A simple question, will it work as described?
Circuit is intended to immediately power on both a 5v and 3.3v supply to an embedded computer and other peripheral devices regulated with LM317 adjustable voltage regulators. Once the computer is powered on, it can switch from the constant supply input power over to the 'UPS' or 'Buffered' power by high-side switching the 12V incoming power with a 3.3V output (see transistors Q1 and Q2).
At that point the device maintains operation with that power and if incoming power is lost all the peripheral devices lose power AND an input to the embedded computer goes low signaling that it needs to finish writing files and begin shutting down. At this point it is operating off of capacitor C1. Once it gets far enough in the shutdown process, it drops out it's output and then power to the device is killed. This is necessary so that if power is re-applied the computer knows to startup, otherwise it would need to drain the capacitor down and would have a required minimum "drain down" time that would be undesirable.
My biggest 3 questions with this circuit are:
1) Is the transistor switching correct? I had to look into how to high-side switch a higher voltage by using the PNP and NPN transistors in together.
2) Is R12 necessary? This is a 'precharge' resistor. I deal a lot with 480V drives for work all of them have pre-charge resistors to reduce inrush current. The idea is in the first 30 seconds while the computer is booting up before it turns on the GPO, the capacitor can be pre-charged at a low current before it is supplied full 12V.
3) The size of capacitor is definitely questionable because it's not clear at all how much time I need and what the current draw is going to be. I figure I need a minimum of 2-3 seconds above 5V to finish writing the files, and current draw is going to be around ~500mA @ 5V. But right now until I've got some measurements I'm taking just a shot in the dark with 500uF.
Thanks for the feedback. You're right about the typo, I meant R13, not R12. But I disagree it was only an inline resistor, there is definitely a clear path to the top of Q2 through D9. However, you're totally right that the diagram was hard to read, so here's a simpler version (I hope):
Also, you did answer my question about the necessity of the precharge resistor. Due to the low voltage, I was hoping it wasn't necessary. In that case we can make things much prettier, like so:
Now, regarding the use of N3904/6 transistors, would the 2N2222 and 2N2907 be more appropriate?