Scope blow, why?

[Chriss]

Hi!

I need a bit of help, I killed my scope but actually I'm not sure what cos the problem.
Here is the diagram how I connected the scope to a unit.

First I connected the crocodile clip from my probe to the ( - ) and after that I put the positive tip of my probe on
the output of the NE555 and BINGO!
PSU, USB ports on PC and scope are gone. 

Here is a detailed description:

My electronic what I tried to measure was a sort of PSU from a FAX machine with an NE555 ic.
I disconnected the NE555 from the circuit and tried to measure with my PC based scope.
The scope was connected to my desktop PC, so I know so far about the problem with the earth of the main power supply...

In my case, I connected first the crocodile clip to the NE555 gnd.
Then I powere up the unit which is powered from the main wall connector where is a voltage of 220V.
At this moment everything was fine.
When I touched the output of the NE555 with my scope probe the hell goes on.

PSU, all usb ports and the scope was gone.
I have to say, the PSU of the FAX machine was a switching mode power supply.
Here is my block diagram:



If I messed up with the earth of the PC and the - of the rectifier diode bridge the stuff would blow up
immediately after I put the crocodile clip to the NE555 gnd. Or I'm wrong?

I know this is a stupid question and situation but I can't figure out the problem. 
Any idea?
Thank's.

[wraper]

IMO issue should be in both PSU and how the scope is made (model?). I would guess scopes's ground connected to USB does not have solid connection with probe ground clip or scope has rather weird schematic. I think what happened it zapped USB data pin(s) with high enough voltage/current and killed the chipset in PC.

[Chriss]

The scope is the Hantek 6022BL.
The PC is working but the usb port's are gone.
The scope is killed, not repairable.

I'm investigate this situation because I'm ordered another scope but now a bench scope, and I wont kill that also
if I made a mistake before.
I wish to figure out how to avoid this problem again...

Do you think I'm connected in a correct way my scope, based on the picture I posted?

Thanks.

[wraper]

Hmm, after some thinking. Why the hell did you connect diode bridge to the output of SMPS? It should output DC, and quiet likely ground ( - ) is connected to mains earth.

[Chriss]

NO, I didn't connected the diode bridge to the SMPS, that was originally there.
I just draw the picture to see in general how that stuff was build up and how I connected the scope.

So to say, I connected the minus from my scope to the NE555 GND and separated the output pin of the NE555 from the PCB
and put the positive tipof my scope probe touched to the NE555 output pin and BUM!

I didn't made any changes to the original electronic...

[wraper]

NO, I didn't connected the diode bridge to the SMPS, that was originally there.
I just draw the picture to see in general how that stuff was build up and how I connected the scope.

FYI, SMPS do not have diode bridge on the output.

[Dubbie]

Post a photo of the power supply thing you were trying to measure.

[rhb]

The most likely culprit is the PSU ground  having a high potential relative to ground on the PC and scope.  There's considerable discussion of this issue in the Feeltech FY6600 thread.  This is apparently a common problem with cheap switching mode PSUs.  They can get away with it in a device which doesn't have an external ground such as a FAX machine.

Don't assume the scope is destroyed. You might be able to fix it.  All depends on what happened. Open it up, take some pictures and post them.

Make sure you understand what happened and why before proceeding.  Measure the voltage between ground and the PSU terminals with a suitable meter in AC volts mode.  You just bought the lesson, make sure you learn it.  I only recently became aware of this problem myself.  So thanks for the warning.

Good luck.

[Refrigerator]

What blew on the PC USB ports?
Those are most of the time protected by not only zener diodes on both data lines but also very low current inductors that blow if your USB data lines become overloaded.
If all of the USB ports are dead then id guess that most likely the 5V rail that feeds into the USB port is gone, or the high side switch that separates USB from the 5V rail is blown.
In that case you had a ground issue and the scope might be repairable, otherwise your scope could very well be cactus.
If you're looking for a new scope i'd recommend the OWON VDS1022, no logic analyzer on that one but the scope front end is much better and you can get an isolated one, for a higher price of course.
I have it an i like it.

[Chriss]

At the moment I don't have access to the unit.

I got the point what Wraper told.
I made a mistake in the drawing.
I extracted the rectifier bridge from the PSU.

The diode bridge is connected to the main voltage source which is 220v.
The DC side of the bridge is connected as I draw.
The NE555 is powered through the resistor voltage divider.
The NE555 is powering the MOSFET....

Here is a more precise diagram:


Of course there are other part's too, but the major case what I tried to measure was the NE555...

The scope is totally dead.
The CPU is in short. Checked after the problem happen.

4 from 6 usb ports are dead on the PC.
I don't checked already what is gone on the PC mobo but I will...

I assume something was wrong with the NE555 and a high voltage traveled through the positive tip of my probe.
That killed the scope and after the cpu gone shorted out the usb port driver on the mobo was overloaded and killed too.

I don't think I made some mistake how I connected my scope probe, but I actually didn't checked the voltage between the
NE555 output pin and the GND.

Does this make sense?

