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Amps available - amps draw ?

Discussion in 'Electronic Basics' started by NJM, Feb 25, 2007.

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  1. NJM

    NJM Guest

    Hello.

    I am trying to teach myself the basics, but I am confused about ampage.

    For instance, say I have a power source that is 12v at 2 amps. I want to
    power a simple LED circuit that I found on the internet that is designed
    for a 12v supply but there is no mention of the current. The circuit is a
    50 ohm resistor and 3, 3.6v ultra bright white LEDs. The resistor value
    was calculated for a 25mA current using ohm's law. What is the resistor
    modifying? The current or the voltage? Will this circuit be usable
    regardless of the supply ampage? The voltage/current relationship has me
    confused.

    TIA for any insight you may be able to provide.
     
  2. Voltage is like water pressure.

    Current is like water flow.

    A resistor is like a tap - the higher the resistance the more the tap is
    closed.

    So if your supply was 12 volts at 1000 amps that would still be the right
    resistor.
     
  3. Jamie

    Jamie Guest

    This has been describe in many ways .. I'll give mine, i'm sure many
    will give you their own version. But to put in a simple means for now.

    Picture your self wading in water (a river maybe), lets say the
    water is moving 5 MPH (Voltage), and you are standing in 1 foot of
    water that is moving against you (Current).
    Now step in 2 feet of water. The mass that is pushing against you has
    now doubled (current), the speed is still the same(speed). Of course if
    you repeat this action and have a big hang over like me, when it reaches
    the mid section, mass will most like go up 10 times. :)

    The Power could be looked at as the combination of mass (current) and
    Speed(Voltage) as Voltage * Current = Wattage.

    using the same analogy with pipes. water moving at a set speed in all
    different size pipes. the larger volume pipe will have more current of
    water in it and will be able to push a heavier weight at the same speed.

    Now, picture a very small pipe with water moving at high
    pressure/speed(voltage). The impact on a surface could cause damage but
    will not be able to push that massive area. if you have ever gotten a
    High voltage arc shock of a few thousand volts with no much amps you may
    have notice a pin hole or some kind of skin damage.

    Now, think of a Resistor as a pipe, the lower R values would be the
    same as large pipes that allow less restriction of mass and the higher
    values would be like small pipes, restricting mass.

    Lets say you have a volt meter(Speed of water), this meter has no
    mass for the water to resist against(very small object in water), you
    can get the same voltage on both sides of that resistor how ever, in the
    real world, the resistor will be connected to an item like the LED's.
    These LED's are lets say the blocking mass, much like you standing in
    the water. the water will slow down as it finds its way around you.
    This slowing down of water (speed)= Voltage will be seen by
    your volt meter when testing the speed (voltage) at that point.

    Now you can increase this flow by using a smaller resistor how ever,
    this is like increasing the volume of water which puts more pressure
    against you in the water. this pressure is being converted to energy
    that you're expending keeping your self balanced, much like the LED
    emitting light.
    At some point, there will be too much mass for you to handle and you
    will fall in the water much like the LED going to LED haven!

    Now lets touch base a little on Speed (Voltage) even though you can
    have high voltage with no mass behind it, it's does not mean it's safe.
    Take a thin sheet of plastic and put it in the river with slow moving
    water but a large area. The plastic sheet will most likely handle that
    how ever, placed in fast moving stream of water (Voltage) with no Volume
    (current) and it can blast a hole in the sheet. just picture this as
    the maximum voltage a component can handle before it breaches or shorts
    out from over voltage(Speed)..

    Ok, I've gave my explanation. Years ago i used this example teaching
    the basics back when they use to have Cub, Boy,Eagle scouts etc... I can
    say that many of them ended up in a electrical/electronic couriers.
    Hope that gave you some insight!.



    "I'm never wrong, once i thought i was, but was mistaken"
    Real Programmers Do things like this.
    http://webpages.charter.net/jamie_5
     
  4. The current rating of a power supply is the maximum current that the
    supply can safely provide.

    The current actually drawn by any load will be determined by the
    characteristics of the load (as long as its demand doesn't exceed the
    capability of the supply).

    I assume that the three LEDs and the resistor are all in series. If
    the LEDs truly drop 3.6 volts, there will be 10.8 volts across them
    when lit, leaving 1.2 volts to be dropped across the resistor. Ohm's
    Law says that you will get 1.2 volts across a 50 ohm resistor when it
    is passing 24 mA.

    The voltage rating for LEDs is often a "typical" value - actual parts
    may vary from that, and the voltage will vary to some extent with the
    current. The rated current for a LED is usually the maximum current,
    beyond which the life of the part may be shortened. LEDs will work
    fine at much lower than rated current, but will be a little dimmer.

    --
    Peter Bennett, VE7CEI
    peterbb4 (at) interchange.ubc.ca
    new newsgroup users info : http://vancouver-webpages.com/nnq
    GPS and NMEA info: http://vancouver-webpages.com/peter
    Vancouver Power Squadron: http://vancouver.powersquadron.ca
     
  5. ehsjr

    ehsjr Guest

    Your circuit looks like this:

    +12---+
    |
    [50] 1.2V
    |
    [LED] 3.6v
    |
    [LED] 3.6v
    |
    [LED] 3.6v
    |
    Gnd---+

    The 3 LEDs hold the voltage at the bottom of the
    50 ohm resistor at 10.8 volts (3.6 + 3.6 + 3.6)
    The voltage at the top of the resistor is 12 volts,
    so there is a 1.2 volt drop across the resistor.
    The resistor limits current through the LEDs by
    dropping voltage - that means it affects both
    current and voltage.

    The current and voltage mathmatical relationship is
    shown by the ohms law formula E = I*R. If the voltage
    is fixed, then current _must_ increase when resistance
    is decreased, and vice versa. Intuitively, think of
    running in water (say 2 feet deep) as resistance. It
    takes more energy and you go slower than if you run on
    dry land with no resistance. If you lower the resistance,
    less energy is wasted fighting the water, and you go
    faster. In the electrical circuit, the resistance
    wastes electrical energy by converting it to heat and
    it lowers the voltage proportionately.

    Ed
     
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