Automatically switch between two power sources

Discussion in 'General Electronics Chat' started by tjohnson, Dec 25, 2015.

  1. tjohnson

    Thread Starter Active Member

    Dec 23, 2014
    I made an LM386 audio amp that can be powered by either a 5V USB port or a 9V battery. A USB port is the primary power source, and the battery makes it convenient to use the amp without one, but I only want the battery to power it when the USB cable is not plugged in. Right now I have a slide switch to manually toggle between the power sources, but I would prefer to automate this.

    From the googling that I have done, it seems that the preferred way to automatically switch between power sources is diode ORing. The problem with this is that if both power sources are connected, the one with higher voltage is used, which is the opposite of the behavior that I want.

    I've seen several schematics for more complicated circuits, but if it's more advanced than adding a few discrete components, I'm not interested in using it for this simple project. Is there a simpler way to accomplish what I want, or would I have to use a bunch of MOSFETs or an IC?
  2. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
    One power MOSFET that acts as the switch, and two small signal mosfets to control it. No ICs unless you want them. The plan is that 9 V battery input is connected to the load through a P-channel MOSFET (sometimes called an active diode), and the USB 5 V input *disables* the 9 V input by turning off the MOSFET.

    Without drawing it up, 1 medium power MOSFET, 2 - 2N7000, 3 resistors, 1 shottkey diode. Is this about the level you were hoping for, or still too complex?

    tjohnson likes this.
  3. tjohnson

    Thread Starter Active Member

    Dec 23, 2014
    Thanks for your reply. I came across this thread, and it seems like the only two options are to use either a MOSFET circuit like you described which I was hoping to avoid, or a SPDT relay. I think I'll look into the second option more.

    EDIT: After further research, I realized that I would still need to have a SPST on/off switch connected to the battery to prevent it from draining due to quiescent current, so I suppose I'll just keep my circuit the way it is.
    Last edited: Dec 25, 2015