[Armadillo]

You are working on "LIVE" and not isolated.
This is very very dangerous.
Let hope the video will help you understand the problems.
Please use isolated transformer in all your experiments. [Not even switch mode power supply].
Always, stop, read, don't rush and it could have saved you a lot of money and trouble and danger.
So, this is a very good reason for you to buy a Lab isolated power supply.   

[amirm]

Does this make sense?

It does.  You are lucky you didn't kill yourself while you were at it!

Besides Dave's video just posted, this would be another one to watch:



Just get a proper DC power supply instead of trying to make your own the way you did.

[capt bullshot]

OK, you've just repeated the experience every EE has to have at least once in his life ...
Your diode bridge isn't isolated from the mains power, PC / scope is earthed - bang!
Can't tell you why this didn't happen at the moment you connected the ground clip (suppose the power supply was turned on then), maybe the clip was bad. Usually the stuff blows up when you connect the ground.
Sorry for your USB ports / scope, I did this with a "proper" scope once upon a time, no problem at all, scope and probe survived, and continued to serve me for many years, but the circuit board was totally blasted.

Next time, use an isolation transformer for your device under test.

[Ian.M]

If you connect an ordinary probe on the input of a conventional CRO or DSO or a USB DSO to the mains, with or without a bridge rectifier, consider yourself very lucky if only equipment dies!
If you haven't watched the EEVblog #279 video above, stop and watch it now.

Even a few volts of ground potential difference between a PC and a USB device can totally trash your PC motherboard, blowing chips and melting tracks off it.  Even if you think something is grounded, if the ground is from a different mains circuit to the rest of your test bench, or from a separate ground rod etc. there can be enough voltage difference to drive a high enough current to do damage.

Unfortunately the Hantek 6022BL uses a High speed (480Mbps) USB interface^{ref} so you cant use an affordable USB isolator with it to remove the risk of blowing your PC.  A High Speed USB isolator is likely to cost more than your scope!    While it is possible to float a Laptop, by running it from battery, with absolutely NO OTHER WIRED CONNECTIONS apart from the scope,  that's a very dangerous way of working, and should be regarded as a last resort.  If you do float a laptop, use a wireless keyboard and mouse, and place an insulating guard over the built-in keyboard and touch pad as a reminder to keep your hands off. 

For a desktop or tower PC, a robust ground strap between the PC chassis, at one of the PSU mounting screws, at the PC end and a suitable ground point on the scope would help reduce the risk of USB port damage and might even save the scope.  However finding a good ground on the scope without modding it internally may be difficult.  Although there is a spring on the underside of the board next to the BNCs grounding the housing, I wouldn't trust a spring contact, so you might have to get a pair of BNC savers (M-F extenders) and solder the ground strap to their bodies then put them on the front panel BNCs.   With the scope properly grounded it should be about as robust as a budget mains powered conventional DSO, and has similar safe usage limitations.

Its probably not economic to buy a Cat II or better rated isolated differential probe for it, so IMHO I wouldn't use it at all on mains circuits or switched mode PSUs, and one should be very cautious on other high current^*  or high voltage circuits, as other test equipment or normal connections may introduce a conflicting ground.

* e.g. anything with a battery capable of supplying more than a few amps if shorted, or with a large reservoir capacitor or other capacitor bank

[Armadillo]

Maybe this video will make you understand the danger of electric shock!

[rhb]

This was the universe telling you you're in too much of a hurry.  Slow down and spend more time thinking and reading about what you are trying to do.   You are *very* lucky to be alive.  The output of your "power supply" was over 300 V with enough current capacity to kill.

The 555 died the moment you powered it up.  The maximum rating is 18 V and you applied 28 V.

FWIW At about 12 or 13 I decided to ground a tube radio so I could get better reception.  Unfortunately I did not understand about AC/DC radios.  These did not have transformers and used a circuit like yours for power.  I don't remember if it blew up when I plugged it in or when I turned it on, but it died in a quite spectacular fashion with lots of smoke and sparks.  I never forgot the lesson.

[Refrigerator]

@Armadillo
At first that video gave me the chills but then it made me sick in my stomach.
My curiosity back at it again making me watch stuff i'll regret seeing.

[Armadillo]

@Refrigerator;

As long as the curiosity is awakened to halt for a moment to ask the brain, the video works.
hahahhahahaha LOL! Cheers;
thanks for feedback.
 

[wraper]

If all of the USB ports are dead then id guess that most likely the 5V rail that feeds into the USB port is gone, or the high side switch that separates USB from the 5V rail is blown.
In that case you had a ground issue and the scope might be repairable, otherwise your scope could very well be cactus.

Nah, more likely it took USB hub built in into the chipset as data lines usually go directly into it. It's a luck that PC switches on at all. And in PC normally there is a separate 5V switch for each USB port (might be dual).

[Chriss]

Hi to all good peoples!

Thank's for the warnings and videos.
I watched before all that videos too, several times.

I know also about the consequence when we measure equipment main powered.

The circuit what I was measured was not modded by me.
It was in an original stat as the manufacturer made it, and the unit was working several years before.

I ordered a new bench scope, the Siglent SDS1052.
And I ordered an isolation transformer too.

