winner10920:
No, I realized that afterwards
all the mosfets I see say rds at 10v, why not give the one for 5v if its logic level?
Did you see my tutorial for parametric search on DigiKey a few posts back?
You can get more 'life' out of the supercap by running it thru a boost regulator such as Linear Technology LTC3252, the cap can drain lower and the regulator can boost it.
LTV3523 is only good for 175mA; browse around to see what else they have.
Yeah and therea a search for rds. Not rds @5v
You can do a search based on gate threshold voltage, then look up the Rds(on) specifications. Low gate threshold voltage correlates with having an Rds(on) specified at 4.5 or 5V.
I thought gate threshold voltage is when it started to conduct, they never really say when @ 5v what it is, maybe they'll say th is 2min, 3, or 4 max and then where does that put 5v at? I haven't look at a ton of datasheets so far but I've never seen it say 22 milliohm at 5v 5 amp or watever
A mosfet with a specification for Rds(on) at 5v or 4.5v will generally have a gate threshold voltage of 2v or less. So do a parametric search for mosfets with gate threshold voltage of 2v or less and a suitable Rds(on), then look at the datasheets for the devices it finds to see which ones have a specified Rds(on) at 5v or 4.5v.
I'd use a larger capacitor on the 12v side, 10 or 20000uF, then use a switching regulator.
Some TFTs have their own 3.3v regulator so 5v is kind of a waste.
Maybe you can modify your design and power off some devices as soon as you detect power loss.
You could even pulse the backlight to save power let say work it at 1/2 brightness.
All the effort just to see a goodbye message?
Quoting myself here:
Vgs 2.8V or lower
Typically, Vgs is the minimum (or typical) voltage at where it will turn on, but you want to drive it harder than that to get the best performance.
Thus, to get a device that performs well at 4.5V (which is what you'll reliably get out of a 5V AVR digital output pin) you want Vgs to be 2.8V or lower.
2.8V is somewhat arbitrarily chosen -- I could have decided to only look up to 2.5V or keep looking up to 3.5V; exactly how far to look is a matter of dedication and experience (which I at best have medium amounts of 
Now, I searched for current capacity from 2A and up, voltage from 30V and up, surface mount, in stock. Sort by price, look for the lowest Rdson on the first page, and find something like the HEXFET from International Rectifier:
Rds 29 mOhm @ 5A, 4.5V
Yeah the goodbye message would just be a bonus, the main goal is datalogging the shutdown
im using a switch reg to get 5v im gonna try finding cheap caps for like 24v or do u think 16 is ok?
I would find an automotive rated capacitor with at least a 25V rating, and protect it with a 16V Zener diode. In cars, the 12V system can see very significant voltage spikes from the various inductive loads (including the starter motor!) For a super-robust system, I'd use a bi-directional TVS at something like 24V, and a Zener at 16V, and probably look for 80V+ rated components on the 12V end, possibly with an inductor/choke at the front, too. Engine systems can be nasty in this regard :-/
If you feed the capacitor through a resistor then the combination will be immune to short duration voltage spikes. The resistor will also limit the surge current when the 12v line is powered up. The 12v line is typically between 13v and 14v when the battery is charging, so a 16v cap is cutting it a bit fine and I suggest 20v or 25v.
I think you said that the total current draw was around 400mA, so if you use a 4.7 ohm 2W resistor then it will drop around 2V, and limit the surge current to less than 3A.
Do I really need to limit the inrush current off the 12v? I've got a 800ca battery so a capacitor is nothing
When using a large capacitor (as you are) and a very low resistance source such as a lead-acid battery, it's advisable to limit the inrush current, to protect the capacitor, the diode, and the wiring. In particular, the diode needs to have a surge rating at least as high as the peak inrush current.
What would be an advisable resitor if in addition to that 400ma load I have a potential 1A that may be drawn from a usb port
If you are using a switching regulator to generate the 5v supply, then for 1.5A or so total load I'd go for something like 1 ohm 3 watts, and look for a diode with a surge rating of at least 15A. If you are using a linear regulator, then I'd go for a higher value (because more power dissipated in the resistor means less dissipated in the regulator), maybe 2 to 3 ohms at an appropriate power rating, and then the diode doesn't need such a high surge rating.