Author Topic: Scope blow, why?  (Read 7007 times)

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Offline TAMHAN

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Re: Scope blow, why?
« Reply #25 on: November 24, 2017, 08:42:59 pm »
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.
Feel like some additional tamile wisdom? Visit my YouTube channel -> https://www.youtube.com/user/MrTamhan for 10min tid-bits!
 
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Offline StillTrying

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Re: Scope blow, why?
« Reply #26 on: November 24, 2017, 09:26:54 pm »
I ordered a new bench scope, the Siglent SDS1052.

Which version.

Quote
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!

Quote
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.

« Last Edit: November 24, 2017, 09:29:20 pm by StillTrying »
CML+  That took much longer than I thought it would.
 

Offline Chriss

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Re: Scope blow, why?
« Reply #27 on: November 24, 2017, 09:29:13 pm »
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.
 

Offline StillTrying

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Re: Scope blow, why?
« Reply #28 on: November 24, 2017, 09:36:17 pm »
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!
CML+  That took much longer than I thought it would.
 

Offline Chriss

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Re: Scope blow, why?
« Reply #29 on: November 24, 2017, 09:42:12 pm »
The Siglent SDS 1052DL+  .
 

Offline StillTrying

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Re: Scope blow, why?
« Reply #30 on: November 24, 2017, 10:11:55 pm »
The Siglent SDS 1052DL+  .

 :) Very similar to my CML+.
CML+  That took much longer than I thought it would.
 

Offline Chriss

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Re: Scope blow, why?
« Reply #31 on: November 24, 2017, 11:42:38 pm »
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

 

Online Ian.M

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Re: Scope blow, why?
« Reply #32 on: November 24, 2017, 11:46:46 pm »
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!

Quote
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  |O :-DD)   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.   
 

Offline SaabFAN

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Re: Scope blow, why?
« Reply #33 on: November 25, 2017, 02:08:37 pm »
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.

Offline madires

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Re: Scope blow, why?
« Reply #34 on: November 25, 2017, 03:50:51 pm »
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.
 

Offline madires

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Re: Scope blow, why?
« Reply #35 on: November 25, 2017, 04:03:32 pm »
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.
 

Offline capt bullshot

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Re: Scope blow, why?
« Reply #36 on: November 25, 2017, 05:06:13 pm »
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.
Safety devices hinder evolution
 

Offline StillTrying

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Re: Scope blow, why?
« Reply #37 on: November 25, 2017, 05:24:00 pm »
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.
CML+  That took much longer than I thought it would.
 

Offline StillTrying

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Re: Scope blow, why?
« Reply #38 on: November 25, 2017, 05:37:38 pm »
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.

Quote
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.  >:D
« Last Edit: November 25, 2017, 05:41:09 pm by StillTrying »
CML+  That took much longer than I thought it would.
 

Offline ArthurDent

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Re: Scope blow, why?
« Reply #39 on: November 25, 2017, 06:26:34 pm »
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.
 
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Offline NeedsMoreFlux

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Re: Scope blow, why?
« Reply #40 on: November 26, 2017, 02:49:04 am »
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.
 

Offline Refrigerator

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Re: Scope blow, why?
« Reply #41 on: November 27, 2017, 12:31:42 pm »
Don't forget to show us what the inside of the scope looks like now  :-BROKE  ;D
I have a blog at http://brimmingideas.blogspot.com/ . Now less empty than ever before !
 

Offline Chriss

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Re: Scope blow, why?
« Reply #42 on: November 27, 2017, 03:06:51 pm »
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.  :-DD ( 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.
 

Offline Armadillo

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Re: Scope blow, why?
« Reply #43 on: November 27, 2017, 03:48:18 pm »
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]
« Last Edit: November 27, 2017, 04:03:00 pm by Armadillo »
 

Online Ian.M

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Re: Scope blow, why?
« Reply #44 on: November 27, 2017, 04:02:12 pm »
The only thing wrong with ArthurDent's drawing is that it doesn't say "Half-wave Rectified" next to the red "220 VAC".
 

Offline StillTrying

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Re: Scope blow, why?
« Reply #45 on: November 27, 2017, 04:33:21 pm »
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 ;D lethal, Do Not Touch, even with probes.
CML+  That took much longer than I thought it would.
 

Offline Armadillo

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Re: Scope blow, why?
« Reply #46 on: November 27, 2017, 04:41:37 pm »
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 ;D 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!.    |O
« Last Edit: November 27, 2017, 04:48:08 pm by Armadillo »
 

Offline madires

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Re: Scope blow, why?
« Reply #47 on: November 27, 2017, 05:07:02 pm »
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.
 

Offline Armadillo

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Re: Scope blow, why?
« Reply #48 on: November 27, 2017, 05:18:54 pm »
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.
 

Offline Armadillo

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Re: Scope blow, why?
« Reply #49 on: November 27, 2017, 05:53:48 pm »
why all the stuff not blow up when I connected the GND clip from my scope probe
 
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