DSO Nano does not like 120v

Just thought you all would get a kick out of this. I have had my V1 nano for some time now and for a little thing it does the trick. Let a friend barrow to test some issues he was having with a controls simulator for an hvac system. Found out that he blew it up. Here is the story. He took the nano and was testing some inputs and outputs with it, come to find out the nano’s battery was getting low so he decided to plug the nano’s into the USB plug to charge on his laptop and continue working low and behold he had the x10 prob selected and an x10 prob connected to the nano due to some of the out put being in the 50v range. During this endeavor he connected the nano to a line that he thought only had 40 or so volt coming through NOPE 120V blew up the nano and fried the USB port on the laptop. Not sure if this was because he was charging it at the time and it grounded though the nano’s USB or not. I remember reading somewhere that with X10 probe the nano’s max vpp was 800 and 100v/Div. If that being the case I would think that the 120 would not have been a problem. Any how so long little friend and now I go to order a new one and sold out ooo well I can wait.

Thanks for sharing!

The problem here is keeping it connected to USB while probes are live. USB ground is directly connected to Nano probe ground and so may damage both your Nano and USB port with probes exposed to high voltages. A big warning might have been approriate, but others may argue that this is “common sense” (it probably is not among Nano users).

Too bad with your Nano and your friends PC. They’re both (not your friend I hope) most likley beyond repair.

Ben you are correct as I deal with Building automation systems and communications the nano never sees voltage past 24 volts. He learned his lesson and has offered to replace it. Unfortunately the laptop didn’t make it good thing it was an older one. now i just have to wait to get a new one. For testing RS485 communications this this has been a live saver.

I am 100% sure about it was because of USB ground. I had some similar problems when I was trying to measure some voltages in a Transformerless Power Supply (directly conected to 220V)… the nano probe just melt… but, nano and USB port are OK becouse I unplug it quickly… Only I noticed that reading small values are now impossible, I have a lot of noice.

Also, I am trying to implement a RS232 reader for nano, just for reading ASCII values, maybe I can try to extend this implementation to RS485 too.

That would be cool if you there was a reader in the nano. That would make it a lot easier to have all in one. If there is anything that I can do to assist you please let me know. I found the nano by mistake. It has actually been my most valued tool in tracing issues with rs485 data communication. I have the use of a larger oscilloscope, but trying to get into some of the places to check signal strength the nano does the job. I like the fact that I can take the information that I have captured and place it into an excel file with information pertaining to the job and show the owners the issues first hand. I agree with many that yes having an expensive oscilloscope is nice to have, but for the price and ease of use this little nano holds its own. With people out there willing to help each other I can see this piece of equipment going many places and not just for the home hobbyist. I have had others in my field ask me where i got my nano then they see the price. I know of at least three of them that have purchased the nano V1.

I know many fluke instruments also have a common ground and are limited in voltage meassured by the isolation capacity of the power adapter which you can replace with more expensive ones for higher voltage isolation. The nano has zero isolation to the power source, I guess seeed could easily provide a special usb cable with a transformer for galvanic isolation as a extra accessory along with a warning that it must be used when charging and meassuring at the same time.

Way cool, I always wished for this :slight_smile: I guess RS232 or RS485 shouldn’t be any different, you just have to set the tigging levels a little different for each. Automatic baud detection would be soo cool too.

I learned the other day that it is reall bad to put your ground from the oszilloskop to a live circuit. With a big bang some copperlines vapourised 30cm in front of my face. Luckyly i had no injuries.
I’d better go outside… its too dangerous at home :smiley: .
(I ordered protectionglasses now :wink: )

I just want to point out that the official 800Vp-p spec is inaccurate. If you refer to my attachment at viewtopic.php?f=12&t=1681, you will see that +/-50V-Pk and +/-500V-Pk are the actual limits to keep the Nano ADC linear. This can be seen in the frontend.xls spreadsheet calculations. It is also interesting to note that the new DSO Quad specs are set at 80V-PP/400V-PP which equals +/-40V-Pk and +/- 200V-Pk respectively.

