# Differential Probes



## LaSwamp (Mar 16, 2021)

Hi guys. Is anyone here using a differential probe with their scope? I was told that testing generators and other VAC sources without using one will blow up my scope. I used my scope last weekend on all sorts of ac sources and did not, as far as I can tell, blow anything up. Is there real danger potential or are the chances of actually blowing anything up fairly remote?


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## tvl (Nov 14, 2018)

I'll throw in my opinion for what it's worth:

1) Differential probes do provide an extra layer of protection. Actually an excellent layer of protection! However, they're somewhat costly if that matters to you.

2) Based on some photos I came across for the Hanmatek DOS1102 , it does appear your channel 2 is CAT II rated ........... meaning it's acceptable to connect to a 120 VAC *outlet*. That is if *ALL* safety precautions are followed!

3) If you're not going to use differential probes, always ensure your probe is set for 10X attenuation. The supplied probe that shipped with your scope should suffice and has already proven to do so. However, *at the least*, I would consider getting a better quality probe that might be less apt to fail on you. One that has 100X attenuation would be a nice addition!

4) On the other hand, if you simply wish to add another layer of safety with your present setup, you could utilize a cheap and proven current limiting device as shown in the video. This device would provide great current limiting protection and is something you can make yourself rather cheap! Video

There's probably more options and methods I could share, but hopefully these will help. There are many individuals out there in the world who are and have been connecting their scope to 120 VAC just as you have. And, I'm sure there are some who have "been bitten". The key is to *always* be alert, *always* ensure everything is connected properly, *always* double check your setup and *always* be prepared for something to go wrong .................. meaning allow some distance between the equipment and yourself and ready with a plan of action if required.

*If you were to ask for my personal recommendations, I would say the following:*

A) Go ahead and purchase the differential probes. One set you might consider and are affordable: Differential Probe

B) Build yourself a current limiter. It's cheap, fun to build and will at least offer *some* protection.

*EDIT*: I should have also mentioned the current limiter will be plugged into the 120 VAC outlet you wish to check (_whether it be your generator or home outlet_). You can then access the hot blade on the current limiter's outlet with you scope probe. This puts the light bulb between the outlet under test and your probe/scope's front end. If you were to inadvertently connect the ground clip incorrectly, which in essence would normally be a dead short, the light bulb then becomes your "load", which will illuminate alerting you that you have done something incorrectly, *BUT* more importantly, avoids a lot of sparks and potential damage.


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## LaSwamp (Mar 16, 2021)

Excellent writeup, thank you for the advice. I wasn't sure if there was a likely danger in the way I was testing the signal with the scope or if it was something that _could_ happen but doesn't unless there's some really huge mix up with how the connection is done. I'm not planning on doing anything exotic with the scope, just testing outlet waveforms. I used Channel 1 for the various tests and I didn't have any issues. From now on, I'll switch to Channel 2. The scope seemed to do fine and I was able to learn quite a bit. Especially about the Coleman and the Firman. And the inverter in my truck. I haven't tested the Tripp Lite UPS on battery power yet, but I'd bet a steak dinner at Ruth's it's going to look like a step pyramid.


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## mtucker (May 9, 2021)

Using a ground cheater on the AC power cord of your scope is very cheap insurance that you won't fry your scope (or more likely your scope probe) if you clip your scope ground lead onto a hot line (or even on a location with a DC potential). A differential probe is even better, but they can be very expensive.


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## tvl (Nov 14, 2018)

*Just as a precaution*: It's not advisable nor is it considered safe to to use a "ground cheater" between your scope and AC outlet. The ground is there for a very good reason ................... and lots of information regarding the subject can be found on the web if interested. 

This is also the reason isolation transformers exist .......................... however, isolation transformers are meant to be connected to the DUT (device under test).

Anyway, just my 2 cents!!!


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## LaSwamp (Mar 16, 2021)

tvl said:


> *Just as a precaution*: It's not advisable nor is it considered safe to to use a "ground cheater" between your scope and AC outlet. The ground is there for a very good reason ................... and lots of information regarding the subject can be found on the web if interested.
> 
> This is also the reason isolation transformers exist .......................... however, isolation transformers are meant to be connected to the DUT (device under test).
> 
> Anyway, just my 2 cents!!!


