EEVblog #279 - How NOT To Blow Up Your Oscilloscope!

[M. András]

the tv must be grounded otherwise i dont think they would use a 3 terminal iec plug the whole thing has a huge metal shield beneath the plastic, as for the coax calbe for the tv it comes from a optical modem a huawei one and the thing runs from 12 volts

[SeanB]

One item must be grounded and the other must have a floating supply. If you have a fibre modem then it is the item floating, most cable modems are grounded via the cable system interface. The TV then is the grounded item, in any case it is due to connecting a floating supply to a grounded on while the power is applied. In extreme cases this will blow the input chips on either the TV or the modem.

[M. András]

good to know, the modem i dont care if it fries  it belongs to the service provider but the 3 expensive sony lcd tv would be a problem, so if i measure the coax to ground it will read few ten volts plus?. any good way to avoid any damage?

[SeanB]

Safest is powered off and plug pulled out of the wall on both. If the modem has a metal case then add an external wire to earth, probably via a mains plug earth pin with the wire in the cable grip properly. otherwise you need to strip the cable insulation off somewhere along the cable and earth the shield exposed there, then tape it up again ( no soldering or you melt the inner wires) and wire to earth.

If you own the house drill through the wall and have an earth rod outside the room deep in ground with a 4mm conductor to the inside and bond all ground wires from equipment to it.

[Mint.]

19:25 If that actually cause the computer to reboot, that is indeed very nasty. USB ports are usually fused and/or current limited. I believe they even have to be according to the USB specification.

I did hear before that over current protection occurs by rebooting your PC ...

Yeah! Just short out the USB port with a key or something metal and the PC shuts down.

[MartinX]

A little off-topic maybe.

Is it best to ground the scope, or having it floating (use a wall-plug without ground) when measuring or cars/engine-management systems?

It doesn't really matter normally,as far as the car is concerned, (BUT see the final sentence!!),because it is standing on four big black insulators,so the car"earth" is not at the same potential as the mains earth! 
If you have other stuff connected to the car which causes the car body/chassis/engine block to be at mains earth potential,you have to be careful not to get your earth clip on the wrong spot.
If the latter is not the case,all the voltages in the car,including the few high ones are only with respect to the car body/chassis.etc.

That said,I must repeat what I said in another thread,"There is no real reason to put the earth lead on anywhere else but a point connected to the body of the car."
If you put the earth lead on say,a +12volt point,& your 'scope was sitting on a car seat,& thus insulated from car "earth",all would be well,but if you did the same with the 'scope's metalwork contacting the bodywork,there would be big "zap",& you could damage your probe,your 'scope,or your car.
If you again insulated the 'scope,& placed the earth clip on one of the high voltage points in the car,you would elevate the instrument's
metalwork to that voltage,& perhaps damage yourself!
Your Oscilloscope mains earth would not protect you in this case,because,as I said in the first sentence,the car"earth" is not at the same potential as the mains earth!

To answer your question:DO NOT remove the mains earth from the Oscilloscope,as if an electrical fault occurred inside the 'scope,causing  external metalwork to be live,& if the 'scope is sitting on a metal part of the car it would elevate the whole car to mains voltage,so everyone working on the car would be at risk.

I pretty much agree with everything vk6zgo says here except "There is no real reason to put the earth lead on anywhere else but a point connected to the body of the car." Well we do not know what kind of car cs.dk are working on or what systems it contain, or what kind of measurements he wants to do, but I definitely agree with the last sentence never just disconnect the earth lead, if you feel the need to isolate the scope always use a proper isolating transformer.

[vk6zgo]

A little off-topic maybe.

Is it best to ground the scope, or having it floating (use a wall-plug without ground) when measuring or cars/engine-management systems?

It doesn't really matter normally,as far as the car is concerned, (BUT see the final sentence!!),because it is standing on four big black insulators,so the car"earth" is not at the same potential as the mains earth! 
If you have other stuff connected to the car which causes the car body/chassis/engine block to be at mains earth potential,you have to be careful not to get your earth clip on the wrong spot.
If the latter is not the case,all the voltages in the car,including the few high ones are only with respect to the car body/chassis.etc.

That said,I must repeat what I said in another thread,"There is no real reason to put the earth lead on anywhere else but a point connected to the body of the car."
If you put the earth lead on say,a +12volt point,& your 'scope was sitting on a car seat,& thus insulated from car "earth",all would be well,but if you did the same with the 'scope's metalwork contacting the bodywork,there would be big "zap",& you could damage your probe,your 'scope,or your car.
If you again insulated the 'scope,& placed the earth clip on one of the high voltage points in the car,you would elevate the instrument's
metalwork to that voltage,& perhaps damage yourself!
Your Oscilloscope mains earth would not protect you in this case,because,as I said in the first sentence,the car"earth" is not at the same potential as the mains earth!

