i need to bypass the 7812 regulator so i just put my 12v on the output leg but then the 7812 gets super hot and starts smoking. there is nothing on the input of the regulator and no shorts. Why would it be overheating like this?

 

Can u show me ur schematic?

Need to see how u have wired it.

 

Apparently the current is passing through the only 2 legs that are connected...the ground pin and the output pin. I'd say, "Don't do that".

Just a guess here but, if you used a 317 chip with resistors to set the output voltage the reference pin wouldn't be at zero ohms to ground. Be sure to read my signature line!

 

The regulator sees this as the output voltage higher than the input voltage. See page 11 in the linked data sheet.

http://www.ti.com/general/docs/lit/getliterature.tsp?genericPartNumber=lm340-n&reg=en&fileType=pdf

 

Isolate the two sources (output of the 7812 and the raw source) with two diodes. That way, whichever has the higher output will supply the load.

 

If there is a problem with the 7812 overheating then remove it. Then you can continue with your troubleshooting with your substitute 12 volt supply, but be careful, there may be a reason the 7812 burned out.

 

Isolate the two sources (output of the 7812 and the raw source) with two diodes. That way, whichever has the higher output will supply the load.

true but this will also drop output voltage by Vf of the diodes (0.7V or so).
one can add also diode between common terminal of the regulator and dc ground to compensate for this. ideally problem could and should be eliminated at the regulator (they could have integrated other diodes too) but that's not going to happen.