Repair temperture board

Discussion in 'The Projects Forum' started by RoelJansen, Aug 15, 2013.

  1. RoelJansen

    Thread Starter New Member

    Aug 15, 2013
    Dear friends,
    Last week I did something stupid: A putted 24 V from an accu on the signal out of my Velleman temperature board (scheme attached). Well, I saw a spark and that was it: the board does not give any signal now (should be 0-20mA current loop). I though, transistors are sensitive things so I replaced the one connected to signal out i.e. the T2 BC557. Still the board did not work. What I noticed was that the current through te board is now 100mA which should be 30 mA according to specs. Also, the resisitor R11 becomes now very hot once the board is powered with 12V (directly turned the power off). Do you have any idea what to replace to get it working again?
    Many thanks,
  2. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    Thank you for providing the schematic! Many people are less organised when asking for help. :)

    Also, welcome to the forum. :)

    Applying 24v DC tot he signal out pin on that circuit, with a spark, has almost certainly killed T2 BC557 and the opamp IC IC1. T1 is probably ok, as are the two input diodes.

    The regulator zener ZD1 12v could also be fried.

    I would remove the IC1 and T2 from the PCB. Then power it up with 12v-16v etc, and check the voltage on the zener diode ZD1, it should be 12v or a bit less. If the voltage on ZD1 is very low and the circuit draws too much current it is likely ZD1 is shorted.

    Or, if you just want to order some parts and fix it I would just blanket replace IC1, T2, and ZD1.
  3. MrChips


    Oct 2, 2009
    Electrolytic capacitor C2 could be shorting. Remove C2 and see if the board works at 12V.

    You can also remove ZD1 and test the board with V+ at 12V.