Schotky diode - help

Discussion in 'General Electronics Chat' started by daveyah2002, Aug 21, 2013.

  1. daveyah2002

    Thread Starter New Member

    Aug 21, 2013
    5
    0
    Hi,

    new here so excuse the question.

    How do I go about selecting the right 2 pin schottky diode for a reverse current protection circuit where the voltage can be up to 12v and the circuit current 5A.

    Does it have to be 12v or will 15/20 do. I really don't know.
     
  2. w2aew

    Member

    Jan 3, 2012
    219
    64
    If you are talking about using a schottky in series with your device/circuit which will:
    - have to pass the 5A that your circuit requires
    - and protect the circuit if the power was hooked up backwards

    Then, the diode will have to be rated for more than 5A forward current. It will most likely be rated by power dissipation, in which case you'll have to look at the forward voltage of the diode at 5A current, and multiply them together to get the dissipation in the application. For example, a schottky may have a forward voltage of 0.55V at 5A, resulting in power dissipation of 2.75W. Make sure the diode you pick exceeds the power dissipation expected by a good safety margin (1.5x-2x or more).

    The reverse breakdown voltage should also exceed your supply voltage by a good margin.
     
    daveyah2002 likes this.
  3. daveyah2002

    Thread Starter New Member

    Aug 21, 2013
    5
    0
    Thanks for your reply.

    I have a rough picture of the circuit attached that I found on the net a long time ago to measure the current drawn by a battery, I don't remember the source. Would you clarify it for me please if you can?

    The PSU they indicated was capable of measuring the circuit current and is not used as a supply.

    I am unsure of the role of the diodes in this case and thus the correct ones to choose. I think they are there just to protect the PSU from reversing the polarity of the battery. But I am not sure. If that is the case then the forward current rating needn't be 5A?

    I would be grateul for your thoughts.
     
  4. #12

    Expert

    Nov 30, 2010
    16,343
    6,828
    That circuit looks wrong. The diodes will short out the power supply. What is its intent?
     
  5. daveyah2002

    Thread Starter New Member

    Aug 21, 2013
    5
    0
    Thanks for replying.

    The battery on the left will be removed from the device on the right (tablet PC) and wired up as shown. The purpose was to place the PSU (used in ammeter mode) in series with the battery/device to measure the current drawn from the battery. I assumed the diodes were to protect the PSU from being fed from the battery if connected in reverse.
     
  6. #12

    Expert

    Nov 30, 2010
    16,343
    6,828
    Sorry about the dumb answer. I didn't read you reply correctly.

    IF the amp meter has a voltage drop that is very low (and that is likely), it will read the current, but it will show a negative reading. I don't know why a Shottky diode is used instead of a common rectifier. Shottky diodes have a lower forward voltage than common rectifier diodes (for a given current).

    The way this is arranged, the amp meter can be removed and the circuit will still work. In that case, a 5 amp diode is the minimum allowable size. If you remove the battery and try to use the power supply as a power supply, the silicon rectifier diode will short it out and the fuse will not protect the power supply because the fuse is outside the current path.

    Is this more on the path you wanted?
     
    daveyah2002 likes this.
  7. cornishlad

    Member

    Jul 31, 2013
    196
    25
    Where can we see an example of a PSU used in "ammeter mode" Or is this something you are assuming can be done..
    If you have some kind of special PSU that has that as a feature then the shottky diode will not conduct in your circuit and the zener will conduct as a normal diode. None of that makes any sense to me. Ignoring the diodes the PSU, would have to have some internal switching arrangement to move the current meter from it normal circuit location, to being connected across it's output terminals. I've never heard of such a thing being implemented. If this is a commercial PSU please show a link to it..

    #12 posted while I wrote ! I think I may be about to learn something new about PSU facilities !!
     
  8. #12

    Expert

    Nov 30, 2010
    16,343
    6,828
    Oops! I thought the diode with tails was a Shottky. It looks like a zener symbol. Since the OP used the word, "Shottky" I ask for clarification.
     
  9. daveyah2002

    Thread Starter New Member

    Aug 21, 2013
    5
    0
  10. daveyah2002

    Thread Starter New Member

    Aug 21, 2013
    5
    0
    It's a Shottky in the picture and I didn't change the pic so it's probably just the originators crappy drawing.
     
  11. #12

    Expert

    Nov 30, 2010
    16,343
    6,828
    OK. I mistook the symbol as the right one for the discussion, so what I said is correct (as far as I know, right now).
     
    daveyah2002 likes this.
Loading...