# \[Bug?\] Arduino Mega 5V Pin Shows 14V Over USB — No Heating, Fully Functional **URL:** \ **Category:** General Electronics\ **Created:** [May 29, 2025, 3:04pm UTC](https://forum.arduino.cc/t/bug-arduino-mega-5v-pin-shows-14v-over-usb-no-heating-fully-functional/1385495 "2025-05-29T15:04:39Z")\ **Posts on this page:** 17\ **Page:** 1
**Author:** ![kshitij\_kulkarni](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/kshitij_kulkarni/32/1219792_2.png) [@kshitij\_kulkarni](https://forum.arduino.cc/u/kshitij_kulkarni)\ **Post date:** [May 29, 2025, 3:04pm UTC](https://forum.arduino.cc/t/bug-arduino-mega-5v-pin-shows-14v-over-usb-no-heating-fully-functional/1385495/1 "2025-05-29T15:04:39Z")
Hi everyone, I'm seeing a bizarre electrical behavior and would love your input. * * * ## Setup - **Board** : Arduino Mega 2560 (Also tried with an Arduino Uno R3, both clones) - **Connection** : USB to: - Raspberry Pi (with official power adapter) - Laptop (tested both plugged in and on battery) - **Multimeter** : Verified with fresh batteries. - **Wiring** : Absolutely _nothing_ connected to the Arduino except the USB cable * * * ## The Problem I'm consistently measuring **10V to 16V** across the adjacent **5V and GND** pins of the Arduino — even when: - Only USB is connected - Nothing is connected to any other pins - Multiple USB cables and ports are used - Host device is changed (Raspberry Pi ↔ Laptop) - Board is swapped out (tried a Mega and an Uno) * * * ## The Weird Part Despite the absurd voltages: - **Everything works** perfectly: - Sketches upload normally - Serial communication works - I/O is responsive - **Nothing gets hot** : - USB ports - Voltage regulators - Microcontroller - **Voltage is _real_** , not phantom: - Connected a **12V PC fan** to 5V and GND — _it spun up normally_ - Connected a **100Ω resistor** — _it heated up_, voltage stayed at ~14V * * * ## Tests Performed | Test | Result | | --- | --- | | Changed host device (Pi → laptop) | No change | | Changed Arduino boards | No change | | Measured all 5V headers on board | Same ~14V | | Loaded rail with fan + resistor | Still high voltage | | Checked for heating anywhere | None observed | * * * ## What could be causing this? - Rogue **boost converter** or backfed **VIN path**? - Faulty or floating **voltage regulator?** - Some insane **USB PD negotiation bug** (especially via USB-C)? - Ground reference weirdness? - Straight-up cursed hardware? I'm genuinely confused — I've tried every sanity check I can think of. If I hadn't seen it with a load (and multiple boards), I'd dismiss it as multimeter error or phantom voltage. But here we are. * * * ## 🧵 Should I Ignore It? - Functionally, **everything is working fine** - No signs of hardware stress or damage - But the electrical behavior is... **physically impossible** Has anyone ever seen this before? I’d rather not ignore a potential silent failure or hidden electrical gremlin down the line. Any advice, insights, or similar experiences would be greatly appreciated. Thanks! **— Kshitij** ---
**Author:** ![van\_der\_decken](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/van_der_decken/32/1006976_2.png) [@van\_der\_decken](https://forum.arduino.cc/u/van_der_decken)\ **Post date:** [May 29, 2025, 3:13pm UTC](https://forum.arduino.cc/t/bug-arduino-mega-5v-pin-shows-14v-over-usb-no-heating-fully-functional/1385495/2 "2025-05-29T15:13:55Z")
A Mega and and an Uno spontaneously generating 14V? As the saying goes, extraordinary claims require extraordinary evidence. You have not provided any such evidence. ---
**Author:** ![jim-p](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/jim-p/32/922840_2.png) [@jim-p](https://forum.arduino.cc/u/jim-p)\ **Post date:** [May 29, 2025, 3:15pm UTC](https://forum.arduino.cc/t/bug-arduino-mega-5v-pin-shows-14v-over-usb-no-heating-fully-functional/1385495/3 "2025-05-29T15:15:49Z")
> [@kshitij\_kulkarni](#): > > tried with an Arduino Uno R3, both clones) Just not possible! I'd say your meter is broken. Do you have a different meter or a 9V battery or AA battery you could measure in order to test the meter? ---
**Author:** ![anon94801460](https://avatars.discourse-cdn.com/v4/letter/a/278dde/32.png) [@anon94801460](https://forum.arduino.cc/u/anon94801460)\ **Post date:** [May 29, 2025, 3:21pm UTC](https://forum.arduino.cc/t/bug-arduino-mega-5v-pin-shows-14v-over-usb-no-heating-fully-functional/1385495/4 "2025-05-29T15:21:56Z")
Did a chatbot write your post? The unusual formatting and distracting, excessive use of bold face type suggests so. ---
