How to (Safely) Measure Full Wave Rectified AC Mains On An Oscilloscope

[@Nisnor31]

The Dave's video about ground loop is excellent for pointing the issue. This one definitely deserves at least as many view as Dave's one, for quickly answering to that specific case (FBR on direct AC). Thank you so much for clearing this out and saving my hands and my 'scope 🥰

[@WaswaSayari]

This video is really helpful to me. I foolishly tried hooking up the gnd probe of the scope on the negative of a rectified HV DC . Within the blink of an eye, 1 track was vaporised and the alligator clip welded on to the negative pin ....now I know the reason. Thank you so much.

[@PatrickHoodDaniel]

Great video!

[@GlennMartinez]

Thanks…I isolate from “main” via 3prong to two prong adapter! But I will build your safty light circuit this morning! I owe you!

[@elmakednos]

The ultimate safety is using an isolated transformer with the device is going to be tested. So you don't have to worry about a thing and you can use both channels with one scope.I am from Greece and we have 220+-10% Volts A/C on the live. So we have to be more carefull.

[@AB-yu2tj]

Thank you for clearing this up. I just made this exact mistake yesterday. I was lucky I had the circuit under test connected to a current limiter just like yours.

[@Jeffrey-kn4oz]

Clearer and easier explanation by far than Dave's.

[@michaeldashnaw9627]

Picky perhaps, but as you are describing what's going on your hand is in the way!

[@stephennixey]

Thank you for this information it shows me how to work on some of my PSU's without buying an expensive isolation transformer

[@davidprice2861]

Very good explanation of a scope killing hazard, I was thinking of this scenario a while ago, did a couple of meter tests to scope ground and yes, 'full mains voltage'. Scope users beware! Moral of the story is, neutral and earth are tied. Don't do it.

[@aliens1192]

I made this damn mistake 20mins ago and was looking to find out what actually went wrong ... LED driver without trafo and Isolation ... Both probe and the driver's pcb exploded badly ... :( Thanks for the explanation! Now I can sleep, although I lost my very new Siglent probe as well as the led driver ... :( P.S.: My oscilloscope still works!

[@cookieboi4449]

Also in Australia, Neutral (common ground) is connected in the meter box to an earth stake ground. Thus with DMM probed into outlet Ground pin and live you will read 240 vac. The differential between N and A (live) will trip a RCD when greater than 15 mA. By the way, I am not an electrician,,, only a hobby guy,,, so check it out yourself.

[@qrptoqro7834]

Great video. I have found the same thing. I smoked a 'scope probe ground lead checking the output of a pair of Astron PS full wave bridge rectifiers (2x parallel) a while back also. That wasn't line voltage, it was post-rectification. However, DC negative on the PS is connected to the chassis which is connected to ground which is connected to neutral... and up in smoke the story goes. For measuring shunt voltage difference (in a current measurement application) placed on a negative DC return lead (not positive) you run in to the same ground problem. To get around it you can float the power supply of the 'scope. The way I have done that is by using a DC to AC power inverter running off of a second DC power supply. I have not bypassed the ground lead of the power wire to the 'scope as you suggested with your tape method. I would not try that method myself, but to each their own. You could also get one of the 3 prong to 2 prong plug adapters so you don't have to cut any ground pins or try wrapping with tape.

[@MetatronicModsLLC]

8:24 - That's really the best way, short of investing in a proper differential probe for doing those types of measurements. (Idk why everyone seems so focused on the tape method lol).

[@JohnPaulGrippa]

Excellent video! I have made that same mistake before. Luckily, It was a very high current bridge rectifier and my circuit breaker kicked in, so no permanent damage.

[@v037_]

finally! some educative video

[@peterjohnson5586]

Just put the probe ground clip to the Neutral (N) line and measure any other point with your probe tip.

[@montelatici]

Split phase 240 volt systems used in North America. In most other parts of the world 240 is derived from a ground to a current carrying lead. No neutral.

[@LovelyKauai]

Thx. Very informative. Thanks for sharing Uncle's video link! A current limiting test fixture is great PPE for one's shop! 😎

[@frankt9156]

Cover the ground has its danger right?

[@surgingcircuits6955]

Good video. Important video. Was concerned watching you hand-hold the rectifier why connecting leads. Be careful!!!

[@MetatronicModsLLC]

Yeah bad example from me =/ Though if you're careful to keep your work space dry and clear of grounded objects, and especially if you use a "safety light" you'll probably be just fine even if your hand slips (I've wanted to do a video just for laughs, but thought better of it since everyone and every work environment is different). Working on misbehaving HV circuits it's inevitable that you'll get bit from time to time, but taking those precautions ensures that nothing serious will happen when you do.

