EEVblog #279 - How NOT To Blow Up Your Oscilloscope!

hi I just read a post on the EEV blog forum where somebody was asking about the oscilloscope probes and how they can be potentially dangerous if you hook this ground lead up to the wrong point in your circuit you can block your circuiting block your scope bang and it really is a big trap for young players and I've mentioned it before but they wanted to know exactly under what circumstances that could happen if I could do some diagrams to explain it and things like that well glad you asked let's try and sort it out so what's actually causing the problem here well it's to do with the fact that the BNC connector the outside ring of the metal outside ring of the BNC connector on almost all benchtop oscilloscopes like this are what's called mains earth grounded their mains earth reference in other words this metal shell on the outside of the BNC is connected directly via a very low impedance path dead short through to the mains earth on the back of the oscilloscope on the ISC connector and hence through to the mains earth in your system so if I've got a power cord here's an IC power cord is plugged happens to be plugged into the same power board as this oscilloscope it doesn't have to be it can be on the opposite side of the room the opposite side of the house or factory or whatever the mains earth will be common and tied together throughout the whole system so I've got my multimeter here set to ohms let's make sure we probe the right thing I'm going to probe earth the center earth pin there and I'm going to probe that B and C and there it is not 0.9 ohms it's effectively a dead short there's some there's some DC resistance in there but it's a very low resistance path effectively well it is a dead short through two mains earth and that can be problematic as I'll explain and because that outside shell of the BNC connector is connected directly through to your earth pin here you're going to get the exact same response on your earth pin there it is it's a dead short look at that and because it's a very low impedance path this path is capable of actually carrying a lot of current if you hook it up to the wrong point and it can do a lot of damage this little ground attachment lead is probably the weak link in the whole system because the shield of the the coax going back in the internal connections all going back to mains oh that's very you know very chunky bits of copper in there but this is probably the weak system so this is going to blow up or vaporize if you attach this to the wrong point and you have enough energy in your system you can blow this probe apart in your hand bang and just as an aside on any multi-channel scope like this be at 2 or 4 channel like this one these both of the BNC connectors are shorted together so there you go they're shorted together and through 2 mains earth so if you've got your two probes like this they're both connected to your Silla scope these two grounds are shorted together you can't just put these willy-nilly anywhere in your circuit because if you put them to anywhere that is not the same voltage potential ie both ground or both plus 5 volts or whatever then if they're different potentials you short out that thing here's a power supply I'll show you if I connect one ground lead to the positive output another one to the negative bang I shot the output that can ruin your day don't do it and not only that if that particular point in your circuit that you're shorting out has a large amount of energy in it and you get a huge current through this by shorting it out bang you can blow up your probes and sparks can fly everywhere real nasty stuff so let's try and take a look at this and figure it out what's happening on a diagram here please excuse the crudity of the model didn't have time to build it to scale or to paint it so what I'm going to show here is three different scenarios this is scenario number one and I'm going to assume a single probe not the dual probe thing I showed you before that's a different thing we're talking about this mains earth referenced problem so what we've got here is we've got a scope okay and it's got the BNC connector here and it's got the i AC mains input connector going down to earth down here and note the different symbols the shezzy mains ER symbol here is different to internal ground symbols that's just a little aside it's not really important for the purposes of this but as you saw with the measurement before that BNC is shorted directly through to mains earth and it's also shorter directly through to the alligator clip on your probe here or your crocodile clip whatever you want to call it now here's your widget your circuit that your probe in and it for simplicity sake let's say it has a positive rail and an internal ground rail as well and various points in your circuit that you're probing okay now let's assume that it's battery powered or what's called floating it's completely floating it is not referenced in any way back to main zorth it's just isolated in its own little world it's not connected to anything else now that means in this scenario here you can connect this alligator clip through to any part of your circuit it can be connected to ground could be connected to your positive rail or any other part of your circuit and you won't do any damage at all why because there's no current flow there's no way there's no loop here it's not going to flow anywhere current just can't magically flow down that alligator clip through the circuit unless there's a return path if