To answer your question:DO NOT remove the mains earth from the Oscilloscope,as if an electrical fault occurred inside the 'scope,causing  external metalwork to be live,& if the 'scope is sitting on a metal part of the car it would elevate the whole car to mains voltage,so everyone working on the car would be at risk.

I pretty much agree with everything vk6zgo says here except "There is no real reason to put the earth lead on anywhere else but a point connected to the body of the car." Well we do not know what kind of car cs.dk are working on or what systems it contain, or what kind of measurements he wants to do, but I definitely agree with the last sentence never just disconnect the earth lead, if you feel the need to isolate the scope always use a proper isolating transformer.

My reasoning was:
(1)If the signal is unbalanced to earth,you will still be able to see it OK,even if it is an artificially created voltage which is negative w.r.t earth (on a negative earth vehicle).

(2)If it is a signal balanced to earth,the massive capacitance of the 'scope case to car "Earth" may unbalance the signal with adverse results.
If it is a 2 channel 'scope,the easiest thing in this case would be to use the "pseudo differential" method of testing.

I haven't worked on car electronics since the 1988 Ford Falcon,so modern vehicles may use balanced signals.
Maybe someone more familiar with current trends could enlighten us.

[ErikTheNorwegian]

To build a USB isolator youself, you just need a ADuM4160BRWZ and a few components.

Data sheet:

http://www.analog.com/static/imported-files/data_sheets/ADuM4160.pdf

The IC is on the bay:

http://www.ebay.com/itm/ADI-ADuM4160-RW-16-USB-5kV-Digital-Isolator-/120905501702?pt=LH_DefaultDomain_0&hash=item1c26878c06

[M. András]

Safest is powered off and plug pulled out of the wall on both. If the modem has a metal case then add an external wire to earth, probably via a mains plug earth pin with the wire in the cable grip properly. otherwise you need to strip the cable insulation off somewhere along the cable and earth the shield exposed there, then tape it up again ( no soldering or you melt the inner wires) and wire to earth.

If you own the house drill through the wall and have an earth rod outside the room deep in ground with a 4mm conductor to the inside and bond all ground wires from equipment to it.

hmm i guess i gonna buy a new pc psu if the damn thing is plugged in the whole pc case floats at half mains voltage according to the meter the tv and the coax from the optic fibre modem has no problem, the cheap chinese usb 3 hub shield floats too, however everything has a perfect connection to earth, i guess i should call the pwer utility company to check their wiring outside the house and the earth connection, as far as i know the neutral and earth have to be connected before the electricity meter, but i cant check that too unless i void the seal then a huge fine will come soon

[Monkeh]

Safest is powered off and plug pulled out of the wall on both. If the modem has a metal case then add an external wire to earth, probably via a mains plug earth pin with the wire in the cable grip properly. otherwise you need to strip the cable insulation off somewhere along the cable and earth the shield exposed there, then tape it up again ( no soldering or you melt the inner wires) and wire to earth.

If you own the house drill through the wall and have an earth rod outside the room deep in ground with a 4mm conductor to the inside and bond all ground wires from equipment to it.

hmm i guess i gonna buy a new pc psu if the damn thing is plugged in the whole pc case floats at half mains voltage according to the meter the tv and the coax from the optic fibre modem has no problem, the cheap chinese usb 3 hub shield floats too, however everything has a perfect connection to earth, i guess i should call the pwer utility company to check their wiring outside the house and the earth connection, as far as i know the neutral and earth have to be connected before the electricity meter, but i cant check that too unless i void the seal then a huge fine will come soon

If it were properly earthed, the case would not be floating. There's a fault in either your PSU, your house wiring, or your electricity supply, and it's a potentially fatal one no matter where it is. If it's in the latter, you'll have to get it dealt with by the utility, for the former two, you can fix it yourself, you just need to find it.

[M. András]

Safest is powered off and plug pulled out of the wall on both. If the modem has a metal case then add an external wire to earth, probably via a mains plug earth pin with the wire in the cable grip properly. otherwise you need to strip the cable insulation off somewhere along the cable and earth the shield exposed there, then tape it up again ( no soldering or you melt the inner wires) and wire to earth.