[@dafpnp]

I liked the poor man's differential probe approach 😄

[@theodore998]

Thanks for this. I have been watching many hindi videos trying to figure out what I'd done wrong :) vapourising my copper tracks made a gorgeous sound though :)

[@grzesiek1x]

4:29 but i don't understand why people always repeat the same thing without thinking....I am new in electronics so maybe I don't get some things but I use logic thinking when I learn etc. And I see one thing nobody mentions - if I connect the clip from osciloscope to "neutral" which is connected to the ground (and that crocodile clip is ground ) so they both are ground and I can mesure something with the probe to "line" right? Or i just make sure that my "crocodile" clip is connected to the same ground or neutral and the probe to the line? I am not talking about how dangerous or not it is but about facts. Because sometimes people are so focused on hasards that they don't explain how things actually works (because in my opinion if we understand something we can avoid mistakes and potential dangerous sitiations!) otherwise we remain still like babies...

[@friedmule5403]

Noob question but would it not be faster and safer to measure the voltage between the two points with a DMM so you are sure?

[@kgsalvage6306]

One important note. Some electric panels do have an isolated neutral not connected to ground. But, even with it that way your still not safe. There could be something plug in that internally has the common and ground connected. Always use an isolation transformer.

[@pierreretief]

Thanks for the video man!

[@stephenholland6328]

What if you didn’t connect scope ground to anything and just probed output of the full wave bridge? The scope probe would be earth referenced. If you want to use the probe ground lead the attach to the neutral side of the outlet.

[@uricohen5463]

very very very good video thank you

[@niceguy60]

Both of my scopes are handheld and I only use a single channel for each. If for some reason I use both channels on a single scope I use a differential for channel two, problem solved.

[@surgingcircuits6955]

Also, what attenuation are you using since 120v is RMS and you are probably connecting +/- 170v to your leads. Thx.

[@MetatronicModsLLC]

I believe I was just using my cheapo ebay x10 probes iirc.

[@VeryMuchBlessed]

Good video for sure. Thanks for these tips. Very good explanation.

[@IsaacOLEG]

nice tip, I was not sure of what may happens of the signal if no ground lead is plugged , if I connect both grounds together, I mosly make an antenna ? (sorry I am thinking slow in those matters ) thanks for the vid

[@Infinitesap]

Could you please make more videos. Why is neutral connected to ground in the breaker box? What the point of three wires of which one is ground?

[@MetatronicModsLLC]

It all boils down to safety: The hot and neutral wires are both current carrying. Even though neutral has a nominal voltage of 0V it still carries the same current as the hot wire in order to complete the circuit. And since any wire will have some finite resistance the actual voltage of a neutral wire connected to a load will be something greater than 0V per Ohm's Law: V=I*R [Neutral Voltage]=[Current]*[Resistance of Neutral Wire] More importantly, the grounding wire does not normally carry any current. Its purpose is to ground any exposed metal on a powered device, so that if a malfunction occurs, for example causing a hot wire to become dislodged and touch against a metal chassis, rather than have the chassis energized at line voltage the connection to ground shorts out the hot wire and causes the circuit breaker to trip.

[@elmakednos]

I don' t care about ground. I use isolation transformer. It is far cheaper than my oscilloscope and my life.

[@1magnit]

So do I

[@crozwayne]

Yep

[@eduardperdomo1038]

me too

[@manny3549]

exactly! differential probes are so expensive

[@BrentLeVasseur]

The only problem is that if your scope and test equipment is floating and you touch it in the wrong place, you can become the ground and then you get zapped.

[@donmoore7785]

How many cases are there where the line powers a full wave bridge? You are most definitely speaking about a special case, and one that the operator should know is a problem. If he doesn't, he should not be in electronics work.

[@TheBaconWizard]

Not everyone doing electronics is "in electronics work" IF the above is a situation one finds, then I'm sure you'll agree it is better to know how to be safe than not for all concerned.

[@xenonram]

@@TheBaconWizard Yeah, what about people learning!?! Not everyone knows every single intricacy of using an oscope.

[@Bodragon]

There are many cases where mains voltage is rectified. Switch mode power supplies for one.

[@roberthousedorfii1743]

Thanks to your vid, and the EEVblog #279, I went and checked the continuity on my TrippLite 250W "Isolator" FFFS, the damn ground is common! Doesn't that mean that if I connect ground, even on an isolated circuit, it will still be a path?

