Afraid to use my oscilloscope? - Page 1
Forum section: electronics / beginners

author: DennisCA
author_rank: Contributor
timestamp: November 08, 2021, 09:55:56 am
content: Let me start by saying I've seen the "how not to blow your oscilloscope up" video as well as the "scopes for dopes video" and while I feel I learned from those videos it wasn't enough to assail my worries.I still don't feel like I understand well enough to know where to safely probe, wheter to use the ground clip or not on the probe. For instance I have this alarm clock radio, the alarm function isn't working, I think the oscilloscope could help me test the IC and other paths on the scope, but I feel hesitant to try.The plug is ungrounded, but if I understand correctly it's still earth referenced via the neutral and because my scope is grounded, I should be able to just use the probe (fixed 10x) on the inputs and outputs of the chip to see what's going in and out? The transformer is 230V > ~8.5V, though it has a center tap for ~4.3 volts so it should be safe from high voltages, but I feel very uncertain about everything I do at the moment.Are there any procedures, or videos or texts pertaining that teach me how to help one determine where it's safe to probe? What to look for, general principles, clues, tips? In addition to the two videos mentioned earlier.I don't think an isolation transformer for the DUT is the answer to every question. Also I think I used an IT that I would need to put the earth clamp on the probe and provide an earth reference that way. But I am uncertain where it ought to go, don't even have a schematic for this. I looked at the chip layout but didn't see an earth pin, that would've been my goto otherwise.I know the proper way is to learn about circuits and electrical theory to the point that it just makes sense, but I am not there yet... Got a book about circuits I started on recently... I feel mucking about with real circuits also teaches me things as well as keeping me motivated to read on.

author: capt bullshot
author_rank: Super Contributor
timestamp: November 08, 2021, 10:14:57 am
content: At the left hand side, you should notice the mains transformer. The "dangerous" mains voltage ends here. Apparently, there are a few components on the PCB that are connected to mains (like these blue resistors). Stay away from these with the oscilloscope. If you can't guess which components from experience, remove the PCB and follow the traces on the other side. This radio looks like a quite old design, they've made some connection through high value resistors to mains - maybe to make "sensor" keys work, but might be something else anyway.The transformer (and the high value resistors) isolates the rest of the radio from mains, so there's no earth connection to the circuitry. As it is isolated, you have to connect the ground clip somewhere, preferably to circuit GND, to get results.So to use your scope, you should first of all find the common GND of the isolated side (this might be the "-" minus) terminal of one of the larger electrolytics and connect the ground clip there. The shields of the tuneable inductors might work as GND as well, usually I'd expect them connected to circuit GND.

author: Nusa
author_rank: Super Contributor
timestamp: November 08, 2021, 10:21:28 am
content: Do sanity checks with your multimeter first...that'll save you a lot of grief. In fact, do all your initial sanity checks with the multimeter. There's little reason to use the scope before you have identified a signal worth watching.

