5V and 3.3V supply

Discussion in 'General Electronics Chat' started by mndless, Jan 14, 2014.

  1. mndless

    Thread Starter New Member

    Jun 1, 2011
    8
    0
    Need some help for a circuts newb... simple question line filtering type question....

    Have a 3.3V IC circut with a component that needs 5V. Rather than adding a second regulator, would it cause issues to simply use a voltage divider for the lower voltage. The 22Ω and 47Ω resistors are there as a voltage divider...

    The 3.3V and the 5V are providing power, neither are a signal line...

    Would the following present me with any issues ???

    [​IMG]


    Any and all input is greatly appreciated.

    Thanks!
     
  2. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
    A voltage divider is fine if you do not draw any current from it. In other words no.

    Actually if you use low enough value resistors and do not mind wasting all that power you can get a low enough output impedance so it can work.

    So suck it up, get another regulator would be my advice.
     
  3. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,290
    1,255
    What is the current needed by your 3.3 volt circuit?
     
  4. mndless

    Thread Starter New Member

    Jun 1, 2011
    8
    0
    Russ, thanks... a second regulator is not an issue...

    And ronv, haven't measured it yet but should be around 50 mA with short maximums of no more than 200 mA...
     
  5. MrChips

    Moderator

    Oct 2, 2009
    12,446
    3,361
    I would use two regulators both being fed from the power jack input.
     
  6. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
    1,190
    156
    Aren't there any better ways to do it than 2 voltage regs or what he posted ???
     
  7. MrChips

    Moderator

    Oct 2, 2009
    12,446
    3,361
    I am doing exactly that on my boards right now, i.e. using two regulators.

    Here is a good reason for using two independent regulators.

    The 3.3V regulator is used to power digital circuits.
    The 5V regulator is used to power analog circuits. It is imperative to keep the digital and analog supplies separate.
     
  8. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    You can buy a bag of fifty 3.3v regulators on ebay for a few dollars. They are fine with a 5.0v input, so they turn 5.0v into 3.3v. Then you are set for life! ;)

    They are usually a SMD package but still with 3 pins and they can be hand soldered without too many problems. :)
     
  9. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
    1,190
    156
    Couldn't he use one of those cheap chinese bread power supplies ??
     
  10. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    Using two 1N4001 and one Schottky diode, if you want then you can in parallel with a 2K false load from the output to GND.
     
  11. mndless

    Thread Starter New Member

    Jun 1, 2011
    8
    0
    Yep... have a pretty tight board and was looking for a way to eliminate a component... still have some work to do...:cool:
     
  12. IcedFruits

    Member

    Jan 15, 2014
    84
    2
    there are 4 options:

    1) use voltage divider, but u need to calculate current u need for ur 3.3v circuit and choose resistances accordingly. it's a very bad idea if the 3.3v circuit needs somewhat higher current anywhere near to 100mA.
    2) use 3x 1N4007 in series before the 3.3v circuit. but the voltage drop each diode offers is current defendant, so u need to check the voltage drop with a multimeter, and a pot before u connect these to 3.3v circuit.
    3) use ldos like LM1117 / AMS1117 as THE_RB suggested, its the best alright, will supply 500-600mA easily, provided the 5v can sustain that much. :)
    4) if 3.3 circuit draws steady current like 50mA, then u can simply add a 1.7v drop resistance. i.e. 1.7/0.05=34ohm resistance in series to 3.3v circuit. wattage requirement for the resistance should be 34x(0.05^2) watts. if the current draw is 100mA, then i need 1/2 w resistance minimum, and it will generate heat.....etc
     
    Last edited: Jan 15, 2014
  13. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
Loading...