Battery powred oscilloscope

[DMack]

Thanks S. Petrukhin. You know, I'm starting to think a battery powered scope might be the best solution for my situation. It sounds like they're a bit slower and less accurate, but a safer bet for beginners. Do you know anything about the Seeed Studios DSO Nano V3?

https://www.amazon.com/Handheld-Oscilloscopes-DSO-Nano-V3/dp/B015X6LZFO/ref=sr_1_1?dchild=1&keywords=DSO150+15001K+Fully+Assembled+Digital+Oscilloscope%2C+DIY+Kit+with+Carrying+Case%2C+Digital+Oscilloscope&qid=1611008009&sr=8-1

[S. Petrukhin]

Thanks S. Petrukhin. You know, I'm starting to think a battery powered scope might be the best solution for my situation. It sounds like they're a bit slower and less accurate, but a safer bet for beginners. Do you know anything about the Seeed Studios DSO Nano V3?

https://www.amazon.com/Handheld-Oscilloscopes-DSO-Nano-V3/dp/B015X6LZFO/ref=sr_1_1?dchild=1&keywords=DSO150+15001K+Fully+Assembled+Digital+Oscilloscope%2C+DIY+Kit+with+Carrying+Case%2C+Digital+Oscilloscope&qid=1611008009&sr=8-1

Yes, it's just a toy of the sound range and the only trigger on the front. sampling is performed by the built-in STM32F103C8 ADC. But it can be powered by powerbank and not be afraid of grounding. Most often, you need to look at the passage of the signal and roughly its shape.

For serious research, of course, it is necessary to use a normal oscilloscope and a differential probe. I have an MSO5074, but no differential probe yet. Therefore, I do not throw out the DSO150.

I can't say anything for sure about the option you indicated, but I think it's $100 thrown away. 

[rstofer]

I use this toy as an isolated oscilloscope sometimes in power supplies. 

I ordered one!  There are times where using my bench scope is awkward and a little low bandwidth scope is all I need.  My AD2 will often do the job but this looks a little more convenient.

I won't see it any time soon, shipping is 20-40 days.

[rstofer]

Most bench power supplies have the outputs floating with respect to ground, and include a third binding post on the output so that you can ground either voltage rail if you need to.

Thanks for the info fordem. So I have a basic bench supply (TekPower TP1803D) that is grounded with the 3-prong plug, but only has binding posts for positive and negative. Assuming my circuits won't have any USB connections, can I still get into a dangerous situation if I connect my scope to it a circuit that is powered from this supply? From what I've been reading, it's the power supplies that have the ground binding post that can cause this situation. I did a continuity test from both of the bench supply's probes to its ground prong and there was no continuity. I'm thinking that indicates that its voltage is floating and should be fairly safe, but I could be wrong.

(Attachment Link)

If the output of the supply is floating, and it probably is, you have no issues connecting the scope ground clip to the black output (assuming you want a +5V supply).  The supply doesn't have a ground jack next to the black jack, that is regrettable.

Use a DMM to measure voltage between the output jacks and the scope BNC connectors (which should be earth ground through the scope power cord).  There shouldn't be any.  If there is and it's on the red output while the black shows nothing to ground, that's probably ok.  It just means the black jack is probably grounded.  In the circuit, connect the probe ground to the black wire or somewhere connected to black.

Draw a picture and ask before guessing.

I wouldn't use those banana plugs on a bet (or dare)!  All exposed non-current carrying metallic surfaces are to be grounded.  That's why the scope BNC connectors are grounded.  Here we have the voltage on a screw that the user is bound to touch.  A really poor banana plug!  Hopefully there is no high voltage feeding back toward the power supply.

[DMack]

The supply doesn't have a ground jack next to the black jack, that is regrettable.

...

I wouldn't use those banana plugs on a bet (or dare)!  All exposed non-current carrying metallic surfaces are to be grounded.  That's why the scope BNC connectors are grounded.  Here we have the voltage on a screw that the user is bound to touch.  A really poor banana plug!  Hopefully there is no high voltage feeding back toward the power supply.

I agree that my cheap power supply's design isn't ideal, and potentially even dangerous. However, I've been using it for years for my relatively low-powered circuits. Some day I will upgrade to something better/safer ...

[S. Petrukhin]

I use this toy as an isolated oscilloscope sometimes in power supplies. 