From my point of view and also it is the same point what made me out of idea was,
does I connected my scope actually to the GND pin from the NE555 to my scope GND clip and the
output pin from the NE555 to the positive tip of my scope probe.

When I checked before the voltage on the VCC and GND on the NE555 there was a voltage around 13V DC.

In that case, I dodnt should have problems.
But! here comes the think, as capt bullshot wrote "Your diode bridge isn't isolated from the mains power, PC / scope is earthed - bang!"

I was thinking about that too, but how is this in relation actually?
It comes as I shorted a diode somehow when I connected my scope into the circuit...

This makes sense to me.
Because I search for the reason why all the stuff not blow up when I connected the GND clip from my scope probe but when I connected the positive tip to the NE555.
This is the point where I cant figure out why that was happend.
If I didn't found out why that happend,  I cant be sure how to make my steps in the future and not to be in danger.

If I connect the unit what I repair to the isolation transformer and the scope too I assume that will be safe to me and my scope too.
Is that enough secure?

I don't worry about my PC and my crap Hantek, but I worry about myself only in the first place.

[Jeroen3]

Measure AC voltage between what you want to measure, and the chassis of the scope.

[capt bullshot]

If I connect the unit what I repair to the isolation transformer and the scope too I assume that will be safe to me and my scope too.
Is that enough secure?

The idea of the isolation transformer is to open the circuit path from mains L to Earth (mains N is connected to Earth here). So use the scope as usual, to ensure it's case is earthed for your personal safety, and place the isolation transformer between mains supply and your device under test. That way there's no more connection from the device mains input to Earth through the power distribution system.

Connecting your scope will put one potential of your device to earth, that is as intended. Often you will use the most negative potential (in your case the "-" of the rectifier). Don't use any switching potential to connect your scope GND to.

Without an isolation transformer, connecting any potential in your mains circuit (no matter if AC side or DC side of the rectifier) you create a short circuit path from mains L through some components, your scope / computer to Earth. That goes bang, as you did.

Connecting both the scope and the DUT to the output of the transformer may work without a bang, but it's at a rist, depending on the actual scope and DUT. Never open up the safety earth connection of your scope (this is what you do when you connect the scope to the isolation transformer).

Think twice before measuring in a mains connected circuit, there's enough voltage and power present to harm you seriously. I'm still very cautious when I need to do such stuff. Most of the time it's just a bite if you touch the circuit accidentally, but there's no guarantee.

[Armadillo]

Why the neutral clip thing??
The schematic is incomplete, the return ground is missing back to the "FULL" bridge or the Cap, fortunately or unfortunately speaking.
Anyhow and irregardless;
That raises Safety into Question.
It is therefore imperative that you check your house ELCB or earth leakage system and the "Earthing" continuity to all the power sockets outlets in your house.

[Armadillo]

Obama in Elkhart: “I Hope You Don’t Mind Me Being Blunt About This”

Do get a licensed electrician to check the house electrical?          

[Refrigerator]

I've been zapped many times to a point where whenever there's high voltage there's also this invisible force field preventing my hand from coming any closer than 20cm to the high voltage.   
Getting zapped is no fun so stay safe.

[TAMHAN]

Hello,
just wanted to say *fart* and register myself into that discussion...

Will try, next week when back from all the tradeshows, to make a more detailed diagram for what went wrong.

[StillTrying]

I ordered a new bench scope, the Siglent SDS1052.

Which version.

When I checked before the voltage on the VCC and GND on the NE555 there was a voltage around 13V DC.

What you thought was GND was -320V, and what you thought was +13V was -320V +13V, ...so bang!

If I didn't found out why that happend,  I cant be sure how to make my steps in the future and not to be in danger.

Draw the diodes of a bridge rectifier, and follow where the main's live +/- 320V goes.

[Chriss]

Hi TAMHAN!
Thanks mate for your interest.
I'm waiting for you.

StillTrying:
What you mean under which version?
If you mean the firmware version I don't know.

My best regards.

[StillTrying]

StillTrying:
What you mean under which version?

I don't think there is a plain SDS1052.
You're going to have to be a lot more exacting and precise, to prevent blowing things up, including yourself!

[Chriss]

The Siglent SDS 1052DL+  .

[StillTrying]

The Siglent SDS 1052DL+  .

  Very similar to my CML+.

[Chriss]

Yea but you're is a 100MHz scope... :-)

And what you think about this scope?
Is it a stable one?
I mean, is it worth the money?

Sent from my GT-I8260 using Tapatalk

[Ian.M]

When I checked before the voltage on the VCC and GND on the NE555 there was a voltage around 13V DC.

What you thought was GND was -320V, and what you thought was +13V was -320V +13V, ...so bang!

If I didn't found out why that happend,  I cant be sure how to make my steps in the future and not to be in danger.

Draw the diodes of a bridge rectifier, and follow where the main's live +/- 320V goes.

When you are sketching a diagram to see if you can figure out what is and isn't safe to connect a scope probe to,  be careful to use the right ground symbol! If you use the wrong one you'll  confuse yourself and think that points with a potential difference between them are actually at the same voltage.

