Multiple Devices on a Single Power Supply

Discussion in 'General Electronics Chat' started by DanRilley, Jan 16, 2010.

  1. DanRilley

    Thread Starter Senior Member

    Jan 13, 2008
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    0
    Hello, I am doing a project for a museum that involves 3 devices requiring slightly different power requirements. Weight and size is an issue so I want to use a single transformer supply and then use voltage regulators to provide 3 separate DC outs. I am just wondering what the issues I may run into with this. I am a beginner at electronics, but from what I gather, as long as the 3 devices don't draw amounts over the VoltAmp rating of the supply I should be fine. Is this correct?

    Here are the details of my project.

    Device 1: Arduino (Atmel AVR 328P with a 5V regulator). Current draw is tough to figure on this but I think 300-600mA should be fine.

    Device 2: Yamaha QY70 sequencer. I couldn't find any direct electrical requirements, but the suggested power supply is 12V 600ma.

    Device 3. Creative TravelSound Speakers. Again only have suggested power supply of 5V 1A.

    Power Supply: 12V 1.67A 20VA

    So far I can run both the QY70 and the Arduino off this single supply by just splitting the power cable (The arduino has it's own voltage regulator so I don't even have to worry about setting that to 5V) Now I want to power the speakers off this 12V supply as well.

    I have a regulator equivalent to an LM7805 (Datasheet) which can provide 5V 1A. Do I just need to run the 12V power supply through the basic suggested circuit of the datasheet (pg. 22 fig. 7) to get this to work, or is there a better regulator circuit that anyone would suggest? Since it's 3 devices off one transformer, what type of power fluctuations do I need to consider? Obviously power for each device needs to be very constant.

    From my calculations (which I don't know if this is how you're supposed to do this:)

    Device 1: 3VA
    Device 2: 7.2VA
    Device 3. 5VA

    Total: 15.2VA
    Power Supply Capability: 20VA


    Does this seem like it will work?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Great that you're helping out the Museum. They're typically cash-starved.

    However, using a linear regulator is very inefficient; it'll waste a lot of power regulating 12v down to 5v.

    Have a look at this:
    http://www.mpja.com/prodinfo.asp?number=17182+PS
    A switching supply that meets your needs for under $5. It will be a lot more efficient than using a linear regulator, which will save the museum on the electric bill over time.
     
  3. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    Using a linear regulator voltage regulator, the critical figure is the total current of the loads, compared to to supply capability, eg:

    Load 1 = 12V 600mA
    Load 2 = 12V 600mA
    Load 3 = 12V 1000mA

    Total current 2200mA
    At 12V, that is 26.4VA

    The reason for this is that a linear regulator takes the same supply current as the load (or a fraction more to power itself); the difference between the input and output voltage is dropped across the regulator, so the 5V 1A output regulator would be dissipating 7W (7V x 1A, the difference between 12V and 5V).

    If you used a switch mode regulator rather than a linear regulator, you would get much nearer to your original figures. Using something like a National Semiconductor 'Simple Switcher' IC, you may well get the 5V 1A output while only drawing around half an amp from the 12V supply.

    That reduces the 12V load to around 20.5VA, so if the other loads are less than 600mA each your 12V supply may work.
     
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  4. DanRilley

    Thread Starter Senior Member

    Jan 13, 2008
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  5. SgtWookie

    Expert

    Jul 17, 2007
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    OK, keep in mind that the switcher will put noise on the 12v input, which might affect the QY70.

    I'd use another 330uH inductor on the input with another cap to make a pi filter; that should help to keep things more quiet.
     
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  6. DanRilley

    Thread Starter Senior Member

    Jan 13, 2008
    107
    0
    Hi SgtWookie,

    Since I'm all for removing as much noise as possible, which input do you refer to when adding the extra inductor and capacitor? The unregulated 12V input (refering to the regulator datasheet example circuit), or the regulated 5V input to the speakeers (the result of the aforementioned circuit), or some other place? I assume the first, but I want to be sure since I know squat about PI filters. Below I just showed what I understood, red or blue?

    [​IMG]

    [​IMG]
     
  7. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,536
    The second drawing will be much quieter.
     
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