Feeding monitor from 2 sources without interruptions

Discussion in 'General Electronics Chat' started by ponferrada, Mar 15, 2019.

  1. ponferrada

    Thread Starter New Member

    Feb 28, 2019
    Hi all,

    Im trying to do something like this:


    What I want is to feed a 12V monitor that is in my car without interruptions. Sometimes this monitor is feeded by a computer installed on my car and sometimes is feeded by another system that is actived when I put the reverse gear. It dependes if the computer is on or off.

    I dont want to use diodes because I want to avoid excessive heat in the board. I have to do that with SUD50P04 P-channel transistor. In the simulation I choosed the FDS4685 because it haves Vds=-40V like the SUD50P04, so I think that this is enough for the simulation.

    I did a lot of differents configurations, but it's really hard for me to fully understand the behavior of a PMOS transistor. So I have no idea how to do it right with it.

    If someone could help me I would be very grateful.

    Thanks in advance
  2. pmd34

    Active Member

    Feb 22, 2014
    Hi ponferrda, you might consider something like this:
    (Assumming its OK if both supplies are on, and they will not cause problems connected to each other.)
    Your own circuit is OK ish, but there is effectively a build in diode in a mosfet, so in your case power could actually flow back into the supply that is at 0V.
    For a P Mosfet: if the gate voltage is less than the source, the FET switches on... so long as this is valid, current can flow either way in the device.
    If the Gate is at the same voltage (or higher) than the source then the FET will be switched off.
    ponferrada likes this.
  3. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
    At the current that a monitor will need the power lost in a diode will be small and so will not generate much heat - especially if it's a Schottky diode.
    ponferrada likes this.
  4. ponferrada

    Thread Starter New Member

    Feb 28, 2019
    Thanks pmd34, its working perfect, thats what Ive was looking for.

    AlbertHall, the power of the monitor is 70W, feeding it with 12V, so the current its about 5.83A, not neglible about heating, I say that because I can confirm that the diode heats up, because was my previous setup, the diode was a Schotky diode of course.