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Sherlock Holmes - Why did the diode fry?

Discussion in 'General Electronics Discussion' started by rockrockmcrock, Dec 20, 2011.

  1. rockrockmcrock

    rockrockmcrock

    41
    3
    Dec 19, 2011
    Hi gurus,

    I'm trying to work out what happened to fry a schottky diode in a second hand battery I've been given.

    The battery has a small PCB that includes a charging port, connection port (for connecting the battery to the equipment, and a diode for reverse polarity protection.

    The pack would trickle charge at 70mA, the diode is rated at 400mA and has a capacity of 1100mAh.
    If trickle charged through the port all current passes through the diode.
    A jumper can be added to the connection port which bypasses (shorts out) the diode, this allows for battery cycling/conditioning.

    I suspect the battery has been cycled at 1C (1100mA) with the jumper attached. The 400mA diode is still in the circuit but shorted. Would current still flow through the diode? and is this what blew it (it is really toasted!)?

    I have to say I'm confused as I thought that if you short something out in a circuit the current works like water in pipes and it all takes the path of zero resistance (i.e. the short)

    Thanks guys!
     
    Last edited: Dec 20, 2011
  2. OLIVE2222

    OLIVE2222

    690
    25
    Oct 2, 2011
    hi rockrockmcrock,

    Indeed if the diode is shorted they can't be damaged in anyway. What it can happen is that the 1C current was applied before setting the jumper, or the jumper was removed before the 1C current. Trash the diode and place a 2Amp one will make the stuff more bullet prof!

    Olivier
     
  3. rockrockmcrock

    rockrockmcrock

    41
    3
    Dec 19, 2011
    Thanks - that's what I thought and you've put my mind at ease as I did in fact replace it with a 3A version (although the size difference is comedic!).

    I really can't understand it though as the standard smart charger for it shouldn't be able to work without the jumper in place as I'm pretty sure it senses back current to judge when the battery at capacity. With no back current it should drop out with a "connection break" error on its LCD. Hence the need for the jumper to take out the diode - I might be wrong though!

    The only thing I can think of is that the jumper was dirty and causing a resistance. Complete mystery - but thanks again - at least I know my 3A schottky was a good choice and will save the worry in future!
     
  4. jackorocko

    jackorocko

    1,284
    1
    Apr 4, 2010
    You might want to double check the continuity of the bypass while your at it to make sure that is working as intended.

    Btw, how did you come up with comedic instead of cosmetic?
     
  5. rockrockmcrock

    rockrockmcrock

    41
    3
    Dec 19, 2011
    :) I'm not an electronics expert - so when I open up the battery case I see a few dainty ports and this little tiny fried diode on the board - I think right, I'll just get one with a bigger ampage - I rock into Maplin's expecting to get roughly the same physically sized diode and I get this massive monster instead that is half the width of the PCB and has garden wire for connectors. I didn't want to look like a complete fool so I acted as if I was expecting something that big&ugly and left thinking "OK it's only 39p but is this thing physically dangerous and how the @£$%£!! am I going to fit it on the board!" :confused:

    Luckily it did - but it looks like the original diode's steroid-mad "Frankenstein" style brother. I didn't even know diodes came that big on PCBs outside of the 1960s classic car. :)

    I really did mean comedic - perhaps I'm childish, but it made me snigger when I saw it anyway!
     
    Last edited: Dec 20, 2011
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