**Author:** ![kshitij\_kulkarni](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/kshitij_kulkarni/32/1219792_2.png) [@kshitij\_kulkarni](https://forum.arduino.cc/u/kshitij_kulkarni)\ **Post date:** [May 29, 2025, 3:28pm UTC](https://forum.arduino.cc/t/bug-arduino-mega-5v-pin-shows-14v-over-usb-no-heating-fully-functional/1385495/5 "2025-05-29T15:28:57Z")
Tried ChatGPT to help figure out what went wrong to see if maybe I was missing something. I didn't want to type the whole story again so asked it to summarise. I have thoroughly examined the post and corrected errors, dw. ---
**Author:** ![sonofcy](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/sonofcy/32/1283617_2.png) [@sonofcy](https://forum.arduino.cc/u/sonofcy)\ **Post date:** [May 29, 2025, 3:34pm UTC](https://forum.arduino.cc/t/bug-arduino-mega-5v-pin-shows-14v-over-usb-no-heating-fully-functional/1385495/6 "2025-05-29T15:34:07Z")
Either a faulty meter or a faulty procedure. The boards do NOT ever create more than 3.3VDC or 5VDC, depending on the board and USB, regardless of whether C and/or PD are present, will not produce more than 5.1VDC without PD hardware. ---
**Author:** ![kshitij\_kulkarni](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/kshitij_kulkarni/32/1219792_2.png) [@kshitij\_kulkarni](https://forum.arduino.cc/u/kshitij_kulkarni)\ **Post date:** [May 29, 2025, 3:41pm UTC](https://forum.arduino.cc/t/bug-arduino-mega-5v-pin-shows-14v-over-usb-no-heating-fully-functional/1385495/7 "2025-05-29T15:41:34Z")
You were right. When I tried to probe across a 1.5V AA cell, the meter read 3.72V. I didn't think the meter was faulty earlier because the 12V fan started up effortlessly when connected it. Maybe the fan starts up at voltage much lower than 12v. Anyways, thanks 🙂 ![image](https://europe1.discourse-cdn.com/arduino/original/4X/3/0/1/3019d10838725df80445df1c313ba6b2fb0d7f58.jpeg) ![image](https://europe1.discourse-cdn.com/arduino/original/4X/2/1/0/210392f3bdb40c723e7bfe5ba36b020796a2c7ce.jpeg) ---
**Author:** ![kshitij\_kulkarni](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/kshitij_kulkarni/32/1219792_2.png) [@kshitij\_kulkarni](https://forum.arduino.cc/u/kshitij_kulkarni)\ **Post date:** [May 29, 2025, 3:43pm UTC](https://forum.arduino.cc/t/bug-arduino-mega-5v-pin-shows-14v-over-usb-no-heating-fully-functional/1385495/8 "2025-05-29T15:43:06Z")
![image](https://europe1.discourse-cdn.com/arduino/original/4X/1/f/6/1f6d902074ddb75cf853894bd22315fc12c81b5d.jpeg) Arduino mega not connected to anything ![image](https://europe1.discourse-cdn.com/arduino/original/4X/f/6/3/f63e0f7d7d0e7d8fcef53fc1e6bc706d1bdf70d6.jpeg) Connecting two wires. ![image](https://europe1.discourse-cdn.com/arduino/original/4X/c/0/8/c08ca07b77fdec45fab185b4276893cf062a6f83.jpeg) Measuring a significant voltage ![image](https://europe1.discourse-cdn.com/arduino/original/4X/d/a/d/dada38a940e2df23f66e9d760536584229276891.jpeg) ---
**Author:** ![van\_der\_decken](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/van_der_decken/32/1006976_2.png) [@van\_der\_decken](https://forum.arduino.cc/u/van_der_decken)\ **Post date:** [May 29, 2025, 3:43pm UTC](https://forum.arduino.cc/t/bug-arduino-mega-5v-pin-shows-14v-over-usb-no-heating-fully-functional/1385495/9 "2025-05-29T15:43:37Z")
> [@kshitij\_kulkarni](#): > > **Multimeter** : Verified with fresh batteries. I guess the chatbot added that bit, because clearly, you didn't. ---
**Author:** ![jim-p](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/jim-p/32/922840_2.png) [@jim-p](https://forum.arduino.cc/u/jim-p)\ **Post date:** [May 29, 2025, 4:05pm UTC](https://forum.arduino.cc/t/bug-arduino-mega-5v-pin-shows-14v-over-usb-no-heating-fully-functional/1385495/10 "2025-05-29T16:05:58Z")
> [@kshitij\_kulkarni](#): > > 1.5V AA cell, the meter read 3.72V. So it multiplied the actual voltage by about 2.5 (3.72/1.5) 2.5 x 5V = 12.5V Makes sense. ---
**Author:** ![kshitij\_kulkarni](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/kshitij_kulkarni/32/1219792_2.png) [@kshitij\_kulkarni](https://forum.arduino.cc/u/kshitij_kulkarni)\ **Post date:** [May 29, 2025, 4:24pm UTC](https://forum.arduino.cc/t/bug-arduino-mega-5v-pin-shows-14v-over-usb-no-heating-fully-functional/1385495/11 "2025-05-29T16:24:56Z")
Heh no by that I meant that the reading persisted after replacing the batteries of the meter. I can see how it could be misleading. Thanks ---
