Regulated DC Power Supply Help

Discussion in 'The Projects Forum' started by mbxs3, Mar 6, 2010.

  1. mbxs3

    Thread Starter Active Member

    Oct 14, 2009
    141
    3
    Hi everyone. I am a beginner electronics enthusiast and I am currently working on a regulated dc power supply. I am building it IAW a book I am reading called "Tab Electronics Guide to Understanding Electricity and Electronics". The power supply construction has gone very well and I thought I was about done when I ran into the problem I am having now. The power supply is built to provide positive and negative regulated dc power. I built the "positive" side of the circuit and everything worked as designed, so I mirrored the "negative" circuit from this, which is also what the book instructed to do. My issue is that the negative side of my power supply is not adjustable. I am getting -37vdc no matter what position P2 is in. I rechecked all my wiring and verified that no solder points were shorted across and everything was wired to where it needed to go. I also verified my transistors and potentiometer read out correctly with my DVM. What am I missing here? Thanks for any help. I have included the schematic I build my circuit off of. I can provide pics of the circuit card itself but I doubt it will be of much help because of all the wiring hiding the PCB.
     
    • PSS.pdf
      File size:
      1.2 MB
      Views:
      46
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    What transistors did you use? I don't see the part numbers on the schematic, or values for any of the other components.

    The negative side is almost a mirror of the positive side. There are differences. If you installed the diodes upside down, that would explain your difficulty. If you installed the caps backwards, they would be destroyed.
     
  3. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,534
    A quick reference of the schematic...

    [​IMG]
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Start by taking voltage readings from ground to the cathodes of D2, D4, and D6. You should get about a 0.7v increase per diode; so about 0.7v, 1.4v, 2.1v - roughly.
     
  5. mbxs3

    Thread Starter Active Member

    Oct 14, 2009
    141
    3
    Sorry for not being more descriptive. C3 and C4 do not have polarity markings on the actual capacitor so I just installed them with out regard to a + and - lead. They are both 0.1uf. The part numbers for the transistors used on the "negative" circuit are as follows:
    Q2=2SA1302(NTE2329)
    Q4 and Q6= TIP 32C(NTE 292)

    As far as the diodes go(D2, D4, and D6), I have the emiiter of Q4 connected to the cathode of D6, the anode of D6 to the cathode of D4, the anode of D4 to the cathode of D2, and the anode of D2 to circuit common.

    I took some more readings while attempting to troubleshoot, not sure if they are helpful but I noted that I had the output of C2-(-37vdc) on the collectors of Q4 and Q6. The reading on my - DC Output Jack is also -37vdc. I took readings on the terminals of P2 and found that the wiper lead had -25vdc.

    Thanks for the responses and also thank you for cleaning up the schematic and making it easy to see on the forum.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    OK, so try taking the readings I posted last. If the cathode of D6/emitter of Q4 doesn't measure close to -2.1v (+/- around 0.3v) then there is where the trouble is.
     
  7. ifixit

    Distinguished Member

    Nov 20, 2008
    638
    108
    The -25V on the wiper of P2 doesn't sound right. Should be approximately -2.4 to -2.8 volts. D4, D4, D6, or Q4 could be bad.
     
  8. mbxs3

    Thread Starter Active Member

    Oct 14, 2009
    141
    3
    Ok, I took a reading from the Q4 emitter lead and it was also -37vdc. I could not safely read the cathode of D6 due to the way I have the circuit designed. With a -37vdc reading on the emitter of Q4, does this mean my collector and emitter are shorted?
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Yep, that's why I had our OP start with the diodes. If he's getting 2.1v on the cathode of D6, then the problem is most likely Q4. The voltage on the pot is off because the output voltage is so high (negative).
     
  10. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If you are getting -37v at the emitter of Q4, then one of the three diodes are most likely open.

    If Q4 is shorted, that could cause enough current flow to burn out one of the diodes.
     
  11. mbxs3

    Thread Starter Active Member

    Oct 14, 2009
    141
    3
    With my circuit card removed so I can access my solder points, I read out all 3 diodes using my DVM and all 3 read around 630ohms when my positive lead was connected to the anode and negative lead to the cathode. With the reverse lead connections, all 3 read open.
     
  12. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    So, what are the voltages you're getting from ground to the cathodes of those three diodes?

    The diodes appear to be good. You may still have an open connection between them, between anode of D2 and ground, or open between the cathode of D6 and the emitter of Q4.
     
  13. mbxs3

    Thread Starter Active Member

    Oct 14, 2009
    141
    3
    Ill take some reading for any possible open connections you mentioned. Ill also try and figure out a way to safely take the voltage readings on the diodes, but it could take me a while. Thanks again for the help, I will report back as soon as Im done with the readings.
     
  14. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Gee, is it really that difficult?

    Measure voltage from Ground to anode of D2. You should measure 0v. If -37v, the connection between the anode of D2 and ground is broken.
    Measure voltage from Ground to cathode of D2. You should measure -0.7v. If it measures -37v, D2 is open. If you measure less than -0.7, something else between the cathode of D2 and the emitter Q4 is open.
    Measure voltage from Ground to anode of D4. If -0.7v, good. If -37v, the connection between the cathode of D2 and anode of D4 is open.
    Measure voltage from Ground to cathode of D4. If -1.4v, good. If -37v, D4 is open.
    Measure voltage from Ground to anode of D6. If -1.4v, good. If -37, connection between the cathode of D4 and anode of D6 is open.
     
  15. mbxs3

    Thread Starter Active Member

    Oct 14, 2009
    141
    3
    Thanks for pointing me in the right direction, Sgt. Found that I neglected to solder the anode of D4 to the cathode of D2. Corrected this and it works like a charm. Thanks again!
     
  16. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Glad you found the problem and got it fixed. :)
     
Loading...