U13 (7805TV) Overheating (150f


#1

Anyone have any ideas as to what may be causing this incredibly high temperature on the 7805? I didn’t see any shorts or scorched points on the board (outside of my soldering iron) but when I did power it on, I saw some blue magic waft off of the coils area.

I still have to review the schematics and do a quick trace, but I thought I would ask to see if anyone may have any pointers to where to look.

Thanks!


#2

I’m just taking a guess but that sounds like you have a short from the collector (output) pin to ground or something downstream of that FET, likely after the transformer due to the magic smoke released, that has a short. The 7805TV is a voltage regulator, which when it is passing alot of current will create alot of heat. The 7805 would be passing alot of current if it’s output pin is constantly lower voltage than the output voltage it’s designed to produce. The 7805 passing more and more current to try to raise the output to the level it’s designed for but never being able to reach it. A second problem could be that the common/ground plane that the 7805 is connected to is above 0 volts. Voltage regulators measure the difference between the output and ground pins to determine if the output is above of below the set value (5V in this case). If the ground is at 2V and the output is at 5V(the level the output should be) the voltage regulator sees a 3V difference and tries to correct it to a 5V difference while the outputs charge is actually already 5V with respect to a true ground and will be raised even higher when the regulator corrects. Long story short I think you should check the circuits which the output and ground pins are connected to, I would expect you to find a wrong connection. I’m also not an electrical engineer but this is my best understanding of whats going on. If anybody knows that I got the right or wrong idea please let me know. Hopefully you’ll have this problem solved by now though