M80/81 with stock PSU?

http://stuff.nekhbet.ro/2006/06/18/how- … -7805.html

Use an N-MOSFET to switch ground off on the to Pi.

The above schematic has the filter circuit to remove ripple..

M80/81 a mosfet will do the trick - Mosfet and 2 resistors (1k for inline with the Gate to limit current to the Mosfet, and a 10k on the Gate to Gnd after the 1k to ensure the mosfet really is off when its off (and gate isn't floating)). But if you're pulling 12.5 Amps - make sure to get a High On Resistance, Logic Level MOSFET thats capable of say 60V/30A , but 100V/75A is probably more preferable...

Using a relay is possible - but you need to be careful as 12V high current relays will draw a quite reasonable amount of current just to turn on - often you'll need a transistor to drive them from a Micro pin.. they also will 'crack' lots as the physical contacts make/break... so much better to use a MOSFET... if you do end up with a relay, make sure its Coil-Resistance is very very high and use Ohms Law to calculate that at 5V you wont draw more than 20ma through the Micro pin... For example, 120Ohm is not unusual to see for the coil-resistance, and at 5v, that will draw 41ma - more than you can safely provide via an Atmel... so you'd need to use a transistor to switch the relay, and the micro switching the transistor... at which point, you may as well have used a MOSFET

Ok, tried prototyping something, but hit a couple of snags:

1. Most serious: is it even possible to do this when the circuit being switched is also the one which controls the MOSFET?  In particular, when the Sanguino digital pin (connected to the gate) goes low, then basically the gate is shorted to the *drain* and, based on googling, this is not good.  (Of course, having the control circuit be the one switched makes no sense, unless it also gets power from elsewhere: in this case, through USB from the RasPi -- my Sanguino is always on in that way, and what I'm trying to achieve is make a digital out pin have the same effect as manually plugging/unplugging the 12V SD PSU, which is what I currently do).

2. Wrt combining an always-on regulator (for the RasPi) in the same circuit: problem is that all USB ports share the same ground, so the RasPi closes the ground circuit (in attached schematic, the RasPi connects USB GND to pin 2 on PS_OUT -- pls ignore the 10K resistor permanently shorting gate to drain, I realized the stupid mistake after taking screenshot, pulled out resistor but was too lazy to take new screenshots -- still, flaw noted above remains even w/o that resistor).

Any hope of making this actually work (with or without the regulator, preferably with)?  Any help would be appreciated, as I'm completely clueless when it comes to even relatively basic circuits...  thanks!

A few specific questions on last iteration (high-side switch), in case anyone is reading:

1. Do I *have* to use a P channel MOSFET for high side switch?
2. Is there any way that a logic-level (5V) signal is sufficient to turn on the MOSFET on high side (it's switching 12V), or is a separate control circuit necessary?
3. How does "logic level" translate into specs? A particular Vgs(thresh) value? Or..?
4. How do you prototype high-current circuits? Anything that doesn't involve soldering/desoldering (which is what I do now)?

Thx!

Edit: FWIW, attaching what I'm going with (pls ignore component names, mostly picked randomly to match package types). Basically the regulator circuit is copy-pasted from datasheet, and the high-side switch is pieced together based on Google results: a P-channel MOSFET with a pullup resistor to 12V, controlled by a smaller, secondary logic-level N-channel MOSFET connected to the Sanguino, with a pulldown resistor. Components won't be here till next week, so if you happen to see anything horribly wrong till then...

The LEDs always flicker, you just may not have noticed.. but its 'normal'...
The steppers sounding funny - you running a fan on your electronics ? a 2hr print will easily see you running the steppers into their thermal shutdown 'as delivered' if you have anything other than a frosty 20°C airconned room (or there abouts...... )

So far, sounds like the board gave you no issues, and its just the 'usual' LED behaviours (it comes and goes.. I could go into a long winded explanation but'll bore everyone to tears...) and potentially overheating steppers...

Ok, prints fine (just completed) -- doh! On plus side, guess I learned what ridiculous overextrusion looks like, I wouldn't have imagined. 

Finally, fully remote operation! Now I can leave printer running, and use cellphone to check webcam and properly switch off printer wherever I am.  If anyone else cares about details, I can post more about circuit and minor firmware mod (basically a #define and 3 lines of code in main loop). Although at ~$15 BOM (plus labor) this is admittedly not the most cost-effective approach unless if you're also doing it out of personal curiosity, and can probably be avoided if you also plan to do further upgrades in future (eg, ATX PSU, electronics, etc -- but if I reach that point, I'll get another printer instead or perhaps build, say, a Kossel.. if only I had the time ).For now, it's nice, though!

Ambient room temp is ~25C (+/- 3C).  If it goes close to 30C, with the humidity in this place, *I* suffer (nevermind Pololus), so I switch on A/C.   Now that I'll probably be doing longer/unattended prints, we'll see.

Thanks!