Help understanding amps and resistors

Discussion in 'General Electronics Chat' started by t44, Jun 20, 2012.

  1. t44

    Thread Starter New Member

    Jun 20, 2012
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    I am building my first electronics project and need help.

    I am building a circuit having multiple loads. Each load has different specs needing different volts and amps, each load is being wired parallel,all from one power supply.
    When I calculate resistors for each load, the equation calls for the supply volts, the load volts and amps. It never ask for the amps being supplied.

    Question: Does the amperage of different power supplies having the same voltage have any effect on choosing the correct size resistor.
    Example:
    two different power packs:
    power supply pack #1 specs are: 12V, 500mA
    power supply pack #2 specs are: 12V, 3A

    I think I am suppose to add up the max amps on the entire circuit to know the correct amperage for the power supply pack. But I don't get the amps part completely. If one load needs a 1/4 watt 1000 ohm resistor and another load on the same circuit calls for 2 watt 150 ohm resistor then won't a large amount of supply amps harm the small resistor vs safe on the larger resistor?
     
  2. bretm

    Member

    Feb 6, 2012
    152
    24
    Don't think of the power supply current rating as how much current it supplies, but rather how much current it can handle and still be able to supply the rated voltage. In the case of 12V supplied to a 1000 ohm resistor, the resistor will draw 12mA. This is less than the either supply is rated for, so the chosen supply will successfully provide the 12mA. The 150 ohm resistor will draw 80mA, and again either supply will be able to handle that. In parallel, both loads together will draw 92mA, and either supply will still work.

    If they're in parallel then they'll get the same voltage, which is the supply voltage. Each load will pull a different amount of current, depending on its impedence.
     
  3. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
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    It's interesting to see how little Ohm's law is actually known/understood.

    Current (Amps) is a result of voltage applied to a resistor/resistance.

    When I started with electronics, I was very young, and not professional at this point of time. But I soon got a feeling of voltage as kind of a tension, and Amperes as some kind of limitation of the power transformers/batteries can output.

    Later computer programming also helped me to understand electronics more, where you need to eliminate instances of fallacious thinking, to get along with software.

    I sometimes think, people who don't have actual programming experience, still have a mindset which largely incorporates instances of fallacious thinking, religious/urban myth based etc.

    I mean what is the logic when we only have one and the same power supply grid, but on the other hand, people repeatedly ask for power supplies that must have specific Amps rating not to destroy circuits? Right, this is absolutely irrational.

    But I can recall this a bit when I think of watches, calculators, SEGA etc, which I have taken apart without knowing how they actually work. Nowadays people have it more easy, can use the internet to get all the information!
     
  4. PaulEE

    Member

    Dec 23, 2011
    423
    32
    To follow up on the above,...

    Voltage, or a potential difference between two points (such as a set of power supply output terminals), is an electric field. An electric field is a phenomena that is able to force charges (read: electrons) through some medium, usually a conductor or semiconductor.

    If a power supply claims to be able to supply "5 volts at a maximum output current of 1 amp", or "5v, 5 watt power supply", these two phrases describe how much charge the electric field (power supply) can move in a given amount of time. This can be interpreted as energy per unit time as well, which is why I mentioned "watts".

    The amount of current that'll flow for a given resistance R is V/R = I. The amount of power dissipated in R resulting from the forcing of charge through R by the electric field is:

    I*I*R = P = I^2 * R or (V*V)/R = P = V^2/R or P = VI.

    Water is a great analogy for Ohm's Law. At the top of a waterfall, water has a high potential (for falling, that is)...and low at the bottom. This potential difference, when water is allowed to fall, causes water to flow from the top to the bottom of the waterfall. This current, from top to bottom, can do useful work as it flows. An example is a dam for hydroelectric power.

    :)
     
  5. t44

    Thread Starter New Member

    Jun 20, 2012
    5
    0
    Is supply output current (amps) forced into the resistor and the resistor dissipates the excess amps as heat? Or the is it the only amps passing through a resistor is what the load is drawing?

    see attached image of lights and resistors IMG_0617.JPG
     
  6. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
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    Volts is static - the voltage is always present and remains the same.

    Amps is dynamic, it's the current flowing through a resistor, depending on the voltage.

    If the voltage remains the same, it will depend on the resistor value.
     
  7. t44

    Thread Starter New Member

    Jun 20, 2012
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    see recent reply
     
  8. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
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    typical instance of fallacious thinking.

    you think there is "the load" (lamp or LED), and "the resistor", eventually dissipating excess amps, or passing through.

    The truth is all components have resistance, and all components are some kind of loads.

    I suggest to look this up in literature, and read until you understand it.

    And in addition, for simplicity, it would be helpful to leave out the "loads", means only to connect resistors in questions to 12V.

    So then obviously the resistor becomes the load.

    If you don't have a use for accurate replies (see recent reply)? And grammar as well to check.
     
  9. #12

    Expert

    Nov 30, 2010
    16,270
    6,786
    At the top of the chat page is a thread called, "Ohm's Law for noobies". Read it.
     
  10. t44

    Thread Starter New Member

    Jun 20, 2012
    5
    0

    Save your snotty attitude for someone who cares!
     
  11. t44

    Thread Starter New Member

    Jun 20, 2012
    5
    0
    I repeat... save your snotty attitude! Just because you understand something and I don't does not give you the right to condescend. If you were on an operating table perhaps I should preach to you and not lift my scalpel.
     
  12. MrChips

    Moderator

    Oct 2, 2009
    12,437
    3,360
    Hold your tongue bud. If you don't like the service provided here I suggest you go shop some place else.
     
    shortbus likes this.
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