Rails for an opamp?

Discussion in 'The Projects Forum' started by rougie, Nov 9, 2012.

  1. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    Hello,

    I have moved onto using an op amp to create my voltage ramps. My rails of the
    opamp LM324, is 5V. However, I noticed that my circuit works perfectly when the rails are exactly 5.05VDC!!!

    What would be the best component that would hold my rails constantly at 5.05 VDC?

    I thought of a voltage regulator but, to adjust to this exact voltage we need to use a pot and I don't know if this will be adequate?

    All suggestions are very appreciated?

    r
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,367
    If the voltage supply is that critical then something is wrong.
    Can you post your schematic?
     
  3. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    5,003
    745
    an LM7805 regulator
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Most LM7805s don't output exactly 5.05V:D
    Of course, as MrChips said, if supply voltage is critical, something is wrong.
    It could also be that some other part of the circuit is sensitive to it.
     
  5. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    hello,

    The real problem was that my DC/DC converter supplying my 3.3 VDC for my circuit was somehow down to 1.6VDC :eek::eek::eek:

    I realized that I had a connected a few pull down resistors of 200 ohms on the inputs of my CPLD to test some things!!!! I unplugged those and the DC/DC went back to normal (3.3VDC)

    However, I just tried my circuit by purposely raising the rails to 5.13 VDC and everything seems to still be normal ...

    But in any case, I still have provided the circuit in case you guys see something that can be potentially wrong?

    Thanks for your help... really appreciated

    r
     
    Last edited: Nov 15, 2012
  6. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    One problem is that you need a resistor in series with the base of transistor to limit the current. The base of a BJT is very low impedance (looks like a forward biased diode). Without a resistor the Op Amp is likely operating at its output current limit, as if the output were shorted.
     
  7. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    so what if I relocate r5 as an re resistor
    won't this limit the current through vbe?
     
  8. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    There are two problems with your schematic.
    There is no current limiting resistor for the output transistor.
    This is already mentioned by crutshow.
    And the reference is dependend of the power supply.
    You can take a reference IC to make the reference stable.
    The TL431 is one, take a look at the attached datasheet how to use it.

    Bertus
     
    Last edited: Nov 11, 2012
  9. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    You could and it would, but that would change the current through the LED. It would now be ≈(Vo-0.7)/R5 where Vo is the op amp output voltage.
     
  10. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    okay thanks guys!
     
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