Prevent desktop from switching off when power inverter switches from mains to backup
Put a decent sized cap on the 12V line. Calculate what the estimated load is  by the power supply rating. If your computer is \$(12V)^2/300W = ~2Ω\$  because  \$ V^2/P = R\$. Then calcualte the 1/RC time constant, if you use a big capacitor like 1Farad then you would be protected to 60% of the voltage for 1/(1Farad*2Ω) = 0.5 seconds, you would probably want to be protected to 90% of the voltage so you would really be protected for much less than that but your switching time is probably in the msec, so a 1Farad cap would do great. 







simulate this circuit &ndash; Schematic created using CircuitLab