you've got a floating system like this there's no way the current can flow through there so you can hook that alligator clip in a floating circuit to any point in that circuit and you won't do any damage what you will do is actually put if you connect it through to this point here in your circuit then you've mains earth referenced that point in the circuit what does that mean oh it just picks it if connected at the street it means if it doesn't it may not do anything in most cases it's not going to cause a problem at all unless there's some you know you're working two audio circuits in this home and all sorts of things but we won't go in pretty much scenario number one isolated widget you're working on completely safe to hook up the alligator clip or the probe to anywhere else in the circuit and because the probe is connected to the tip of the probe is connected to your 1 Meg input impedance on your scope let's not talk about times 10 probes and things like that is a 1 Meg resistor you the worst your going to do is put a 1 Meg resistor across any point in your circuit usually not a big deal you certainly can't blow anything up not a problem alright let's look at scenario number two we've got one of these isolated main supplies or a plug pack we've got our 110 or 240 volt mains coming in here and there's no earth pin we've only got our active and our neutral there and we've got a isolation transformer 50 60 Hertz and there's physical isolation electrical isolation between the primary and the secondary windings so there's no direct electrical path is just a little bit of capacitance really but you know there's no direct electrical connection so it's an isolated supply so I've just pictured here it's a you know half wage bridge rectifier but it can be a switching power supply switching plug pack or a regular transformer based plug pack makes no difference if it's got that electrical isolator supply and no earth no earth connection at all then it's exactly the same as scenario number one with our battery supply no different you can connect your alligator clip up to any point and you'll circuit any rail anything and you're not going to blow anything up no turns going to flow because current can't flow DC current can't flow through this transformer in any sort of loop at all completely safe now these isolated plug packs pretty easy to spot because they've only got the two pins on them active in neutral there's no third earth pin some of them might have a third earth pin just for mounting or they might have it to connect through to the internal transformer as a safety but they will still be isolated and if you're unsure if it does have that third pin measure it take the output here use your meter measure it between the ground pin or both pins just to be sure and the earth pin if you don't measure that direct short then it's an isolated supplier and that can be a switching type or the old-fashioned direct transformer type and the other type might be one of these switching plug packs another dead giveaway it's only got the two pins the active and neutral it doesn't have the third pin going in there now here's an interesting one this is my dell notebook power supply and as you can see it's got three pins on the mains input cord and it certainly has the earth pin on the connector so let's measure it let's see what we get look at that it's 1k instead of a direct short this one for whatever particular reason is our only 1k and that that's pretty down low and that can ruin your day that I would classify that as quite low impedance not as low as a dead short of course but still you wouldn't want to go probing in such a system with your oscilloscope if you had your notebook you're trying to repair your notebook and your repairing it from this then well you know you've got that 1k through two mains earth I'd be disconnecting this when I worked on it if I had to probe it live and power it directly from its battery so it was completely isolated now let's look at scenario number three what's called a mains earth referenced power supply remains earth referenced product or mains earth reference system it's the same as before it might have an isolation transformer in there but it has that third earth connection which shorts out the secondary side it's almost always the negative line but you know it doesn't have to be but it almost always is the negative line is shorted out and could be shorted to the primary so you lose that isolation there so this is now connected through to this mains earth back here back at your power point in your I swore in your office war in your lab wearing whatever it's all connected right back and then that is connected through to your negative terminal and your negative terminal is connected up here so effectively your circuit ground is now almost always once again it doesn't have to be the negative point in your product but it almost always is so that is not only just a common ground inside your product it's also reference to mains earth now why is this an issue well look what happens now if we connect this alligator clip up to this positive rail up here you know how it wasn't a problem before no current flowed no danger whatsoever well what happens now this positive rail current can flow through here through the shield of your BNC through the oscilloscope down through mains earth down here back through your power board back into your the lead the IC Li coming into your product all the way through here and up through your ground bingo congratulations you've just shorted that point there