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I've designed a circuit that I've already had tested it on breadboard and it has worked fine. When I did it on a PCB a new issue has appeared. The voltage regulator (I'm currently using a 7805) gives right values at no load, but when I attach the Atmega328 on it the output voltage goes to 1V instead of 5V and input goes to 2.5V, instead of 12V at no load. I've already try to change the capacitors. As specified on DS, after rectify it I put a 330nF on INPUT and another of 100nF on OUTPUT, both ceramics. Can someone, please, figure out what's going on?
Here is the datesheet of lm7805:http://www.componentschip.com/details/ST/LM7805.html
Where's the schematic?
LIke Abe said a "bad cap short" kind of situation. Try increasing the input resistors to ground. See if the reading change in the "right" direction.
Without seeing your setup I can't be sure but it sounds like you might be placing a dead short or very nearly a dead short on the regulator output.
Have you tried a known fixed load on the output of your regulator like a fixed value power resistor.... maybe 100 - 10 Ohms which should draw about 50mA - 500mA?
I would use one of the larger resistance values for test purposes unless your regulator has a heat sink on it.... or place the resistor load on it momentarily just long enough to measure the output at the regulator.
If the output voltage from your regulator is 5v with the resistor load, then something is wrong with your circuit after the regulator.
It would help if you provide the schematic. If there are any resistors in series to the source or load of the 7805, measure the voltage across them, which will tell you how much current is being drawn. The problem is you are drawing more current that you are sourcing, possibly a problem in your pcb layout or an incorrect component.
Sorry, wrote that in short hand. Your circuit is designed to provide up to some amount of current to the input of the 7805. Since the voltage is being pulled down on both sides of the 7805, you are drawing more current than the input side is designed to supply. You should be able to find the cause of the problem with an ohm meter. You will probably see a low resistance reading from the output of the 7805 to ground. You can lift one leg of the components on the load side of the 7805 until you find the problem.
If you don't see a low resistance to ground on the load side, you could have a problem on the supply side that doesn't allow enough current to be supplied to the 7805. That is why I suggested measuring across a resistor in series to see how much current is being supplied when the voltage is being pulled down. I = V/R
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