I was a bit confused about the suggestion to use a cheater plug for the scope itself, given the probes ground through the ground plug. My guess is that it would create a floating neutral condition on the scope power? I'd definitely like to upgrade the probes themselves. As you can imagine, the ones that came with it are probably not high grade. What's out there that's considered good?


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## mtucker (May 9, 2021)

No argument that a differential probe is the preferred way, but one will likely cost the same or 2X more than an inexpensive scope. 



LaSwamp said:


> I was a bit confused about the suggestion to use a cheater plug for the scope itself, given the probes ground through the ground plug. My guess is that it would create a floating neutral condition on the scope power? I'd definitely like to upgrade the probes themselves. As you can imagine, the ones that came with it are probably not high grade. What's out there that's considered good?


 There is no need to have your scope "ground" lead actually grounded to the earth ground on your AC wall outlet for it to work. Think about it this way.... your battery operated volt meter has a black negative (aka ground lead). The black lead is not connected to earth ground on an AC wall plug since it runs off batteries. The meter leads are floating and simply measures the V potential between the two leads. You can put it across line to neutral on your AC outlet or measure DC. 
The problem you run into when using a single ended ground lead without a ground cheater plug is your scope ground lead will be connected to the AC receptacle earth ground and if you then put your scope ground lead on the hot wire..... pop.... lots of current flow from shorting line to ground. Now if your scope is connected to house power and you are measuring your generator output (that is not connected to the house power) you are OK because the generator is isolated from your house. With a ground cheater plug, you don't have to worry about it.


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## LaSwamp (Mar 16, 2021)

I was thinking about a better probe, something like this 100x attenuation

Would that give me a good level of protection from ground loops or anything else that might cause a problem?


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## tvl (Nov 14, 2018)

1) I feel the probe your referenced would be better at probing 120 VAC over the probes that shipped with your scope. The reason is because it has the 100X attenuation and it's designed for higher voltages, which your current probes are not.

2) However, this probe is not the same as a differential probe and is not meant as a substitute ............... just so you know!

3) If you're *certain* you will *always* make the correct connections with your *ground clip*, you should not have an issue with the probe shown. Actually, that goes the same for your current probe set, but then they aren't really designed for the higher AC voltages you wish to check. I do feel the new probe would serve you much better in this situation!

4) It's so easy to get wrapped up in what you're doing and make a simple, but stupid mistake. Having stated that, the current limiting device I mentioned in an earlier post would help in this situation and is easy & cheap to build. You really can't mess up if you utilize this device correctly when making measurements. An incorrect ground clip placement in this case only illuminates a light bulb. Anyway, just another thought for you!

5) Although it isn't a common occurrence, any scope probe can fail at any time ......... a cheap one or an expensive one. Always be on the alert and ready for some type of failure when working with higher voltages. I've been lucky and have never experienced a probe failure, but I have read of a handful of instances. Again, just my 2 cents!

*EDIT*: The type failure I'm referring to in the above instance is where the attenuation switch no longer functions correctly. In this case, the probe may be set to 10X, but in actuality, it's in the 1X mode because of switch failure. The probe your looking to purchase has only *ONE* setting, therefore this won't be an issue with this probe. While there are other types of probe failure, I've *never* heard of one blowing up in one's face or causing any type of serious injury ................... they just simply cease to operate properly. Again, it's the *ground clip on the probe* that can play havoc when not connected correctly ................. and in this case, sparks fly and maybe even a small electrical explosion!


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## LaSwamp (Mar 16, 2021)

I went ahead and ordered that probe. I can always use it with a differential probe if I get one and I like the 100X attenuation. I feel a bit safer with it, although the probes the scope came with seem fine. 

I do have a question about the ground clip on the probes. When are you supposed to use them? I've read conflicting ideas on when, or if, to use them. Isn't the probe already grounded via the BNC port? I could see a problem if you use the ground clip on a hot leg. I'm sure that would cause a problem. I didn't use the ground clip when I did the generator tests a couple of weeks ago and there were no issues. I simply clipped the probe to the hot leg and it gave me the waveform. Obviously, the ground clip is there for a reason, though. If you need to use it, what, exactly, does it clip to? 

