How much damage can I do with a 20v battery and a USB scope? - Page 1
Forum section: electronics / beginners

author: SantaClaus
author_rank: Newbie
timestamp: October 10, 2022, 06:54:45 pm
content: My first scope is arriving this week and I’m really excited! I’m also nervous about learning a hard (expensive) lesson. I’m coming from an arduino hobbyist background, not EE. My understanding of circuits is murky and I don’t even know what I don’t know.The scope is an Analog Discovery Studio which I will have plugged into a laptop.I’ve only ever wasted a few dollars of magic smoke in my time but now I feel like I have some skin in the game. My computer is very precious and the Analog Discovery Studio is an extravagant purchase for me. I’m worried I’m going to send some volts somewhere they shouldn’t go and I haven’t got the budget for those kind of lessons.I’ve watched Dave’s video about blowing up scopes, and I’ve been finding and reading a lot of good resources on scopes. These all presume an ability to identify voltage potentials, which I, umm, am not yet confident in my ability to do. I was hoping to have a sanity check before I go diving in with probes.The Analog Discovery gets power from a barrel jack, so I don’t think it has a ground reference. The laptop will also be floating. I don’t understand the implications of having the whole setup floating like this.The project that I want to test has a brushless motor driven by a speed controller module controlled with user input collected by an ESP32 MCU. It is powered by a 5 cell li-ion tool battery (~20v, 3Ah). I use a buck converter module to provide a 3.3v rail for the ESP32 and connected sensors.If I want to faff around and look at the phases of the brushless motor on the 20v side of the project, am I raising the potential of the whole test setup? Can I ground this accidentally by touching the wrong thing? If I were to power the project from a grounded bench PSU, is that better or worse? Should I be protecting my equipment in some obvious way that I don’t know about?What’s the absolute worst place I could put the ground clip of the probe? What single thing can I do to cause maximum damage, to my computer, to my test equipment, and to my pride? How did you blow up your first scope?Am I being paranoid? The scope is rated for +-25v, but I don’t have any idea of the black magic that goes on in brushless motors. If I active-brake the loaded motor while it is drawing >1a, what does that do to the voltage on the individual phase wires? What kind of stuff in a DC circuit can cause voltage spikes or transient voltages? Is this stupid to even think about going near the 20v side? Is it dangerous to scope the project while it is connected to this chunky battery at all? Should I be limiting the current to a range much lower than the battery can provide while the project is under test?I bought the scope ostensibly to look at the serial communication and signaling between the two MCUs in the project, so I’m happy to stick to stuff under 5v. But I also bought it to explore and learn with, and I kinda want to stick the probes anywhere they will fit (on the project). I’m hoping someone can warn me off any obvious dangers they see here.Much thanks to anyone willing to read all this and to anyone willing to offer some guidance. Cheers,

author: rstofer
author_rank: Super Contributor
timestamp: October 10, 2022, 08:12:40 pm
content: You can use 10X probes to reduce the voltage seen by the scope to 2V from 20V.You can always measure the voltage with a DMM before using the AD Studio. ETA: you might also measure relative to earth ground.You could also put a 100k resistor in the scope ground lead. This will alter the readings but limit the ground current. After things start to look right, remove it. I haven't tried this but it seems like a reasonable way to isolate ground systems.Most of the time we have a schematic and this helps identify what and how to measure things. Just poking around is reckless. Far better to start with a disposable DMM.You could use a powered USB hub to isolate the Studio. You might be able to use a USB isolator if it is fast enough.https://forum.digilent.com/topic/20373-usb-galvanic-isolation/Nice choice on the AD Studio. I have a couple of AD2s but I still want one of the StudiosI tend to limit my use of the AD2 to voltages <= 5V. If I really want to measure higher voltages, I use a real scope with 10X probes. I find the AD2 most useful for 'educational' types of projects. I can spend all day looking at a 10k resistor in series with a 0.1 ufd ceramic capacitor as a low pass filter in both the time and frequency domains. It is reinforcing in that I go back to the math and work forward to waveforms.

author: SantaClaus
author_rank: Newbie
timestamp: October 10, 2022, 10:04:09 pm
content: Appreciate your input! I read some of your other posts about the AD2 and intent to perform the experiments you outlined! I have some switchable probes on the way, and I think I'm going to tape the switch in the 10x position until I have any reason to change it.I’m really stoked about the AD Studio! I have a longer term goal of designing and bringing up a custom PCB and this seems like the ideal platform to support this goal. It also is just what I need to debug some issues with my current ESP32 project. I didn’t know the thing existed until last week. I saw an article on Hackaday about someone building a laser microscope around an Analog Discovery 2 and was immediately sold.I was set to buy the Analog Discovery 2 and was finding some good deals on eBay. But after looking at the Studio I had to follow my heart. The built in connectors seemed to tidy everything up a lot. The swappable breadboard platens also got my imagination running. I was instantly picturing custom 3d-printed test rigs with pogo pins or a platen with built in vices for holding awkward work. Lots of opportunity for fun cyber-deck/circuit-deck designs with a Raspberry Pi too.The sticker price is rough for hobbyists, but I can’t really blame a company for not chasing the retail market. I could find ways to debug my project issues without this, and I could collect equivalent or better functionality with cheaper tools. On the other hand, FPGAs are cool as hell and a hackable and well-documented portable lab for learners really hits a sweet spot between my practical needs and my equipment fantasies.

