Author Topic: A digital scope for audio circuits, what do i need?  (Read 18251 times)

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Offline marmad

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Re: A digital scope for audio circuits, what do i need?
« Reply #50 on: June 11, 2013, 12:09:24 am »
Battery powered scopes without isolated channels, especially those with metal BNC connectors, scare me. If I'm going to clip probes to a DUT involving mains, then I want any exposed metal connected to these probes solidly bonded to earth, since I'm likely connected to earth somehow.

But it seems a bit as if you're mixing two different things: the safety of test equipment - and the safety practices of the operator.

I have a workshop filled with dangerous equipment: band saw, table saw, radial arm saw, etc. The machinery itself is designed to be as safe as possible - but there all kinds of things that operators could do with the machinery that would be unsafe.

Battery powered scopes without isolated channels are designed to be used floating with CAUTION to 30Vrms max - and if there's any doubt whether more than 30Vrms is present or not, the DSO should be grounded and floating measurements shouldn't be attempted. In other words, I wouldn't go probing in an unknown DUT with it until I had done some thorough checking.
 

Offline vk6zgo

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Re: A digital scope for audio circuits, what do i need?
« Reply #51 on: June 11, 2013, 08:02:46 am »
In many countries of Europe three wire AC plugs are not used at all, and people use oscilloscopes and every other type of appliances form PC's to vacuum cleaners to any other imaginable type of devices all the time. Instead of PE connections GFI Ground Fault Interrupters are used for safety and work very well. 

I repeat: It is deadly dangerous to ground an oscilloscope to an earth ground, both for you and the scope.  If you touch a live AC mains circuit and the scope with another part of your body you will suffer a serious shock that would likely not occur if a two wire connection was used.

Whenever using a scope or a refrigerator, it must be used with caution if you think you might connect somehow directly to the AC mains.
In all situations, this means don't work on equipment barefoot on a wet cement floor, don't use the scope in the shower or swimming pool, you don't play with an oscilloscope outside in the rain. Don't work on electrical equipment with wet clothes or shoes. Don't sit on metal benches or chairs while working on equipment or high voltages,  don't work on any electronic equipment naked or barefoot. Always try a single hand, a one-handed approach when working with mains connected equipment or with any high voltages. Above all, don't ground yourself to a PE connected piece of equipment with one hand and touch any other equipment with the other you are not sure about.

This is just common sense.

Secondly, ground loops between PE connected grounds can damage the circuit you are working on or cause you to experience mild shocks.  Even though PE connected devices should eliminate this problem, difference in ground resistance and generated currents from separate AC mains outlets connected equipment, even in the same room can create small voltage, but high-AC current connections. These low voltages with high  currents can blow out USB and printers etc, and you might even a low-voltage spark when connecting these grounds.

Connecting yourself between PE grounded equipment and equipment with faults or AC mains circuits might give you possibly fatal shocks that would not happened if the test equipment you are using ware using two wire AC connections and (and it helps if you have it) GFI.

If there is any suspicion that a piece of equipment could be damaged and presenting an AD mains hazard, this can be quickly checked with an AC handheld voltmeter. If you are getting high voltage readings to an earth ground, you should not connect your scope to it until you've repaired the trouble.

In most instances, these types of AC mains faults do not exist, to be sure you can always check with a multimeter before you risk connecting yourself to any equipment with your hands.

When repairmen work on high-voltage main lines, they deliberately isolate themselves and any other metal around them from an earth ground, or else they could be killed.  How do you think it is possible for repair technicians to work on 500000 VAC lines if they didn't isolate themselves completely from AC grounds.

Paul,the lack of an Earth pin on the mains socket (General Purpose Outlet,or GPO in OZ-talk),does not in any way imply that the Mains supply to your house is balanced with respect to Earth.

It simply means that there is no connection from the GPO back to where the Neutral line is earthed at the point of entry("Fusebox" in Oz-talk).

As it is,you will still be able to measure the full Mains potential between the Active line & any earthed object (water pipes,etc).

Non earthed GPOs were ,& are still,to some extent,used in the USA,but they make no mistake about the fact that if you get  connected between Active & an earthed object,you will suffer an electric shock.

