Arduino UNO only supplys 3.9?V

Hi folks!

I’m experiencing the weird effect, that my Arduino UNO R3 suddenly only supplys about 3.9?V from its 5?V header pin. Also the digital HIGH on the other pins is only about 3.9?V.

Could it be that I fried something on the board or on the ATmega?

To be more specific: when the Arduino is powered from USB with 4.6?V the output is only about 3.8?V – 4.1?V max. When I power it from an external supply via the Vin pin and GND, there is the same effect: the output on the 5?V pin and all digital pins repectively seems to be about 0.9?V – 1.0?V below the input voltage.

I’m not really sure what has happened, but I have the feeling, that two resistor legs accidentially shorted on my breadboard arragement for half a sec and that fed back to the Arduino. It’s the only explanation I have.

The question are:
Is anyone familiar with this kind of “undervoltage effect” I described?

Can someone please confirm, that an USB powered Arduino really does supply 4.5?V from its pins – not 3.9?V?? (For a reference: my Mini Pro compatible does supply on its pins quite exactly the voltage I feed in.)

I’m pretty sure that I have a fault here. What could it be? How to trouble shoot from here to find out what it is?

Thanks a lot!
Darkwing

You are very likely trying to pass too much current through the board / processor.

Darkwing: that two resistor legs accidentially shorted on my breadboard arragement for half a sec and that fed back to the Arduino.

No, nothing got "fed back."

However, depending on what those resistors were connected to, it could have caused a short circuit.

Darkwing: when the Arduino is powered from USB with 4.6?V

The minimum for USB is 4.75v. What's the source? A computer or an external 5V supply?

What else is connected to your Arduino.

Check your Multimeter first! - Usually a low battery makes the meter read too low.

// Per.

Darkwing: Could it be that I fried something on the board or on the ATmega?

Not if it's powered via USB.

Darkwing: Can someone please confirm, that an USB powered Arduino really does supply 4.5?V from its pins

I hope nobody can. USB is supposed to be 5.0V.

Darkwing: I’m pretty sure that I have a fault here. What could it be? How to trouble shoot from here to find out what it is?

Step one: Check your measuring device.

Since the USB alone is significantly higher voltage than when Arduino is connected
you are trying to draw too much current. What’s on that breadboard?

Did the polyfuse get hot BTW? They take a few minutes to recover from over-current.

Hi!

Thanks for the response y’all!
There are a couple of things mentioned to address, I will go through it step by step. I hope you’re prepared for the plenty of text you demanded for.?:wink:

  1. The Meter Question
    Since almost everyone bought into the idea that my meter is wrong, I feel kind of forced into ‘proving’ to you first, that my meter is right. Not only that it is completly new (ok, that’s no argument), it also measures what it is supposed to measure. See for yourself:

Measuring a litte power supply with a built-in display: 5?V

Measuring another litte power supply with built-in display: 12?V

I hope that little countercheck convinces you, that my meter is not way off the track, but quite accurate.
(Btw: It’s really complicated to take pictures of taking measurements?…?^^)

  1. The USB Confusion
    Ok, here is something I actually researched and learned because of your questioning: the voltage on my USB hub powering the Arduino in this case is actually really 4.6?V. It’s a hub connected to a hub connected to the PC. On the PC the voltage is 4.99?V – 5.04?V, on the first hub it is 4.79?V and on the second hub it then is 4.68?V.

Ok, fair enough – but if I understood this quite correctly, that’s well within the spec in this type of situation. So nothing’s wrong with the USB power (and the meter, as you see again?;)?). One could argue, that this is not the most fortunate constellation. Yes – and since I now know about that, I’ll see if I can change this in the future. But anyway this doesn’t really matter with respect to the problem I described – as will become more clear in a few seconds.

So: maybe I initially confused some people with „USB is 4.6?V“ –?but now that’s settled, I hope.

  1. Measuring Arduino output
    Now that we covered the basics and cleaned up misunderstanding, let’s move on to the real topic.

Instead of text, I again will present to you pictures of measurements I took, so that anyone can see for oneself, what am I measuring and what I’m talking about. Let’s go:

Measuring the pin output of an Arduino compatible Mini Pro powered with 5?V from the power supply you saw before:

You see, that it sais round about 5?V again.

Measuring the pin output on ‘5?V’ of the original Arduino UNO R3 powered with the same 5?V power supply via Vin:

You see, that there is a quite significant drop. I don’t think that’s normal. Can anyone confirm this?

Measuring the pin output in ‘3.3?V’ of the original Arduino UNO R3 still powered with the same 5?V power supply via Vin:

You see that it sais round about 3.3?V – just like one qould expect. I this that’s normal. Can anyone confirm?