But really, I'm just trying to figure out when (and how) the ground clip needs to be used if the probe is already grounded.


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## tvl (Nov 14, 2018)

Read some of the following messages from the following site. Possibly the will help you!

What's the point of the ground

Basically, when you're working with AC and the scope is plugged into an AC outlet, then yes in this case, the scope *and* probe are already grounded and they both share the same ground reference. Therefore, the ground clip is* not required* when taking measurements such as you were doing because your scope *and* probe are already referenced to ground. In these situations it's best to simply remove the ground clip to avoid any issues .................. don't even let it dangle, because it might just touch a hot leg ..................... and you know the rest of that story!

Now, lets say I have a piece of AC equipment connected to a *true* isolation transformer. In this situation, I don't have a ground reference, so the ground clip will be required and can essentially be safely placed anywhere.

And, then there is electronics - review some of the post from the this site as well:

Oscilloscope ground clip | Electronics Forums (electronicspoint.com)

I'm thinking this little bit of information is what you're looking for. However, simply Google any of your concerns and you should find more answers.


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## LaSwamp (Mar 16, 2021)

That makes sense. I don't plan to do deep dives into testing electrical signals, just mostly testing to see if a VAC waveform is clean or not. So far, the scope has provided a wealth of information about the generators and inverters I have. I now know which ones I can use with no problem and those I should only use as backup or to loan out in an outage. I'm still trying to figure out all of the settings. 

I finally figured out how to download a waveform to a USB drive. There's a certain way the drive has to be formatted or the scope won't use it. I downloaded some waveforms. They were not in typical image format like .jpg, .gif, or .tif. They were in .bin format. When opened, they look like text files. I'm guessing there's some software I need to use to interpret the .bin files into an image of a waveform. It's been fun trying to figure it out, but at this point, I'd really like to just solve the puzzle and move on to the next spin of the wheel. I figured out how to get the scope to display Hz, but it's been a bear trying to figure out which setting shows VAC. I though I had it figured out, but then the setting switched to some amp reading. And the voltage reading was high at 123 VAC when it was showing voltage. So I don't know if I actually figured it out. I got so frustrated last night that I just reset the scope to factory setting and will try again from scratch. But overall, it's been fun figuring it out.


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## LaSwamp (Mar 16, 2021)

Okay, I figured out how to output the data to a bitmap image. My house power:



















That voltage seems a bit high. I'm only reading 118 VAC on my UPS. Frequency looks spot-on, though.


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## tvl (Nov 14, 2018)

You wrote: * "That voltage seems a bit high. I'm only reading 118 VAC on my UPS. Frequency looks spot-on, though."*

Just guessing, but I'd think the scope is more likely to give a correct voltage reading versus the UPS. I'm assuming you don't have a digital voltmeter handy?

As a matter of fact, my home voltage runs about 125 to 126


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## iowagold (Jan 22, 2020)

LaSwamp said:


> Okay, I figured out how to output the data to a bitmap image. My house power:
> 
> View attachment 9702
> 
> ...


that peak looks clipped to me....
or is that a setting on the scope resolution?


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## LaSwamp (Mar 16, 2021)

iowagold said:


> that peak looks clipped to me....
> or is that a setting on the scope resolution?


I saw that clipped peak but didn't know what to make of it. I don't think it's the scope, but I'm not really sure. I had always assumed that we had pretty clean power. I could try to retest and see if I get the same result.


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## LaSwamp (Mar 16, 2021)

tvl said:


> You wrote: * "That voltage seems a bit high. I'm only reading 118 VAC on my UPS. Frequency looks spot-on, though."*
> 
> Just guessing, but I'd think the scope is more likely to give a correct voltage reading versus the UPS. I'm assuming you don't have a digital voltmeter handy?
> 
> As a matter of fact, my home voltage runs about 125 to 126


Actually, I have a couple of DMMs. I didn't think to pull one of them out and compare to the RMS I was getting from the scope. I guess the VAC could be 125 since it's summer and the power company might be giving us an extra boost?


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## iowagold (Jan 22, 2020)

are you on a manual range setting or auto range on the scope?


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## LaSwamp (Mar 16, 2021)

iowagold said:


> are you on a manual range setting or auto range on the scope?


I'm using auto ranging on the scope.