Having an earthed instrument protects you against the case where the body of the 'scope becomes connected to the Active line,either due to an internal fault,or connecting the probe incorrectly to a Mains line.

This is a far more likely scenario than touching an Active line while also touching an earthed instrument body.
You will get a (less severe) shock anyway, in the latter case,whether you are actually touching an earth or not,as your shoes,etc,are not perfect insulators.

I have never seen the adverse results you attribute to a  system where the Earth connection is present at the GPO.

All of the above said,thousands of Americans & Europeans have used Oscilloscopes operated from 
non-earthed GPOs to fault-find Electronic equipment,because they know what they are doing & don't do silly things.
 

Offline MrSadman

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Re: A digital scope for audio circuits, what do i need?
« Reply #52 on: June 11, 2013, 08:26:26 am »
As it stands I will have to limit my measurements to around 24 volts. I could use either the Rigol(or any other scope that has a SMPS with similar filters) that has 115V on the chassis when not connected to anything or with a scope that doesn't have that. I know that both the USB scopes and the handhelds are a different thing to use. Getting a 2 channel Fluke scopemeter is out of my pricerange. Something like a Picoscope 5000 series has got a high vertical resolution and could thus be useful after I get a real scope down the line.
 

Offline marmad

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Re: A digital scope for audio circuits, what do i need?
« Reply #53 on: June 11, 2013, 01:13:37 pm »
As it stands I will have to limit my measurements to around 24 volts. I could use either the Rigol(or any other scope that has a SMPS with similar filters) that has 115V on the chassis when not connected to anything or with a scope that doesn't have that. I know that both the USB scopes and the handhelds are a different thing to use. Getting a 2 channel Fluke scopemeter is out of my pricerange. Something like a Picoscope 5000 series has got a high vertical resolution and could thus be useful after I get a real scope down the line.
The point is, even with USB scopes (which have metal BNC connectors tied together and back to laptop GND), you will have to be very careful when probing devices running off the mains (in case of unexpected faults).

OTOH, if you get a handheld with isolated channels (such as this Hantek or this Owon), you can safely probe any circuitry running off the mains. And it could also still be useful down the line as a portable.
 

Offline Tepe

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Re: A digital scope for audio circuits, what do i need?
« Reply #54 on: June 11, 2013, 10:25:58 pm »
In many countries of Europe three wire AC plugs are not used at all
Until a decade ago I had never had any in the various places I have lived - now I have a couple of outlets with PE in the kitchen and nowhere else.

and people use oscilloscopes and every other type of appliances form PC's to vacuum cleaners to any other imaginable type of devices all the time. Instead of PE connections GFI Ground Fault Interrupters are used for safety and work very well. 
Have had one for like 40 years now.
ceterum censeo systemd-inem esse delendam
 

Offline marmad

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Re: A digital scope for audio circuits, what do i need?
« Reply #55 on: June 11, 2013, 10:38:23 pm »
In many countries of Europe three wire AC plugs are not used at all
Until a decade ago I had never had any in the various places I have lived - now I have a couple of outlets with PE in the kitchen and nowhere else.

and people use oscilloscopes and every other type of appliances form PC's to vacuum cleaners to any other imaginable type of devices all the time. Instead of PE connections GFI Ground Fault Interrupters are used for safety and work very well. 
Have had one for like 40 years now.

Yes, this seems to be common in some parts of Europe. It just means that it's likely that if you have devices with SMPS' (PC, modern DSO, etc) you have ~110VAC present on the metal case (or any metal connected to it). Not necessarily a major problem - but it can be an unpleasant one some times.
 