And I hope you can clearly see, that I’m taking measurements while nothing is connected to the Arduino. Nothing draws ’too much current‘. It’s just the available voltage. And as I see it, a voltage drop out on the pins of the blue down to 4.10?V is not normal. Or is it? Can anyone please confirm?

And hopefully to erase all doubts in advance, I took a measurement on a little real world example: powering an LED from the Arduino and measuring the voltage drop across that arrangement:

I hope you can see clearly that it sais ‘not round about 5?V’. I thinks that not normal. Can someone please reenact if neccessary and confirm?

Thank you very much!

I don’t know if it is significant, but it seems that something ’has recovered‘ from around 3.9?V up to 4.1?V. Or is it just my imagination?…

  1. The Breadboard Project
    In fact, I don’t think it’s relevant at the moment, but since you’re curious of course, I want to show you the breadboard and the assumed spot where – I’m not really sure! – the two contacts touched and maybe (!) fried something on the Arduino.

(Bigger version in the attachements.)

If I recall the situation right, I saw these two resistor legs in the circle touching each other for one moment, and so connecting the 12?V collector of the BC639 to its 5?V base. There was a short on the 12?V power supply, because I recall that it ‘rebooted’. Also it seems that the top side of the D-type flip-flop (SN74HCT74Naa) got fried: almost all pins are shorted providing VCCand some HIGH levels are only 1.8?V since then. That’s why I now connected the lower side (it’s a dual flip-flop) – and this works.

So, now you know everything. :wink:

What’s to say about shorting the collector to the base? Could such a short be a plausible reason for my ‘undervoltage problem’?? If yes: what could be damaged and is it repairable? I hope you can see the wiring clearly; maybe a foreign breadboard looks confusing at first glace, but it’s actually not very complicated. :wink:

Thanks for reading, thanks for participating!
Darkwing!

Ahh, i see the problem now.

You cannot power the Arduino from the VIN-pin or the DC-jack UNLESS you supply it with 7,5V or higher.

If you need to power your Arduino from a 5V Supply (Say, USB) - You need to connect that 5V supply to the 5V-pin.

Problem solved.

// Per.

Really?…? I spent the last three hours writing this post to get that information?! :astonished: DANG! :grin: Now I call that learning –?I’m sure I’ll remember this for all my life. ^^ gmpflmhmpl?…

Before you ask: I did read the product information (some weeks ago?…) ^^ I did now again.

So I can use the Vin, the 5?V and the 3.3?V pins as either inputs or outputs if required. Thanks for pointing that out (again). Silly me – it’s all in the description. ^^

But, well, me powering Vin with a 5?V supply is new – it wasn’t always the case. I did that just randomly today, when I composed the pictures –?not knowing that 7.5?V fact, I know now since your post. 7.5?V at Vin results almost in spot on 5?V on the output pins. So concerning this: problem solved.

I also just rearranged the cables in my room and I’m now powering the Arduino directly via a 5.05?V supplying USB port from my PC. The output voltage on the Arduino pins reaches a value of 4.56?V. The crucial question: is now this ok? Can you confirm?

I noticed: when powering with 7.5 ?V via Vin pin or when powering with 5?V via the 5?V pin, I receive an initial short in the power supply (it has short protection and resets itself). But the short is just for a jiffy, then it’s back to normal and seems to work. Is thisnormal, do you encounter the same effect??

I never checked this before –?but now, since the assumed incident, I am suspicious:
The ATmega, the polyfuse and the surrounding components are getting unusually warm, not to say hot, without the Arduino being connected to anything, just running a quite idle blink program. The polyfuse is at 65 °C and rising after about ten minutes. Also the little 7CE50Z (must be the voltage regulator) besides the DC power jack raised almost up to 80 °C! The question: is this normal?

I don’t think so. It even smells warm, if you know what I mean. I find this quite alarming! What can be wrong?

I switched it off now. I will try, let’s say tomorrow, powering it just via USB and letting it run an idle blink program for an hour. I’ll measure the temperature and post the finding.

Meanwhile, if anyone has any ideas: please let me know.

Thanks
Darkwing!

Darkwing:
I noticed: when powering with 7.5 ?V via Vin pin or when powering with 5?V via the 5?V pin, I receive an initial short in the power supply

Not at all normal.

Darkwing:
The ATmega, the polyfuse and the surrounding components are getting unusually warm, not to say hot, without the Arduino being connected to anything, just running a quite idle blink program.

The polyfuse is only “in circuit” when powered by USB. If powered by Vin/Barrel jack, the Polyfuse isn’t used. It is possible the nearby regulator is warm.

If the ATmega328 is warm, you like have a damaged ATmega328.

Darkwing:
The polyfuse is at 65 °C and rising … almost up to 80 °C! The question: is this normal?

