5V on board really always 5V?

I did a silly thing yesterday. I loaded a program expecting my arduino to do one thing but it didn’t do it, well, because I loaded the WRONG program. Silly me. On the other hand, I lost my trust to the 5V lead on the board. While the wrong program was loaded, the 5V lead failed to power anything. I didn’t have a multimeter but I tested the output with an LED and resistor. It failed to light up. When I loaded the correct program, just for comparison I connected the same LED and it lit up just fine. I wonder if the 5V on board is set by program (bootloader?) since if it’s always 5V it should work regardless the program I load. Any explanation? BTW, my wrong program outputs on several digital pins that are not connected. Thank you.

If you look at the schematic, the +5V comes straight out of the power supply section, and is the same point that powers the ATmega MCU.

This voltage is not in any way under program control and should always be +5V. Unless there is some other problem such as shorting out the power supply or drawing too much current and causing the voltage to drop.

The +5vdc is always on and not under any programmable control. If you wired the anode of a LED to the +5vdc and then wire a current limiting resistor in series, the LED will light if you hook the other end of the current limiting resistor to a ground pin. For you program to be able to light the LED then the resistor must be wired to a I/O pin and your software program must program that specific I/O pin as a output pin and finally your program must set the specific I/O pin to LOW.

Lefty

My circuit has a couple of 595 shift registers and leds with current-limiting resistors. It's a working circuit, under the right program. I'll check for possible shorts. During the problem I was still able to receive decent communication from arduino. If the 5V were shorted I expect the mpu should not work, weird.