Two +5 power supplies in one circuit? Some other questions as well.

Discussion in 'The Projects Forum' started by drx, Jun 16, 2007.

  1. drx

    Thread Starter New Member

    Jun 16, 2007
    3
    0
    Hello,

    I am a newbie when it comes to electronics, and I searched the internet for a circuit like that and couldn't find it. It might be because I lack the proper terminology or something.

    I am trying to build a circuit that can be powered by either of the two +5V supplies (one being an USB connector, the second being a custom connector, also 5V). What would be the best way to implement something like that?

    I figure it would be smart to implement some sort of signal to know which of the power supplies is currently supplying power to the circuit. However, I don't know how to pull that off, since logic gates need a power supply as well, so I can't just do A & ~B or something like that.

    On an unrelated note, does anyone have any tips for assembling PCBs? I will need to solder an IC with fine pins, spacing 1mm from each other. Any advice soldering them manually?

    Also, what is the best equipment for soldering PCBs? Soldering iron-wise, that is. I have heard that using eutectic alloys helps, how true is that?

    Thanks in advance, hope I don't ask too many questions :)

    Luke
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    You can isolate the power inputs with diodes:

    View attachment 2x5v.bmp

    Eutectic not only melts at a lower temperature, but solidifies without precipitating constituent metals. (English translation: It won't booger up when solidifying.)
     
  3. drx

    Thread Starter New Member

    Jun 16, 2007
    3
    0
    The problem with diodes is that they have a huge forward voltage, 0.6V is too much for my circuits (some of the ICs I will be using can't be used with voltage lower than +4.7v, so even Schottky diodes are out of question...)

    Is there any other component that acts in a similar manner but doesn't have such a large forward voltage?
     
  4. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    IC that allows minimum loss switchover.

    Maybe you could use a device such as this and then adjust your custom 5 volt supply to minimize the loss from that voltage source.

    hgmjr
     
  5. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
    4
    If the absolute acceptable minimum voltage is 4.7V, then you are setting yourself for failure. USB Vbus could vary from 4.4V minimum during enumeration to 5.25V maximum. Even under one full unit load, the minimum is 4.75V, which doesn't leave you with enough safety margin.

    How much current does the circuit draw?
     
  6. Tube Tech

    Active Member

    Jan 11, 2007
    46
    0
    "I figure it would be smart to implement some sort of signal to know which of the power supplies is currently supplying power to the circuit. "

    http://en.wikipedia.org/wiki/Comparator

    "The problem with diodes is that they have a huge forward voltage, 0.6V is too much for my circuits "

    So use a 6V power supply...
     
  7. a_kent

    Active Member

    Jun 12, 2007
    30
    0
    That is a cool part!
    Kent
     
  8. drx

    Thread Starter New Member

    Jun 16, 2007
    3
    0
    Thanks a lot! That's exactly what I"ve been looking for.
     
Loading...