               
            Earth Ground           Chassis ground   Common or Signal 'ground'

Only Earth ground is a true ground.  That's the one where eventually you can trace it back to a large chunk of metal driven into the earth, (and not into a plastic flowerpot  )   Its what's your incoming mains supply is referenced to.  In many countries there will be a Neutral to Ground bond somewhere in the supply system, and Neutral will *NORMALLY* be held within ten volts or so of Ground, except if there is a fault.   In other countries the Neutral to Ground relationship wont be so well defined, and you may even be supplied with two phases of a three phase supply.  You can bet there is a ground somewhere even if both your supply wires are at a high voltage relative to it.  Learn what the local electrical code requires and what the actual situation is at the socket supplying your bench.

The other two symbols are chassis ground, and common or signal 'ground'.   Chassis ground is the right one to use if there is a connection to a metal frame or case that may not actually be Earth grounded.  You don't expect it to be reliably connected to ground, but you also don't expect there to be a significant voltage between it and true ground.  Common or signal 'ground' is a much woollier concept, and is where the danger can be.   E.g. in a single supply OPAMP circuit, your signal 'ground' is typically held at half the supply voltage.

On the primary side of a SMPSU, the Common (lets stop calling it a 'ground') is almost invariably the negative terminal of the bridge rectifier.   Draw it out with the bridge rectifier, reservoir capacitor, and any transformers and optocouplers shown in full detail as individual component symbols (including all the diodes inside the rectifier), and the rest shown as functional blocks.  Include the supply with the supply ground, and if you have a grounded neutral, show that but with a resistor between the earth Ground symbol and Neutral to remind you that Neutral is probably not actually be at 0V with respect to Ground.

Now you can work out what voltage difference to expect between your scope input, which normally has the connector shell, and thus the probe ground clip grounded to chassis, and the chassis connected to the supply Ground wire, traceable all the way back to earth Ground, and the circuit you want to probe.

If you draw it out, in many European countries, you'll find a SMPSU common is banging up and down at 50Hz between somewhere near 0V and about -310V with respect to earth Ground.   In the USA it will be banging up and down at 60Hz between near 0V and -160V unless the PSU uses a voltage doubler, in which case its probably sitting at -160V.  In other countries its banging up and down between less well defined voltages, but still more than 1.4 x your RMS AC supply voltage apart.   In all cases shorting it to a true Ground is a *BAD* *IDEA* and hundreds of amps can flow for that brief period before the supply breaker trips or fuse blows.

Even if you *expect* the circuit to be truly Grounded, unless you are 100% certain its connected to the same Ground point as your scope, check the voltage between your scope ground and where you are going to clip the ground lead to, on your meter on *both* AC and DC voltage ranges.  If there is a difference you should investigate further - if its a few volts AC or DC, try a small torch bulb between a known good true Ground and your scope ground, and also between your true Ground and the circuit.  If it glows at all there is a ground offset and a low enough impedance to drive significant current  if you short it, so *STOP*.  If it doesn't light or glow at all and the bulb filament still tests good, its OK to connect your scope ground to the circuit.  If you see an AC voltage of around half your supply voltage, (give or take a large margin), but no DC voltage, its probably AC leakage through class Y EMI filter caps.   Take a small mains appliance bulb and check as above.   If you see a large DC voltage, *STOP* immediately.   

[SaabFAN]

I'm a bit surprised the FI-Breaker didn't trip the moment GND was connected (as far as I know, FI-Breakers are mandatory on every circuit of an electrical installation in Germany).

Are you sure your PC-Chassis was GROUNDED? Is the PC equipped with a proper ATX PSU and the outlet correctly wired?

Also if you try to supply a circuit from a Voltage-Divider, you're asking for trouble. The Supply-Voltage will jump all over the place, depending on the dynamic current draw of the device. For very low power-devices it might work with a large enough capacitor, but usually you need a regulated voltage - Zener Circuit, or active Voltage Regulator / PSU.

[madires]

I'm a bit surprised the FI-Breaker didn't trip the moment GND was connected (as far as I know, FI-Breakers are mandatory on every circuit of an electrical installation in Germany).

The current is limited by a 100k resistor I = V / R = (230V * SQRT(2) / 100kOhms) = 3mA. And our typical RCD (type A) triggers at 30mA RMS.

[madires]

Thank's for the warnings and videos.
I watched before all that videos too, several times.

This is not to discourage you or blame you, but please stay away from mains as long as you don't fully understand the videos and why your USB scope got toasted. The same for an isolation transformer. It's a useful tool when you know how to use it properly. However, it's not a magic safety device.

[capt bullshot]

I'm a bit surprised the FI-Breaker didn't trip the moment GND was connected (as far as I know, FI-Breakers are mandatory on every circuit of an electrical installation in Germany).

For new installations, yes that may be true. You don't have to upgrade existing installations.

[StillTrying]

The current is limited by a 100k resistor I = V / R = (230V * SQRT(2) / 100kOhms) = 3mA. And our typical RCD (type A) triggers at 30mA RMS.

That only applies when the live is on a +ve peak. When a -ve peak comes along the BR connects the live directly to the circuits GND.
A diode in the BR might have blown in a fraction of the -ve a half cycle, before the RCD had time to react.

