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Voltage regulator voltage too high

Discussion in 'General Electronics Discussion' started by Chumply, Nov 12, 2019.

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  1. Chumply

    Chumply

    6
    0
    Nov 12, 2019
    I’ve bought 1000+ L7805CP voltage regulators - made by STMicroelectronics. On all the data sheets it say there is a +-10% voltage tolerance yet the voltage I’m getting is +55%. It says it’s input voltage range is 35v - 7v and its output is a nominal 5v (4.5-5.5v) yet I’m getting 7.85vdc. My power source is a 28.8v battery. Is this just a faulty batch or am I overlooking something? Thanks

    thanks.
     
  2. bertus

    bertus Moderator

    2,060
    777
    Nov 8, 2019
    Hello,

    Do you have the decoupling capacitors mounted on the regulators?
    Without them, the regulator may oscillate and show a to high voltage.

    Bertus
     
  3. Chumply

    Chumply

    6
    0
    Nov 12, 2019
    Hi Bertus,

    I am not using a decoupling capacitor. I have used the same voltage regulators before the only difference being before they were 12v output & not 5v. But I never had to add anything else to the circuit. Could you recommend a decoupling capacitor I could use? Thanks
     
  4. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    Why not look up the data sheet?
    Try a 100nF cap on the output and more on the input.
     
    Chumply and Martaine2005 like this.
  5. Martaine2005

    Martaine2005

    3,563
    969
    May 12, 2015
    Hi Chumply, linear regulators like the 78xx series usually require a load in order to regulate correctly.
    The datasheet will have this information. Try a 1K resistor as the load to verify whether this is the case.
    Also try without the caps. Caps are usually only required if the input voltage is a small distance away. The output caps are usually small ceramics if required at all.
    Read the datasheet. It will have sample circuits showing approximate cap values.

    Martin
     
    Chumply likes this.
  6. Chumply

    Chumply

    6
    0
    Nov 12, 2019
    Thanks Duke37,
    I have read the 57 page data sheet - but not being too confident in these matters I thought it best to ask. Thank you very much for your suggestion - I’ll give it a go.
     
  7. Chumply

    Chumply

    6
    0
    Nov 12, 2019
    Thanks Martin,
    I’ll definitely try this and let you know how I get on.
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,839
    Jan 21, 2010
    If they were really cheap, you might not have what you think you have. I found some cheap lm337's that were not lm337's :-(
     
  9. Chumply

    Chumply

    6
    0
    Nov 12, 2019
    Hi Steve,
    They were bought from Mouser for around £0.30p each.
     
  10. bertus

    bertus Moderator

    2,060
    777
    Nov 8, 2019
    Hello,

    Here is the mentioned picture in the datasheet about the capacitors:

    3-pin_low_power_regulators_with_capacitors.png

    Bertus
     
  11. Chumply

    Chumply

    6
    0
    Nov 12, 2019
    Thanks once again Bertus,
    I have bought some caps and will experiment tomorrow.
     
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