Andrew & Tonga,
No problem with measuring that on elevated potential.
there's nobody who forces you to connect the scope's chassis ground terminal to your circuit at all.
Each probe usually has a "live" pin and the "ground"/negative pin. Connect these tro pins at each end of the resistor, and the scope will display the voltage across it.
The trouble only starts when you measure across this resistor with one probe/channel, and want to measure on a different component (with differnt potential) with the second channel.
If these two omponents have one common potential, this does not need to be at chassis/ground potential. If they both have one lead connected to (example) the positive line of your PSU, just connect the "negative" of one probe to this level, and the positive leads of both probes/channels to the other end of both components. then you'll measure the voltage relative to the supply voltage. Just make sure that you have common potential on both channels' "grounds"/negatives.
Does this make sense to anyone but myself?
Jennice
No problem with measuring that on elevated potential.
there's nobody who forces you to connect the scope's chassis ground terminal to your circuit at all.
Each probe usually has a "live" pin and the "ground"/negative pin. Connect these tro pins at each end of the resistor, and the scope will display the voltage across it.
The trouble only starts when you measure across this resistor with one probe/channel, and want to measure on a different component (with differnt potential) with the second channel.
If these two omponents have one common potential, this does not need to be at chassis/ground potential. If they both have one lead connected to (example) the positive line of your PSU, just connect the "negative" of one probe to this level, and the positive leads of both probes/channels to the other end of both components. then you'll measure the voltage relative to the supply voltage. Just make sure that you have common potential on both channels' "grounds"/negatives.
Does this make sense to anyone but myself?
Jennice
Hi, Sy & Jennice,
Can you sort out the safety issue & then I'll try & pick up the pieces (or hold the jackets).
I want to measure the current flowing in the Vrails of an amp or between filter stages of a PSU and examine the ripple due to mains or due to signal loading the output.
But this temporary resistor could be inserted anywhere in a circuit to give a better idea of pulsed current flows.
Can you sort out the safety issue & then I'll try & pick up the pieces (or hold the jackets).
I want to measure the current flowing in the Vrails of an amp or between filter stages of a PSU and examine the ripple due to mains or due to signal loading the output.
But this temporary resistor could be inserted anywhere in a circuit to give a better idea of pulsed current flows.
SY,
I think we should make sure that we use the same terms here. I may have a problem since english isn't native to me.
Let me try to re-phrase my thoughts.
You can ground the secondary side of your circuit all you want.
You can also ground the Supply side of your scope all you want.
If the scope has a "class I" PSU (it has three pins, and requires grounding for formal safety reasons), using this ground pin is a good idea. Countries like Denmark (where I live) don't have safety ground in all the outlets before the 1970s. Therefore I don't have my class I scope grounded, whereas personal protection is made by a residual current breaker in the house's fuse panel.
However, there is no point in grounding the secondary side of your scope. One should check if the measurement input BNC's are connected to mains safety ground by design. Often, these inputs are floating (a differential input stage). They are not connected to mains safety ground in my scope.
The only thing you achive with "safety grounding" the scope inputs, is to make all non-zero related measurements impossible.
As long as the mains side of your scope is correctly connected, you'll be safe up to 2 - 3.5 kV, depending on your scope's design. (This is the isolation (insulation?) voltage of a power supply built according to EN/IEC 61010.) These requirements
must be met if you (a manufacturer) want to legally put a CE mark on your device in order to sell it within the EU. In the US, Underwriters Laboratories (UL) have adopted this standard as UL61010-B (for measurement and laboratory equipment), as a replacement for their UL 3111 standard.
To make a long story short... If your scope is connected properly to the mains, and the secondary (measurement) side is still floating relative to chassis GND, your scope is designed with a differential measurement input. In that case, there is no problem connecting the probe terminals on each side of a resistor, which is connected to the secondary (low voltage) supply rails of a supply of a design.
Jennice
I think we should make sure that we use the same terms here. I may have a problem since english isn't native to me.
Let me try to re-phrase my thoughts.
You can ground the secondary side of your circuit all you want.
You can also ground the Supply side of your scope all you want.