[StillTrying]

Yea but you're is a 100MHz scope... :-)

A lot of the time I've got the 20MHz limit on, and/or even the probe on X1 ~7MHz, you'll find 50MHz enough, near enough all the time.

And what you think about this scope?
Is it a stable one?
I mean, is it worth the money?

You should find all the very basics fine, but for many of the extra settings and options, you're probably asking the wrong person. 

[ArthurDent]

StillTrying is correct. With your non-isolated power supply, your 555 ground is only at earth ground every other half cycle when the L lead is going positive. On the negative 1/2 cycle when the L lead is going negative, the negative voltage from L flows through the upper left diode (in the bridge I've drawn) to your 555 ground making that 'ground' -220VDC. The attached crude drawing shows this path in red. If you now connect your scope ground (which is also power line ground or zero V) then you have -220VDC half wave connected through your circuit and then your scope to power line ground and this is why your scope was fried. As others have said you were lucky that it was just equipment and not you that got destroyed.

I would NEVER make a power supply like the one you have shown nor use any piece of equipment that was built that way. Every piece of equipment I own has a secure earth connection connected through the power cord and also connected through their ground connections to other pieces of equipment they would be connected to.

[NeedsMoreFlux]

I scrolled through pretty fast, but I don't see that you even took it apart for a visual check. Is that right?

Have you been trying to diagnose it by checking the easy things first?

Fuses?

Shorts to ground?

Visual inspection for fried components?

Probing to see where the electricity stops flowing?

Good luck, buddy.

[Refrigerator]

Don't forget to show us what the inside of the scope looks like now   

[Chriss]

Hi!
The most imoprtant question for what I asked was here actually what ArthurDent showed through his drawing.

I know about that problem and if there were no ne555 I would never put my scope to that bridge rectifier and measure that way.

But I was driven by my mine does the NE555 is a component which have to use DC voltage Umax around 18V and
have nothing to do with the main power supply directly, but is  driving the FET.
So I tried to check for a live signal from the NE555 with my scope between the the GND and the output pin of then NE555.
That should be an ok connection if the circuit would be a better quality or the circuit would be powered from battery.
I don't know how should I connect my scope to measure the output from the NE555 than as I did.

Conclusion:
- I didn't tracked back the whole trace of the pcb from the NE555 to the main input. ( my failure )
- I would say this design of psu is a very cheap and crap design, I even don't know how that worked in the paste ...
- A bad connection was the reason why the scope was not killed when I was connected the alligator clip to the NE555 gnd.
But when I moved the scope probe and the connection on the alig. clip was made, that was the moment when I touched the NE555
output the big bang was started.  ( that is why I was in confusion, why was my stuff blow up when I touched the NE555 )
- exactly the two diodes was blown which feeds the ( minus ) wire from the L and N on the rectifier bridge.

Guy's you are great and thank you very much to all of you.
My best regards.

[Armadillo]

Hi!
The most imoprtant question for what I asked was here actually what ArthurDent showed through his drawing.

Except that the concept is WRONG!....
If you are already referenced to the lowest of the potential level at that point, then you can never be lower than the lowest reference level, regardless of how the Live and the neutral invert.
Think of how the ac power is generated by the generator, its just rotating about a center. It floating except that its tied to earth reference.
Nobody did bothers to correct ArthurDent except you should not take it as being correct.

Edit: After the rectifier bridge, you should begin to think only as DC and the capacitor negative is already at the lowest of the lowest potential. [no such thing as -220V anymore]

[Ian.M]

The only thing wrong with ArthurDent's drawing is that it doesn't say "Half-wave Rectified" next to the red "220 VAC".

[StillTrying]

Edit: After the rectifier bridge, you should begin to think only as DC and the capacitor negative is already at the lowest of the lowest potential. [no such thing as -220V anymore]

That's wrong, you've got 0V-13 VDC riding on top of +0.7v to -300V directly connected to the mains, there's a lot more AC there than DC.

Anytime a BR is directly connected to mains it's potentially lethal, Do Not Touch, even with probes.

[Armadillo]

Edit: After the rectifier bridge, you should begin to think only as DC and the capacitor negative is already at the lowest of the lowest potential. [no such thing as -220V anymore]

That's wrong, you've got 0V-13 VDC riding on top of +0.7v to -300V directly connected to the mains, there's a lot more AC there than DC.

Anytime a BR is directly connected to mains it's potentially lethal, Do Not Touch, even with probes.

Utterly rubbish. More AC than DC, so if you put your multimeter into AC mode, you can measure the -300V AC at the DC capacitor?

Edit: Even on fundamental full wave rectification or half wave rectification will easily proof you WRONG!.   

[madires]

The current is limited by a 100k resistor I = V / R = (230V * SQRT(2) / 100kOhms) = 3mA. And our typical RCD (type A) triggers at 30mA RMS.

That only applies when the live is on a +ve peak. When a -ve peak comes along the BR connects the live directly to the circuits GND.
A diode in the BR might have blown in a fraction of the -ve a half cycle, before the RCD had time to react.

A RCD is triggered by the unbalance of L and N. The polarity doesn't matter.

