Tweaker's Asylum

• I use a magnifying glass - Batman 11:47:34 02/25/02 (0)
  In Reply to: How do you measure Diodes with a DVM/Multimeter? posted by BrianM23@Msn.Com on February 25, 2002 at 04:29:45:

  There's no practical way that you can use your DMM to measure the parameters of a diode. The best way to determine replacement values is to use a magnifying glass (with the power off :-) ) and read the part number from the diode body. The part number you're looking for usually begins with "1N". You can then find lots of places on the web that will give you the operating characteristics of that diode are.

  For power supply diodes, you could measure the input voltage and that would give you some idea of the PIV required, but not the current. For the little fellas in the tuner, you need to know it's part number so you can find out whether it's germanium or silcon and then find its operating parameters.
  
  

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• Re: How do you measure Diodes with a DVM/Multimeter? - mikedale 08:34:32 02/25/02 (2)
  In Reply to: How do you measure Diodes with a DVM/Multimeter? posted by BrianM23@Msn.Com on February 25, 2002 at 04:29:45:

  Testing diodes is easy IN THEORY, particularly out of circuit. Using a multimeter on a low ohms range should show low resistance in one direction and very high in the other. But there is a snag - a diode will not conduct (even in the forward biased direction) unless a certain voltage is applied to it (its threshold voltage). This is around 650mv for a basic diode (schottkys are less). So on a modern multimeter the test voltage applied by the meter to make a resistance reading may be less than this and the diode will appear open circuit when in fact it is okay.

  This is why modern multimeters have a seperate diode test position which actually measures the voltage drop (not the resistance) across the diode when a test voltage is applied to it. So your meter will read O/C or infinity in one direction and something like (say) 634 (mv) in the other.

  Although it would be preferable to test out of circuit, you should be able to test a diode like this in-situ in most cases (with the power OFF!) probably worth not testing your glass signal diode in circuit though in case the surrounding circuit is a bit sensitive. What this will not tell you though is the max voltage and current ratings. For this you'd do best to simply look up the type numbers in a manufacturers data table.

  I hope this points you in the right direction...

  Good Luck,

  Mike
  
  

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  • Re: How do you measure Diodes with a DVM/Multimeter? - Corbett 10:17:34 02/25/02 (1)
    In Reply to: Re: How do you measure Diodes with a DVM/Multimeter? posted by mikedale on February 25, 2002 at 08:34:32:

    > This is why modern multimeters have a seperate diode test position which actually measures the voltage drop (not the resistance) across the diode when a test voltage is applied to it

    The voltage drop should be the same as the test voltage, regardless of polarity, so that's probably not what's being measured. However, significant current flow only exists in one direction.
    
    

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    • Re: How do you measure Diodes with a DVM/Multimeter? - . 02:50:40 02/26/02 (0)
      In Reply to: Re: How do you measure Diodes with a DVM/Multimeter? posted by img on February 26, 2002 at 02:47:07:

      In diode test mode (found on most DMM's nowadays) a current is
      injected into the diode and the resulting voltage drop is read
      out on the meter. The meter will show a fraction of a volt
      (nominally .6 for silicon) in forward mode, and open when reversed.
      (Assuming the diode is good). As others have pointed out, this will
      not tell you the voltage rating or current capability of the diode.
      An Ohmmeter will show a low reading when the diode conducts, and
      a high or open reading for the other polarity.
      CAUTION!!! Some Ohmmeters can damage a PN junction especially on
      the lowest range where more current is available. This won't happen
      with power type diodes, though. The time to be careful is when testing
      a diode-connected transistor, or a small detector type diode.
      hth, Eric
      
      

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