5v only puts out 1v on the pad-234 analog/digital trainer

Discussion in 'Analog & Mixed-Signal Design' started by Prowlin, Oct 2, 2011.

  1. Prowlin

    Thread Starter New Member

    Oct 2, 2011
    I have recently received a pad-234 analog/digital trainer and promptly got to work on knocking out my lab assignments when i noticed that my 5v power supply was only supplying 1.3v. I checked the other power sources and they checked out fine. I also checked the fuse and that good also. Is there a way to fix this issue. I am not sure what might have caused this but it worked fine for my first two labs.
  2. SgtWookie


    Jul 17, 2007
    Is this PAD-234 trainer your personal property, or does it belong to a school? If the latter, don't attempt to repair it yourself; notify your instructor immediately that your trainer has a faulty 5v supply.

    The Operator's Manual can be viewed/downloaded from here:
    Schematics for the unit begin on the 9th page.
    The schematic for the power supply is on the 10th page.

    This 5v supply uses the LM7805K fixed 5v regulator, which comes in a TO-3 steel package. The design of the trainer 5v power supply is not in compliance with manufacturer's recommendations, which require an 0.33uF cap from the IN terminal to GND, and an 0.1uF cap from the OUT terminal to GND. Extra capacitance can be added, but these small caps are required to be very close to the regulator itself to prevent oscillations.

    If the capacitors specified above are not present, the regulator may oscillate at frequencies in the MHz range, and the output voltage will usually measure somewhere between 1.1v and 1.7v with a meter set to DC when this oscillation is occurring. It is difficult to predict exactly what conditions will cause the oscillations, but ensuring that the recommended capacitors are present will prevent them from occurring.

    It could also be that either C9 or C12 has become faulty (open).

    If you happen to have some capacitors, with the trainer off, try connecting one across BP6 and BP3; observing proper polarity if it is a polarized electrolytic (stripe by negative lead) or tantalum (positive lead indicated) capacitor. Then power up the trainer, and see if your 5v supply stabilizes.