If the scope has a "class I" PSU (it has three pins, and requires grounding for formal safety reasons), using this ground pin is a good idea. Countries like Denmark (where I live) don't have safety ground in all the outlets before the 1970s. Therefore I don't have my class I scope grounded, whereas personal protection is made by a residual current breaker in the house's fuse panel.
However, there is no point in grounding the secondary side of your scope. One should check if the measurement input BNC's are connected to mains safety ground by design. Often, these inputs are floating (a differential input stage). They are not connected to mains safety ground in my scope.
The only thing you achive with "safety grounding" the scope inputs, is to make all non-zero related measurements impossible.
As long as the mains side of your scope is correctly connected, you'll be safe up to 2 - 3.5 kV, depending on your scope's design. (This is the isolation (insulation?) voltage of a power supply built according to EN/IEC 61010.) These requirements
must be met if you (a manufacturer) want to legally put a CE mark on your device in order to sell it within the EU. In the US, Underwriters Laboratories (UL) have adopted this standard as UL61010-B (for measurement and laboratory equipment), as a replacement for their UL 3111 standard.
To make a long story short... If your scope is connected properly to the mains, and the secondary (measurement) side is still floating relative to chassis GND, your scope is designed with a differential measurement input. In that case, there is no problem connecting the probe terminals on each side of a resistor, which is connected to the secondary (low voltage) supply rails of a supply of a design.
Jennice
Unless, of course, you touch the BNC connector... high voltages in places which are usually safe to touch are a hazard! 
I checked two scopes that I have on hand. Both connect the ground lead to the chassis. Think BANG! if you try this. That's why floating differential inputs were invented.
Thomas, I do not have a manual, though I've seen them for sale on the 'Net.
I checked two scopes that I have on hand. Both connect the ground lead to the chassis. Think BANG! if you try this. That's why floating differential inputs were invented.
Thomas, I do not have a manual, though I've seen them for sale on the 'Net.
SY said:Unless, of course, you touch the BNC connector... high voltages in places which are usually safe to touch are a hazard!
I checked two scopes that I have on hand. Both connect the ground lead to the chassis. Think BANG! if you try this. That's why floating differential inputs were invented.
Thomas, I do not have a manual, though I've seen them for sale on the 'Net.
Yes, I once took an old hot chassis TV into the lab and chose the wrong way to plug it in. Quite exciting.
SY,
Well, mine have common level on the two BNC's, but this isn't connected to mains supply ground.
The Tektronix TDS224 at work isn't "ground" connected either -
they're floating individually.
Another thing that struck me... The need to ground your DIY only exists if you either make your own transformer, or use one which isn't class II (double insulated). I am getting the strange feeling that you expect most DIY'ers to use class I transformers or other transformers which aren't safety tested!? The normal, 100% (item-by-item) saftety test involves the high voltage test (dielectric strength). If this is passed, what's your concern?
Jennice
Well, mine have common level on the two BNC's, but this isn't connected to mains supply ground.
The Tektronix TDS224 at work isn't "ground" connected either -
they're floating individually.
Another thing that struck me... The need to ground your DIY only exists if you either make your own transformer, or use one which isn't class II (double insulated). I am getting the strange feeling that you expect most DIY'ers to use class I transformers or other transformers which aren't safety tested!? The normal, 100% (item-by-item) saftety test involves the high voltage test (dielectric strength). If this is passed, what's your concern?
Jennice
Transformers are one source of shock hazard. In interconnecting equipment, there are many others. That's why any piece of equipment with a metal chassis needs to have a firm earth ground. Equipment with insulated chasses need to be approached with GREAT caution when probing around.
And floating exposed metal of the sort which is quite commonly touched is an extreme hazard! Do not float the ground of a scope at hazardous voltages!
And floating exposed metal of the sort which is quite commonly touched is an extreme hazard!
Do not float the ground of a scope at hazardous voltages!Especially if you take a +/-50V transformer, and ti the mid point to ground. The secondary side is way beyone what is considered a non-hazardous voltage in offivcial terms.
Are you really better off, touching +80 relative to ground, than touching the same point, if it is floating???
No, metal enclosures and grounded equipment have nothing to do with each other. It all comes down to the clearance and creepage distances on (and between) circuits, pollution degrees (of which pollition degree II includes normal household/office environment), and safety insulation (normal, double, or reinforced). Your transformer is part of this safety insulation consideration. This is partly why transformers are divided into differnt categories, depending on their intended use.
