Lm79xx voltage regulator

Discussion in 'General Electronics Chat' started by Copey84, Aug 5, 2015.

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  1. Copey84

    Thread Starter Member

    Jul 27, 2015
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    Hi all, was hoping someone could answer a question about the 7912 I have used in my power supply. The pin out for the 7912 is
    Ground, vin, vout, basically should there be continuity between vin and the metal tab on reverse of regulator? Doesn't seem ok to me as this will cause a short to any metal it comes in contact with. I think there is an insulating washer that goes on back of metal tab when heatsinking, but should it really be live?
    Also Included pic of voltage reg, maybe someone could tell me if it is a descent brand or a dud, thanks..

    7912.jpg
     
  2. absf

    Senior Member

    Dec 29, 2010
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  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Very many power devices in such packages have some element connected to the metal case. It is that way so there is some way for the power to get out of the little package; the actual chip sits directly on the tab and thus is electrically connected. Insulating the device from the heatsink is typical.

    However, there is a trend these days to keep the part isolated is at all possible so you may get luicky.

    When you have questions of this type just go to the device data sheet for the part you are using. I found the answer to your specific question right there on the first page:

    https://www.fairchildsemi.com/datasheets/LM/LM7905.pdf
     
  4. Copey84

    Thread Starter Member

    Jul 27, 2015
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    Thanks for that Allen, will have to get insulating washers as I have to heat sink the regulators to metal casing of power supply housing.
    The 7912 regulators I have tried have all been unable to output a stable-12 volts, getting very frustrated with this project. Checked all the usual suspected faults, connections, components, nothing wrong.
    Set up a variable resistor and when voltage from bridge rec was reduced down to -20 from -27, then the 7912 was able to output correctly. Problem now is I don't want to have to include variable resistor in circuit to get correct voltage.
    As the type of transformer i'm using outputs 5v higher than its rating,(10va 15v rating, 20v output not underload) I thought that if I replace transformer with a lesser rating this might solve problem. Do you think I could go to a 8v 10va transformer? this would allow -20 after rectification, or would this cause problems maintaining steady voltage level underload? would like 500ma at a constant 12 volts.
    The only other solution I can think of is replace the regulators, the ones I have used were quite cheap maybe they don't like such high input voltages.
    Appreciate any replys.
     
  5. Copey84

    Thread Starter Member

    Jul 27, 2015
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    Hi ErnieM, thanks for reply. Have checked data sheets to find that the Max input is -35 volts, yet when I input -27volts it will only output around -18v and not the -12v it should be. Only when I use variable resistor to bring voltage down to -20v will the regulator output the correct -12v.
    I have a dual supply +-12v , the 7812 can output correctly but why can't the 7912, both have same voltage being fed in at -27v but the 7912 will not output correctly. Any ideas why this would be, have tried all the obvious things many times ,connections components all ok.
     
  6. ISB123

    Well-Known Member

    May 21, 2014
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    Did you try putting a load?
     
  7. Copey84

    Thread Starter Member

    Jul 27, 2015
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    Voltage will drop a bit when underload, but I need it to be -12 v for op amp circuits.
     
  8. ISB123

    Well-Known Member

    May 21, 2014
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    Tell me the values of capacitors used and their type.
     
  9. #12

    Expert

    Nov 30, 2010
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  10. Copey84

    Thread Starter Member

    Jul 27, 2015
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    Hi there, the caps are of the electrolytic type and are rated at 25v 2200mf and 16v 22mf I've included circuit diagram. IMG_20150729_191624.jpg
     
  11. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    #12 is trying to tell you that you will need decoupling/bypass capacitors for the correct functioning of the regulators.
    I pasted the images from the datasheet for you:

    bypass_capacitors_on_78XX_and_79XX.png

    This looks very much the same as your other thread:
    Dual rail virtual ground power supply

    Lets continue over there!

    Bertus
     
    Last edited: Aug 5, 2015
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