Another consideration here is that 120VAC is an RMS value, whereas the spec is a P-P value. Multiply RMS times 1.41 to get Peak (Pk) or multiply 120VAC times 2.82 to get P-P. This can change the expectations considerably.

The attachment of the Nano ground lead and the associated ground loop current is what most likely caused the problem, not the Nano lead attached to the 120VAC. No mention was made as to where the Nano ground lead was attached. It would be interesting to measure the voltage between where that Nano ground was attached and the ground connection of the PC.

A safer approach is to just leave the Nano ground lead disconnected in this situation. Sure, the measurement will be off by the ground voltage differences, but that small error is better than smoking things with a ground loop current.

To prevent this bad experience while the Nano is charging and you must use the Nano ground lead, you should always use a multimeter to make sure that the voltage between the Nano ground and the intended Nano ground lead clip point is 0VAC, before attaching the Nano ground lead. You may be surprised at the amount of voltage difference that can exist between electrical outlet ground points, especially in a larger building. This small ground-voltage-difference and the associated current is what makes things smoke. For example, if the ground voltage difference is only 1VAC, that can generate 10Amp of current through a ground continuity circuit of only 0.1-ohms.

lygra sound advice I will have to keep that in mind when I purchase my new nano. As I deal with 7 to 12 volt 24 volts at the max. This is good information by far.

nano ground loop?

Maybe I can clarify a bit further. If you ever work with sound systems, you will quickly learn that the hum from the sound equipment is due to mismatched ground potential between sound components. In this situation, you make sure the system grounds have nearly zero resistance to prevent this audio ground loop current that results in speaker hum. Ground loop currents exist in our physical electrical world and sound systems are a classic example. These small audio ground loop currents do no damage because of their small milli-volt differences involved. They just annoy. AC mains ground differences on the other hand are not protected by circuit breakers and can provide significant amperage when they exist. The further you get from the building power input panel, the greater these ground voltage differences can become.

When you pass several amps of AC current through the Nano ground path as in the above post example, this current can end up passing through the Nano circuit board via the ground foil (trace). In this example the ground current enters on the Nano test lead ground, and exits on the Nano USB jack ground. These two points in the Nano are connected via the Nano circuit board. The traces on this board were never designed to pass this volume of current so they can heat up enough to lift from the circuit board and move to who knows where before they burn open.

If you are lucky, the hot lifted circuit foil just burns open and the ground current is stopped with just a burned open ground path on the circuit board. In this case just repair the opened ground foil and you are back in business.

If you are less fortunate, this hot lifted circuit board trace moves enough to touch other circuit board components and shorts them to ground while the Nano is still providing circuit power and before this trace burns open. This is usually catastrophic with respect to circuit board damages.

This same business takes place inside the laptop USB port. Those traces are also subject to the same heating effects from this ground loop current. In the laptop the ground loop current flows from the USB jack ground pin via the circuit board to the power adapter jack ground pin, via the power adapter to the wall outlet ground.

I refer to all of this as ground loop current. So the Nano sees this ground loop current and so does the laptop USB port. Any potential difference between two points will provide current flow when a path is connected. AC mains ground voltage differences are no exception. This AC ground potential difference will in fact pass through any ground path connected between this potential difference.

Hope this clears things up. You got a very high return for your three words. :slight_smile:

Also you should be very careful when measuring something you know have a high ground offset voltage, for example when doing phase to phase measurement. I would simply say this should be avoided when using the nano if the phase to ground voltage is high where you intend to ground the probe. Also the back plate on the nano is made of metal and is very close to the battery cord (which has a ground wire) and I can say that the battery cord is not meant to isolate like 400V so it is not safe to hold the nano while having the probe ground connected to high voltages.

