Electronic – 7805 regulator outputting 4V instead of 5V

5vpower supplyvoltage-regulator

I followed a tutorial on Sparkfun to put together a 5V power supply. But for some reason I'm only reading 4V coming out of the regulator. I know it's not the regulator because this is the second one I've tried. I checked the data sheet to make sure it's connected right and yet the problem still persists. I'm pretty sure I'm giving it enough power because it outputs 4V with a 9V wall wart input as well as around 15V from two 9V batteries in series. Eventually I gave up and decided to move on, even though 4V is below the minimum rating of the ATmega328p (4.5V,) and now I'm facing problems trying to program the chip, which I assume is because of the chip not getting enough voltage.

Does anyone know why the regulator isn't regulating properly?

The schematic for the power supply is:

enter image description here

from here

Best Answer

If you are doing everything correctly then the chances of it working correctly are extremely high (99.999%)

SO it is very likely that you are doing something other than you say or imply. Follow the following to see if we can find out what the problem is:

Are you using C1 and C2?
You MUST to start.

Are they close to the regulator?
Must be close to start.

What is your load?

What is Vout with no load?

Have you put a new battery in your voltmeter ?
("Low battery" in meters is a too common cause of apparent voltage errors.)

What are you using for the PTC resistor?
If it is too high a resistance it will cause problems.

The PTC is wholly unnecessary to start and reasonably unnecessary in any case.
If you are using one, short it out.
Were you using one?

If the circuit is not your EXACT circuit please modify it to show EXACTLY what you are doing.

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