Measuring mains powered devices with usb scope, some questions.

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Hi! I have some doubts about this and rather ask before i do anything that is potentially dangerous to me or my equipments.

Is it safe to ie. measure phone charger output with usb scope? I've read many discussions/articles about measuring mains with usb scope and how it is potentially dangerous, but did not find an answer if it's fine to measure the low voltage side of such devices without issues.

If it makes any difference i have "active" USB hub which is powered with external source from mains and not directly from USB port of PC.

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The big question is whether the charger output is isolated from the powerline connection. Otherwise, it could, indeed, be quite hazardous to both body and equipment.

 

I would start by checking DC continuity between each of the three line pins and each of the two output "pins". If there is continuity, don't even think about it. If there is no continuity, then you are PROBABLY ok to proceed, but you must do so under your own judgement. Nothing anybody here writes is likely to fit your situation perfectly and you have to be the final arbiter in the matter.

 

Can't really answer about active hub. Have not checked any.

 

For what it is worth, I am willing to bet that some/many USB-based chargers are NOT isolated. They get connected to things like iPhones where there is no exposure of electrical contacts to the user. But, when experimenters get involved with live circuitry, all bets are off.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

Last Edited: Thu. Aug 24, 2017 - 06:55 PM
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Thx! So I was on the right track in the sense that it might cause permanent damage if not done with caution.

Could isolating transformer and active/differential probes for the scope aid the situation? Damn this hobby burns all my monies so fast, but better safe than sorry.
http://www.trafox.fi/en/products/transformers/isolation-transformers/adjustable-isolation-transformers-kls/

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Use of an Isolation transformer is ALWAYS a good idea when working with unknown supplies,  I have one on my real scope just for that reason.....

 

 

Jim

 

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Agree. If this is the sort of thing you are likely to be doing in the future (that is, similar power levels), then a transformer rated at 10W or maybe 25W should more than do it. Here is one at Allied for under $15. Just an example. Its 50VA (50W). If you use a  transformer like that, protect the terminals VERY well and make yourself a little box (standard electrical wall utility box) with a duplex outlet and cover. Also, connect a standard power cord permanently and insulate its connections to the transformer very well.

 

https://www.alliedelec.com/triad...

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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JoniS wrote:
Is it safe to ie. measure phone charger output with usb scope?
Maybe; most phone charger outputs are isolated.

To confirm, research the labels of the standardization organizations on the phone charger's wall wart.

JoniS wrote:
If it makes any difference i have "active" USB hub which is powered with external source from mains and not directly from USB port of PC.
PC - USB is not isolated (bus power, ground, shield, D+, D-); in your case, the active active hub is not isolated due to ground and shield.

Notebook PC - USB is isolated floating if power from battery or an isolated wall wart.

There are USB hubs with an isolated upstream USB port :

B&B Electronics, now B+B SmartWorx

Advantech B+B SmartWorx

Industrial Grade USB 2.0 Hub

http://www.bb-elec.com/Products/USB-Connectivity/USB-2-0-Hubs/Industrial-Grade-USB-Hubs.aspx

and USB isolators :

http://www.bb-elec.com/Products/USB-Connectivity/USB-Isolators.aspx

https://www.olimex.com/Products/USB-Modules/USB-ISO/

 

Why for HDMI :

https://olimex.wordpress.com/2014/01/22/bevare-non-grounded-tv-monitors-can-damage-your-olinuxino/

and the Olimex USB-ISO has been mentioned for when one shorts their target that's attached to their notebook PC (USB overcurrent)

 

Edit : strikethru

 

"Dare to be naïve." - Buckminster Fuller

Last Edited: Thu. Aug 24, 2017 - 10:19 PM
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I recall a Thread that discussed whether or not common wall warts were isolated these days, and Nard tested a bunch of them.

Sorry, I can't locate the Thread at the moment.

 

Bottom line, back in the old days wall warts had small transformers in them, and were isolated.

 

These days the small wall warts are often transformerless, and were not isolated.

 

+1 on using an Isolation transformer for powering the device under test.

In addition to Jim's comments about making a nice box for it, I'd add a fuse.

 

JC

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Jay,

 

I could not find the 'Freaks thread either, but over on Nard's own website he has summarised what was done and found.

 

http://www.aplomb.nl/SMPS_leakag...

 

 

Ross McKenzie ValuSoft Melbourne Australia

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Jay,

 

I could not find the 'Freaks thread either, but over on Nard's own website he has summarised what was done and found.

 

http://www.aplomb.nl/SMPS_leakag...

 

 

Ross McKenzie ValuSoft Melbourne Australia

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Hi JoniS, what are you trying to measure with the scope?

 

Be aware that an SMPS is really 2 circuits, a primary (connected to mains) and a secondary connected to your device. The 2 sides should be isolated. Isolated means small/harmless currents due to capacitive and inductive coupling. An isolated SMPS does contain a isolation transformer, but physically small due to high frequency switching.

 

Non-isolated SMPS can potentially kill, and so can a faulty SMPS. I have seen both. Non-isolated SMPS for mains voltage is not legal to sell.

 

Measuring across mains and device side of SMPS can kill the input of some scopes if isolation fails. There are solutions:

 

An isolation transformer as suggested by others is a good idea (does not kill you, but could still kill your scope).

 

Isolate the scope from mains and use rubber gloves. Beware if 2 or more probes have common ground. Most scopes have common ground.

