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Posted: 6/24/2021 7:04:27 AM EDT
Per advice I received from you guys, I finally broke down and bought an oscilloscope. This one was on sale for Prime Day and I picked it up:  Scope

I have a Wen GN400i inverter. THD should be around 1.2%. For some reason, I have been unable to get a stable reading of Hz using a DMM. The reading jumps all over the place. That was not expected, given that the Wen is an inverter and should be producing a clean signal.

Now that I have the scope and it's calibrated, I don't know how to connect it to the generator in order to capture the waveform. The videos I've seen, the guys use waveform generators. I did find a video of a guy using some sort of electrical plug. It looked like he was clipping the probe from Channel 1 into one of the blades of the plug. It looked like some kind of diagnostic plug. It was hard to tell. Can I just use something like a surge protector and plug it into the generator outlet? Or a bonded-neutral plug? I'm new to all of this, so it's a bit alien to me.

Once I get a snapshot of the waveform, I was going to post it here to see if something weird is going on with the Wen power output.
Link Posted: 6/24/2021 1:53:29 PM EDT
[Last Edit: 6/24/2021 1:53:49 PM EDT by pavil58ar]
Not going to comment on the scope as I don't know the answer. But for the inverter generator I wonder if you need a load on it, even a small one, to stabilize the output? Just a WAG...
Link Posted: 6/24/2021 1:57:38 PM EDT
do not under any circumstances connect the scope probes directly to the the generator/inverter output.
you can damage the scope and damage yourself.
Link Posted: 6/24/2021 6:00:51 PM EDT
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By ar-jedi:
do not under any circumstances connect the scope probes directly to the the generator/inverter output.
you can damage the scope and damage yourself.
View Quote


This x1000.  The alligator clip dangling from the scope's probe is connected to AC mains safety ground.
Link Posted: 6/24/2021 7:03:24 PM EDT
[Last Edit: 6/24/2021 7:03:53 PM EDT by Skibane]
The simplest solution is to connect the generator to the 120 VAC terminals of a 16 VAC doorbell transformer, and then connect the scope to the transformer's 16 VAC output terminals.

The transformer serves two purposes: It reduces the generator's AC output to a voltage the scope can safely handle, and it prevents ground loops (i.e., isolates the generator's neutral connection from the scope's ground connection).
Link Posted: 6/24/2021 8:00:33 PM EDT
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By ar-jedi:
do not under any circumstances connect the scope probes directly to the the generator/inverter output.
you can damage the scope and damage yourself.
View Quote


Really?    I used to do this all of the time with the scope that we have at work.   I should try to find the specs on that one to see what the input voltage is rated for.  I am not sure that I will be able to Google it.   That scope was old when I started working there over twenty-five years ago.....

But hooking it up to a smaller transformer is a good idea.   Step the voltage down to a safer level without sacrificing the wave form.  
Link Posted: 6/27/2021 4:00:03 PM EDT
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By Skibane:
The simplest solution is to connect the generator to the 120 VAC terminals of a 16 VAC doorbell transformer, and then connect the scope to the transformer's 16 VAC output terminals.

The transformer serves two purposes: It reduces the generator's AC output to a voltage the scope can safely handle, and it prevents ground loops (i.e., isolates the generator's neutral connection from the scope's ground connection).
View Quote

Just make damn sure you connect the transformer up the correct direction
120 to 16 VAC transformer becomes a 120 to 900 VAC transformer if connected backwards.

The best thing to do is just go buy a 10X probe. This will bring the 120VAC down to 12VAC and make it generally safe for the scope input.
Link Posted: 6/27/2021 8:27:29 PM EDT
Thanks, guys. I wasn't sure what I was doing and I wanted to make sure I didn't fry my scope, or myself. I was able to attach the plug to a spare USB a/c charger and get my readings. I encountered a strange issue that I could really use some help with.

The Wen unit I tested was fine. Very nice, smooth sine wave. No issues at all using it for anything. I also have a Champion 200913 open frame inverter. I tested it on the scope as well. I was expecting the same sine wave that I got with my Wen. But what I got was this:



I tested several, different ways. From the outlet direct, from an extension cord, from a surge protector, the same waveform. However, when I plugged in one of those bonded-neutral plugs, I got a regular, normal sine wave like the Wen.

So, why would the signal out the box look so bad like that, and why would a bonded-neutral plug clean it up? My thought is that there may be some grounding issue someplace. Is that possible? I'm really stumped on this one.
Link Posted: 6/27/2021 8:30:08 PM EDT
Once I plugged in that bonded neutral I just told you about, this is what the waveform looked like:



I would think that the Champion inverter would look like that without having to use a special plug. It's got a floating neutral, but so does the Wen and the Wen looked fine without any plugs.
Link Posted: 6/28/2021 12:29:54 AM EDT
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By TheLASwamp:
I was able to attach the plug to a spare USB a/c charger and get my readings.
View Quote

what does this mean?
Link Posted: 6/28/2021 1:17:06 AM EDT
[Last Edit: 6/28/2021 1:17:50 AM EDT by AR15Texan]
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By ar-jedi:
do not under any circumstances connect the scope probes directly to the the generator/inverter output.
you can damage the scope and damage yourself.
View Quote

From the Amazon link the OP provided there is a picture of the supplied probes and it has a 1x/10x switch in the body of the probe.  You don't think it would be safe to attach the positive lead to the 120VAC hot leg with the probe in the 10x position?  If not why?
Link Posted: 6/28/2021 6:37:20 AM EDT
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By ar-jedi:

what does this mean?
View Quote


I used to hot leg of a USB charger plug.
Link Posted: 6/28/2021 4:08:00 PM EDT
[Last Edit: 6/28/2021 4:12:33 PM EDT by Rat_Patrol]
Link Posted: 6/28/2021 4:09:40 PM EDT
[Last Edit: 6/28/2021 4:10:50 PM EDT by Rat_Patrol]
Link Posted: 6/28/2021 7:34:12 PM EDT
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By Rat_Patrol:
A scope, or in your case a DSO (digital storage oscilloscope) measure voltage over time.

