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  • Powering a load from two sources

    Backbon3

    Member

    Updated: Oct 22, 2024
    Views: 1.1K
    Hello!
    I have a problem that I hope you will help me resolve.

    I have two sources of power and I want to power a light bulb using them.
    This sounds silly now but the thing is that one system is powered (power source) when other is not, or they both can be powered at the same time and my wish is to power this bulb when at least one of them has power. I am getting negative wire (-) and positive wire (+) from one supply and I get in the same way two wires from other supply (+ , -). Voltage difference is 12V when supply is active. Positive wire from one suppy is connected with positive from the other, the same is with negative wire. OK, now the problem is that voltage potential (on/off) from those supplies are controled by a MOSFET that cuts the negative wire at one supply and positive at the other. So I do not have a common wire that will be at one point at the whole time. If those two supplies would be active all the time MOSFETs would give connection to negative wire on the one side and positive on the other and it would be easy with a few diodes to direct current over the bulb. Is there a way to resolve this. Oh I hope that I have made myself clear 😀

    Power supply 1:
    ------------------ (-) MOSFET controled
    ------------------- (+)


    Power supply 2:
    ------------------ (+) MOSFET controled
    ------------------- (-)

    Thanks
    0
    Replies
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Replies

  • Munguti

    Member • Jul 30, 2009

    Either i have not understood your question, or the solution is simple in that you can have the two supplies powering your system in parallel, this might increase the total current you can achieve compared to using one supply kind of like solar cell, i.e. no need for a mosfet to switch power supplies.
    I don't know if this has helped.
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  • Backbon3

    Member • Jul 30, 2009

    Thanks for your attention and reply.
    I should have noticed that I do not have any control of these power sources that I am using. These system is on a car, and I have pulled these wires out. One pair of wires powers one light bulb, the other pair powers the other light bulb (this is standard). Now I want to add third light bulb that will be on if at least one of those two is on. The problem is that switch (integrated in car control unit) on the first light bulb gives (+) and (-) is always connected. And it is reversed on the other so (-) is over a switch and (+) is constant.

    P.S. If (-) would be constant on both pair of wires then I would put two diodes from (+) wires to my light bulb (+) and connect minuses together.

    Thanks
    Bye
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  • reachrkata

    Member • Aug 5, 2009

    Since i cant draw an actual circuit, let me try it this way -

    1) You would need two more MOSFETS to realise this. It is very simple to get an AND logic, but what you need is an OR logic.

    2) Connect the + and - of each source, the Gate and source of two n-channel MOSFETs respectively (though a resistor divider if needed) such that the corresponding MOSFET achieves saturation mode when the corresponding switch is closed. The final case is that both the MOSFETs achieve saturation when both switches are closed.

    3) Short the Drains of both the MOSFETs and connect your third bulb in series between Battery (direct) and the shorted drains.

    4) This way when either or both swithes are closed, the third bulb will glow.

    - Karthik
    😁
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  • Backbon3

    Member • Aug 6, 2009

    Wow, that is it. So you understood my problem😀 I think I understand what you are saying, I'll try that. Thank you very much for a solution!
    Bye
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