If you own the house drill through the wall and have an earth rod outside the room deep in ground with a 4mm conductor to the inside and bond all ground wires from equipment to it.

hmm i guess i gonna buy a new pc psu if the damn thing is plugged in the whole pc case floats at half mains voltage according to the meter the tv and the coax from the optic fibre modem has no problem, the cheap chinese usb 3 hub shield floats too, however everything has a perfect connection to earth, i guess i should call the pwer utility company to check their wiring outside the house and the earth connection, as far as i know the neutral and earth have to be connected before the electricity meter, but i cant check that too unless i void the seal then a huge fine will come soon

If it were properly earthed, the case would not be floating. There's a fault in either your PSU, your house wiring, or your electricity supply, and it's a potentially fatal one no matter where it is. If it's in the latter, you'll have to get it dealt with by the utility, for the former two, you can fix it yourself, you just need to find it.

if its wiring then its in the house other parts which belongs to the utility company cos i rewired 3 years ago the whole flat from the switchbox. but i bet its  a psu fault, however i rechecked it today and it has nothing between the case and earth its a 4 flat  house with common wiring from the pole

[M. András]

found it the extension outlet wasnt grounded, it had a single 2 condutor wire, its installed to the desk for atleast 10 years or so, never checked it, grounded plug grounded 3 way outlet. i feel lucky now

[JimmyM]

Sorry to admit it but I fell victim to the earth ground. I guess that puts me in the "young players" category. I was looking at AC mains wave form while running on my generator.
As soon as I touched the scope's ground lead to the "hot" wire there were some sparks but I pulled it as soon as it happened and didn't seem to do any damage. So I connected the scope's ground to the "neutral wire" (bonded neutral main panel any way). Yeah, yeah. I know it ain't kosher but for a few seconds to get a look, it worked.
Now I'm more aware of it.

[Kevlarious]

Nice vid. I had my Dad back in the day to show me the do's and do not's. My Welch Scientific silly scope, dang the gurl is still here with me, all vacuum tube are good to go... She did her time at McGill U, Montreal. This scope was a boon to the many radio clubs across Canada back int he day. Dad worked on the Mid Canada Line and Pine Tree line.

[Ed.Kloonk]

Thanks for this video. I remembered that I own that god-awful portable Vellerman scope that Jaycar sold. Was handy cos I was running around in those days and the Vellerman could be chucked in the bag but carrying a full 20mHz CRO on the off chance needed was just impractical. Whilst it is easy to get used to the Vellerman being floating, like a MM, it was hard to force myself to remember to be careful what to measure when I found myself back on the CRO.

Having the Vellerman changed me, I have to say. Bringing something home and putting it on the bench and using the grounded CRO began to grate me a little. I actually considered a permanent isolating transformer so I would stuff myself up. Talked myself out of it tho, because of the dual input and figured the second probe's earth would still cause the magic blue smoke anyway...

[PeteInTexas]

Thank you for this.  Hope you still get "credit" for views because I'm just not into clicking "likes" and thumbs-up or whatever.

[sakujo7]

A bit after 20:0 I think I did actually hear slight static in the audio.

Yep. The scope triggered as well - note the green LED blinking at the same time as the static!

It's probably from discharging the parasitic capacitor in the power supply (mentioned at 7:42, and unfortunately not expanded upon).

[didi268]

Thanks for the video Dave, and keep on the excellent work!
I had a issue with oscilloscope ground many years ago.
I have bought a regulated power supply 30 V, 3 A, linear regulator, nice big transformer.
So everything should have been well isolated...
I found out the hard way as I had to put the ground of my probe to the V+ line one day, and bang, the power supply was shorted.
Luckily the current limit was set quiet low, so nothing catastrophic happend.
But of course it was enough to scare a "young player ... )
And what has caused the problem? The power supply has a case of sheet metal, and the wire that goes to the - Terminal was by accident squeezed between 2 pieces of the sheet metal when it was assembled and the isolation was damaged.
So the - Terminal was grounded, causing the short over the ground connection.
After isolating the wire and taking care  that it was not squeezed again the power supply still serves it's duty.

Dieter 
   

[chrisstra84]

Thank you for this Blog!

I tried to do something like that on this video and so I have figured out, that my ground connection to my “New-Old” Telequipment S51B was damaged.


Thank you so much!!! Now I have fixed it!

Christian

[knks]

I have two different laptops, I checked the power adapters for them (3 wires in, 2 wires out), when adapters are off.

Resistance between earth and negative output is about 1K for both of them. Is it normal?