Unexplainable Voltage Drop

Discussion in 'The Projects Forum' started by Dammi, Apr 12, 2011.

  1. Dammi

    Thread Starter New Member

    Feb 15, 2011
    Hi guys

    I'm using MSP430F2274 which needs a power supply of 1.8V - 3.6V.
    I have a computer charger which gives 19V and maximum 1.5A. I regulate (correct word?) that with L7812CV down to 12V which I then use in a motor controller. I then regulate those 12V to 5V with L7805CV and use a voltage divider with 4.7kΩ and 9.4kΩ to get it down to 3.3V (I have some capacitors also connected just in case)

    When measured without the MSP430 connected, it is just as predicted, 3.3V across. But when I connect the MSP430, it drops down to 1.04V which the MSP430 doesn't run on.

    Any suggestions on what could be wrong?
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    The voltage drop is completely predictable. The voltage divider limits current to 355 μamps. When the IC draws any current, the voltage naturally sags.

    Voltage dividers only work for one level of current. You need an active regulator - http://www.national.com/mpf/LM/LM2937-3.3.html#Overview
  3. t06afre

    AAC Fanatic!

    May 11, 2009
    How about using LM317 (or equivalent) voltage regulator. But remember to use a cooling fin on it. Even if your MSP430F2274 do not draw much current.
  4. Dammi

    Thread Starter New Member

    Feb 15, 2011
    Alright, I'll try that :D

    I was just being lazy with the votlage divider, I didn't want to drive all the way just to get another regulator :/

    Thanks, I'll let you know if/when it works ;)
  5. John P

    AAC Fanatic!

    Oct 14, 2008
    If you operated the MSP430F2274 in its lowest-power mode, the maker claims that it draws only 270uA. That should work fine on your voltage divider, though it's a strange way to run a processor. Are you doing something that makes it draw more power?
  6. Dammi

    Thread Starter New Member

    Feb 15, 2011
    I'm trying to run a DC motor control circuit (Rough sketch http://rafnem.hi.is/ieee/media/howtos/2010/11/10/ras.jpg)

    So when a button is pushed, the MSP will give a signal to for example In1 on the diagram and the motor would turn clockwise, then another button is pushed and the motor turns counterclockwise.

    But I don't think (well.. at this moment at least) that's not causing the problems because when I'm connecting the MSP430, I don't have the motor control circuit connected and I still get the voltage drop.

    I haven't tried the above methods yet, still waiting on the regulator
  7. SgtWookie


    Jul 17, 2007
    Have you considered converting an ATX form factor computer power supply to a bench supply? It has 12v, 5v, and 3.3v already available.

    Google "ATX bench supply" for lots of ideas.
  8. Dammi

    Thread Starter New Member

    Feb 15, 2011
    Nice 1 SgtWookie!

    Wish I'd seen this earlier, It would have been alot cooler to use that :D I'll definetly use it in my next project :cool:

    But beenthere
    Thanks alot ! It worked like a charm :D