[@MetatronicModsLLC]

So the ground pin on the outputs connects through to the ground pin that plugs into the wall? If so, I suppose "isolation" in this case is more just referring to isolation from line noise coming through your outlets. That seems inappropriate to call isolation though. Is there input to output ground pin conductivity without anything plugged in? what about input to output neutral with nothing plugged in?

[@imark7777777]

Ground is supposed to be carried through a Transformer although tying the ground and neutral on the output of the Transformer is technically required and that's what makes it a different form of isolation Transformer. There's a lot of marketing going on as well.

[@rickrooney7812]

What do you do with the ground clips when you connect one channel to each side of the bridge rectifier?

[@joeblogs8204]

I dont get it. at 2:20 the circuit on the secodary transformer side is surely referenced to earth/mains potential at point B, the chasis ground? Really do not understand why we earth the secondary transformer circuit to the chasis . Surely if we earthed the chasis to protect us from a primary mains transformer malfunction, then did not connect the secondary side to the chasis, that would be safer? It would basically be the same as usinf an isolation transformer. Totally confused

[@MetatronicModsLLC]

Sure, but that can lead to interoperability problems whenever devices share a connection (eg. audio or video signals).

[@myelectronicsworld]

The alligator clips and the probe tips were so close to each other... weren't you afraid of shorting them accidentally? By the way covering up the ground pin with an isolator (8:10) may not be the right solution as the Neutral line is connected to the ground in circuit boxes in some countries as you mentioned earlier.

[@MetatronicModsLLC]

To your question: No, not at all. That's the beauty of using a safety light, if you short the hot and neutral nodes together, rather than being a dead short, there's always at least the impedance of the light bulb -plus you have a super obvious indicator light that makes it very clear when such a short has occurred. Regarding your comment: True, but if you are planning on connecting the ground clip to neutral anyway, then you don't even need to worry about any of this. Covering the ground prong (or taking differential measurements) is only necessary if you're trying to probe around a non-isolated circuit and your desired scope-ground node is at some non-zero potential.

[@myelectronicsworld]

@@MetatronicModsLLC That's true. I need to make a jig like yours with a bulb.

[@MetatronicModsLLC]

@@myelectronicsworld Uncle Doug's video I linked gives a pretty thorough how to. Just keep in mind you'll want to select bulb wattages based on how much current your test device uses - higher watt bulb for higher watt devices (I've even used 3 way bulb splitters to bring the impedance down low enough for some devices). If the bulb is too low wattage ie. it's resistance is too high, you might not have enough volts present on your device's power input for it to operate properly (some SMPS will crap out prematurely if you starve them like that -but then the test light isn't really for working on that sorta stuff.)

[@myelectronicsworld]

@@MetatronicModsLLC Many thanks.

[@СерафимГончаров-в5ш]

What about the case when I'm using an usb oscilloscope with a laptop that isn't connected to anything?

[@marks6292]

Cant you just isolate the input to the scope, like with an isolation transformer?

[@MetatronicModsLLC]

if you have a scope with isolated inputs then yes you can. but that's somewhat of a special case

[@emilylengyel6856]

When using two oscilloscope probes, what do you do with there ground clips

[@MetatronicModsLLC]

Remove them or clip them up out of the way

[@sajiktm]

In this scenario if cut off the third pin ( ground pin) of the oscilloscope will protect the probe and measuring circuit components?

[@MetatronicModsLLC]

Don't cut the pin off. It does serve a functional purpose as well as protection for the equipment and you. The tape should only ever be used while taking these sorts of measurements and removed immediately following. Really the best thing for you to do is get a second probe (doesn't have to be anything fancy, just a standard scope probe). 8:24

[@cookieboi4449]

if you have a RCD connected to your scope mains in,,, would that not trip at 15 mA ? before any damage could occur? I have those on all my 20 + circuits, also I use an isolation transformer when working high voltage stuff.

[@MetatronicModsLLC]

Assuming your scope's internal wiring is all in order (maybe not always a great assumption for newer budget brands) yes. But here in the states you usually only find GFCIs/RCDs in bathrooms and kitchens. ¯\_(ツ)_/¯

[@AdamJPollockLiveProductions]

Good video. Hey can I ask what would happen on the first schematic, if the secondary winding of the transformer was centre tapped?

[@donmoore7785]

If the center tap is tied to ground, nothing would "happen" unless you put your ground clip on either hot leg of the secondary - shorting that winding.

[@Bodragon]

Can you not just disconnect the ground connection to the oscilloscope, i.e. just power the scope using hot and neutral without the ground?