So does everything connected to the mains. Lets elevate this debate to a relevant level, shall we?
Could you please be more specific, and give an argument as to why? Even a sailing ship fits your definition above.
No, again it depends on the insulation barriers provided within.
If your claim is universally true, then why is all my commercially purchased (Onkyo, Harman Kardon, Philips etc) audio/video equipment allowed, when it's in metal enclosures, and still class II equipment (not grounded)? Even my Roland music keyboard is with metal enclosure, and has a class II PSU. Could you please tell Roland why they should obey SY's rules and not apply IEC norms?
Again... depending on the construction of the scope, the ground and probes "negative"'s are not always tied to the same potentials. Why do you think some probe BNC connectors are insulated with plastic? Maybe because some scope manufacturers have realized that floating differential probes can be built into scopes as well.
Jennice
BNCs are NOT commonly insulated. I've got a few dozen of them here and not one of them has a knurled ring that's insulated. They're all metal and they're all attached to the shield. And if that's floating at some elevated potential and the user casually touches it while touching something at the DUT ground (a not-unlikely scenario), injury can occur. Let's not even mention other exposed parts, like other BNC jacks on the scope that don't have anything attached to them.
There is a reason that current probes and isolated diff amps exist. There's no reason not to use the proper tools for the job. There's no reason to recommend a hazardous procedure when there are safe ways to accomplish the same thing.
There is a reason that current probes and isolated diff amps exist. There's no reason not to use the proper tools for the job. There's no reason to recommend a hazardous procedure when there are safe ways to accomplish the same thing.
SY,
Could you please anwser my questions, or is that too much to ask?
My post included floating differntial circuits for that purpose... but if you want to measure something precisely, you will always be at some risk if you don't know what you're doing.
Janneman,
That's exactly my point!
If I touch 80V relative to a fixed ground, and I am floating (not grounded), nothing happens (the bird scenario).
If I touch 80V relative to ground, and I am grounded, I can get hurt.
If I touch 80V of a floating supply, then what current is supposed to hurt me, regardless if I am grounded or not?
Can you please tell that so SY, and ask him to read about the differences of class I and class II appliances?
Jennice
Could you please anwser my questions, or is that too much to ask?
My post included floating differntial circuits for that purpose... but if you want to measure something precisely, you will always be at some risk if you don't know what you're doing.
Janneman,
That's exactly my point!
If I touch 80V relative to a fixed ground, and I am floating (not grounded), nothing happens (the bird scenario).
If I touch 80V relative to ground, and I am grounded, I can get hurt.
If I touch 80V of a floating supply, then what current is supposed to hurt me, regardless if I am grounded or not?
Can you please tell that so SY, and ask him to read about the differences of class I and class II appliances?
Jennice
I know this isn't pertinent to the direction this thread has taken, but I'm still looking at getting a scope.
I'm wondering if the B&K Beckman 1590A 4 channel 100 MHz portable scope is any good for what I need and whether or not the probes are easily acquired?
I have been watching the Tektronics but they are all over the board and many are for sale that seem to need repair. That worries me a little. If the guy who owns the scope can't fix it, I wonder what he was using it for.
So, are the B&K scopes any good?
Thanks, Terry
I'm wondering if the B&K Beckman 1590A 4 channel 100 MHz portable scope is any good for what I need and whether or not the probes are easily acquired?
I have been watching the Tektronics but they are all over the board and many are for sale that seem to need repair. That worries me a little. If the guy who owns the scope can't fix it, I wonder what he was using it for.
So, are the B&K scopes any good?
Thanks, Terry
Jennice, I am familiar with safety standards. But that's not the question at hand. The question is, with an indeterminate DUT, is it safe to float the return lead of a single-ended scope probe? And the answer is, "No!" It is far too easy for the user to contact high voltage points that can deliver significant currents- notice how often you contact more than one device when doing measurements, plugging/unplugging, etc.
The other question at hand is how to measure ripple currents. And the answer is, "With a current probe."
The other question at hand is how to measure ripple currents. And the answer is, "With a current probe."
