three voltages, one circuit

Discussion in 'General Electronics Chat' started by FingerPrint, Mar 8, 2010.

  1. FingerPrint

    Thread Starter Member

    Jul 24, 2008
    21
    1
    Hi,

    I'm trying to do get this circuit round my head before I try to build it.

    I was stuck on vref, but thanks to this
    http://forum.allaboutcircuits.com/showthread.php?t=28442&highlight=dual+voltage I can work that out.

    How do I get 3 different voltages on 1 board?
    30v pin 4 (L298) and steppers
    5v pin 12(L297) pin 9 (L298)
    vref pin 15 (L297)
    Can I use two PSU or would it be better to use one PSU and use voltage regulators.

    I have a home made PSU from a computer (3v 5v 12v) and a desktop psu 0-30v.


    Bob.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You can use two PSU's for testing or running. You will need to connect their returns together (V-) so that they have a common reference point.

    Attempting to make an efficient 5v supply from a 30v source may be more than you wish to take on at the moment. A 7805 regulator might be used, but most of the power will be dissipated in the regulator as heat, instead of the 5v load.

    For example, let's say that your 5v load requires 100mA current.
    With your source being 30v, The regulator will have to drop 30v-5v=25v across itself.
    Since power = volts x current, total power consumption for the 5v circuit will be 3 Watts, 2.5 of which will be in the regulator itself.

    This is ignoring the 5mA (nominal) "overhead" current that the 7805 regulator requires to regulate the output voltage, which adds another 125 milliwatts power that the regulator must dissipate. It will help to keep the room warm. You would need a heat sink on a 7805 regulator. Efficiency would be a rather dismal 16%.
     
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