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12VDC Power Supply outputting 19V?

Discussion in 'Electronic Repair' started by Glenn Ashmore, Jan 18, 2005.

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  1. I have a Nippon America DVP-3212 power supply that I fear has had an
    encounter with a power surge. Now instead of 13.8VDC it is putting out
    20.4V. Checked it with the 'scope and it is still smooth and ripple free
    but the voltage fine adjust pot has no effect. It looks like the voltage is
    controlled from a little PC board on one side.

    I have the schematic from NA but I don't know where to start looking.

    Any suggestions?

    --
    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at: http://www.rutuonline.com
    Shameless Commercial Division: http://www.spade-anchor-us.com
     
  2. Jerry G.

    Jerry G. Guest

    If this is supposed to be a regulated supply, then the fault is in the
    voltage regulator circuit. Any component going defective in the regulator
    circuit, including the output stage, can cause a fault condition.

    The only way you can fix this is to properly troubleshoot it. You will be
    required to have a descent background in electronics theory and have a DVM,
    to troubleshoot this.

    Most of these simple regulated supplies have a regulator board that feeds an
    output transistor or output IC. Some of these low end small current rated
    supplies use a single power IC to do the regulation, and output together.

    As for ripple in the output of a power supply, the ripple will show more if
    the supply is loaded. When there is no load, there may still be no ripple
    showing on a scope, even if the filtering is not fully working.

    --

    Jerry G.
    ======


    I have a Nippon America DVP-3212 power supply that I fear has had an
    encounter with a power surge. Now instead of 13.8VDC it is putting out
    20.4V. Checked it with the 'scope and it is still smooth and ripple free
    but the voltage fine adjust pot has no effect. It looks like the voltage is
    controlled from a little PC board on one side.

    I have the schematic from NA but I don't know where to start looking.

    Any suggestions?

    --
    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at: http://www.rutuonline.com
    Shameless Commercial Division: http://www.spade-anchor-us.com
     
  3. Some of these can be tricky but the first thing to check assuming it doesn't
    need a load to regulate, would be to see if the pass transistor is shorted.

    --- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
    Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
    +Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
    | Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

    Note: These links are hopefully temporary until we can sort out the excessive
    traffic on Repairfaq.org.

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name is included in the subject line. Or, you can
    contact me via the Feedback Form in the FAQs.
     
  4. Where are you guys putting up temporary graphics? I tried posting to the
    a.b.s.e newsgroup but it hasn't shown up yet and most of what I see there is
    BS.

    --
    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at: http://www.rutuonline.com
    Shameless Commercial Division: http://www.spade-anchor-us.com
     
  5. No ICs on the regulator board. Just 4 transistors with Korean codes that I
    am trying to figure the equivelent values for. Three TO-92 transistors
    marked 3DG9014 which I believe to be .45W 50V general purpose NPNs and a
    TO-220 marked JDD313 of yet to be determined function.

    I realize that ripple usually comes under load but the best I could do was
    to put a couple of 24V light bulbs in the circuit when I looked at it on the
    'scope.

    Output is an array of six 2N3055 transistors and the regulator board is
    feeding them 20V.

    --
    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at: http://www.rutuonline.com
    Shameless Commercial Division: http://www.spade-anchor-us.com
     
  6. Franc Zabkar

    Franc Zabkar Guest

    It looks like a linear supply with six pass transistors. Like Sam
    says, one more of the 2N3055s are probably shorted C-to-E. Check the
    base drive for any additional problems.


    - Franc Zabkar
     
  7. But he says the driver is feeding the 2N3055s 20 V, so need to check
    the feedback. Need to see circuit as someone else requested.

    --- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
    Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
    +Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
    | Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

    Note: These links are hopefully temporary until we can sort out the excessive
    traffic on Repairfaq.org.

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name is included in the subject line. Or, you can
    contact me via the Feedback Form in the FAQs.
     

  8. Finally figured out what was happening to my web site. My 5 year URL
    registration expired Sunday! Back up now so I put up the rather fuzzy
    schematic that Nippon America sent me at:
    http://www.rutuonline.com/dvp-3212.gif

    --
    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at: http://www.rutuonline.com
    Shameless Commercial Division: http://www.spade-anchor-us.com
     
  9. Measure the voltage at the base of V28. It should be somewhere around
    14-15 V when the regulator is working. If it's that or less, check the
    check each of the driver transistors for shorts. If it's higher, check
    the components in the feedback circuit from the output.

    Sometimes it's as simple as a bad or dirty adjustment pot.

    --- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
    Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
    +Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
    | Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

    Note: These links are hopefully temporary until we can sort out the excessive
    traffic on Repairfaq.org.

