If I connect a RB450 toa POE power supply and a to a DC power supply via the dc jack, would this cause a problem or provide power redundancy to the unit?
RB use highest voltage source.I not see reason for this connection.
I used second connector to power a switch
The reason is power redundancy, one power supplies dies, still have power
Simply stick two switching PSU outputs in parallel - no need additional LAN cable or POE injector.
That is bad practice. You should not connect the output of two DC power supplies together in parallel.
They will fight each other and cause an early failure.
Dual power supply systems have extra circuitry to either switch between main and hot standby or to take different inputs into a common regulator.
You are not doing that with two supplies plugged into the RB450. They are fighting each other and will fail prematurely.
Tom
If we inspect typical switching PSU AC input, diode bridge, power MOSFET , PWM controller , transformer, one half wave rectifier , tl431 IC like programmable voltage controller and feedback via optocoupler. What part exactly can damage exceeding what maximum ratings? What part exactly can be out of work condition?
I would think anything after the rectifier would be able to be paralleled. If you still must isolate each power supply from each other a simple diode after the power supply would work to prevent backflow. Every solar panel has this, so that solar panels don’t discharge your batteries at night.