to that point there with a very low impedance connection through that alligator clip that crocodile clip and your scope probe and what happens bang it blows up or you know if if this rail up here has a lot of energy can supply a lot of energy you can vaporize your earth lady can explode in your hand real nasty stuff if it's a real low energy thing where your product might just shut down and it's not inherently like not massively dangerous if your power supply is not capable of actually delivering a lot of current then while your product is short out you power rail and it doesn't blow up but you see it's not going to work anymore okay so you want to avoid that so that's why when you're probing around with a mains earth reference oscilloscope like almost all bench oscilloscopes are if you're dealing with a mains earth referenced product be very careful where you put that alligator clipped that crocodile clip in your circuit because if it's not at the same point which is equipotential is the technical term or the same potential equal potential in there as your ground line you're going to short out that particular ever a voltage rail or a signal wire or something like that it's kind of blow up ruin your day so there you go that's what happens with a mains earth reference system beware now let's have a look at some bench power supplies here you'll note that any good bench power supply will have three output terminals positive negative and what's usually a green one which is a dead giveaway which they call ground okay it might be call grant that's actually mains earth back on the mains input lead on this thing so normally if you don't join these together if you know if you haven't joined them together at all these power supplies are floating they're completely safe you hook up your positive negative like this to your positive and negative terminals and you can power your circuit you can probe around you're completely safe but if you strap these two terminals together like on this power supply over here you see a lot of them will come with one of these straps that allow you to strap the mains earth through to the negative terminal and that instantly turns that isolated power supply into a mains earth reference power supply and the product you're powering you need to be very very careful where you probe with an earthed oscilloscope now why would you want to join these together I won't go into the reasons because it has to do it depends on the system your your actually are designing and things like that so I won't go into it but if they are joined together and you mains F reference it beware but I don't think I can leave it there without giving you a bonus one number four USB products and it can be other connections as well not just USB but USB is a common example these days let's say you have your widget and it's battery-powered okay it's normally completely isolated and completely safe to probe any way you want with your oscilloscope but once you connect the USB lead through to your computer you're in deep trouble why because the us the ground pin on the USB connector is almost always connected unless you're going to isolated USB which are very rare and very specialized it'll be connected through to the ground point of your circuit in there and then that lead will go back to your the USB port on your computer down here which will be connected through to the ground point on your circuit board in your processor and everything else but you're the ground point in your PC is also mains earth reference almost all pcs are and it goes through two mains earth directly through and you can measure that so bingo your isolated product has just become mains earth reference by the mere fact of connecting up that USB cable and you've got that dangerous loop scenario happening again and you've got to be careful where you pro where you connect the alligator clip or the ground lead on your oscilloscope if you connect it up here bang you've just shorted the whole thing out again beware now I know you won't believe me unless I practically demonstrate it so it will we've got an Arduino compatible board here in this case it's the free Tronics 11 board and if you're powering your little Arduino board from you plug your isolated plug pack power supply remember no mains earth pin and you're powering it then you're completely safe because it's isolated or you're pairing it from the battery completely safe it's isolated but if you take your USB cable here and it's hooked up to my desktop computer here and we plug that in and we power our board there it is it's turned on repairing our board from our USB what happens we've got our mains cable here let's measure mains earth and see what we get here's the ground pin on our circuit look at that not mine ohms there you go because it's got to go through or dicky stuff in the computer and things like it's shorted to mains earth bingo so you sweet little innocent Arduino board that you thought was isolated has just become a real potential problem and if you try and probe around here with your oscilloscope or if you try to probe your circuit or your shield connected on to that with the with the alligator clip on your crow probe and you put it anywhere other than that ground terminal is screwed you're going to short out the power supply the Arduino shut down it won't blow up in your face because the little power supply on the Arduino can't generate you know huge amounts of current and provide a large amount of energy but it's going to shut down short out your power supply you might