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Not getting correct voltage from voltage regulator

Discussion in 'General Electronics Discussion' started by Farticus, Sep 2, 2015.

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  1. Farticus

    Farticus

    36
    1
    Jun 21, 2015
    Just fiddling with a breadboard and voltage regulators.

    Attached is a rough schematic of my layout.
    I am not getting the correct output from the regulator.
    Have tried 5V 9V and 15V Volt regulators but the output is always the same 25.3V

    Obviously I doing something very basic wrong. Tried 2 different Voltage sources but same result.
    Can someone please point me in the right direction?

    Volt Reg.jpg

    Thanks
     
    Last edited by a moderator: Sep 2, 2015
  2. Old Steve

    Old Steve

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    Jul 23, 2015
    What type of regulator? A part number would help. The fact that you're getting 25.3V out regardless of whether you use a 5V, 9V or 15V regulator is odd, to say the least.
    It's also a good idea to put bypass capacitors from the input and output to ground, as close as possible to the pins, especially on the output or oscillations and a seemingly high output would result, but your problem goes deeper than this I expect, going by the constant 25.3V output.
    Edit: Are you dead sure you're connecting the regulators the right way around?
     
    Last edited: Sep 2, 2015
  3. Farticus

    Farticus

    36
    1
    Jun 21, 2015
    Steve they are all EBay specials with no numbers but all are brand new.
    I am sure it something very very basic that I am just not understanding.
    I am very new to all this not yet worthy of the "NOOB" title
     
  4. Old Steve

    Old Steve

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    Jul 23, 2015
    Do you have a link to the eBay listing? What you appear to be getting is a forward voltage drop equal to that of two series diodes, rather than a regulator.
    How do you know which way to connect them, too, without numbering and/or a datasheet?
    A photo of these 'regulators' might clear things up.

    Since you're here in OZ, I'd recommend looking at Jaycar or Altronics, maybe even Dick Smith, for standard LM78xx series regulators.
     
  5. Old Steve

    Old Steve

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    Jul 23, 2015
    Last edited: Sep 2, 2015
  6. davenn

    davenn Moderator

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    Sep 5, 2009
    I would be making sure you have the regulator connected correctly .... those voltage reading I have seen on a reversed connection
     
    Martaine2005 likes this.
  7. Farticus

    Farticus

    36
    1
    Jun 21, 2015
    I Think I may have found my problem.

    I was using wires with alligator clips to connect everything to the breadboard and it seems the connections may have been "iffy".

    I checked the 15 V reg physically holding the "inlet" and "ground" directly to the battery terminals and measuring the output.

    26V in gave me exactly 15V out

    18V in gave me around 14V out

    Thank you all for your help.

    One step closer to being a fully fledged "NOOB"

    Appreciated
     
  8. cjdelphi

    cjdelphi

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    Oct 26, 2011
    If it's a 7815 you need a load on the output, use a 1 - 10k between out and gnd (the load)
     
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  9. Old Steve

    Old Steve

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    Jul 23, 2015
    Don't forget those bypass capacitors, too. Some regulators won't work without them. (The LP2950 and similar MUST have an output capacitor.)

    Incidentally, if you're using a plug-in breadboard, a regulator fits OK in the holes without needing to use flying leads. (Excuse the dirty, dusty old regulator - I've been using it on and off for over 10 years.)

    100_6165.JPG
     
    Last edited: Sep 2, 2015
  10. Farticus

    Farticus

    36
    1
    Jun 21, 2015
    Thank you guys. All information is appreciated and noted.
     
  11. cjdelphi

    cjdelphi

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    Oct 26, 2011
    I will have test this out with several regulators ... switching regs i understand why you need to place a cap across its output or to use an inductor .

    But linear regs should be happy with just a small load (i'll have to have a play sometime)
     
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  12. Old Steve

    Old Steve

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    Jul 23, 2015
    Try an LP2950. A 2.2uF output cap is recommended for the 3V, 3.3V versions and 1uF for the 5V version. I accidentally left out the output cap on an LP2950-3.3 a few weeks ago, (or I might have put a small 0.1uF cap on it, I forget), and the output was high according to my DMM. I added a 10uF tantalum cap to the output and the voltage dropped to a steady 3.28V.

    From the LP2950 datasheet:-
     
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  13. davenn

    davenn Moderator

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    Sep 5, 2009

    NO ...... its important to have caps on the input and output of linear reg's, it for one thing helps with internal oscillation stability
    2 x 0.01uF or 2 x 0.1uF are the common values to use. its also common to use a electro/tantalum cap on the output
    10 uF to 100 uF

    this is all stated in the datasheets for the 78xx and similar series linear reg's

    Dave
     
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  14. cjdelphi

    cjdelphi

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    Oct 26, 2011
    Davenn....

    My post was not saying you don't need them, what i am saying is, i would like to do some tests using a small resistive load (without the cap)

    Not for any other reason except to discover the results...
     
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  15. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Common 78 series regulators that use an NPN output stage are stable without any capacitors. LDO types using an PNP output stage will require some capacitance. But not ceramics because the ESR is too low and won't introduce the phase lead needed for stability. A small value tant or electrolytic will be fine. Always best to consult the data sheet.
    Thanks
    Adam
     
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  16. davenn

    davenn Moderator

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    Sorry Adam, I disagree

    If that were really the case then they wouldn't be specified in datasheets
     
  17. davenn

    davenn Moderator

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    Sep 5, 2009
    that's not what you said/implied ... the poor OP would be confused with your comments
     
  18. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Thats fine Dave. They are stable without capacitors. Does the data sheet say they are for stability? I would be suprised if it did. But have not used one for a long time. I am doing DIY at the moment. Let me check out a data sheet when I get 5. But just think, whats the voltage gain of an emitter follower?
    Adam
     
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  19. davenn

    davenn Moderator

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    yes it does ... the input one for stability and the output one for transient suppression

    78xx regs.GIF


    I use the 78xx series regs by the dozens in the commercial products I build
    I never use them without caps
     
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  20. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Define stable operation? I have never heard of an input capacitor that stops internal oscilations. Thats like saying the bigger the battery the less risk of oscillation due to lower resistance, not sure about that. I will look into this later. And yes I too use capacitors on my regulators, but that was not the origional question.
    Cheers
    Adam
     
    Martaine2005 likes this.
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