combining two 12v power supply

Discussion in 'The Projects Forum' started by ell, Sep 27, 2011.

  1. ell

    Thread Starter New Member

    Jul 26, 2011
    hi, i'm doing a project which involve 2 supply of 12v to a programmer circuit. Difference is in day, the power is supply from battery with solar panel as charger, while at night, the power is supply from an AC to DC power supply.

    Problem is I'm using a LDR with relay to switch the supply depending on day or night, but when in the process of switching where the relay switch from normally close to normally open, there is a delay gap where NO power is supply to the programmer circuit.

    to overcome this matter, i plan to combining both supply to the circuit with adding diode on each supply so that the current would not reverse or effect to each other.

    BUT this things is I'm not sure if i using this method, the input to circuit will become 24v or still remain 12v. Because the circuit needs only 5v which i used 7805 to convert it.

    Ques: will the voltage become 24v or remain 12v? if not to combine both power supply, what kind of connection i should use?
  2. debjit625

    Well-Known Member

    Apr 17, 2010
    That process will happen within some milli seconds,anyway it will depend on the type of relay you are using and their are fast switching relay avaliable you can use them.But still their will be some time gap so for that moment we use a parallel capacitor with the load and at that moment it will discharge itself as a power source.The value of your capacitor will depend on the current and voltage rating of your load and the time relay needs to switch over.

    The diode way will also go, I do it many times, but I don't know how exactly you are connecting the diode so it will be better to give us a schematic before you proceed.

    Good Luck
  3. t06afre

    AAC Fanatic!

    May 11, 2009
    You also have something named make before break relay. (Google it) However I would have used an electrolyte cap as energy reserve for the switcing period.
  4. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    Just use diodes to to create the redundant supply. Less parts to fail.
  5. wayneh


    Sep 9, 2010
    Some cap will be needed anyway to avoid odd behavior when switching happens.

    The only problem with the diode approach is that the voltage drop that will occur across them. If that's not a problem, ditch the relay and just OR the supplies together. They'll be in parallel, so you won't get 24V. Current will flow from the supply at a higher voltage into the load. The lower voltage supply will not contribute.
  6. SgtWookie


    Jul 17, 2007
    As has been already mentioned, you can avoid the relay alltogether by using diodes.

    Here's one way to do it:

    Relays are power hungry, bulky and noisy.
    Using diodes will make for a smooth transition between supplies; whichever supply has the higher voltage is what will be providing current for your load.

    In the simulation, the solar system is providing the power while the sun is shining. Normally, a wall wart will be on all the time it's plugged in. If your solar system is providing more output voltage than the wall wart, the latter will be consuming very little power.
  7. wayneh


    Sep 9, 2010
    Actually, the AC-to-DC supply might ALWAYS provide a higher voltage than even the charged battery, so that the solar panel might never really contribute except to keep the battery charged, maybe overcharged. The battery will draw the panel voltage down to the battery voltage, which may be below the AC-DC voltage.

    The diode solution "switches" supplies based on voltage, but that might not work for your setup. You need to provide more info about the battery, the charging setup, the wall wart ratings, etc.