[Armadillo]

The current is limited by a 100k resistor I = V / R = (230V * SQRT(2) / 100kOhms) = 3mA. And our typical RCD (type A) triggers at 30mA RMS.

That only applies when the live is on a +ve peak. When a -ve peak comes along the BR connects the live directly to the circuits GND.
A diode in the BR might have blown in a fraction of the -ve a half cycle, before the RCD had time to react.

A RCD is triggered by the unbalance of L and N. The polarity doesn't matter.

That's what I am saying, nobody bothers to comment doesn't means you are right! Generally, if you are too wayward, nobody will give a damn, generally speaking.

[Armadillo]

why all the stuff not blow up when I connected the GND clip from my scope probe

[Chriss]

What do you mean with this missing return gnd?

[Armadillo]

See attached.
The current must return back to the capacitor or the bridge.
The schematic was wrong or I don't know if that was the actual wirings.
Without the return connection, it could answer your original question regarding the negative probe clip.

Edit: The Germany already answered, less than 30mA, the RCD will not tripped [apprehended the missing return line].   

[Chriss]

Yea, that's correct.

I think I shorted out the BR through my GND of my scope probe through the PC USB port, where the GND is connected to earth.
The delayed big bang was just a coincidental situation because of a bad contact of the alligator clip.

[Armadillo]

I am glad that you found the answer to your question.   

[ArthurDent]

There seems to be confusion or misunderstanding about how A.C. works and is referenced to ‘N’ and what function the diode bridge performs.  First, the ‘L’ is 220VAC rms referenced to ‘N’ , earth, ground or whatever you want to call it. If you connect a scope probe ground clip to ‘N’ and the tip to ‘L’ you will see a sinewave that is symmetrical around the ‘N’ or zero reference so you will see the voltage swing smoothly from zero to about +311 peak and then drop through zero to -311 peak. The effective heating or rms value of this waveform is the rated 220VAC.

The diode bridge isn’t magical but can be thought of as a voltage controlled very fast double-pole double throw reversing switch. A diode will conduct in the forward-biased direction and not conduct when it is reverse-biased. If you draw a bridge rectifier as 4 separate diodes, one pair of opposite diodes will be ‘on’ during the positive half of the cycle and the other opposite pair will be on during the negative half of the cycle. What a full wave bridge does as opposed to a single diode is increase the efficiency of the rectifier by using the energy from both halves of the cycle and make filtering the pulsating D.C. easier because the ripple frequency is doubled.

Armadillo should look at the first schematic posted by Chriss where the ground connection is not missing and I'm assuming that is the way Chriss wired it. Ian M. is correct in that I wasn’t concise in my drawing as I only meant to show that the negative on the capacitor and the 555 were connected through the diodes in the bridge to the 220VAC line, not that there would be 220VAC at that point. It’s immaterial because there is a short of the L line through the 555 circuit and the scope with no fuse in the line for protection. The attached photo should make it clearer. 

[Ian.M]

Actually, the bottom right circuit with scope ground connected to neutral has a high risk of blowing your scope.  You'll probably get away with it in a single dwelling (with no wiring faults - are you feeling lucky?) in the USA, because for the usual US domestic and small business supply the NEC specifies that Neutral is to be bonded to Ground at the main panel, and the US split phase 115V system typically has relatively low neutral currents, but for countries that don't use split phase supplies, heavy loads can result in enough Ground-Neutral differential to burn tracks off boards etc.

[Armadillo]

Yeap, unfortunately there are misunderstandings.

Let's start from the beginning the original OP Questions;

why all the stuff not blow up when I connected the GND clip from my scope probe but when I connected the positive tip to the NE555.

Which implies he knows that connecting the Gnd Clip to the rectified DC ground will create a big bang!. He is not asking why it bangs but why it did not bang!.

Ok let me explain why I said your concept was wrong. Firstly, it was not the question asked. Secondly, the rectified DC voltage at the capacitor negative rail is at the lowest of the lowest voltage [don't agree?]. So the concept that another -220v will flow to the negative rail of the capacitor is a wrong concept leading to some people thinking AC voltage more than DC voltage, a confused concept.
However, it is a fact that the rectified "DC" voltage at the negative capacitor rail/circuit ground is lower than the Neutral line. So regarding whether it will bang when the neutral ground lead of the probe touches the rectified DC ground, I will refer you to the underlined and highlighted text above, because no one had denied it.

Cheers, let put the matter to rest. thanks

[Ian.M]

Without a photo of both sides of the fax board with connection points marked we can only speculate.  One possibility is that the O.P. was using a x1 probe, got the ground clip on a reasonably safe secondary side 0V point, then probed a point on the primary side, grossly exceeding the scope's input voltage rating, which blew right through its MCU and took out the host USB interface.

[Chriss]

I asked a friend of mine to send me a schematic about the section of the psu where the problem was.
He draw it by hand and I'm draw colored lines on it, which should represent my thinking where the problem was.

Check the picture.