**Author:** ![BitSeeker](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/bitseeker/32/1241632_2.png) [@BitSeeker](https://forum.arduino.cc/u/BitSeeker)\ **Post date:** [May 29, 2025, 4:38pm UTC](https://forum.arduino.cc/t/bug-arduino-mega-5v-pin-shows-14v-over-usb-no-heating-fully-functional/1385495/12 "2025-05-29T16:38:13Z")
Just goes to show that reading real volts requires a real Fluke.... 😉 Seriously though, it might just be dirty tracks on the rotary control. I think a faulty MOV would cause a low reading rather than high. Occasionally a capacitor can spill its guts (electrolyte) which damages or shorts adjacent tracks. The meter might still be salvageable. Fans can indeed start spinning at much lower than the rated voltage and often spin quietly although it should reach max designed speed and airflow at the rated voltage. ---
**Author:** ![kshitij\_kulkarni](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/kshitij_kulkarni/32/1219792_2.png) [@kshitij\_kulkarni](https://forum.arduino.cc/u/kshitij_kulkarni)\ **Post date:** [May 30, 2025, 1:55am UTC](https://forum.arduino.cc/t/bug-arduino-mega-5v-pin-shows-14v-over-usb-no-heating-fully-functional/1385495/13 "2025-05-30T01:55:30Z")
The meter had undergone a battery leak recently and I'd cleaned out the contacts using a hard-water-stain-removing solvent which had worked remarkably well. Maybe I underestimated the extent of corrosion. Besides, when I tried to measure the resistance of a 100 Ohm resistor, I get a reading of 115-120 Ohm. I am certain that my resistor has a tolerance of not more than 10%. I was thinking that maybe some part of the circuitry that deals with the AnalogReference voltage might have had a failure. The fact that the failure was sudden and not gradual is noteworthy. I took some measurements 12 hours before this and it was all absolutely fine. That's another reason why I didn't suspect it was a meter-issue initially. One day I will be able to afford a fluke 🙂 ---
**Author:** ![Wawa](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/wawa/32/1211303_2.png) [@Wawa](https://forum.arduino.cc/u/Wawa)\ **Post date:** [May 30, 2025, 5:26am UTC](https://forum.arduino.cc/t/bug-arduino-mega-5v-pin-shows-14v-over-usb-no-heating-fully-functional/1385495/14 "2025-05-30T05:26:27Z")
> [@kshitij\_kulkarni](#): > > The meter had undergone a battery leak A leaking battery in a toy can be fixed, but not in a precision instrument. > [@kshitij\_kulkarni](#): > > One day I will be able to afford a fluke I have been a consumer electronics tech for more than half a century an never owned/needed a Fluke. If you spend at least US$50 on a DMM then you can't go wrong. Currently I wouldn't spend any money on an American business anyway. Leo.. ---
**Author:** ![BitSeeker](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/bitseeker/32/1241632_2.png) [@BitSeeker](https://forum.arduino.cc/u/BitSeeker)\ **Post date:** [May 30, 2025, 6:51am UTC](https://forum.arduino.cc/t/bug-arduino-mega-5v-pin-shows-14v-over-usb-no-heating-fully-functional/1385495/15 "2025-05-30T06:51:34Z")
There is nothing wrong with the alternative cheaper brands of meters. Even the humble DT830 does an adequate job for low voltage applications and hobby work. However, for something a bit more long term and serious as a replacement I would probably spend a little more as suggested by Wawa. On the other hand, a Fluke would be overkill just for working with Arduinos and low voltage projects, so please don't feel that you need to go spending that kind of money. I was only having a bit of a joke due to the lookalike appearance of the meter shown in the picture. Its just unfortunate about the battery leak. The corrosion caused by the gunge that leaks from batteries can have unpredictable consequences as if it leaks inside it can cause damage to tracks on the PCB and to components. It could of course also be that the meter has co-incidentally suffered another fault. ---
**Author:** ![mikedb](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/mikedb/32/1098613_2.png) [@mikedb](https://forum.arduino.cc/u/mikedb)\ **Post date:** [May 30, 2025, 9:07am UTC](https://forum.arduino.cc/t/bug-arduino-mega-5v-pin-shows-14v-over-usb-no-heating-fully-functional/1385495/16 "2025-05-30T09:07:30Z")
> [@Wawa](#): > > I have been a consumer electronics tech for more than half a century an never owned/needed a Fluke. I agree , the only Fluke I owned , the Liquid Crystal start to go black around the LCD and slowly growing to the center of the LCD. ---
**Author:** ![system](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/system/32/1140315_2.png) [@system](https://forum.arduino.cc/u/system)\ **Post date:** [November 26, 2025, 9:07am UTC](https://forum.arduino.cc/t/bug-arduino-mega-5v-pin-shows-14v-over-usb-no-heating-fully-functional/1385495/17 "2025-11-26T09:07:45Z")
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