[@jonking7345]

You could, but it removes the safety function of the ground

[@naturalmilkhoneyandzenretr9508]

We need your assistance. Please help with my solar

[@roberthousedorfii1743]

Another question. If I only want to see a 60 Hz waveform on my Rigol DS1054Z, can't I simply connect the probe tip to the hot wire after REMOVING the ground clip? ( You know, I mean physically REMOVING the ground clip from the probe, entirely. My Rigol probes ground clips are removable. Not sure if all Manufacturers do that... ) I just ordered a 100x Probe to do this with. That should be perfectly safe, right? ( All other precautions having been taken of course... )

[@MetatronicModsLLC]

yeah that's fine

[@we-are-electric1445]

Or just clip the ground clip to the outer sheath of the probe cable - stops it accidentally touching anything and causing a short. You are already referenced to earth as the scope and power supply to the rectifier are the same. My response is a bit late but maybe someone will benefit if they are reading this now !

[@strangescience3414]

Awesome thanks

[@qddk9545]

Why on earth would anyone put mains directly into a bridge rectifier ???

[@marsa74]

Or as a rule of thumb. Never connect the ground clip to anything else than earth when working on the mains directly as you are always shortening something. Use a differential probe or connect the circuit to be inspected to a separating transformer. And keep in mind that once you have connected the ground clip of the first probe a separated circuit is mains earth referenced, too, and the rule re-applies for the second probe.

[@MetatronicModsLLC]

Yeah, that's a good rule of thumb when you're not sure what all is going on in the mains powered circuit you're testing. But I'm not sure I know what you mean by working on mains "directly." And I think that might be part of the point I was getting at with the video. One of the traps for newbies is knowing when a node in a mains powered device or system is earth referenced or floating.

[@marsa74]

@@MetatronicModsLLC "mains directly" means a mains powered circuit as you already wrote. It was meant in contrast to a circuit where you definitely know that it is galvanically isolated.

[@PowerScissor]

I've watched this whole video twice and I'm still lost on how to look at my mains AC wave. I've only ever looked at DC stuff with my cheap scope....and am now less confident on what to do for AC mains. Anyone have a succinct version of where each probe goes (mine only has 2)

[@MetatronicModsLLC]

if all you want to do is look at the line voltage on a power outlet, just connect the scope's signal in to the hot side of the outlet. you don't need to connect the ground clip, but you do need to make sure that your scope and probe are rated for the voltage in question.

[@PowerScissor]

@@MetatronicModsLLC Thanks!

[@karlosp02]

Thanks u really helped 😊👍

[@andywander]

This info IS correct, but....how many devices are there out there that actually tie a bridge rectifier directly to the AC line?

[@MetatronicModsLLC]

You don't buy too many Chinese electronics off of ebay/alibaba do you? lol

[@MetatronicModsLLC]

It's probably more of a concern if you're experimenting/building your own stuff, just good stuff to be aware of. But it is very common in LED rope lights, as well as many import electronics designed and manufactured just as cheap as anyone possibly could.

[@ihatenumberinemail]

Every switching powers supply ties a bridge rectifier directly to the AC line. The transformer goes *after* the diodes and inverter (transistors) in an SMPS.

[@jasoneyes01]

Neutral is not supposed to be connected to ground in your breaker box. Neutral goes to the center tap of the transformer on you power pole in North America. Ground goes to a copper earth rod in North America.

[@MetatronicModsLLC]

Neutral and ground ARE bonded at the main breaker box. They aren't bonded in any sub/auxiliary breaker boxes you might have in commercial or some large apartment buildings. Regardless, the location of the connection is less important than realizing that neutral and ground do indeed share a low impedance connection.

[@donmoore7785]

You are not correct. Metatronic's clarification is correct - only in sub panels is this true (and required by code). In the main panel they are bonded.

[@lbrieden1]

Cover the ground plug with tape? Seriously?

[@MetatronicModsLLC]

Yup, it's a fairly common practice. Most new scopes have plastic cases, so the risk of shock in the event of an internal fault is low. But if you aren't comfortable with that, just use the two probe differential measurement method.

[@roberthousedorfii1743]

Dude, just go to home depot and get one of those 3 prong - 2 prong converters.. MUCH safer than trusting tape! Or am I missing something?

[@packratswhatif.3990]

Ahhhh, No ! Just find the cash and purchase an isolation transformer that can handle the power rating of your test subjects, say 100 to 500 VA rating. That or run your scope off of a battery pack system that is floating. Guess you could always fuse the ground lead of the scope ( which I think is illegal).

[@cocosloan3748]

@@MetatronicModsLLC So by covering ground of the scope plug - I will avoid any DMG that can happen to my scope ? Ty for answering in advance...

[@ColocasiaCorm]

Stop sniffing