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Two power sources...

Discussion in 'General Electronics Discussion' started by Michael Quiñones, Aug 23, 2019.

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  1. Michael Quiñones

    Michael Quiñones

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    Dec 6, 2016
    [​IMG]
    I have this two power sources, can I connect them directly, or do I need extra components?

    Any operation may occur:
    Adaptor A is OFF while adaptor B is ON
    Adaptor A is ON while adaptor B is OFF
    Adaptor A is ON while adaptor B is ON
    Adaptor A if OFF while adaptor B is OFF
     
    Last edited: Aug 23, 2019
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    in 99.99 % of cases this connection is not possible. A typical adapter has no provisions to prevent inflow of current from the secondary side if another power source is connected. The reverse current may damage the adapter. Although when both adapters are ON, the load current is unequally shared between the adapters as no two adapters have exactly the same output voltage. Special provisions for load sharing are required.

    If the output voltage is uncritical. simply put a diode into each output path:
    upload_2019-8-23_7-29-56.png
    This will decouple the adapters at the cost of an output voltage one diode voltage drop (~0.6 V or 0.3 V for Schottky diodes) less than the 12 V you expect.
    Otherwise a more elaborate scheme using controlled switches (relays, transistors, ...) is required.
     
    Tha fios agaibh and davenn like this.
  3. Michael Quiñones

    Michael Quiñones

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    Dec 6, 2016
    Thanks for your idea but what diode number can I used for ?
     
  4. dave9

    dave9

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    Mar 5, 2017
    There are many suitable diodes, what you choose could depend on availability and cost in your region. Since the voltage is only 12VDC, you should not have trouble finding a schottky diode rated for higher than this, but you did not mention the current of the circuit.

    A generic suggestion would be something like a couple of MBR2515 (available from multiple brands) if the load is less than 25A, then if they are handling more than "roughly" 4A current (each), you should mount them on a heatsink, which is easy to do since they are TO-220 package.

    http://www.onsemi.com/pub/Collateral/MBR2515L-D.PDF
     
  5. Michael Quiñones

    Michael Quiñones

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    Dec 6, 2016
    Thanks for your reply. But need a diode rating 500ma - 1A. It just supply a current to a 12v relay.
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    1N4001 (50 V, 1 A) or similar is an inexpensive choice.
    1N5818 or similar is an inexpensice Schottky diode alternative (less voltage drop: 0.33 V vs. 0.85 V for 1N4001).
     
  7. dave9

    dave9

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    Mar 5, 2017
    In the future it would be helpful if you would start out mentioning things like this low current requirement.

    You could test the relay, it will probably work fine with the ~0.6Vf drop of a common silicone diode.

    However there seems to be still more to this than you mention, as 500mA, let alone 1A, is very high current for activating a relay unless it's a quite big one for very high current. Typical 12V relay coil current is usually 100mA or less.
     
  8. Michael Quiñones

    Michael Quiñones

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    Dec 6, 2016
    This was the relay I am taking about.
    I dont know how much current it takes?

    Please suggest me a diode number for this.
     

    Attached Files:

  9. dave9

    dave9

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    Mar 5, 2017
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