Zener Diode - Worn Markings

Discussion in 'The Projects Forum' started by louye2, Jul 28, 2012.

  1. louye2

    Thread Starter New Member

    Jul 28, 2012
    My Sharp microwave oven (R-4850) took a power surge and stopped working. I did the usual microwave oven parts tests (fuse, magnetron, cap, etc.) and finally found a broken Zener Diode on the control board. It may be on the circuit that controls the digital display.

    Unfortunately, the markings on the Zener diode are worn off and I have not been able to find a schematic online for this microwave oven. I tested the diode with my Harbor Freight multimeter and it show a forward voltage drop of 765 mV.

    What advice do you have on identifying the right diode to put into the circuit?

    Thank you very much.

  2. takao21203

    Distinguished Member

    Apr 28, 2012
    Maybe the control board is fried too.

    I would eventually try to supply it with 5V artificially and see if it still works.

    If it is based on a specialized VFD chip (vacuum fluorescent display), voltage might be higher eventually 12V.

    Difficult to say without a picture.
  3. WBahn


    Mar 31, 2012
    I'm surprised it is acting like a diode at all. Have you tried seeing what the reverse voltage is?

    Can you see any standard parts on this board? That might tell you something about have voltage you might be needing. Can you track the Zener to see what parts it connects to?
  4. louye2

    Thread Starter New Member

    Jul 28, 2012
    The board is not fried because I can get the oven to work with a jumper cable inserted where the broken diode used to be. However, the jumper cable has to be inserted after the oven is plugged in. If it is in place before the oven is plugged in, the control board and display do not work. Strange?

    The reverse voltage on the Zener diode is 1 mV. Is the part possibly still good? I could put it back into the circuit using a wire connected to the stubby end of the glass filament. Should I try this?

    Thanks. I'll try to add some pictures to my next post.