author: 2022-seo
author_rank: Newbie
timestamp: October 11, 2022, 08:27:50 am
content: This post is very useful to me, thank you very much for the platform

author: rstofer
author_rank: Super Contributor
timestamp: October 11, 2022, 10:57:43 pm
content: Quote from: SantaClaus on October 10, 2022, 10:04:09 pmI bought a pair of FIXED 10X probes specifically for the AD2. The allowable input voltage is low and the replacement cost is high. Not that I ever work with higher voltages but, still, I don't want to take a chance.ProbeMaster 3904.2 I believe. About $100/pair so they are quite pricey. But so is the AD2...https://probemaster.com/3900-series-scope-probes-100-mhz/

author: SantaClaus
author_rank: Newbie
timestamp: October 18, 2022, 07:02:12 pm
content: I received the device and got everything set up and wanted to follow up on the post.Initial impressions of the ADS are good. The form factor is nice and the case is sturdy. I've never had a proper breadboard that I've actually used, so I'm really enjoying that aspect. The integrated breadboard was one of the big selling points for me after having seen this video from CuriousMarc about restoring his awesome retro deluxe breadboard kit.https://youtu.be/enMAp99udqwThe ADS really seems like the direct descendant of that device -- a lab attached to a breadboard. The mess of wires and circuits inside are tidily replaced with an FPGA in the modern version.The only hurdles so far have been in Waveforms. I got myself pretty flummoxed with the window management. It obviously doesn't use platform native APIs, and the behavior was obtuse to me. I finally got the hang of it, but just getting two instruments on the screen together took me minutes of fidgeting and experimenting. Also, as a total scope newbie, it has taken a while to learn the ranging and viewing parameters. Watching people operate traditional scopes -- they seem just twiddle the knobs and get the waveform in view. I was trying the mousewheel everywhere and just completely losing the view. It took a long time for the view to redraw, so I wasn't getting good feedback about what my inputs were doing. This really has more to do with my ignorance than any real software limitations, but I wish there was a software control in Waveforms that was as intuitive and responsive as a knob. Otherwise I'm super glad that the software is cross-platform. I'm excited to get the device running with a Raspberry Pi and an old monitor in order to have a more permanent bench setup.In terms of my use case, it has been great so far. I built the simple RC filter that rstofer suggested in another post. It was a great intro to the device and software. I also put together a quick circuit with a thermistor and had fun measuring temperature with an oscilloscope.Right now I'm using it with an older laptop, which eases the anxiety of blowing up my primary computer. I also decided not to power my project with the external battery and use the 12v supply on the ADS instead. No magic smoke or even magic smells. I was able to see the communication over the two UARTs with the logic analyzer, and I decoded it with the protocol analyzer. It has been really cool to have a log of sample traffic to dissect, and it really compounds the learning I'm doing on the coding side.I'm excited to have this really cool tool in my workspace and have no regrets with my purchase. I still think it is too expensive, but the value proposition will be different for everyone.

author: james_s
author_rank: Super Contributor
timestamp: October 18, 2022, 08:50:31 pm
content: As long as you use a 10X probe you should have nothing to worry about. For reasons I have never understood, most cheap probes have a 1X/10X switch, which I recommend gluing in the 10X position. It is very rare that you will ever need a 1X probe and if you do, just spend 20 bucks to get a dedicated 1X probe.

author: Old Printer
author_rank: Frequent Contributor
timestamp: October 19, 2022, 05:54:20 pm
content: Love working with my AD's, every so often I am able to snipe one of the older black AD's off eBay on the cheap, like $50! I pass them along to friends. With the low prices on decent computer monitors the last few years I have a acquired a dual 25" monitor setup that is great to work with my old eyes. Congrats on your ADS, I am sure you will love it.

author: rstofer
author_rank: Super Contributor
timestamp: October 19, 2022, 10:58:59 pm
content: Quote from: SantaClaus on October 18, 2022, 07:02:12 pmI have done this and it works well. I seem to recall a requirement to install the drivers first. This is for the AD2 but I'm sure there is a page for the Studio as well.https://digilent.com/reference/test-and-measurement/analog-discovery-2/getting-started-guideThe cool thing is in developing some kind of widget to hang off a Pi, you can run the project code and watch the output from the ADS at the same time. A Pi 400 works well here! A 27" monitor is also a plus. Dual screens, space available, is the way to go. Dual 27" monitors is even better!I'm glad you liked my experiment. There's a lot of education in that simple setup.

author: py-bb
author_rank: Regular Contributor
timestamp: October 22, 2022, 01:08:06 am
content: Oh wow that is a fair bit of money. I recommend you keep an eye out for a 2nd hand old analogue one but stillreally really important thing:It's not a volt-meter, the ground clip is actually grounded. That means if you touch it to something that isn't floating or grounded you'll have created a shortIf it was a voltmeter (this would be a "differential probe") you could stick it on say +3.3v and the probe on +5v and see the circa 1.7v difference - no you'd have shorted 3.3v to ground!A poor man's differential probe takes 2 channels, clip the ground clips of the probes together, put your scope in "add" mode and invert channel 2, now it's doing C1 + (-C2) = C1 - C2 - bam the difference.Otherwise respect voltage ratings, like if it says DC 20v or whatever stick within that, this includes surges/switching noise (you can probably get away with it) and whatnot.This is what x10 probes are for though really.Someone else pointed this out to me in another thread but if you AC couple it and inspect a large DC offset'ed waveform, that capacitor can stay charged for a while and if you touch the probe to a circuit you risk discharging into it, my scopes have switches that go AC<->GND<->DC - so to switch you have to go through ground (which discharges it)finally, if you do select GND, verify this disconnects your probe (but it'll leave the ground clip connected of course) - I'vehearda story where this grounded the probe tip, so it became a short. I personally doubt this ever happened as that'd be a stupid design but check once and know forever.