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name is included in the subject line. Or, you can
    contact me via the Feedback Form in the FAQs.
     
  10. Well, the base on V28 is barely 20mV with 30V on the collector and the
    emmiter is outputing about 5V out to V27. By driver transistors you mean
    the ones feeding V28?

    --
    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at: http://www.rutuonline.com
    Shameless Commercial Division: http://www.spade-anchor-us.com
     
  11. Here are some pictures of the board that might help:

    Back side of PCB: http://www.rutuonline.com/dvpbottom.JPG

    Component side of PCB: http://www.rutuonline.com/dvptop.JPG

    Orange wire at lower left goes to V26 on the heat sink and reads 22V to
    ground Not sure where the purple one goes yet but it reads 30.4V. Blue
    yellow and brown go to the 4.7K voltage adjust pot. Red and black go to the
    output terminals and the fiber insulated wires go to the transformer.

    I am getting some strange readings off the pot. As I turn the knob the
    yellow center tap varies from .72V to .79V, brown goes from .73V to .26V
    and blue goes from 8.6V to .8V. Checking the pot's resistance it goes from
    3.8Kohms to .3 ohms. I would have thought that when the pot was all the
    way over the voltages would be the same.

    --
    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at: http://www.rutuonline.com
    Shameless Commercial Division: http://www.spade-anchor-us.com
     
  12. << Output is an array of six 2N3055 transistors and the regulator board is
    feeding them 20V. >>

    Glenn-

    In another post you mentioned very low voltage on the base of V28 with 5 volts
    on the emitter connected to V27's base. It would appear that V28 is cut-off,
    and the base-emitter junction is reverse biased, possibly limiting voltage to
    its zener value.

    That leaves V27, V26 and one of the 2N3055s as a potential culprit.

    I suggest you measure the voltages on these transistors. Pay attention to the
    base-emitter voltages. I'm not sure you said it, but it appears that the
    output voltage is close to the same as the 2N3055 base voltage. If that is
    under load, one of the 2N3055s could be shorted. The base-emitter voltage
    should be around 0.7 volts.

    Fred
     
  13. ..
    V27 is 5V on the base, 30V on the collector and 22.3V on the emmiter. V26
    is getting 22V at the base from V27 and feeding 22.75V to the bases of the
    other 2N3055 transistors.

    One more lead. The emmiter sides of the 2N3055s are tied together in groups
    of 3. I separated the two groups and checked resistance between the
    emmiters and base on each. One group is 6.5M ohms and the other is 790
    ohms. So I guess everybody that said check the output transistors first
    was correct.. Unfortunately the group with low resistance is burried deep
    in a double layered heat sink that will require complete disassembly to get
    to. :-(

    --
    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at: http://www.rutuonline.com
    Shameless Commercial Division: http://www.spade-anchor-us.com
     
  14. Can you just run with the group that works as a test? The current rating
    will be cut in half but that's the main difference.

    --- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
    Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
    +Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
    | Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

    Note: These links are hopefully temporary until we can sort out the excessive
    traffic on Repairfaq.org.

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name is included in the subject line. Or, you can
    contact me via the Feedback Form in the FAQs.
     
  15. Gentlemen, and especially Sam, I thank you! Found the bad transistor,
    removed it and I am back down to a nice healthy 13.8V. I will put a couple
    of 2N3055s on my list for the next Digikey order.

    OTOH, I also have a Samlex RPS1215 that uses 2N3055s and is totally dead.
    Unless one of you has the patience to guide me through fixing that one too,
    I will scavange a transistor from it.:)

    --
    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at: http://www.rutuonline.com
    Shameless Commercial Division: http://www.spade-anchor-us.com
     
  16. Totally dead might be easier. :)

    --- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
    Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
    +Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
    | Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

    Note: These links are hopefully temporary until we can sort out the excessive
    traffic on Repairfaq.org.

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name is included in the subject line. Or, you can
    contact me via the Feedback Form in the FAQs.
     
  17. James Sweet

    James Sweet Guest


    If you need them quick, even Radio Shack carries the 3055's.
     
  18. Glenn-

    You reversed the leads on your meter between the two groups. Do both forward
    and reverse polarity resistance measurements on both groups, and I think you
    will find them to be comparable.

    Is it easy to remove V26? If you did, and connected a 1000 Ohm resistor from
    the 2N3055 bases to ground, the output voltage should go low with a load. If
    it does, then the 2N3055s are probably OK and V26 may be shorted
    collector-to-emitter. If output stays high then there is likely one or more
    shorted 2N3055s.

    Fred
     
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