author: Ian.M
author_rank: Super Contributor
timestamp: November 08, 2021, 10:22:57 am
content: Without a schematic (and preferably a full service manual), its difficult for beginners to assess such a board.Start by getting the board out and taking better*photos of both side of it and posting them here so we can help you identify if there is an isolation barrier, and if so, which areas of the board are mains 'live' and which are reasonably safe low voltage.The four resistors near where the mains lead is connected to the board are suspicious - they may indicate the existence of a non-isolated zero crossing detection circuit for timekeeping.* PCB photos need to be well lit (avoiding shadowing), in focus, high enough resolution to read component markings, and in most cases taken with the camera lens centered over the board to avoid perspective distortion.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: November 08, 2021, 10:27:12 am
content: 1. First unplug your gadget (it probably is unplugged, but can't see the mains plug).2. Unscrew the PCB from it's housing, so you can also examine the bottom.3. Follow the mains wiring. (It probably goes directly to the transformer)4. Make notes (metal or with a felt tipped marker) of where the mains voltage goes.5. Isolate that part of the circuit, so you can't touch it.This can for example be done by first cutting out some corrugated cardboard to even out the underside of the PCB combined with some plastic or a few layers of duct tape over the cardboard. Then clamp your custom made isolation to the PCB (screws if mounting holes are available, or use duct tape / isolation tape or a few small dabs of hot glue)Usually anything after the transformer is safe to poke at with your oscilloscope, especially when you use a 1:10 probe.The biggest risk is the crocodile gnd clamp. There often is not a good attachment point, and it gets pulled in all kinds of directions while you are measuring. I often identify the "GND" connection of the circuit, and then solder something to it which I can securely grab with the crocodile clamp.The danger here is that if the crocodile clamp slips of the GND connection then it has the potential to short two parts of the circuit together on the PCB.If you want to be extra safe, then you can modify the GND lead of the scope probe and put a small fuse in it.For general safety you have to be aware that some big high voltage capacitors (primary circuit of SMPS power supplies) can hold hair raising charges for some time after a circuit is unplugged.There are also some circuits that inherently work with high voltages.* Nixie tubes -> Few hundred volts.* CCFL backlight upto a few kV (Keep your scope away from this).* Fluorescent displays 60V to 150V (approx, I think so somewhere in that region).* more ...Edit:The big resistors are indeed suspicious, and they are likely used for detecting the zero crossings, but for that you need both the "phase" and the "neutral". It's a bit weird they did not just connect this to the secondary side of the transformer.Idea:It's quite easy to make a small isolation transformer if you have two identical transformers. For example use one for 230Vac -> 12Vac, and then use the other backwards for 12Vac ->230Vac.Yet another idea:You can analyze the power supply section, and then solder some wires just after the rectifier and then inject a DC voltage from an external power supply. This way you do not have to plug your gadget into mains voltage at all. If your gadget has multiple secondary windings this is more difficult.

author: DennisCA
author_rank: Contributor
timestamp: November 08, 2021, 10:48:17 am
content: Thanks for all the suggestions so far, however I noticed nobody remarked upon what I mentioned of not using the ground clamp and instead relying on the common earth reference between the DUT and oscilloscope, I have seen youtube videos where they do that and also seen posts on forums where people say that's how they mostly use their scope when looking at circuits. Is this a bad practice vs using an IT? (I don't own an IT either, for now).I do believe I probed that big resistor with my DMM and it delivered 230V.

author: capt bullshot
author_rank: Super Contributor
timestamp: November 08, 2021, 11:10:29 am
content: Not using the ground clip of the oscilloscope probe is in general a bad idea. A common earth connection can provide the signal ground for some rather rough measurements, but this is not the normal use case. The "normal" way is to always connect the ground clip to a suitable circuit GND.There are special circumstances that will give you halfway usable results (gnd clip not connected), including low frequencies and high signal levels. In general, not using the gnd clip will bring all kinds of interesting distortion and interferences to the scope if the signals are small (less than maybe 5V) and of higher frequency than maybe audio range.Most circuits don't require an isolation transformer, as they are safely isolated from mains anyway (by their power supplies). If the circuit isn't, I'd recommend to stay away until you've gained more experience.The big resistors provide safe isolation from mains, although this technique isn't common anymore. Don't know whether it's common, but from what I've seen, this kind of radio uses the very small current from mains through these resistors to make "Sensor" buttons work. If you touch the button, a small current flows from mains through your body that is detected by a simple circuit. Because the resistors are of high value (1 MOhm ballpark), the current is small enough you won't notice it. For the same reason (very high value resistor) these resistors don't provide a suitable ground path for scope measurements.

author: SilverSolder
author_rank: Super Contributor
timestamp: November 08, 2021, 11:51:36 am
content: Using an isolation transformer is a good idea - it will make you feel much more confident and comfortable, even if you "could" measure without one...

