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Trying to understand AMPs, and loads on outputs.

Discussion in 'Power Supplies' started by Baf05, May 9, 2018.

  1. Baf05

    Baf05 Full Time Elf

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    I was just taking a look at Davids 4 way mains distro boards, and it says that it can handle a max of 10amp over the 4 outputs.

    Then I was looking at 8 way distro boards and it says 80amps in total?

    I simply do not understand stand this, ever since I started this hobby, this part has confused the hell out of me! Having said that I have not ever had a fuse blow or tripped a breaker in my house so I maybe doing some thing right?

    The reason I ask, is to try and work out a box build, I see a lot of people build their boxes or daisy chain boxes together with power boards the use one mains power point? (which is rated at 10amps?)

    When I built my first box, I wired in 5 power supplies to one power cord (for ease of plugging in) and the still plugged that into a power board.

    THEN I add the radio for music, the switch power, the box for the DC lights, I think there was another power board too....all back to one power point. When I spoke to an electrician about getting a 15amp power point he basically laughed at me?

    If I was going to the mini I think I would be in the ear of someone trying to understand this all day!!! LOL
     
  2. David_AVD

    David_AVD Bite my shiny metal ass! Community Project Designer Generous Elf

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    You need to know / learn about Wattage. Basically, the Wattage is defined as the Amps multiplied by the Voltage.

    So a mains power example is 240V x 10A = 2400W.

    If you have a 5V power supply that can supply 80 Amps on it's output, that's 5 x 80 = 400 Watts of DC power.

    So the current (Amps) on the low voltage side is 80A, but on the 240V side that's still only 400W, so just 1.6A.

    The conversion of high voltage to low voltage transforms the current from low to high.

    I'm simplifying it for now, but do you get the general idea?
     
    Last edited: May 10, 2018
  3. OP
    OP
    Baf05

    Baf05 Full Time Elf

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    Ahhh, ok that does help somewhat :)
     
  4. fasteddy

    fasteddy I have C.L.A.P Global Moderator Generous Elf

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    Amps values vary based on the voltage for the same power used

    So for example if you have a 30 amp load at 12vdc then the load at the 240v outlet will be much less than 30amps because of Ohms law
    so lets start with the equation used to calculate power (watts)
    P (watts) = E (Volts) x I (current)
    So in the 12vdc example the wattage used at 12vdc 30 amps would be
    P (watts) = 12vdc x 30 amps = 360 watts

    But if we used the same current at 240 volts then the power (wattage is much higher
    P (watts) = 240vdc x 30 amps = 7200 watts

    So as you see the voltage plays a big role in how much current is needed for the same power

    So now if we wanted to know the actual current we would be drawing from a 30 amp 12vdc supply (360 watts) plugged into a 240 volt outlet then we would use the below equation
    I (current) = P (watts) / E (volts)
    I (current = 360 watts / 240 volts
    So that 30 amps at 12vdc will draw 1.5 amps at 240 volts
     
  5. David_AVD

    David_AVD Bite my shiny metal ass! Community Project Designer Generous Elf

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    Of course, in reality there are electrical conversion losses.

    This means that a power supply putting out 5V at 80A (400W) on it's output will actually draw more than 400W from the 240V side.

    A typical power supply efficiency would be 80% - 90% so the actual power draw could be 500W (400W / 0.8) on the 240V side.
     
  6. fasteddy

    fasteddy I have C.L.A.P Global Moderator Generous Elf

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    Yes you are correct David, losses also play a part but didn't want to over complicate things for the theory as losses would depend on the power supply used and the load on the power supply, generally the cheap Ray Wu power supplies have higher losses then quality Meanwell power supplies and I would factor in what you have being only 80% efficient to get your 240v current draw
     
  7. videoman3857

    videoman3857 Full Time Elf

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    How do you get 5 amps ??
    400/240 = 1.666666666666667. Add a power factor of 80 % and I get 2 AMPS
    240 VOLT X 5 AMPS = 1200 WATT Add power loss factor and I get 960 watts

    What am I not seeing here ?
     
  8. Kimbo3000

    Kimbo3000 New Elf

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    If you DO NOT include the losses from converting 1 voltage to another.. then 400W @ 240V is still 400W @ 5V.. its just the current (amps) that changes.
    Power = Volts x Amps.. so Amps = Power / Volts
    .. so 400W / 240V = 1.67 Amps
    and 400W / 5V = 80 Amps

    This isnt exactly true as one voltage is AC and one is DC.. but for this discussion its good enough :)

    If you have an Android phone/tablet there is a handy app called 'ElectroDroid'.. its free and includes calculators for the above mentioned things.. and also voltage drop (how long the cable can be before your pixels turn red). It also includes pinouts for ethernet cables etc.
     
    Last edited: May 9, 2018
  9. David_AVD

    David_AVD Bite my shiny metal ass! Community Project Designer Generous Elf

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    Sorry, that was a typo. I started writing a different example and forgot to go back and change the 240V current. 1.6A is correct. :)
     
  10. videoman3857

    videoman3857 Full Time Elf

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    Ha ha ha.. All good David. I thought I was missing something - I never was real bright at Math.


    @Kimbo3000 . Thanks for the info on the App. Downloading it now
     
  11. JasonC

    JasonC New Elf

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    That handy little App is also available on the Apple store. It's probably worth more than the $4 something that they're charging for it.
     

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