That is all it does.

The measurement of voltage is a measurement of difference in potential, essentially. What is the potential difference at the point of the probe compared to the reference. The reference is that short little alligator clip on the probe.

If you had nothing to measure reference to (your alligator clip is just dangling in air), your reference was "floating" and giving you crap data. Once you secured a reference, you were GTG and got a true measurement.
View Quote


Yeah, I was finally able to get some readings. What surprised me was the fact that my Champion inverter was making some weird looking waveforms until I bonded the neutral. I was expecting the waveform to look good bonded or floating. I haven't been able to figure that one out.
Link Posted: 6/29/2021 7:08:18 PM EDT
I was able to reach Champion tech support this morning. They are sending me a new fuel valve to replace the leaking one. I asked him about the strange scope readings I posted here. He asked how I was testing the signal and I told him I was clipping onto the hot leg of a plug. He said that to get an accurate reading on Champion inverters, I need to ground the probe connection. He said that attaching only to the hot leg and no ground was going to give a noisy reading. That's why when I bonded the neutral, I was then showing a clean sine wave. So, according to his explanation, it was a instrumentation issue since I was attaching, ungrounded, on the hot leg.
Link Posted: 7/1/2021 3:00:42 PM EDT
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By Rat_Patrol:
Why not? I see the scope is rated for 300v, and I would ASSume the scope has leads that match/exceed that, so there shouldn't be an issue at all, other than you are playing with mains voltage that can kill you if you touch a no-no
View Quote View All Quotes
View All Quotes
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By Rat_Patrol:
Originally Posted By ar-jedi:
do not under any circumstances connect the scope probes directly to the the generator/inverter output.
you can damage the scope and damage yourself.
Why not? I see the scope is rated for 300v, and I would ASSume the scope has leads that match/exceed that, so there shouldn't be an issue at all, other than you are playing with mains voltage that can kill you if you touch a no-no



This is a common mistake new 'scope users make.  The common/ground lead of each probe is connected to the common lead of the other probes AND the chassis of the scope, which is grounded to earth.

Some folks measure floating ground devices without blowing up the scope, but if the generator is make contact with the ground, you can form a ground loop amd shock yourself or blow up the scope.

The acceptable way to measure mains voltage is with a differential probe.

Also keep in mind 120Vac is 120 V  RMS, which is 343 Vp-p
Link Posted: 7/1/2021 3:28:50 PM EDT
Link Posted: 7/1/2021 3:41:19 PM EDT
Don't go messing with mains voltage without using a Differential Probe.
Like this one...

Seen too many college grads almost kill themselves in the lab while poking around a power supply.
Link Posted: 7/1/2021 8:48:21 PM EDT
Would bonding the neutral eliminate the risk of a ground loop?
Link Posted: 7/1/2021 9:57:13 PM EDT
[Last Edit: 7/1/2021 9:58:09 PM EDT by Rock_Ranger]
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By TheLASwamp:
Would bonding the neutral eliminate the risk of a ground loop?
View Quote



Nope, it would help guarentee a ground loop.  Right way is with a differential probe.
Link Posted: 7/2/2021 7:18:47 AM EDT
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By Rock_Ranger:



Nope, it would help guarentee a ground loop.  Right way is with a differential probe.
View Quote


The differential probes, as they they only way to be completely safe? The ones I saw were major $$$. A simple ground wouldn't suffice?
Link Posted: 7/2/2021 3:56:13 PM EDT
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By TheLASwamp:


The differential probes, as they they only way to be completely safe? The ones I saw were major $$$. A simple ground wouldn't suffice?
View Quote



I used this probe

Probe.  $168

Link Posted: 7/3/2021 8:45:08 AM EDT
Discussion ForumsJump to Quoted PostQuote History


That's a cool little device. From what I can see, it appears that it connects to the BNC channel on the scope and converts the leads into what you'd find on a DMM. The probes that came with the scope have x10 switches on them, but apparently the problem is not a voltage issue but a ground issue? I didn't have any issues last weekend testing all sorts of VAC sources from generators to inverters to wall power with nothing blowing up that I could tell, but I certainly don't want any trouble in the future.
Link Posted: 7/9/2021 2:50:47 PM EDT
[Last Edit: 7/9/2021 2:51:25 PM EDT by Rock_Ranger]
OP, this video is a little long, but explains very well why hooking a scope directly to mains voltages is dangerous for you and the scope.

Failed To Load Title


The video has diagrams starting at 5:00
Link Posted: 7/10/2021 10:11:48 AM EDT
[Last Edit: 7/10/2021 3:14:28 PM EDT by TheLASwamp]
The scope came with two probes that allow switching to x10 attenuation. I bought a better probe with x100 attenuation. That lowers the voltage significantly. I was looking at a differential probe, but the price jumped up on me. I'm still trying to figure out all of the settings.

I did have a question about saving waveforms. It took forever to create a USB thumb drive format that the scope recognizes. I finally created one. When I downloaded some waveforms, I was expecting screen images. Instead, it looked like text files in .bin format. I'm guessing I need a proprietary program to interpret the data? I was reduced to snapping pics of the scope screen so I could post them. I was hoping for a more elegant solution. I can't find a setting to change the format of the data to .jpg or any other proper image file format.

ETA:  I found the setting that allows for saving in bitmap format. Let the good times roll.

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