author: Renate
author_rank: Super Contributor
timestamp: November 09, 2021, 06:42:00 pm
content: Normally such a small radio with a transformer would be safe, but it looks like they are feeding some clock signal from off the line so you should be careful.In a normal repair facility you'd have an isolation transformer which you'd use even in normal circumstances (in case there was a primary secondary short).If you can afford it, pick up two transformers, like 230 VAC to 12 VAC (or 24 VAC or 6 VAC or whatever).Connect the primary of one to line, connect the two secondaries together, connect the primary of the second one to the power cord of the radio.The efficiency is not good and if the transformers are small you won't be able to play at full volume, but that's not a big deal.Since you want to fix the clock the line signals may be where you have to start and an isolation transformer will make you happier.

author: DennisCA
author_rank: Contributor
timestamp: November 10, 2021, 01:30:10 pm
content: I found this on a 2nd hand site, sold as an isolation transformer, 30 euros:Put it in an enclosure and hook up two outlets and I should have an IT right?

author: SilverSolder
author_rank: Super Contributor
timestamp: November 10, 2021, 02:09:01 pm
content: Good find! - it's not often that you see a toroidal isolation transformers - they are superior to the normal types.Warning - When you mount it in your enclosure - don't accidentally create a 1 turn loop with the bolt through the middle and your enclosure!

author: Old Printer
author_rank: Frequent Contributor
timestamp: November 10, 2021, 02:30:30 pm
content: Make sure you are using at least a 10X probe, and if still concerned get a 100X one for good measure.

author: Renate
author_rank: Super Contributor
timestamp: November 10, 2021, 02:53:09 pm
content: That's a nice transformer and 1000 VA to boot.You're going to have to start up your own repair shop!

author: Brumby
author_rank: Unknown
timestamp: November 11, 2021, 07:53:38 am
content: Please be VERY careful if you are thinking of using an isolation transformer.The obvious benefit brings with it some risks that are not immediately obvious. The general wisdom is to NOT use an isolation transformer if there is any other way of performing the measurement ... and this starts with the question: "Do you reallyneedto measure this on a scope?"A set of appropriately rated differential probes isby farthe better option.

author: DennisCA
author_rank: Contributor
timestamp: November 12, 2021, 06:26:35 am
content: Differential probes are kind of expensive though. But I don't know I need to use an oscilloscope on the high voltage circuit, I was mostly considering using that on the low voltage parts.But I also know that on the whole I will most likely need an IT for future projects as well, so I might as well get this one. I have written a list of tools to keep a look out for on the 2nd hand market, such as a variac, signal generator (into the RF range) and current limiter seems like some basic useful tools. I have no funds for buying new equipment so I am willing to spend a long time waiting for them to pop up, like this transformer did.I have fixed 10x probes for this scope. It's an analog 40mhz scope (hameg 404-2).

author: HB9EVI
author_rank: Frequent Contributor
timestamp: November 12, 2021, 02:31:55 pm
content: Indeed you can cause you trouble with the usage of an isolation transformer - most obvious is the fact, that as soon as you clip the GND of the scope in a circuit, there is no isolation anymore. That's not a big problem, if you're probing in a low voltage circuit, but doing so in a 230V circuits means you and the scope are in the same danger like without an isolation transformer.like it was said already, the right way to go working on mains voltage level is a differential probe.if you're probing the on secondary side of the device's transformer, so there where you measured 8-9V not much can happen to you or the scope; but beware that there is mains voltage on the same pcb and sometimes the way to get in touch with it is shorter than it seems.

author: freda
author_rank: Regular Contributor
timestamp: November 23, 2021, 06:41:45 am
content: Don't you normally power the scope AND the DUT from the isolation side ?what bothers me most, is that say you had a brain fart and did something stupid,the RCD protection on your mains won't trigger, will it ?

author: james_s
author_rank: Super Contributor
timestamp: November 23, 2021, 06:57:40 am
content: Quote from: freda on November 23, 2021, 06:41:45 amNo, why would you do that?It's true that an isolation transformer defeats the RCD. I don't consider this a serious issue though, in much of the world we don't even use RCDs except for outlets in wet locations.