I made a 10X probe in accordance with the principle in the link cromwell-intl.com/radio/probes.html and then proceeded to measure/display 120 vac sine wave and modified sine wave (140 watt inverter). The sine wave was from the motorhome outlet and looked like a sine wave (well, DUH!). The modified sine wave was kink of a shock since it was really a blocky square wave, sorta. All this was on the internal battery and the nano still works!

A note about the link: Those probes assume scope input resistance/impedance of 1 megohm and the nano is only .5 megohm so the resistances are gonna be different. The really neat thing was finding really small coax to make the probe lead fit into a 3.5 mm (.125 in) mono plug. The wire on the yellow video RCA plug cable IS skinny coax! I must have a short ton of A/V cables so I sacrificed one for the project.

Karl

There is some excellent advice in this thread, but also some conflicting information. Here’s a go at a summary on my part.

Attached is a spreadsheet that models the DSO Nano input stage. This is to the best of my knowledge an accurate model and is also how V3 firmware calculates voltage. Note that input resistance depends on selected gain range (choice of V/Div).

When using a 10x probe, 800V is within safe range for the Nano input stage (only 43V seen on connector input) as long as we operate on battery power. The issue is not the voltage itself, but potentially high ground loop currents when operating the Nano from USB power (see explanation from lygra). The second issue is with limited isolation against absolute voltage levels as pointed out by dnordenberg.

Lessons learned may be as follows:

  • Never use the Nano for measurements (disconnect test leads) when operating from USB unless the measurement target and Nano/USB port shares a common ground.
  • Avoid touching the Nano when test leads are exposed to a high ground level offset and keep the Nano at a safe distance from other conductive material.
    Nano Input Stage.zip (8.03 KB)

I think seeed should added a safety notice in the nano box about these issues as they might be potentially deadly and I really don’t think they want to have that on their conscience.

http://en.wikipedia.org/wiki/CE_mark might be a good idea to have on the product. It does not cost money as the manufacturer puts it on by their selves if they think the product lives up to certain requirements. It declares that the manufacturer has understood and followed EU consumer safety, health or environmental requirements so the product should be safe to use. All products which might be dangerous in some way must have a CE mark to be legal to sell in EU and EFTA countries. It applies to for example small toys that can be swollowed by small children that must follow some size guidelines for the age group so it can’t be swollowed, electronics, lifting equipment etc. As a private person I can import non CE products but I can’t sell it as a business. The CE making doesn’t make the product absolutly safe to use in any extreme way but it tells you the manufacturer has done what they can to make it safe according to EUs requirements which is based on a lots of research funded by the EU committee trough their member countries. The product must also include a text “certificate of conformity” which is usually on the first page in the instruction manual or included on a very small paper. Many asian countries put the CE mark on non conforming items to be able to market it in EU but that is another problem.

To make thing simple I would say that if you are going to use the nana on voltage within the ranges either with an x1 or x10 probe that you should isolate the nano and make sure that you ave a full battery before you begin testing. After some conversation with my friend I found that as he was doing some testing the battery became low. He knew enough to shut of the nano and before connecting to the laptop, but forgot and maybe didn’t realize that with the ground from the probe connected that the minute he turned on the nano pow everthing went creating a ground loop. lygra and BenF are correct. This was a VFD that was spiking due to a faulty internal board. When he was testing with the nano disconnected he didn’t have an issue. It was only when the nano was grounded through the USB port on the laptop that the nano got destroyed as well as the laptop.

I concur with your findings of 800V-pk. I have revised my front-end files to correct an error and to also provide additional information concerning probe effects upon the results and Nano actual input resistance calculations.

The revised attachment can be found at viewtopic.php?f=12&t=1681&p=5450#p5450

thanks for all your finding and suggestion… we will consider adding extra warning in the product ,besides thinking over how to avoid these situation in some other way…

we are considering adding the warning on the product specification/manual .

Sorry to bump this thread.

Is it safe (both for the device and the user) to measure 230V/50Hz mains voltage with USB unplugged and x10 probe? I have the DSO Nano and want to use it to test UPS outputs.