Diode substitution - Is "Reverse Current" something I need to consider?

Discussion in 'General Electronics Chat' started by bteddy, Apr 12, 2010.

  1. bteddy

    Thread Starter New Member

    Apr 12, 2010
    3
    0
    Hello, great site.

    I need to replace a diode that is not available anymore. I have its data sheet and have found close substitutes but nothing exact.

    The specs of the original are:

    Vrm - 400V
    Vf - 1.3V
    If - 1A
    Ifsm - 25A
    Ir - 10ua
    trr - 50ns

    My problem is the reverse current.
    All the close matches I have found are 5ua. From what I have read, the "ideal diode" would have a reverse current of zero, the lower the reverse current the better.

    This makes sense, If I am applying 350VAC at .5A continually, the voltage and current do not change when in reverse bias to the diode. Meaning the reverse current is a "Rating" of how much current the diode will let flow through it, not a "limit" of how much reverse current it can handle. So, I should be able to replace it with a diode with a lower reverse current rating.

    My Question:
    Is my logic in the above statement correct?
    If not, why? Please provide details or point me relevant references.

    Thank You
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    With those easy specs, a 1N4005 will do quite nicely as a general purpose rectifier.

    Unless you have some situation where reverse current is critical, it's not something to worry about.
     
  3. bteddy

    Thread Starter New Member

    Apr 12, 2010
    3
    0
    I wish it was that easy. The original is a fast recovery diode, trr = 50ns. This is a critical specification. I can use one faster but not slower (<= 50ns).
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    That does make a difference. Take a look at Digi-Key part #UF1004DICT.
     
  5. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,536
    You want fast find a Schottky diode. They come in decent currents, but they only drop around 0.2V or so. This might be a negative for your application, and they aren't your normal diode in that their basic theory of operation is a bit different. They approach the ideal a bit closer than conventional diodes, but blow faster if you exceed the specs.
     
  6. AlexR

    Well-Known Member

    Jan 16, 2008
    735
    54
    The problem with Schottky diodes is that have low Vmax, in the order of 40V-60V and they they have really bad reverses current characteristics Ir=.5mA-1mA is not uncommon.

    Lower Ir is generally better. Some applications can tolerate relatively high reverse reverse current but I can't think of a situation where you would go out of your way to have extra reverse current.

    If you are running the diode at 350Volt then using a 400Volt diode is sailing a bit close to the wind. Probably that is why the original diode failed in the first place. I would go for a diode rated at 600Volt or greater.

    Something like a BYV26C should fit the bill and should not be too difficult to get.
     
  7. bteddy

    Thread Starter New Member

    Apr 12, 2010
    3
    0
    The information is great, but I need an answer to the question:

    With all else the same, can I replace a diode with a Ir (reverse current) of 10ua with a diode that has an Ir of 5ua?
     
  8. AlexR

    Well-Known Member

    Jan 16, 2008
    735
    54
    YES!

    I thought that's what para 2 in my post effectively said.
     
  9. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    The reverse current is temperature dependent and could be anywhere between 0 - 50uA depending on the actual diode and temperature etc.

    A new one with a specification of 5uA is simply in the same range, but never so bad.

    The UF4004 is a very close match to the original - the Fairchild data sheet gives reverse leakage at 10uA (at 25' C), and 50nS recovery.

    The 4005 - 4007 are higher voltage but the reverse recovery time is slightly longer at 75nS.

    Datasheet:
    http://www.fairchildsemi.com/ds/UF%2FUF4004.pdf
     
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