author: Kalvin
author_rank: Super Contributor
timestamp: November 23, 2021, 07:12:09 am
content: You could use a lab power supply to feed voltage into the clock's circuitry. In this way you would not have to deal with the mains voltage at all while probing the circuitry.

author: freda
author_rank: Regular Contributor
timestamp: November 23, 2021, 08:01:30 am
content: Quote from: james_s on November 23, 2021, 06:57:40 amwell mains powered scope, mains earth lead connected to chassis, well is on my old tektronix, and I did that on the very rareoccasion I was checking non isolated mains powered stuff in way past.Sure I got battery powered scope now, even the power brick is isolated, no earth, so nothing to worry about with these as theyare by default "floating".

author: Ian.M
author_rank: Super Contributor
timestamp: November 23, 2021, 10:41:35 am
content: If a scope has a ground pin on its power in connector:*NEVER* float your scopeCompetent engineers who thought they knew what they were doing, and who had taken what they thought to be reasonable precautions against contacting a live part have been killed doing exactly that. e.g.https://groups.io/g/TekScopes/message/10754A CAT II 600V rated differential probe from a reputable manufacturer isn't likely to be cheap, but is still far cheaper than a funeral!If it does not have a ground pin on its power in, read its manual carefully. It *may* be rated for floating measurements up to 50V from ground, but*NOT* for mains voltages.Very few scopes, even handheld, are rated for full floating measurements Cat II 600V or better on mains voltage circuits. If you can touch the metal shell of any BNC connector or any other connector with a single channel connected via its original probe lead, you can bet your handheld scope is not CAT II rated.Except in the rare case of scopes designed and intended for fully floating operation (e.g. Fluke ScopeMeters), your scope was designed under the assumption that no hazardous voltages would be present on its chassis (or front panel PCB ground plane). You don't know how good the insulation is between the scope chassis or ground plane and your finger at the supposedly insulated control you are touching. e.g. conductive dirt may have built up on the sides of a button, or in the hole for the grub-screw of a knob, from fingerprint oils and ambient dust. If your scope isn't properly grounded, and you are using it on a non-isolated mains derived circuit, all it takes is one bad mains spike at the wrong moment i.e. while you are pushing a button or holding a knob, and there's a significant risk of one dead engineer.

author: m k
author_rank: Super Contributor
timestamp: November 23, 2021, 11:37:15 am
content: Long long ago I had to connect the protective ground to get anything out from an old Tek and single ended probe.Possibly the whole measured circuitry was totally unknown.Maybe the isolated one was the scope and not a DUT.I've also measured mains without any protection but that is finally pretty stupid thing to do.It's not the mains but the probe and where it actually is when it isn't.BTW, around here and for repairs the isolator transformer used to be a law.

author: PKTKS
author_rank: Super Contributor
timestamp: November 23, 2021, 12:55:18 pm
content: Quote from: m k on November 23, 2021, 11:37:15 amIT is ... the best rule of them all.ISOLATE all your gear on the bench to repair nasty stuff..you do not know how bad the DUT is ..And safe rules in these cases are not handy.I have my bench ISOLATED (floating) for decades..so far so good nothing weird happened.Just think ahead and be safeREPAIR and R&D are 2 very different thingsPaul

author: freda
author_rank: Regular Contributor
timestamp: November 23, 2021, 01:16:54 pm
content: I guesQuote from: Ian.M on November 23, 2021, 10:41:35 amI guess I generalized too much in my post. As yeah I was referring to a scopemeter as I have, actually its a bit old, since its labeled by Phillips.But I do have another scope, with 4channels, color display and a lot of fun to use and intended for digital and general low voltage scoping.I appreciate your post Ian.M to remind myself never to use it in any voltage stressfull way.