I ordered one!  There are times where using my bench scope is awkward and a little low bandwidth scope is all I need.  My AD2 will often do the job but this looks a little more convenient.

I won't see it any time soon, shipping is 20-40 days.

There is a free space inside the case, where you can put a flat battery, a charge board and a converter.
But it is more convenient for me to connect powerbank through the converter wire 5V->9V. This toy still works after triple power conversion. 

Inside, the LCD backlight is powered via a resistor from 9V input voltage, then an LDO of 3.3 V for the processor and a capacitor-forming chip of +/- 9V for powering the op-amp. Maybe I have already forgotten a little and do not describe it exactly. The scheme of this oscilloscope is available on the Internet.

Be careful when connecting the boards, if you disassemble them. I have an old version, where the MSU outputs do not have protective resistors and an offset of 1 pin (which easily occurs - there is no key) burns out several MCU outputs. 

[DMack]

Thanks everyone for your responses. Not to oversimplify things, but is it safe to say that if I make sure to never connect the scope probe's ground clip to anything other than the negative rail in my circuit – which is actually ground if I use bench power, I should be okay, or am I still missing something important?

[ledtester]

I just want to nitpick on the way you worded this...

Thanks everyone for your responses. Not to oversimplify things, but is it safe to say that if I make sure to never connect the scope probe's ground clip to anything other than the negative rail in my circuit – which is actually ground if I use bench power, I should be okay, or am I still missing something important?

Simply connecting the oscilloscope's ground clip to the negative rail in your circuit will cause it to be connected to earth ground - regardless of what power supply you are using. I.e., even if you are using batteries.

From what you have said about your Tekpower 1803D it appears it is a floating supply. Therefore, using it to power your circuit will not ground reference it. The words above in bold make it sound like your circuit will be earth grounded if you use the Tekpower, and I don't think that will be the case.

[DMack]

Thank you ledtester, you made a good point. If I omit that part, does the basic premise still hold together as a general rule of thumb?:

“If I make sure to never connect the scope probe's ground clip to anything other than the negative rail on my circuit, I should be fine.”

[ledtester]

It all depends on your power supply's relationship to earth ground.

If you use a floating supply -- batteries or a floating bench supply -- you are safe to connect the ground clip to anywhere in your circuit.

If your power supply is referenced to earth ground then there will be nodes in your circuit which are at earth ground, and you should only connect the ground clip at those nodes.

For instance, there are some power supplies where the positive terminal is at earth ground. In that case the oscilloscope ground clip should only be connected to the positive rail of a circuit powered by that supply.

Perhaps until you're more comfortable using your scope and power supplies, just power your circuits with batteries / a floating linear power supply.

Update: Besides your power supply's relationship to earth ground, you also need to consider your circuit's relationship to earth ground. For instance, if your circuit has a wire extending out to a water pipe, or if it sitting on and touching a metal desk which is grounded, etc. then there will be nodes in your circuit which will be referenced to ground and you have to be careful where you put your ground clip.

[DMack]

Will do ledtester. I’ll stick with testing battery-powered circuits. I had no idea how complex this subject was when I first posted. Hoping I’ll start understanding with more experience. Thanks for the info.

[rstofer]

Thank you ledtester, you made a good point. If I omit that part, does the basic premise still hold together as a general rule of thumb?:

“If I make sure to never connect the scope probe's ground clip to anything other than the negative rail on my circuit, I should be fine.”

Unfortunately, this 'rule' won't work either.  There are circuits, like op amp designs, that use both a +15V and -15V power rail.  Grounding the -15V rail might be bad if the center of the voltage rails is already considered a system ground which it usually is.

It's really difficult to sit thousands of miles away and propose solutions of a general nature that apply in all conditions.  At some point, you just have to know what the circuit does.

One thing that might be true:  We can measure the voltage between the BNC ground and the point in our circuit where we propose to put the ground clip.  If it is 0V, good to go!  If not, we would need to find a better point.  If our op amp PS doesn't have an earth ground, clipping to the -15V supply wouldn't really hurt anything but the waveforms would be bizarre.

I like the idea of testing with a DMM.

[rstofer]

In the real world, this kind of problem just doesn't show up all that often.  We 'know' where to put the ground clip and we 'know' where to probe.  Why wouldn't we?  We just designed the thing...

