The +5volt rail varies with different power supplies

I feed my Arduino with a regulated 9 Volts into the power socket on the UNO. I then assume the on-board 5V regulator does the rest.

However, when I measure the +5v on the header on the UNO I see....

4.8volts when running from USB
6.9volts when running from external 9v supply.

Everything seems to be working ok but this gives lots of issues when trying to measure on the A/D ports.

Any ideas (possible "fake" Arduino??)

mlines:
4.8volts when running from USB
6.9volts when running from external 9v supply.

USB voltage is "5.00±0.25 V (pre-3.0)" so anything from 4.75 to 5.25 should be expected.
6.9V is well above the absolute maximum (6.0V) for the processor chip so either your regulator is bad or you measured incorrectly.

I think the regulator has failed in a strange way. 9 volt supply gives just over 8 volts on the regulator input which is about right as there is a diode in the path. The output of the regulator has 6.9 volts on it :frowning:

Even onboard regulators can be 0.25volt off.
But practically, it’s <0.1volt, so 4.9-5.1volt.
I guess the DMM is not reading correctly. Common problem with cheap DMMs when the battery is almost flat.
I assume you have used a proper GND point for your negative lead.
And you didn’t measure Vin (pre regulator).

A mains power supply is best connected to the DC socket (reverse protection).
And a (9volt) battery to Vin.

A 9volt block battery shouldn’t be used to power an Uno (too weak).
Leo…

mlines:
I feed my Arduino with a regulated 9 Volts into the power socket on the UNO. I then assume the on-board 5V regulator does the rest.

However, when I measure the +5v on the header on the UNO I see....

4.8volts when running from USB
6.9volts when running from external 9v supply.

Everything seems to be working ok but this gives lots of issues when trying to measure on the A/D ports.

Any ideas (possible "fake" Arduino??)

Clearly the on-board regulator is fried and you have put the chips beyond their maximum voltage ratings
so they may be malfunctioning too.

Fortunately the Arduino itself seems to have survived as it runs various sketches ok when on USB, so it looks like the regulator has failed.

Thanks for the inputs

Martin