author: Kalvin
author_rank: Super Contributor
timestamp: November 24, 2021, 06:59:01 am
content: In my understanding the DUT (device-under-test) shall be floating and thus connected to mains using an isolation transformer, and the measuring instruments shall be connected to the protective ground. NOT another way around.And do not solder live high voltage circuits even when the DUT is floating (or, you THINK it is floating).

author: Ian.M
author_rank: Super Contributor
timestamp: November 24, 2021, 09:09:23 am
content: Quote from: Kalvin on November 24, 2021, 06:59:01 amThe problem is *keeping* the DUT floating. If its really floating, accidental contact with it with one hand cant electrocute you - there is no path for current to flow through your chest and heart. You can still get a nasty shock of course with the risk of electrical burns and even local nerve damage, but you will be alive to regret it. However you loose that small amount of safety from a floating DUT as soon as you connect anything grounded as although still isolated, it is no longer floating as soon as you make that ground connection,, which could be via your ordinary scope probes, most of your test equipment, or even your bench PC and it isn't always obvious *what* will introduce a ground to the DUT. e.g. roof mounted aerial systems are often grounded and almost invariably so if they feed multiple outlets.Quote from: Kalvin on November 24, 2021, 06:59:01 amDo not solder live circuits full stop.Even if its low voltage, you can get in big trouble. Most soldering irons and soldering stations have the tip grounded to prevent leakage currents damaging highly ESD sensitive components, and its usual for the tip to be hard-grounded with no series resistor to limit discharge currents. If you attempt to solder anything connected to one terminal of a LiPO battery pack (or other high current battery) with the other terminal grounded, e.g. via your scope's ground clip, you will regret it as the battery shorts out through the ground loop formed by your solder station and scope.If batteries are involved *ALWAYS* totally disconnect and isolate the board or pack while soldering.

author: Steffalompen
author_rank: Contributor
timestamp: November 26, 2021, 06:12:55 pm
content: I wish all the videos on the subject would stop talking about "the probe" and call it the ground clip, to lessen confusion.As for me, I'm sat in the second floor of my wooden house, and there isn't a ground wire to be found on this floor.So then I suppose at worst the floating charge potentials could zap me or my gear? (Unless I manage to ground clip the scope chassis to L or N and contact the other)

author: tautech
author_rank: Super Contributor
timestamp: November 26, 2021, 09:30:07 pm
content: Quote from: Steffalompen on November 26, 2021, 06:12:55 pmWelcome to the forum.Actually it's more correctly called the probe reference lead which for any signal you probe is referenced against.Most scopes of course are mains ground referenced and their BNC shells are at that potential for safety reasons which brings us to your concern that your scope and other instruments may not be grounded which indeed due to the X/Y caps in the mains input filters could couple a small charge into the instruments chassis.This could certainly potentially damage sensitive circuits and also not having a mains earth close to the bench gives you no way to manage ESD so a dedicated lab earth feed could be well worth considering.

author: Steffalompen
author_rank: Contributor
timestamp: November 26, 2021, 10:58:54 pm
content: Thank youSo like an ESD strap I could have the entire feed high impedance and still referenced to mains earth? I like that. Feels safe.

author: cdev
author_rank: Super Contributor
timestamp: November 27, 2021, 02:36:13 am
content: Get a voltmeter with a sensor for high voltage built in. Its often called NCV (non contact voltage) probe. It will sense voltages high enough to give you a shock or damage your scope with a light or a beep. No contact, just proximity is all the probing you need to do. My Uni-T clamp meter includes this handy feature. It beeps to warn me of any place where a dangerous voltage is. Its very handy. Often it warns you of problems you would likely never have found any other way, except being shocked or similar. .

author: m k
author_rank: Super Contributor
timestamp: November 27, 2021, 08:35:45 pm
content: Back in the day we had CRT monitors that occasionally had problems.Many times those problems didn't behave and on top of that the majority of their numerous heatsinks were live.I personally do not miss them very much.

