Dual polarity power supply problem

Discussion in 'The Projects Forum' started by jj_alukkas, Aug 20, 2009.

  1. jj_alukkas

    Thread Starter Well-Known Member

    Jan 8, 2009
    751
    5
    The DVM I made along with a power supply failed after a few hrs of use which is based on 7107. The display always reads 10. something even if I vary the VRPS. So tracing it lead me to the 7805 & 7905 regulators which is only giving me 3.6V despite the 6.3V input with the 7107 removed and no load. It used to give + & - 5's when I built it. Both the regulators are cool without heatsinks , but is there any chance that the regulators have burnt out? Or some other problem? How to verify without removing it?
     
  2. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
    40
    The 7805 and 7905 regulators typically need at least +7V (7805) and -7V (7905) on the input to regulate probably, check the datasheet for the manufacturer that you are using. If you are guaranteed only a +6.3V input voltage you will need to find a LDO (low dropout) regulator which typically requires less than 1V above the regulated voltage.
     
  3. jj_alukkas

    Thread Starter Well-Known Member

    Jan 8, 2009
    751
    5
    The 7805 and 7905 needs +7 & -7 only for Guaranteed Regulation with load. It would still regulate with lesser voltages and I got +5 and -5 when I built it first. But I dont know what happened now.
     
  4. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    Do you have reasonable size decoupling capacitors at the input and output of each regulator (and close by).

    I'd always use at least 10uF at each point, & for some regulators even 50 - 100uF, plus 0.1u on the outputs.

    If they are oscillating due to lack of decoupling, you will get strange voltage readings!
     
  5. jj_alukkas

    Thread Starter Well-Known Member

    Jan 8, 2009
    751
    5
    I have 100uF on the inputs and 0.1uf on the outputs.
     
  6. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    the 7805 & 7905 regulators are only three pin jobbys so are very easy to remove and test properly out of circuit.
     
  7. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    If it's not instability, the only thing left is input voltage??
    - Do you have adequate smoothing after the rectifier to keep the ripple down?

    Quote from a 7805 data sheet:
    The input voltage must remain typically 2.0 V above the output
    voltage even during the low point on the input ripple voltage.

    Can you try a higher voltage as a test, even from a battery or whatever?
     
  8. peranders

    Well-Known Member

    May 21, 2007
    87
    0
    ... and the load must be a few mA also. An unloaded 78xx/79xx can give other voltages than desired.
     
  9. jj_alukkas

    Thread Starter Well-Known Member

    Jan 8, 2009
    751
    5
    The regulator on post was taken for check when it failed to supply + & - 5V when loaded with a 7107 chip and 4 seven segments.

    I tried loading it with another circuit with a chip and a few transistors including 2 TIP's, but readings are 3.6V loaded or unloaded. I will test it with a different supply and reply. What I dont understand is how the output voltage of the transformer dropped to 6V now as when the supply was built, it gave over 8.5Volts. I suspect a short in diodes of the rectifier. Let me check everything briefly.

    I will test everything completely first and reply.
     
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