actually damage your power supply on your Arduino or you might damage your plug pack or something else so that can be a real issue even if it's battery-powered you perón this thing from the battery or from the you--it once you connect that USB lead up if instantly mains earth connected it you've ruined your day and don't try this at home I'll actually demonstrate it we've got our Arduino it's working there's the LED is flashing away and it's hooked up to our mains earth reference computer via the USB and if I hook up the ground lead to the ground the earth lead on my oscilloscope up to the ground point on the circuit no problem still continues to work I can probe away everything's safe but if I connect this through to the power rail the ground lead watch this ready don't try this at home bang and you might be able to hear the computer reboot because the device is shut down I've just I've just shorted out my power supply nasty and if you do that to a high energy circuit you can blow the thing up blow up your scope probe nasty but if I do the exact same thing again powering it from the isolated plug pack it's not mains earth referenced anymore so I can plug my ground lead on to the ground and it's fine of course and I can plug my ground lead anyway else on the circuit that's the same point that was causing it to short out before and it doesn't do anything because this board is floating it's not it's isolated it's not Maine's earth reference I can put that ground lid on any point in my circuit that's not going to affect it so what do you do if you've got one of these mains earth reference products and you can't avoid it being Maine's earth reference and you want to probe it safely working on you know a high voltage high energy switch my main switch mode power supply and you want to safely probe everything well the way to do it is with a a high voltage differential probe and I'll show you some examples of this here's a beak a precision PR 60 Pro Braley talking you know three hundred and something dollars for these are type of probes and as you can see it converts the single-ended BNC input for the oscilloscope through to an isolated differential probe system like this so you can input positive and negative anywhere in any product at all mains are referenced or not and there and be completely safe and usually these probes are actually high voltage probe so they actually have a times 10 so this one's switchable between times 10 and times 110 uation so it actually replaces your traditional times 10 single-ended oscilloscope probe but now is differential completely isolated and they can be isolated to our you know several their you know hundred many hundreds of volts or several thousand volts in this case our common mode plus/minus 700 volts and the differential can be plus minus 700 volts as well and there's other ones here's an Agilent one same thing you know single-ended oscilloscope input through to a differential probe output there it is that's a high 100 megahertz high voltage differential probe and there's lots of other ones on the market there's lacroix one you know there's a whole bunch yockeygal make them every man this dog makes this high voltage differential probes and I highly recommend you are pick one up it should be pretty much your standard kid especially if you're working on switch mode power supplies or something like that they're a little bit on the expensive side but safety first and there are other ways to get around the issue as well and that is to use a mains isolation transformer like one of the ones shown here it basically it does exactly what it says it isolates the mains are 240 volt in earth 240 volt out isolated and that's designed to power your product under test and it tells you here isolate test equipment like crows or protect switch mode power supplies and things like that but this can be used to isolate your oscilloscope so that your oscilloscope is not means earth reference anymore but that's not really the recommended way to do it your oscilloscope should remain for safety reasons a grounded a mains earth grounded product so you should use the isolation transformer to power your particular product under test and this can be an essential bit of kit in any art test or repair lab and if you've got a USB product and you want to isolate that you don't want your USB cable to cause any issues then you can get one of these rather an obscure product a USB isolator there's not too many of them around but that basically Goods got a transformer in there with some high speed to what couple not only the power lines but the data lines as well at full USB our speed these things aren't particularly are cheap there are very fairly nice thing but it isolates us being USB out the output is completely isolated data and power so there you go I hope you found that interesting so just be careful and watch out for mains earth reference systems next time you're probing your oscilloscope you don't want to blow up your oscilloscope you don't blow up your probe and most of all you don't want to blow up yourself so if you liked the video please give it a thumbs up and I'll catch you next time you you

Channel: EEVblog

Views: 769,869

Rating: 4.9611053 out of 5

Keywords: oscilloscope, probe, clip, lead, ground, earth, loop, short, danger, isolation, transformer, isolated, usb, port, arduino, circuit, electronics, computer, bench, power, supply, battery, dangerous, probing, floating, measurement, scope, cro, dso

Id: xaELqAo4kkQ

Length: 24min 25sec (1465 seconds)

Published: Wed May 16 2012