Where it might be a problem is in the repair business when schematics aren't available.  I remember when I was a kid, around '60 or so, we could buy schematics for the TV sets we wanted to play with.  They even had sample waveforms and lots of voltage test points.  Today?  Not so much...

[Ian.M]

If you get into MCUs and programmable digital stuff, even at as basic a level as using an Arduino, you also need to be aware that almost invariably whenever you connect your circuit to a desktop PC or to many laptops, no matter whether its via a serial or USB lead or a programmer or debugger, it grounds the 0V rail of your circuit via the PC.   

Exceptions that don't introduce a ground:  Connections via a USB isolator (which usually limits the max USB speed),  Non PoE at both ends RJ45 Ethernet, or specifically isolated programmer/debugger and USB UART cables.

Grounds introduced via USB are particularly problematic because if you goof and blow up your scope it will almost certainly take out the PC as well, so you've probably doubled the cost of the lesson!

Another thing to be concerned about is ground loop currents.  These can become high enough to do damage if there is significant potential difference between supply grounds, or between supply ground and true earth ground.

e.g. if you are working on a radio receiver connected to an external antenna and ground rod, and your mains supply is TN-C-S, which uses a single conductor for both Ground and Neutral (aka: PEN or CNE conductor) upstream of the customer's utility meter, there can be enough voltage difference at a low enough impedance between your scope grounded to supply ground and the receiver chassis grounded to the ground rod to melt your scope probe leads!   Its also very dangerous as you have introduced a non-bonded grounded object into your electrical installation's equipotential zone, and if the PEN conductor gets broken you could get full mains voltage between the chassis of any grounded equipment on your bench and the radio ground.   It only takes ONE idiot with a backhoe half a mile away to kill you!  If you have a TN-C-S supply, seek qualified expert advice before introducing *ANY* other ground to your bench.

Other ground loop problems can occur due to voltage drops in high current DC circuits.  They are rarely dangerous to you, but can be very destructive to equipment.

[DMack]

One thing that might be true:  We can measure the voltage between the BNC ground and the point in our circuit where we propose to put the ground clip.  If it is 0V, good to go!  If not, we would need to find a better point.

This sounds like a promising solution for my situation rstofer. Thanks!

[Ian.M]

Unfortunately, due to the Y capacitor in non-grounded SMPSUs, you get a relatively high mains supply (line) frequency leakage current of up to 0.75mA with a driving potential of approx half the mains supply voltage.  A normal 10Meg input impedance DMM will thus show approx half the mains voltage between the scope ground and any point (that isn't very high impedance) in an ungrounded crcuit fed by such a SMPSU.  To know whether or not its safe to connect the scope ground to a node, you can't just check voltage.   0V is *always* safe to connect but 120V AC may be either 'will blow up your scope' or a phantom voltage that will vanish as soon as you load it down a little.  A 3W 10K wirewound resistor connected to insulated test prods is a useful thing to have.  Touch it between the  suspect voltage and probe ground, and if the voltage drops to under 7.5V RMS, 0V DC, you know its only Y capacitor leakage current and is safe to ground.  Worst case, assuming a 5:1 pulse overload rating, the resistor will survive for one second connected across the primary side DC bus of a 240V mains SMPSU. 

[wizard69]

As others have said posts like this scare me seriously.    This is the first post that sums up my feelings.

I'd go even a bit further and say don't even plug in the scope until you understand how it works and how the circuit you intend to work on functions.   Work on the right equipment and you can blow up even an isolated scope.

Interestingly; briefly after reading the title for this thread I immediately thought about the AC batter operated power source that Sportsman Warehouse had on display that I saw on a visit yesterday.   Such a unit would certainly power a small scope and might be useful for field use.   However after reading the post starting this thread alarm bells, whistles and sirens went off in my head.   Somebody is fixing to kill them selves if they don't take the time to learn what is going on with respect to connecting test gear of any type frankly.   

Hi king. Welcome to the forum!

Bottom line: A grounded oscilloscope is not designed to be used without its ground conductor connected to earth ground. Life is better if you "get this idea out of [your] head," as you said in your original post.

If you need a floating scope, then buy one that's designed to operate that way. If you're going to use a grounded scope, then learn how to use it as it was intended, including how to properly measure high voltages, perform differential measurements (e.g., use a differential probe), etc.