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## iowagold (Jan 22, 2020)

we are at 128 vac on grid right now


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## iowagold (Jan 22, 2020)

try a 140 volt range if it is in the menu...
it could be getting clipped in the range setting.


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## LaSwamp (Mar 16, 2021)

iowagold said:


> we are at 128 vac on grid right now


I know the grid is taxed pretty heavily right now. Lots of a/c action going on.


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## LaSwamp (Mar 16, 2021)

LaSwamp said:


> I know the grid is taxed pretty heavily right now. Lots of a/c action going on.





iowagold said:


> try a 140 volt range if it is in the menu...
> it could be getting clipped in the range setting.


Okay, I had a chance to take some more readings. I took out the DMM this morning and tested wall power. It showed 121.7 VAC at just under 60Hz. Nothing out of the ordinary. 

I did an auto-calibration of the scope just to make sure that wasn't an issue. The voltages shown on the scope differed depending on how I was looking at the wave on the screen. It ranged from a high of 128 VAC to a low of 122 VAC. The 122 reading appears to be the correct one. I don't know why the values changed depending on the wave view. I had the scope set for auto ranging, but I was still getting that weird clipping on the form. I don't know what that's all about. See below:










I noticed when I tightened up the view, it would lower the voltage shown. The frequency reading agreed with the value on the DMM. 










When I spaced out the wave, like above, the VAC shot up to close to 128 volts. You can still see that weird clipping thing going on. 










The highest VAC reading I got, at over 128 volts. But it's a spread out wave on the monitor which got the highest readings. 










Tightening up the wave brought the VAC reading closest to what I saw on the DMM. As with the other readings, Hz was spot on. 

So yeah, I'm not sure what's causing the clipping. The scope is auto-ranging. I haven't seen a menu which allows me to change the range, although I thought auto ranging was supposed to eliminate the need to manually adjust it. Let me know your thoughts.


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## Airstreamer67 (Oct 3, 2020)

Yeah, I commonly see ~125VAC at exactly 60Hz from the utility grid at home year-round.


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## iowagold (Jan 22, 2020)

LaSwamp said:


> Okay, I had a chance to take some more readings. I took out the DMM this morning and tested wall power. It showed 121.7 VAC at just under 60Hz. Nothing out of the ordinary.
> 
> I did an auto-calibration of the scope just to make sure that wasn't an issue. The voltages shown on the scope differed depending on how I was looking at the wave on the screen. It ranged from a high of 128 VAC to a low of 122 VAC. The 122 reading appears to be the correct one. I don't know why the values changed depending on the wave view. I had the scope set for auto ranging, but I was still getting that weird clipping on the form. I don't know what that's all about. See below:
> 
> ...


yea.... scope operation can be tricky...
kinda like spectrum units....
lots of tricky settings, probe issues....
and not all scopes and spectrum units act or perform the same....
I have seen low cost units perform better or have easy read outs easy to use than the $60 k units 

we use 1 to 1 isolation transformers for inputs....
they can be tricky as well as you need full spectrum shielded transformers with no tanking or trap issues...
we bought those in the late 1960's early 1970's. i think they were thordarson (sp) brand.
these says maybe hammond has them...


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## LaSwamp (Mar 16, 2021)

iowagold said:


> yea.... scope operation can be tricky...
> kinda like spectrum units....
> lots of tricky settings, probe issues....
> and not all scopes and spectrum units act or perform the same....
> ...


I'm still figuring it out, but I've come a long way since I got it. I don't have any formal electrical training so it's been an education learning the terms and nuances of the science. When I ordered my first generator, I had no idea about THD or waveforms. I thought as most do, that a/c power is a/c power. Once you get a DMM and a scope, you realize it's an entirely different world in the signal. As far as the VAC readings on the scope, I can always test them with my DMM. I'm learning how to use the Math function on my scope. It has an FFT function which seems to show distortions at various frequencies. I haven't tried it yet on my generators, but it looks like that might provide the answer to the original puzzle that started all of this, i.e., why the Hz readings from my Wen inverter were so wonky. The FFT readings from the Coleman and Firman generators should be quite interesting as well. I ran out of weekend before I could play around with those, but I look forward to those projects. I'll post them here when I get the screen captures.


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