I assume:
Scene1.:

The whole circuit is not galvanic separated from the main,so when I put my probe to measure the NE555 that was the same as I would measure a BR direct with my scope, putting the alligator clip to the DC gnd and the DC positive of the BR.
That would finish up in a big bang if the alligator clip of my scope is connected to earth.
The green line on my drawing is the path where the current was travel on every half cycle of the L line, also on the N line.
That means from my opinion does I shorted out the D1 and than the D2 or vise verse through the earth, and so I was connected directly to the main.
Al these happend even if I connected the scope to the GND of the NE555.

The pcb is from the upper side filled very hard with some white rubber think, so it is very hard to identify the component labeling etc.
The bottom side is clear.

Scene2.:
Maybe an over voltage was traveled through my scope tip.
But, I don't really think so.
The max voltage of the NE555 should be not more than 18V.
My tip should handel much more voltage around 600V DC.
So, that should not be the case, only if the NE555 was bad and on his output was more voltage then suspected.
But, in that case, I assume the NE555 would be burned, I mean exploded.

Conclusion:
I stand on the theory does I made a mistake and shorted out one of the diodes in the RB through earth connection of my PC.
The reason why the big bang didn't happened immediately  after I put the alligator clip to the minus connection of the capacitor
and powered on the psu, was because a bad connection on the alligator clip and the minus pole of the capacitor.

After moving the probe to connect the tip to the NE555 output i finaly made a nice connection on the alligator clip and the capacitor,
but at that point I was very close to the NE555 output pin, even maybe I touched or not, I don't know because the big bang begin. 



So, what you think about my theory?
From my side, it is a simple and common mistake when it's comes to measuring with scope on mains.

[capt bullshot]

So, what you think about my theory?
From my side, it is a simple and common mistake when it's comes to measuring with scope on mains.

Looks plausible to me.

[Ian.M]

So, what you think about my theory?
From my side, it is a simple and common mistake when it's comes to measuring with scope on mains.

.... and to me.   

However, because its so simple and common, you need to give some thought to methods of avoiding repeating it!   

If you are going to do further experiments with mains,  you need CAT II or better isolated differential probe, which takes most of the worry out of measurements in non-isolated or on the primary side of PSUs, as long as you do all connecting/disconnecting with the D.U.T powered down and stand clear before applying power.   They aren't cheap, but are far far cheaper than your funeral!

It is possible to get by with an isolating transformer, but that has several safety disadvantages that are too complex to go into here - there have been *many* threads on the subject and I doubt this forum needs another one! 

Whatever you do you need an effective and reliable method of deciding if its safe to connect your scope and non-isolated probe to a circuit.   The problem here is isolated (floating) mains powered circuits, where the permissible AC leakage current through the class Y HF return cap between primary and secondary sides of a typical SMPSU and simple inter-winding capacitance in linear PSUs, tends to leave the output floating at some fraction of the mains supply voltage when measured with respect to ground on a high impedance voltmeter like a modern DMM. 

The cure for that is to connect four 4.7K 3W wirewound resistors in series, and connect them across your DMM input.  They'll swamp the leakage current and drag the floating PSU down to ground, but if you inadvertently probe a high voltage node, they'll pass a safe 12mA or so if you are measuring mains, or typically 18mA if you are measuring the high voltage DC bus in a SMPSU.   They are also good for discharging SMPSU reservoir capacitors in a controlled fashion.    Its worth getting a small project box and butchering a set of economy DMM leads to add such a shunt resistance across the input.  You should check the resistance is close to 18.8K before and after use.

Test between the point you want to clip the scope ground to and a good ground point (not the scope itself if its a USB one without its own mains lead and ground wire).  If you see more than a few volts AC or DC, *STOP*, do *NOT* probe that circuit with the scope.  If you get a few volts AC or DC, connect a torch bulb of a similar voltage between the point you want to clip to, and ground - if it glows at all there is a real voltage differential with enough current available to be a problem, so again STOP.   Also, without the 4x 4.7K shunt, (switch back to ordinary leads), you should check that the point you want to probe is within the input voltage ratings of your scope and probe, testing between there and the point you want to clip ground to.

Hopefully you'll avoid future scope destruction and 'ElectroBoom' style excitement on the test bench

[Chriss]

Ian.M:
Can you give some drawing how to actually use that four 4.7k/3w resistors?
I'm not really understand your advice about.

What do you think if I would connect a bulb in series with the unit on the main side?
If I get a short in the circuit the bulb will go on and consume the current but nothing should be killed.
I do this when I repair a device and I'm not sure does there some other shorted parts too in the circuit.

You know what I mean... 

[Ian.M]

A bulb in series reduces the peak current if you short something, but as the cold resistance of a tungsten filament is typically an order of magnitude less than the hot resistance, a 40W 220V bulb can still let through a surge of nearly 2A, which can easily blow sensitive parts.   Higher wattage bulbs make the problem worse.  Also if the D.U.T. is starved for power, waveforms may be distorted making many scope measurements pointless.

I've attached the meter + resistor hookup.   Why so many big resistors? - well that will let them survive gross overloads.  Most wirewound resistors can handle a x5 overload for at least one second, so the resistor values and types chosen will survive brief connection to a 1000V source, and as there will only be 250V across each resistor they wont flashover and create a further hazard.

