Hello, I am using Arduino Uno for a soldering station project. I am powering the Arduino through USB port using a L7805 regulator and I saw that if I remove the USB power, the ON led still lights up when I am powering the soldering station. Also, the other led's are semi light up. The Vin and 5V power pins are not connected.
Please find attached the schematic.
Can you please explain why is happening this phenomenon ?
Called "phantom powering", something in the soldering station to which this is connected is applying some voltage - perhaps 5 V - to one if the I/O pins on the Arduino which is then being conducted by the protective diodes in the chip, to the power rail.
It is not healthy!
If we knew to what you were connecting it, we would be able to see.
I also have the attached schematic for displaying the temperature to which the Arduino is connected.
J1 (1 to 8 ) is directly connected to J2 of the first schematic. The J3 (1 to 3) is connected to pins D8, D9, D10 of Arduino and J6 is connected to +5V rail coming from the 7805.
I disconnected the +5V that was powering the 3 digit 7 segment display (J6 from the schematic) and I removed from the socket the LM358, and the LED's from the Arduino did not lit up anymore.
If I connect back into the circuit the +5V to the 7 segment display or the LM358, then the Arduino LED's light up again.
How bad it is this problem and what should I do ?
Are you trying to power the Arduino from +24V through the regulator? If so, there is some open connection and 5V power from the regulator is not reaching the Arduino. You really shouldn't be routing 5V through the USB port if that is what you are doing. Apply it to the Arduino 5V pin instead.
Yes, I am powering the Arduino from 24V through the 5V regulator into the USB port. I used this powering method because I read that powering through the 5V pin is not recommended.
Could you please be more specific when you say "there is some open connection and 5V power from the regulator is not reaching the Arduino" ?
I observed this phantom powering accidentally, when I disconnected the usb power from Arduino. But the 7805 output will always be connected to the USB port of the Arduino.
the_bear:
Yes, I am powering the Arduino from 24V through the 5V regulator into the USB port. I used this powering method because I read that powering through the 5V pin is not recommended.
Right, but disregarding which power input is used on the Arduino, it is powered from the soldering station, right? So does that work correctly? When +24V is on, the Arduino and display is on and functioning normally?
Yes, the Arduino is powered from the soldering station.
Yes, the soldering station is working correctly.
Yes, when +24V is on, the Arduino and display is on and both (Arduino and display) are functioning normally.
the_bear:
Yes, I am powering the Arduino from 24V through the 5V regulator into the USB port. I used this powering method because I read that powering through the 5V pin is not recommended.
Yes, well, you sadly can't believe everything you read! 
There is a caution that it may not be a good idea to connect 5 V to the "5V" pin while the USB is connected to a PC - especially a laptop - for uploading code or using the serial monitor.
There is absolutely no suggestion that this can damage the UNO itself; transfer of power beyond 500 mA in either direction might trip the polyfuse.
The concern is that if the external 5 V is higher than the USB 5 V, you might "back-feed" the USB port on the PC or laptop. This is in itself, a valid concern as there seem to be reports of USB ports being damaged in this fashion.
The point against this being likely however, is that many or most powered USB hubs do in fact, connect the accessory 5 V supply rated at something like 2.5 A directly to the 5 V terminal on all outputs and the input because without the accessory 5 V supply, the input must feed those outputs and you do not want to insert any device - such as a diode - which will cause a voltage drop.
So given the relative number of Arduino UNOs (some even genuine) and powered USB hubs, I feel if this were a frequent problem, it would be announced to a far greater audience than this forum!
The UNO is a poor choice for "real world" applications - the Nano is smaller, cheaper, and easier to make connections. And the "back-feeding" concern does not apply. 