Offline nctnico

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Re: A digital scope for audio circuits, what do i need?
« Reply #56 on: June 12, 2013, 01:00:40 am »
Yes, this seems to be common in some parts of Europe. It just means that it's likely that if you have devices with SMPS' (PC, modern DSO, etc) you have ~110VAC present on the metal case (or any metal connected to it). Not necessarily a major problem - but it can be an unpleasant one some times.
Actually it sucks pretty bad because when you connect two devices you can have 230V at input pins. Lots of stuff gets wrecked that way.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline Paul Price

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Re: A digital scope for audio circuits, what do i need?
« Reply #57 on: June 12, 2013, 02:46:44 am »

When people are talking about voltage potentials between well-functioning electronic equipment, such as oscilloscopes and PC computers, for instance, they are frightened because  they can measure a high voltage with a sensitive multimeter. There can be a considerable voltage between their respective chassis. However, this small, too small to cause any injury, somewhere in the high micro-amps, but it sometimes it can cause a mild electrical shock that should be considered more unpleasant rather than dangerous. If there were any dangerous AC current, the GFI would instantly kick open the AC circuit in the room.

When using my digital  and analog oscilloscopes, voltmeters and the likes under these situations, (which I have been doing for over 25 years doing electronic design and repair work.), I always connect the ground connection of the oscilloscope or other test instrument first to the D.U.T.  This means that the probe tip of the oscilloscope or signal generator, etc. is at the same ground potential before I connect a signal or a meter or an oscilloscope probe and can do no harm to the D.U.T.

I am not a lone maniac who chooses to work in this way, hundreds of other technicians in service shops in this tiny country operate in this very same way.

In fact, almost every house in this country has 2-wire AC plugs and people connect things together all the time without any injuries to themselves or their equipment, thanks to GFI. Once in a while people bring in equipment that they have burned out an USB port  because of connecting equipment where there are power ground loops. I always remind people to unplug at least one of the devices when connecting USB. But even different AC grounded circuits in the same room can have a nasty low-voltage AC but with a high-current that can also damage equipment and cause a tickle.

I haven't had any damage to any equipment that I've serviced, nor to myself, nor to the oscilloscopes and other test equipment in my work area.  I have been occasionally been tickled a bit by the bite of this inter-chassis AC  potential.

Before I plug anything in that I might be asked to repair or modify, I always make visual check of the equipment for rust or corrosion or other water damage or just check if the equipment shows any dampness, water drops, etc.  If I am unsure of the equipment, I use a DVM to check the AC power cord for resistance to the chassis ground before plugging it in. That is just common sense.  Electronics is about working with electricity and can be dangerous, so caution must be a thing to  learn and practice. I am not against grounding equipment if it is something like an electric washing machine, a refrigerator or a high-powered circular saw that is not double-insulated.

On the other hand, having a grounded oscilloscope on my workbench could put me in great danger. It could allow me to get a serious shock from some other piece of equipment that has a AC mains short to the chassis if I am touching the metal ground of the scope and the chassis of some piece of defective or unsafe equipment. I wouldn't be in danger if the oscilloscope was ungrounded.

In servicing thousands of devices, I can recall only two or three that ever had a AC mains short to ground. These few items could have been really dangerous, but a visual inspection and a DVM resistance check revealed the danger of the problem before I even turned them on. One was a miswired 3-wire AC EU-style line cord the other was due to someone trying to repair the item and connecting one side of the AC line to the chassis.

I have checked by oscilloscope and other equipment with an AC ammeter and after years of use, I can detect no breakdown and AC line leakage after been used in this way.  From tear downs of the power supplies of many pieces of equipment, and of my test equipment, I can always see that there are 1 or two 1000VAC to 2000VAC rated small valued capacitors (about .001 or less usually) connecting the hot AC mains circuits to the chassis grounds and with these capacitors, there is very, very little chance that these capacitors would break down by connecting their chassis to other equipment. I have never seen this happen. In fact the worst case stress on the power supply capacitor to ground is the normal operation of these capacitors since they are  connecting on side of the 240V AC line to the 3-rd wire ground to while they are serving their purpose to lower RFI from the switching power supply.
« Last Edit: June 12, 2013, 03:02:13 am by Paul Price »
 

Offline MrSadman

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Re: A digital scope for audio circuits, what do i need?
« Reply #58 on: June 13, 2013, 08:32:30 am »
Neither the Hantek or Owon are avaialble in Estonia. From cheaper brands there are Tenma and Uni-T, also 2 models from Digimess. I could go for the Agilent U1602B but the higher priced Agilent and Fluke devices just seem expensive.
 


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