[Armadillo]

They bridge the Y capacitor there for EMC compliance and this leakage current is not fixed value, so I am wondering by altering the impedance of the meter will results in how many degrees of measurement accuracy of the meter right there left?
Any forms of documented papers there?

[Ian.M]

You aren't concerned about accuracy, you just want to be able to tell the difference between Y cap leakage and either faulty insulation, or one side of a direct from mains bridge rectifier.   e.g. for any double-insulated (Class II) D.U.T. the indicated voltage will be under 4.7V if its due to leakage, as the safety limit is 0.25mA.   The only thing that could be ambiguous is a direct short to Neutral, and the torch bulb test will indicate if there is currently an excessive Neutral-Ground differential.   N.B. Check the torch bulb before and after use!

There's a chance this testing method will trip an up-stream GFCI (RCD), but that also tells you the proposed ground clip connection point is unsafe.

[Armadillo]

With that un-toleranced resistors selection, I should figure.
At a max of 4.7volts and weighing between faults situations and this code compliance small leakage current.... ehm... oh yeah....some cannot differentiate and would definitely need it.
thanks for sharing.   

[Chriss]

If you see more than a few volts AC or DC, *STOP*, do *NOT* probe that circuit with the scope.

How many voltage is that few volts?
I assume 3-4V or maybe more voltage could be a trigger not to scope..

If I understand your idea of your drawing:
1. the resistors should be connected in series with the GND of the unit where I should connect the GND alligator clip from my scope.
2. the scope alligator clip should be connected then to the resistors in series too.
3. with my dmm I should check for AC voltage
4. if few voltage is present I should stop scoping.

Ok, but at the moment of the voltage checking with the dmm, the alligator clip is connected, and if few voltage is present something can goes wrong.

I feel I don't understand good enough your idea.
Sorry, the language difference make it for me a bit harder to understand the theory . 

[Ian.M]

The series string of resistors need to be in parallel with (across) the meter input jacks.

Take a small plastic project box and a pair of cheap multimeter leads.  Lay the leads across the box  and check the meter end is going to be long enough to reach the meter sockets without straining the wires.  Mark the leads in the center of where they cross the box.  Cut the leads there.  Drill the box half way up its sides, two holes in each long side, one at either end to feed the leads in and out the other side.  Rough up the inside of the box round the holes with sandpaper.  Thread the cut lead ends in through the holes and pull through enough length to work with.   Strip 1/2" of the end of each lead.  Solder the four 4K7 3W  wirewound resistors together in series, twisting each joint before soldering, then slip some glassfibre heat resistant sleeving over the whole lot.    For heat resistant supports for the resistor, take two pieces of thin ceramic tile, cut rectangles that just fit inside the box vertically and take up the full width, and drill one hole through the middle of each of them (use a small glass drill).   Thread them onto the resistor assembly end leads.  Twist and solder both red wire ends to the protruding resistor lead, then the same with the black wires, to the other resistor lead.   Glue the tiles in place with Epoxy or silicon caulk so the resistors cant touch the sides of the box.   Pull the wires tight, and fit a cable tie right up against the box inside wall at each hole so they cant be pulled out.  Secure each wire to the box wall at the cable tie with a blob of epoxy or silicon caulk.  Let all the glue set.  Screw the lid on the box and label it:

Low Impedance Voltage Test Lead for DMM
Max 500V for FIVE seconds
Check resistance before use: 18.8K (nom)

Usage is as described earlier.

The risk is minimal as long as you use a known good earth ground point for the tests.  If you aren't sure if a ground point is good don't use it.   Worst case: take a mains plug, and a length of hookup wire, connect the hookup wire to the ground pin, and screw the other two terminals down tight so the screws cant come loose, bulk up the hookup wire with rubber tape till the plug's cord grip can grip it securely, reassemble the plug, and use that, plugged into the same wall socket plate (or an extension off it) as your test bench equipment, and use the free end of that hookup wire as your test earth ground, AFTER checking the socket with a socket tester.

How much voltage is too much?  Well that depends.  Once you get below about 10V its not the voltage that's the problem, its how much current it can supply, which is why you then need to do the torch bulb test.  If the bulb lights, even dimly (or blows), *DONT* proceed with connecting the scope ground to that point.   Don't try to measure the leakage current with a multimeter - that's a good way to blow $expen$ive$  multimeter fuses if its more than expected.

If the above description of the apparatus required and methods to use it is still incomprehensible to you then *PLEASE* do not attempt to do any work on mains circuits or PSUs except under the direct supervision of a trained and qualified Electronic or Electrical appliance repair technician, with teaching experience.

[Chriss]

Thank's for this nice explanation, I read and translated several times your writings and now I'm on the road with it.
I'm familiar now with your thinking and suggestion.

I don't have many times to repair such thinks with mains power supply, but enough times to kill myself or my equipment.
So, It is better to ask and learn than to smell the flowers from below. 

My best regards.

[StillTrying]

Have you blown the DL+ up yet?

[Chriss]

If you mean my new scope?  No. I even don't get it for now. I have to wait 30-45 days for delivery.  :-)

Sent from my GT-I8260 using Tapatalk

[StillTrying]

I thought being near the center of The Universe Europe you'd get it in days, it's only ~£250 inc. in EU.