Measured with what?

Regulators are designed to get hot and depend entirely on input voltage and current running through them.

News!

I couldn’t wait and took a measurement this morning. ;) I ran an idle blink program and concentrated on the polyfuse:

24?°C ambient 40?°C polyfuse after 45?minutes 44?°C Atmega after 45 minutes.

I measured with my multimeter of course; it has temperature measuring via such a little K-type probe, I’m sure you know these. It’s fairly accurate – and even if it wasn’t and had an accuracy of ±?10?K, I’d still be able to tell, that the following is a problem:

By chance I then touched the little chip besides the oscillator – mine sais ‘2CTI 5R5’:

I flinched surprisedly and guess what: it’s at 96?°C?! Now I’d call that more than serious.

It drops immediately back to about 30?°C when unplugging the power supply (USB). And it raises immediately (45?sec.) up to 90?°C; and we know it stays about there (45?min., 96?°C).

I didn’t notice that yesterday, but now it’s quite obvious that the high temperatures of all the surrounding components are from this little thing, radiating a roundhouse kick of heat.?;)

[u]So, the questionnaire:[/u]

  • What kind of a component is that? (opto-coupler?)
  • Is it plausible that the damage there was caused by some kind of short on the breadboard, like I described in point 4 above?
  • Is it plausible that this component is the source of the voltage problems I described earlier (4,56?V out of 5.05?V USB power), or is it itself just a symptom?
  • Anyone experience with repairing such a thing? (Or should I just accept that I officially fried my Arduino??^^)
  • Or: is this normal?

Thanks!

the circled component is the dual op-amp used in the voltage selection circuit. It should NOT be getting hot!

Darkwing: Hi folks!

I’m experiencing the weird effect, that my Arduino UNO R3 suddenly only supplys about 3.9?V from its 5?V header pin. Also the digital HIGH on the other pins is only about 3.9?V.

Here's what I see on my board (USB power only, nothing else connected, UNO R3):

Directly from the USB V+ pin: 4.96v 5V pin: 4.93v 3.3v pin: 3.31v Pin 8 (digitalWrite HIGH): 4.91v

Sounds like your voltmeter is out of calibration (or maybe a weak battery?)

(Why does everyone think that my meter is out of calibration? I’ve proven that most likely it is not. Or is this some kind of running gag?)

Thanks for the answers, and for confirming the voltages!

Anyway, I now was able to find out that it is an ‘LMV358IDGKR’ by examining the EAGLE file.

Well, ok, seems like there will be some SMD soldering action in the near future. Let’s see if I can manage this.??:wink:

Thanks so far!

When I power it from an external supply via the Vin pin and GND, there is the same effect: the output on the 5?V pin and all digital pins repectively seems to be about 0.9?V – 1.0?V below the input voltage.

You would totally expect this to happen, that is the way regulators work.

westfw: the circled component is the dual op-amp used in the voltage selection circuit. It should NOT be getting hot!

Correct. The second op-amp in this IC is used to buffer the pin 13 led in the Uno rev 3 version of the board. This symptom of a very hot running op-amp was a somewhat frequent problem with several new users about a year ago or so (maybe sooner or later, I'm old and time has puzzled me for a while now). I suspect there could of been a bad batch of op-amps that got into the arduino production stream for awhile. The only other explanation of a possible cause is if a voltage higher then +5vdc had ever been wired into the shield pin 13 it could have damaged the opamp, but the fact that several users reported the same symptom around the same time makes me favor my first guess.

Thanks for that interesting information!

Can you do me a favour? Could you please read my big picturesque post here at point 4 (scroll down a bit) and give me your opinion whether it could be really my fault and I fried the board/op-amp, and the assumed accident really did happend on my breadboard?

Even though I don’t ‘favor’ your second guess, I tend to believe that this is the right one.?;)

THX!

Darkwing:
Thanks for that interesting information!

Can you do me a favour? Could you please read my big picturesque post here at point 4 (scroll down a bit) and give me your opinion whether it could be really my fault and I fried the board/op-amp, and the assumed accident really did happend on my breadboard?

Even though I don’t ‘favor’ your second guess, I tend to believe that this is the right one.?:wink:

THX!

Well if it is the second guess, then live and learn. It’s happened to all of us at one time or another and will happen again. A loose wire, a miswired placement, or just a brain fart is all it takes to let the magic smoke out.

It’s all about having fun with this hobby, so as long as you didn’t bleed all over yourself you should be good to go.

retrolefty: Well if it is the second guess, then live and learn. It's happened to all of us at one time or another and will happen again. A loose wire, a miswired placement, or just a brain fart is all it takes to let the magic smoke out.

It's all about having fun.....

An engineering friend of mine calls it "learning by burning".