author: Ian.M
author_rank: Super Contributor
timestamp: November 27, 2021, 09:59:00 pm
content: Yep, up to about 35KV EHT on a really big color set and typically over 12KV even on small B&W sets at the final anode just waiting to bite you if you had to remove the cap connecting the LOPT EHT output lead to the side of the CRT bowl. A big tube had a lot of capacitance and could deliver a shock that could throw you clear across the room. Also static would build up on the face of the tube when you switched it on or off, as the final anode charged or discharged, so if you switched a set off, disconnected it, and grounded that static by picking it up with the tube face against your chest (which you had to do for balance as that's where most of the weight was), that would charge up the guts of the set to several KV, just waiting to bite you if your grip touched a connector at the back or the mains plug.There were also various voltages on the tube pins associated with the electron gun and focus electrodes of up to several KV.

author: james_s
author_rank: Super Contributor
timestamp: November 27, 2021, 10:30:38 pm
content: The EHT sounds scary but the B+ of 180V or so is more dangerous because it can deliver so much more current. I've been zapped by both on a few occasions and the EHT feels like a powerful static zap, the B+ feels like a properly dangerous shock. A regular switchmode power supply is arguably as dangerous to work on as a CRT display.

author: Gyro
author_rank: Super Contributor
timestamp: November 27, 2021, 10:53:05 pm
content: Quote from: Ian.M on November 27, 2021, 09:59:00 pmI remember doing that exact thing with a MicroVAX II workstation colour monitor that I had just powered down. Picked it up and my finger found one of the BNC connectors round the back. The punch in the chest was so strong that I don't think I noticed anything in my finger. Luckily I kept my footing... and the monitor, but those things weighed a ton!

author: Ian.M
author_rank: Super Contributor
timestamp: November 27, 2021, 11:46:44 pm
content: It was also enough voltage to cause your isolation transformer to flash over if you got bit by the EHT while the set was actually floating, which could cause insulation breakdown, ruining the isolation transformer. I expect that back in the bad old days, a few scopes floated by cheater plugs died that way as well.

author: SilverSolder
author_rank: Super Contributor
timestamp: November 29, 2021, 03:12:21 pm
content: Quote from: james_s on November 27, 2021, 10:30:38 pmWhen I was around 7 years old, I had the brilliant idea of researching the question, "Wonder if one can feel electricity with the fingers?". So I promptly stuck my finger into an unpopulated light bulb socket (in a 240V mains country) to find out. I found out...Ever since then, I have had a healthy respect for working with high voltages!

author: de.leon
author_rank: Contributor
timestamp: December 11, 2021, 02:33:46 pm
content: I was wondering why Dave did not include the multimeter sanity check thing in his video. I'm new to all this but it seems like a "best practices" thing to at least confirm your logic and assumptions as to where to ground a scope with a meter first. My question is how much potential difference between grounding point and scope ground is too much current/voltage or does it need to be 0? Would any voltage throw out any readings or waveforms or would it just be bled off by the mains earth?

author: m k
author_rank: Super Contributor
timestamp: December 12, 2021, 09:55:05 am
content: First you must understand what you are doing, otherwise you can miss a point somewhere.It doesn't mean you can't still miss it after decades of understanding.One example is isolation transformer.Maybe you have not needed it and its function has faded.Then you get an ancient machine with a used to be a 0-class plug.With luck you burn only a fuse.I've measured 10A from me to ground and some real amps from RS-232 GND.I'd say, as a rule of thumb, if you need a meter don't do it, get a guaranteed differential probe instead.(no, I didn't)On the other hand, connecting different cases together was a norm once.But so was waking a battery with a crowbar and connecting bendix gear using the other method and nobody should do those eighter.On the 3rd hand, I've learned to trust the chassis ground, but a note, do not do the same with an unknown extension cord.And don't trust that ancient machine has a chassis even if something looks like one.Maybe one more thing.Nowadays buildings have 3-phase wall stuff, it means that different wall jack groups can be in different phase.So to avoid that part of the equation use only one group of jacks or single extension cord.