Testing diodes question

Discussion in 'General Electronics Chat' started by FastEddie, Aug 29, 2009.

  1. FastEddie

    Thread Starter Active Member

    Jul 14, 2007
    35
    0
    Hi,
    I'm testing a VFD of an unfamiliar brand. The instruction sheet http://www.hitachi-america.us/supportingdocs/forbus/inverters/UserManuals/SJ300_L300P_NBS611CX.pdf
    (page 33) that I'm going off says I should be reading a certain value and I'm reading pretty much the opposite. Is this testing procedure correct? It seems backwards to me.
    Even when I go by their directions I get nothing remotely close to what they call for (all readings OL). If I set my meter to autorange the lowest reading I got was around 11K.
    Thanks
     
  2. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Warning: The above link is a 6.5Mb pdf.

    Congratulation, you had been caught up in the classic pitfall between checking a diode using a digital tester DVM and using a multimeter(with a pointer).

    The reason is that DVM is measuring voltage across diode and the analogue meter is measuring current through diode.

    If you have both types of meter, try testing a good diode and satisfy yourself that the color of the meter leads that give a reading are opposite to each other.

    The pdf is obviously giving you a test result based on trouble shooting with an analogue meter. That's why it seem reversed.

    If you can access the point marked "A" (anode of thyristor), it is much better to check the diode as shown, bypassing the thyristor.

    [​IMG]
     
  3. awright

    Well-Known Member

    Jul 5, 2006
    84
    7
    The polarity of some VOMs is the reverse of what you would expect based upon the color of the leads. That is, the black lead is positive relative to the red lead. Not common, especially with DVMs having more sophisticated circuitry than VOMs so common these days, but I have seen it and been confused by it. Check your tester on a known good diode. You can also measure the voltage and the polarity on the tester leads with another VOM or DVM.

    Some ohmmeters are not very good for testing diodesTry the diode test function that gives you a reading of the voltage across the diode in the forward and reverse directions. In the reverse direction on a good diode the voltage will read either overload or several volts, depending upon the design of the meter. In the forward direction it should show around 0.6 to 1 volts.

    After pontificating above, I printed out page 5-4 (33rd page) of the manual you provided the link to and checking a few points in the table. I agree with you that it seems to have the entries in the "Measurements" column reversed.

    awright
     
  4. FastEddie

    Thread Starter Active Member

    Jul 14, 2007
    35
    0
    Hi,
    Thanks for your answers. I never would have thought of using an analog meter. I'll look around and see if I can find an analog meter so that I can retest the diodes and check the polarity of my DVM leads. Unfortunately I don't have access to point A so I can't check that.
    Thanks
     
  5. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    The thyristor is normally not conducting and this will prevent you from checking the diodes D1~D3.

    However, the manufacturer has anticipated this and thus has connected a resistor across the thyristor between A & K, shown in the image above. This would at least give you certain reading(that of R plus diode conducting) and no reading(diode reverse biased) when the test leads are reversed.

    The main idea here is to look for consistency in the readings of D1 ~ D3. If they are the same, then the diodes D1~D3 are good.

    Be aware that if measured result is comparable to that of a single diode, i.e. you don't see any effect of a series resistor, then the thyristor might have shorted.
     
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