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switching power sources with mosfets

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Hello everyone. First, I apologize in advance if this is a really stupid question. I'm a software guy attempting to build my own pcb. I have two +5V power sources that I want to switch between. I also have a +8V source that is used to determine which power source to use. I've put together a circuit (attached) using mosfets, and I'd like to know if it will work, if there are any pitfalls, etc. If I understand correctly, if I pick mosfets with small RDSon values, the voltage drop through either mosfet should be minimal?

err how are you switching between sources ? they are both triggered from the same input ?

I updated the attached schematic to be more representative of what I'm trying to do. The idea is if the external +8V is connected, I'll get power from it. If it is not connected, but the secondary source is connected, I'll get power from the secondary source. If both are connected, I'll get power from the external +8V.

EDIT: I should mention I'm trying to do this as cheaply as possible with as few components as possible. The ICs that do this for me are fairly expensive, which is why I'm trying not to use one.

A pull-down on the 8V might be a good idea.

The N-MOSFET needs to be able to pass the required current when gate voltage is only 3V. If you can't find a suitable device, you could use P-channel and connect the gate to 0V using an NPN BJT when the 5V is on.

I see two problems with your design:

* If the 8V source is disconnected, the gate of P-type MOSFET will be floating.
* N-type MOSFETs are normally used to connect the load to ground, not to power. Since you have a higher gate voltage, this might work, but.. Maybe you can leave the N-MOSFET out altogether and rely on disconnecting the 5V source.


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