I just noticed that my original Arduino UNO on pin 3V3 have 4.98V output. That's odds. That pins should have 3.3V. So I measured the Vout on the voltage regulator LP2985-33DBVR where that pins is connected. And the voltages on Vout are the same of what I have measured on that pin. Does that voltage regulator need to be replaced?

1 Answer 1


I guess either the regulator has failed creating a short or the 5v rail is shorted to the 3v3 rail elsewhere. Removing the regulator and checking the 3v3 rail is no longer 5v should tell you.

The only time I broke a 3v3 regulator, it failed open, so I don't know how likely it is for a LP2985 to fail short.

You could briefly attach a small load, say a LED and 270 or 330 ohm resistor, to be more certain of the voltage reading.

  • I tested pin 3v3 and pin 5v5, but there is no continuity between them. they seems not shorted together
    – tommy
    Commented Mar 27, 2023 at 15:32
  • I removed the regulator and the 3v3 pin it's around 1,5-2v
    – tommy
    Commented Mar 27, 2023 at 16:34
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    A Uno with no shields and no connections to offboard components should have nothing connected to the output 3v3 regulator. See updated answer - though I wrote that before seeing your comment and 1.5v won't be enough to light most LEDs. I suspect your voltmeter is now seeing a meaningless floating value. Commented Mar 27, 2023 at 19:07
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    I agree about testing it with a small load since sometimes a regulator can actually show a higer voltage(typically input of the regulator(so around 5v in this case)) when no load it attached to it. essentially the voltage won't drop, and so the small spikes keep it high. Commented Mar 27, 2023 at 20:15
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    also try testing it with the arduino uno's analog ports, so get 2 resistors of the same value(for example both 1k resistors, 1 in 3.3v, 1 in gnd, and the middle in a analog port. if you read a value of around 500(half of analog range then you get a reading which keeps fluctuating, or is close to 0 then it is broken most likely, if you get a value of around 350(actually 341)(340(+-30) of 1024 range) then it should still work. Commented Mar 27, 2023 at 20:23

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