Using a scope to test USB 3.0 of my laptop I read 12V instead of 5V

Thread Starter

FranceBB

Joined Apr 16, 2021
5
Hi there!
I was really happy to get my very first scope and I was measuring things on my laptop, but I'm an idiot and I might have got something wrong. My laptop is NOT grounded as it's connected to the power supply via a charger which doesn't have ground, just phase and neutral. I set the scope to DC Voltage and I connected the probe to the USB 3.0, expecting to see 5V but I saw 12V instead, so I got really scared and I immediately removed the probe. Then I tried it on the USB C on the side of the laptop and it was showing 12V, same goes for the HDMI and the Display Port, so I was like: Crap, something is wrong.

What I think happened is that the laptop was floating and when I connected the probe to the USB it found a direct path to ground so it was shorting the power supply of my laptop to ground. I removed the probe, rebooted the laptop and everything was working fine. I've been using the USB port for a few hours and it's working just fine, so... what happened? Why was I seeing 12V? Was I harming the ports and I was just lucky that they probably have overcurrent/overvoltage protection built-in?

Thank you in advance,
Frank
 

MisterBill2

Joined Jan 23, 2018
8,772
It was probably an adjustment error, or it was thinking that you were measuring AC and giving the peak value. AND ticking a probe into a USB connector is a bad choice,on a portable computer if you damage that connector it is very complex to repair. Not a smart choice at all.
 

MisterBill2

Joined Jan 23, 2018
8,772
MC is right!!! The various connectors on a laptop are quite easy to damage and also the circuits connected to them are a bit sensitive to short circuits.
An safe interesting source of signals to examine is a small transistor radio. which has the added advantage of being an isolated source with no shock hazard.
 

SamR

Joined Mar 19, 2019
3,410
PS The O'scope is not the instrument to accurately measure voltage with. Your multimeter is much more suitable.
 

MrChips

Joined Oct 2, 2009
23,535
Build a 555-timer circuit in astable mode on a breadboard and use your oscilloscope to probe all the IC pins.
This would be a much safer sandbox to play around with your oscilloscope.
Change the value of the timing capacitor C to orders of magnitude lower values. This will increase the oscillator frequency for viewing on the oscilloscope.

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BobTPH

Joined Jun 5, 2013
3,316
Most scopes have a variable gain knob. It has to be in the detent position to get a reading based on the range you have selected. I suspect the gain was turned up.

Bob
 

Thread Starter

FranceBB

Joined Apr 16, 2021
5
Thanks to you all for the comments.
Yep, I value the laptop and I guess I'm gonna be better off with a breadboard before I use the scope for anything real, especially 'cause it's directly connected to ground and bad thing can happen with ground referenced circuits, but thankfully the laptop wasn't. Anyway it's better for me to get used to it on a breadboard before I mess something up, so I'll try to get familiar with it.
About the laptop, everything is working just fine. I guess you're right, it probably thought I was in AC somehow and picked up the peak voltage for whatever reason or maybe it was adjusting, I don't know. Anyway, everything is working fine, USB works, HDMI and Display Port work, so I won't mess around with it any longer.
 

BobaMosfet

Joined Jul 1, 2009
1,776
@FranceBB Value your Scope as well. Understand that a scope does one thing- it measures voltage in relationship to its ground. The probes are the interface between the scope and the world, so you also need to understand exactly what you probes do for you... and cannot do for you.

In a DC circuit, you should connect both the probe to (+), and the ground-lead on the probe (or the other probe) to (-) ground pin on USB or any other connection. Do not use your scope on AC at all until you are really familiar with its use, how it grounds itself, and so forth- DC is easy, just be sensible, but AC can mess you up if you don't really grasp current moving in both directions in AC (and shorting through ground, issues).

If you did not have a ground properly connected to the signal source, your scope may have internally limited the voltage level to 12V, as a self-protection measure.

WARNING/REMEMBER: Voltage measured is NOT voltage existing- it's voltage dropped (or _run through_) the measuring instrument. In other words, the voltage necessary to overcome the resistance/impedance of the measuring device's circuit to get current to flow through it. It's critical to understand this. Voltage and current never operate independently of one another unless manipulated to do so, and their relationship is inverse. One gives so the other can take, and vice versa, according to Ohm's Law, because Ohm's law is a circle-formula that describes a ratiometric relationship between the two.
 

MisterBill2

Joined Jan 23, 2018
8,772
One more thing to understand about oscilloscopes is that often, but not always, they are used with attenuator probes that normally provide a 10:1 attenuation. That can cause confusion if you are not aware of it.
 
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