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Thread: Question about Electricity

  1. #1 Question about Electricity 
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    Are volts dependent on ampers or ampers dependent on volts?

    Lets say, as an example, that I have a device which takes maximum 10 volts and 5 amps...And my power source supplies 8 volts and 7 amps....what of the 2 following situations will happen:

    1) The amperage taken by the device will lower to 4 amps (because 8V / (10V/5A) equals 4A...) because the amperage is dependent on the amount of voltage supplied... so in the end the device will take 8V and 4A.

    or

    2) The voltage will change to 14 volts (because 7A / ( 5A/10V) equals 14 volts...) because the voltage is dependent on the amount of amperage supplied...so in the end the device will take 14V and 5A.

    Basically what I am asking is will the voltage change according to the amperage supplied or will the amperage change according to the voltage supplied?

    Thank You


    :-D


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  3. #2  
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    Volts x amps = watts

    Volts are independent of amps unless you say how many watts you have. If you have 5 watts, then by varying the volts, you must also vary the amps and vice versa to achieve the same wattage.


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  4. #3  
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    But a guy before told me that amps were dependent on volts and that i should use my first calculation in number 1 of my example above
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  5. #4 Re: Question about Electricity 
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    Quote Originally Posted by jpfalet
    Are volts dependent on ampers or ampers dependent on volts?

    Lets say, as an example, that I have a device which takes maximum 10 volts and 5 amps...And my power source supplies 8 volts and 7 amps....what of the 2 following situations will happen:

    1) The amperage taken by the device will lower to 4 amps (because 8V / (10V/5A) equals 4A...) because the amperage is dependent on the amount of voltage supplied... so in the end the device will take 8V and 4A.

    or

    2) The voltage will change to 14 volts (because 7A / ( 5A/10V) equals 14 volts...) because the voltage is dependent on the amount of amperage supplied...so in the end the device will take 14V and 5A.

    Basically what I am asking is will the voltage change according to the amperage supplied or will the amperage change according to the voltage supplied?

    Thank You


    :-D

    It totally depends upon your device and your power supply. A power supply that will output 7 amps, could possibly output 20-30 amps surge. Or maintain 14 amps at lower voltage. There is almost no way to be sure until you actually try it.

    Induction devices can do that. An induction device during a brownout, can start to draw its lock rotor amps, or full run load amps. This can cause serious issues.

    You can burn contacts, and relays during a brown out. Because instead of breaking the normal, 12-20 amp load you could be breaking 35-75 amps, or more.



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    William McCormick
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  6. #5  
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    I was talking in terms of just the equations without considering the actual circuit or resistance (which is not realistic anyways). For a certain amount of resistance in a circuit, a higher voltage will, due to I=V/R, increase the amperage. Depending on how your circuit is built, using more amps can decrease the voltage, this is called voltage drop.
    Of all the wonders in the universe, none is likely more fascinating and complicated than human nature.

    "Two things are infinite: the universe and human stupidity; and I'm not sure about the universe."

    "Great spirits have always found violent opposition from mediocrities. The latter cannot understand it when a man does not thoughtlessly submit to hereditary prejudices but honestly and courageously uses his intelligence"

    -Einstein

    http://boinc.berkeley.edu/download.php

    Use your computing strength for science!
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  7. #6  
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    If the load is just resistive, the answer in your example is that the current will reduce to 4 amps.

    But William is right, it all depends.

    The current rating of the power supply is just the maximum rating. The power supply will normally be designed to put out a fixed voltage and the current varies with the load. But voltage will also vary somewhat with the load.

    A-c electric motors turning at a nearly fixed speed and with a constant torque will try to keep the horsepower constant. That means they will tend to draw more current as the supply voltage drops. On the other hand, the inrush of motor starting current will be less at lower voltage, because that just depends on the resistance of the windings.
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  8. #7  
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    Quote Originally Posted by Cold Fusion
    I was talking in terms of just the equations without considering the actual circuit or resistance (which is not realistic anyways). For a certain amount of resistance in a circuit, a higher voltage will, due to I=V/R, increase the amperage. Depending on how your circuit is built, using more amps can decrease the voltage, this is called voltage drop.

    Believe it or not, there are run and start ohms. For many types of elemental loads and induction loads.

    Some heaters do maintain their ohms though, hot or cold. Start or run. I call them constant ohm heaters. They are nice because they will blow fuses if you get a voltage boost at night.

    I have seen multiple shops where the voltage at different times during the day, went from 460 to 540 volts. That means a lot, if you have heaters in your equipment. Sometimes it just pays to use a buck/boost transformer to buck the voltage down all the time.

    The three phase formula for amperes are, Total Watts, divided by, Volts times 1.732, this will give you the amperage draw of each leg.

    Think about a three phase delta system drawing 18,000 watts, at 460 volts. The amperage of constant ohm heaters, is going to be around 22 amps a leg 22.618 to be exact.

    If the voltage goes up during the night to 540 volts, your watts will be 24,805.293 not a proportional difference if you know what I mean. Ha-ha. The amperage will be 26.521 amps a leg.

    These numbers are something that I actually worked with on a couple real projects over a couple years, and triple checked the readings. Checking the amps as well.

    The funny one is "Wye" formulas. Even though the Wye system, has much higher ohms across any two legs, using the same elemental device as wired in delta. The amperage is much, much higher then Delta. I know I did this one day too. Ha-ha. I could not believe the amperage. It goes up around 35-37 amps a leg.

    But on induction loads this is the opposite. On induction loads the amps go down, and the motor can barely turn, compared the same induction load wired Delta. Freakish. You can get into arguments with the best in the world about this. Because it is just so abnormal.




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    William McCormick
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