bat are connected in series ?

    2) If i increase the number of bat from 2 AA to 3 AA, the same bulb B1 will become even brighter (as long as the power output does not blow out the bulb)
     
  5. crutschow

    Expert

    Mar 14, 2008
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    Not correct.
    See my answer in red.
     
  6. Res Earch

    Thread Starter New Member

    Jul 4, 2016
    4
    0
    thanks for the correction.

    i was reading parallel and series connection sections in the link :
    http://www.mpptsolar.com/en/batteries-series-parallel.html

    and i thought the power will be the same.
    So, if i have a bulb B1 and 2 AA bat, to make B1 brighter, i have to connect the bat in series.
    Does that make sense in electronic terms ?
     
  7. crutschow

    Expert

    Mar 14, 2008
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    Yes.
    Most loads are resistive and it's that resistance that determines the current.
    So increasing the voltage by putting batteries in series with increase the load current and thus power.
    Putting the batteries in parallel doesn't increase the voltage, so the load current and power are also unchanged.

    Your confusion seems to come from thinking about two batteries in series or in parallel.
    The power available from two batteries in series is indeed the same as two batteries in parallel, at a given battery current , but that means you must adjust the load resistance so that the load current for the batteries in series in half of the current for the two batteries in parallel.
     
  8. BobTPH

    Active Member

    Jun 5, 2013
    782
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    Your problem is that you think a battery produces a rated voltage and current. This is not correct. A battery produces a voltage. The load determines the current.

    Your car battery can probably produce 200A to start the car. But if you are sitting in the car with it off, and listening the to radio, the battery is not producing 200A, more like 1A.

    Bob
     
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  9. wayneh

    Expert

    Sep 9, 2010
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    It may be worth noting that a typical light bulb has a handy feature, in that the filament resistance increases with temperature, giving it a negative feedback. The brighter it gets, the more it resists additional current. This feature allows the bulb to work over a range of voltages without burning out. We don't need to use a regulated power supply.

    An LED does not exhibit that behavior and is quickly destroyed if the supply is not somehow regulated.
     
  10. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
    1,135
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    The incandescent lamp is a "current dependent resistor". Tungsten has large temperature coefficient. The bulb may have 10x less resistance when cold than hot (full brightness).

    Designing a bulb would include a bit of Thermodynamics. Power losses and temperature and the required resistance to get that temperature.

    You actually have to model the battery as an ideal battery with an internal series resistance, A 3 V CR2032 can't deliver the same amount of power as two 1.5 V D-sized DC batteries in series. Consult the battery datasheets.

    Generally, we talk about "ideal voltage sources" or ones that have 0 series resistance. We also assume that wires have zero resistance. In electrical distribution we cannot make these assumptions.
     
  11. DGElder

    Member

    Apr 3, 2016
    344
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    2) If 2 x1.5V, 15A battaries are connected in
    a) parellel, the voltage remains the same but current is x2.
    b) series, voltage x2 but current remains the same.


    hence power from parallel or series batteries connection are the same.


    None of those statements are correct, hence the errors in your statements that followed #2).

    Presumably, 15A should have been written 15Ah which is amp hours, i.e. the total energy capacity of the battery expressed as the current (amperes) times operating time (hours) - at that current. Ah is not current or power, it is energy capacity. Power is the rate of delivering energy expressed as Joules per second or Watts. The higher the power delivered, the quicker the energy capacity will be depleted.

    Compared to a Single battery into a given load:
    a) 2 Parallel batteries: voltage is the same, 1.5V, the current delivered is the same and the delivered power is the same, but the total available energy is doubled (30Ah).
    b) 2 Series batteries: voltage is doubled (3V), current is doubled, power delivered is quadrupled and the available energy is doubled (30Ah).

    The parallel connection will operate twice as long as with a single battery. The series connection will operate half as long as the single battery connection.
     
    Last edited: Jul 4, 2016
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