View Full Version : 7805 and 7812

07-23-2006, 02:05 PM
I have a transformer / rectifier setup driving both a 7805 and 7812 regulator. The 7805 powers the IC's in the circuit. The 7812 powers 12 volt electric motors. The problem, both regulators become burning hot. I did not put the motors in yet, but my volt meter registers proper functionality. I have a feeling you are not supposed to wire a 7812 and a 7805 in parallel together, maybe the 5 volt should be Wired To the 7812.

Well well, I'm seeing things, three of them.

-Stanley Blystone

Beau Schwabe (Parallax)
07-23-2006, 02:20 PM
"I have a transformer / rectifier setup"

How much voltage and current does this supply?

Beau Schwabe (mailto:bschwabe@parallax.com)

IC Layout Engineer
Parallax, Inc.

PJ Allen
07-23-2006, 08:26 PM
Burning hot means inadequate or little or no heatsinking.· A little aluminum wings thingy isn't going to·make it, mate.· You'll have to screw them down to some mass, like an aluminum extrusion (something thick, not a 1/16" sheet or anything.)

The 7805 & 7812 can share the same input voltage source.· The thing you have to keep in mind is, whatever is not on the output is across the regulator.· Say you're using a 24V input and have a 5V output, then you have 19V across the regulator.· Same circuit, if you are drawing 1A from the output it's because you're pulling that through the regulator, too (and more.)· What all that means is that you have 19V * 1A = 19W·burning off the regulator (19 watts, try keeping your finger on a 20W light-bulb some time).

What all this means is that you want to·minimise the·difference between VIN & VOUT to the extent possible (in your case, VIN must be 3.5V greater than VOUT.)

T Chap
07-24-2006, 03:11 PM
I use 7805's and 7812's on the same board, both off a 3 amp 24 volt regulator from a battery, they aren't hot at all, maybe slightly warm. In my case, when they are burning hot, that means they are shorted, but that is obvious with a meter. When I have semi shorts that result in say 4.8 volts instead of 5, then I get lots of heat.

something smells like it's on fire