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.