Trouble Seeing 120VAC on Oscope

[cobcobbs]

Hi,

I haven't used a scope in well over 20 years, but some recent projects got me to try out a Hantek 6022BE USB model.

As I've been using it, I wasn't seeing what I was expecting on the output of my project, so I checked the input, which is just a standard US 3 prong 120VAC wall plug.

My DMM showed 120VAC.  The scope shows, well, this (yellow is supposed to be 120VAC):


I was expecting a nice sine wave.  This looks like a clipped triangle or sine wav.  I've tried 1x and 10x on the probes, autorange and manual range, and I still get this waveform every time.  The green waveform is the calibration line on the scope and I have no problem with it or other DC measurements.

Any ideas?

[Ice-Tea]

For the love of God, please don't probe mains...

Check Daves video:

https://youtu.be/xaELqAo4kkQ

[Cliff Matthews]

Maybe get familiar with some better software too. https://github.com/OpenHantek/OpenHantek6022

BTW, A/C coupling is a problem. User Horo has devoted a lot of personal time documenting everything in a large PDF:
https://github.com/OpenHantek/OpenHantek6022/raw/main/docs/OpenHantek6022_User_Manual.pdf

Search for the 6022be/bl threads on this forum too.

[james_s]

Yikes, you should *never* try to measure the AC line without a HV differential probe. What on earth are you expecting to find there anyway? You measured with a multimeter and you know that there's 120V, what are you hoping to see? Why do people always seem to want to probe the AC mains? You can see the sine wave just by capacitive coupling by touching your finger to the end of the probe.

[bdunham7]

You really just hooked your new scope up to mains voltage?  If you haven't burned it up already, read the manual.  Apparently it is good for +/- 5V in 1X, +/-50V in 10X, so you are badly clipping the sine wave either way.  You need at least a 100X  regular or 50X differential probe to read mains voltage. 

[BrokenYugo]

Mains normally looks like a clipped sine, in this case it sounds like the protection circuitry of the scope is clipping it farther, note that the limits of a scope are generally expressed in peak voltages, not RMS, 120AC is more like 170V peak to peak. For example shown are my normal mains waveform (via the power transformer in the scope), and the crappy no AVR backup generator feeding the house for a really nasty one. Somewhere on the forum is a whole thread of mains waveforms if you want to see more. If you value your scope and your life DO NOT try to probe mains directly, even if the scope/probe can handle the voltage it isn't designed to handle the spikes and shit you can get with mains, use a differential probe or a low voltage power transformer.

[ejeffrey]

As I've been using it, I wasn't seeing what I was expecting on the output of my project, so I checked the input, which is just a standard US 3 prong 120VAC wall plug.

Everybody has covered why you shouldn't use a scope to directly measure mains voltage, but to cover your diagnostic steps: checking the inputs is a good plan, but don't go straight for the mains.  I don't know what your project is, but most hobby projects have some sort of low voltage power supply.  Check the low voltage power supply first, if it works you don't have to worry about the AC input.  Even if your project works directly with mains voltages you should be able to confine most of your diagnostics to low voltage measurements, such as measuring the coil of a relay or the control signal of an SSR.  If you have a transformer with low voltage secondaries you can safely measure the output voltage waveform with an oscilloscope.  For the most part you don't care about the mains waveform.

For the cases where you do need to measure line voltages, most of the time use a suitably rated hand-held DMM not an oscilloscope.  Other useful tools are a DMM with non-contact voltage detection and a current clamp meter.

[rstofer]

The way I read the User Manual, the scope vertical has a maximum of 5V/div with a 1x probe.  But times 8 divisions, the maximum voltage that can be displayed is 40V but the icon next to the BNC connectors shows 35V max AC + DC combined.

It's right there where you attach the probes.

These scopes just aren't made for higher voltages.

The Rigol DS1054Z has a 100V/div scale but the inputs are limited to 300 Vrms

[james_s]

Technically with a 10X probe 35Vrms max is 350Vrms at the probe tip, but I would still NEVER try to probe such high voltages with an ordinary scope probe. A differential probe is essential.

[rstofer]

As a side issue, I would want to protect my laptop or PC.  I might run the scope through a powered USB hub and I might also consider a USB isolator.  I would want USB 3.0 speed so something like this:

https://www.amazon.com/Intona-SuperSpeed-Isolator-Protection-Supports/dp/B089YB3XQM

In many ways, this scope is a lot like the Digilent Analog Discovery 2 which I use a LOT.  But I don't use it at 'mains' level and rarely over 5V.  Even though I have a set of 10x fixed probes specifically for the purpose, I don't use them to gain voltage range.

[rstofer]

Technically with a 10X probe 35Vrms max is 350Vrms at the probe tip, but I would still NEVER try to probe such high voltages with an ordinary scope probe. A differential probe is essential.

Yes, of course!  But what do you want to bet the range is switchable?  Which means that, inevitably, high voltage will be measured with the X1 range.

As I said above, I bought a pair of fixed X10 probes for my Analog Discovery 2.  I just couldn't afford to smoke the thing at the new and improved price.

At least the DS1054Z can handle a reasonably high input level even with the X1 probes.

[Jwillis]

rstofer Is correct about the input range .  Maximum input is ±5V with 35V pk protection . 120V service goes well beyond the measurable range at 10:1 (50V) .
Putting aside the potential saftey issues  for a moment  , you would need at least a 200:1 probe to measure voltages that high with the 6022BE .
If you want to see the sine wave of mains AC use a transformer and look at the secondary side . You don't really need the accuracy of a differential probe for that  . Any anomalies will appear just as well and is much safer for you and your equipment . 
As mentioned here already the AC wave from mains will not be a perfect sine . There will be noise and deformation of the wave coming from a variety of sources  from your home , service lines ,sub stations , other customers and environmental . 
If noise from Mains is problem there are suppression filters available  . If you use a lot of inductive loads that adding reactive power charges to your bill you can use power factor correction . But in most household conditions neither of these are really necessary or any benefit .
So in a lot of ways theirs little to see and not much you can do about it .

[james_s]

Yes I should have mentioned that, for low frequency work an ordinary iron core power transformer makes a reasonable poor man's HV differential probe. You can use any transformer with a 120V or 240V primary and a secondary voltage within the range your scope can handle. It will even work at higher frequencies to a point, and it should provide a reasonably accurate result.