 

Use isolated probes. But they may cost as much as the scope.

 

Use scope with isolated inputs. Example: Fluke 190 series. Those can also measure high voltage, and probe grounds are isolated too.

 

I use to test SMPS's with a megohm meter across the isolation. 500V or 1000V limited to max. 1mA. Any SMPS that fails this test is dumped.

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Waauw, nice writeup.

Just over halfway down aplomp links back to avrfreaks with:

http://www.avrfreaks.net/index.php?name=PNphpBB2&file=viewtopic&t=82257&postdays=0&postorder=asc&start=0

Is this the link? It's from 2009.

Paul van der Hoeven.
Bunch of old projects with AVR's:
http://www.hoevendesign.com

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peteralarsen wrote:
Hi JoniS, what are you trying to measure with the scope?

From time to time I have some small projects powered from mains via power supplys which I may need to diagnose when it's not working how I intented(this was not really proplem while i still had my old analog scope), but now with only the USB scope(picoscope 2207b) available this is becoming an issue for me.

The phone charger was just an example, and I was actually curious to see how old Nokia chargers compare to new phone chargers, in terms of how clean power it does give.

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A USB 2.0 high-speed isolator :

Amazon

HuMANDATA

USB2.0 Isolator Compact (USB-029L2)

https://www.amazon.com/HuMANDATA-USB-029L2-USB2-0-Isolator-Compact/dp/B01L6C5BDG

 

"Dare to be naïve." - Buckminster Fuller

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I use an ATX power supply, they are bulky but always have a ground-earth connection.

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JoniS wrote:
... (picoscope 2207b) ...
fyi

Pico Technology

Test and Measurement Forum

isolation pico 300o sries

...

if there is no isolation, then how is the external power supply supposed to help against ground loops?

...

The web site states:

"In normal operation the PicoScope 320x is powered either from a USB port on your computer or from a powered hub. It is not possible to power this device from an unpowered USB hub. If both the PC and the device under test are referenced to the same ground (earth) then noise/offset problems may occur. This problem is only likely to occur when using a mains powered desktop PC whilst measuring DC coupled (earth referenced) signals of a small amplitude. If this situation occurs a mains power adaptor (supplied) will have to be used to power the unit."

The scope is protected against overload / misconnection by self resetting fuses. Like most USB scopes powered from the PC an earth loop can be created in the above circumstances. Powering instead from the external mains power supply avoids the earth loop as the power now comes from a floating supply.

...

For safety reasons, all varients of the PS320x have resettable 0.3A fuses between the grounds of the BNC connectors (please note however that some early PS3206 versions have non-resettable 10A fuses).

...

 

"Dare to be naïve." - Buckminster Fuller

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Got an 0-250v 3a adjustable isolation transformer from a friend to use, one consern less now.

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Hi!

 

I see many good suggestions posted.   My small addition is how I personally handle instrument protection if I feel there are no ground differences between the scope and the circuit, but I'm not 100% positive.

 

In series with the ground lead of the scope I insert a (physically) small 100 ohm resistor.   In the event of some ground currents between the device and the scope, the 100 ohm will either protect by limiting the current or possible simply burn up.   I would caution you to use a film resistor, not a wirewound or old carbon composition resistors as they could fail shorted.

 

Good luck.

 

 

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I've got a 2207B pico as well; really nice scope.  My go to tool for measuring mains in one of those pintek ADP25's:

 

http://www.ebay.com/itm/Professi...

 

I'm going to ask the question - is the problem with measuring mains a problem/matter of grounding?  USB ground is the ground of the instrument, but is USB ground in any way grounded to mains?  If this a matter of confusing the hot and neutral causing a problem, or a problem of you can't connect the scope ground lead to the neutral?

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Just wanted to throw in that making an isolation transformer for your workbench is real easy.  Just grab two identical power transformers, hook them up back to back (connect all windings) and there's your line voltage nicely isolated.  Get a socket from Home Depot or somewhere.  It's what I use on my bench (and when I don't things go bang).  S.

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One solution is to use a cheap (USD50) USB oscilloscope (Sainsmart, Hantek 6022BE, etc) (These are about EUR50 on Ali / Ebay) and you can use them with an USB isolator (Again EUD 20 to 50). Even if you burn the scope it's not that much of a loss.

 

And for about USD 150 to 200 you can buy an USB scope with built-in Wifi.

Paul van der Hoeven.
Bunch of old projects with AVR's:
http://www.hoevendesign.com

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alank2 wrote:
I'm going to ask the question - is the problem with measuring mains a problem/matter of grounding?  USB ground is the ground of the instrument, but is USB ground in any way grounded to mains?  If this a matter of confusing the hot and neutral causing a problem, or a problem of you can't connect the scope ground lead to the neutral?
 

The average PC is grounded via the mains cable, so there's a nice earth path from your USB device back to earth. Touch the probe earth onto active and you'll see smoke. Things won't work quite the same way.

Laptops are a different story - you can run them off batteries.

I know my Tektronix 2465 is reasonably tolerant of touching the probe clip on mains - I've done it a few times, but I would not recommend it. Not sure how the newer digital scopes are constructed to tolerate such abuse.

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Isn't the probe high impedance?  Is it when you try to connect the ground to the DUT where the problem might be (allowing massive current to flow through the ground of the notebook)?