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Wiring consistency check

Discussion in 'Electronic Basics' started by Rikard Bosnjakovic, Dec 5, 2005.

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  1. I got a pcb-layout from a magazine, but since I've only got veroboards at
    home I had to relayout it to fit a veroboard.

    I've now completed the circuit and construction, but obviously there's an
    error somewhere that I cannot find. I've double double checked my layout
    but still cannot find it.

    The original, flawless, pcb-layout from the magazine can be found here:

    http://bos.hack.org/tmp/vero_pcb/pcb.jpg

    And my veroboard-version for the same layout can be found here:

    http://bos.hack.org/tmp/vero_pcb/Veroboard.pdf


    I would be very grateful if there is anyone that's got some spare time
    (around 5 minutes) to "proofread" my layout and see if there are any errors.

    Thank you.
     
  2. I can see no errors in your layout. I would be looking for sneak
    paths under C2, or a solder bridge where a hot line runs between two
    pins of the connector row. Are the chip, bridge and diode oriented
    correctly?
     
  3. Thank you for your reply.

    I've now checked, thrice, the places you suggested without being able to
    find any solder bridge or what so ever. Also, the IC, bridge and diode are
    using correct orientation.

    The board was residing in a box for a fresh project I build straight to
    the number from the magazine. It doesn't do as supposed, to I used my DMM
    to measure around on the board. The magazine tells me that the Vref on the
    723 should be around 7 volts. When the board is in the board, Vref is
    around 28 volts, and that's the reason I plugged it out and tested it
    outside the box using only the mains-wires.

    The weird thing, is that when the board is outside the box, Vref is 7
    volts as it should. I take this that it means the board is okay, no solder
    bridges, but the other components (connected to the board via the
    connector pins) could possibly be erronious, so I guess I'll have to
    connect one cable at a time to find the error source.
     
  4. Ken Taylor

    Ken Taylor Guest

    And have you cut the tracks under IC1?

    Cheers.

    Ken
     
  5. The wiring on the veroboard are obviously correct so I can remove it as an
    error source.

    There is however one thing that I find tricky for the wiring in the case.
    Have a look at this image (schematic from the magazine):

    http://bos.hack.org/tmp/vero_pcb/fig1.jpg

    And this case wiring schematic (also from the mag):

    http://bos.hack.org/tmp/vero_pcb/fig2.jpg

    P2 is supposed to change the output voltage over the pole terminals (+ and
    - just under P2). The schematic is for a lab power supply and is rated to
    supply between 3 volts and 25 volts, but what happens when I turn P2 is
    that the voltage does not change at all except for a few millivolts.

    Using my DMM for the pole screws (+ and -), it reads 3.350 V. If I turn P2
    to its extents, the voltage goes from 3.350 to 3.380, and that's it.

    What I'm unsure is about the consistency between fig1.jpg and fig2.jpg. In
    fig1.jpg, P2 should be in series with the amp-meter, but in fig2.jpg, it
    isn't (I think).

    Other people I've talked to that have built this supply have had no
    problems, so there is probably nothing wrong in the schematic, but I
    cannot understand what's wrong. All the wiring (both for the veroboard and
    for the case) are entirely to the number according to the schematics in
    the magazine, but still it won't work.

    In the magazine, there's a table of "testpoints" for measuring with the
    DMM. Pins 6 and 5 on the 723, for example, should be around 7 and 3.5
    volts respectively, and that's what I can read on my multimeter. The
    voltage over the base and collector on T1 are also correct, as well as
    every other testpoint in the table.

    Could anyone shed some light on me for this problem? Are there for example
    any obvious tests I can do to try to find the error source?
     
  6. P2, R5 and R6 form a voltage divider that must reduce the desired
    output voltage to about 3 volts to be compared to the divided down 7.5
    volt reference voltage. You don't provide the resistance values for
    Either divider, so I can't check if they are right.

    You may have P2 and R6 interchanged.

    I would have preferred a different arrangement for the divider that
    used the wiper as the tap that connects to IN-. That wouldn't pass as
    much current through the wiper contact for a longer life, and I think,
    better range.
     
  7. I'm sorry, I just now realized that I should have written the values since
    they are hard to read in the pdf in the OP.

    R1 - 2.2k
    R2 - 1.8k
    R3 - 1.5k
    R4 - 1k trimmer
    R5 - 330
    R6 - 1k trimmer
    P1 - 1k lin pot
    P2 - 4.7k lin pot

    P1 and P2 are the ones on the case, R1-R6 are all on the board.
     
  8. That means that the reference voltage is 7.5*R2/(R1+R2) or about 3.375
    volts.

    So with R6 at maximum resistance (1k), The range for the output should
    be between 3.375 minimum to 3.375*(P2+R5+R6)/(R5+R6) = 15.3 volts.

    With R6 set to zero, changes to 3.375 to 3.375*(P2+R5)/R5=51.4volts,
    which would allow only a small part of the rotation of P2 before the
    output saturates high.

    So no setting explains the low output voltage. I would ohm out the
    resistance from IN- to the negative rail, to make sure something is
    not open in the R5 R6 pair.
     
  9. I have now followed your steps and couldn't find any problems.

    During the ohming, I did however check the copper tracks once more just in
    case I had cut the wrong tracks or similiar. And guess what, I found a
    microscopic size of solder bridging between two tracks. Removing it with a
    small screwdriver helped, because now the power supply works!

    Thank you John for your time and valuable pieces of information. Without
    your posts I would have assumed, wrongly, that the copper strips and the
    board itself was no source of an error, and would have never looked there.
    Your tip about checking R5/R6 made me find the solder bridge.
     
  10. Excellent. Congratulations.
     
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