Jennice said:
Are you kidding??
NOBODY "tells" the Overlord ANYTHING!Seriously, SY has a point, even with fully double isolated equipment, if you open it up to work on it its another ball game, as in "dangerous". It is so easy to touch two points with a large potential delta, and the chassis in a double isolated case is probably just as good a ground as any, if only because it doubles as screen and is connected to system ground.
There was also the point of BNC's and scope ground leads, these are normally connected to mains ground. Trying to use a normal probe to look at the signal between two points neither of which is ground normally results in equipment malfunction, permanent or otherwise.
Jan Didden
still4given said:
Yes, they are "any good". Why not? Thousands are built and sold and used. They spec not as hot as a Tek, but who need 2% time base accuracy when testing an audio amp?
The inportant thing is that it works OK. And if this is your first scope, don't set your sights too high. You'd like two channels, bandwidth upwards from 40 or 50Mhz to be able to see those oscillations, and yes a vertical sensitivity of 2mV/div would be nice, but you can settle for 5 or 10mV.
Try to get a demo before buying or the right to return it if it doesn't work.
Scope probes are fairly universal, most have a BNC connector and like to see a 1MegOhms//10pF input. I'd expect that's what the type you mentioned has. 4 channels @ 100MhHz is a great scope for audio, I never had one that good. Hameg 2 channel @ 60MHz.
Jan Didden
Terry,
I apolegise for the turn this has taken, but it is actualy somewhat relevant - and would find a conclusion, if SY would be polite enough to answer the questions asked, and give valid arguments for his own claims. I wonder what his profession is...?
...because he is afraid of the truth????
What a pile of (cencored)! Since when has the role of moderator entitled anyone to be a dictator, and refuse to answer the questions asked, just because they prove him wrong?
Dictatorship is generally not recognized as an accepted ruling form these days. I don't know if it's still in fashioon at SY's home, but a great deal of the remaining world has found diplomacy and reasoning the way to go. (I wouldn't be surprised if I would be binned for this, but that would just prove my point).
With a class II system, there is no point in tying the secondary side to mains ground (that's the whole idea of the class II definition/requirement)
You can, if you want, tie the chassis of a class II appliance to mains ground, in stead of keeping it floating, but the secondary side of class II equipment is normally floating. For safety precaution, SY could always measure it's potential.
I would like SY to tell me what fault should make the class II equipment's secondary side dangerous to touch.
(If that's at all possible. So far he has refused to answer most of my other questions... a very odd way to moderate.)
With the secondary floating, you are free to have the scope grounded, and still measure that voltage drop on the EUT supply resistor, regardless of it's potential relative to EUT supply zero. also, this is the other way of keeping SY's probes at earth potential. All that happens is that
That is also the case if you touch the rails of a (not rarely found) +/- 50 V supply, which has the mid (zero) connected to ground.
I have never claimed that one should not be careful when working with elecronics, but unless you don't DIY at all, there is always a risc. You can never avoid it, and there's no excuse for not taking care.
A system ground, which for class II equipment is "grounded" for shielding on the secondary side. Thus the chassis floats relative to mains ground.
Not if the EUT is floating. One should never play with class I PSU's unless the person really knows what he/she is doing.
Jennice
I apolegise for the turn this has taken, but it is actualy somewhat relevant - and would find a conclusion, if SY would be polite enough to answer the questions asked, and give valid arguments for his own claims. I wonder what his profession is...?
janneman said:
Are you kidding??
...because he is afraid of the truth????
What a pile of (cencored)! Since when has the role of moderator entitled anyone to be a dictator, and refuse to answer the questions asked, just because they prove him wrong?
Dictatorship is generally not recognized as an accepted ruling form these days. I don't know if it's still in fashioon at SY's home, but a great deal of the remaining world has found diplomacy and reasoning the way to go. (I wouldn't be surprised if I would be binned for this, but that would just prove my point).
With a class II system, there is no point in tying the secondary side to mains ground (that's the whole idea of the class II definition/requirement)
You can, if you want, tie the chassis of a class II appliance to mains ground, in stead of keeping it floating, but the secondary side of class II equipment is normally floating. For safety precaution, SY could always measure it's potential.
I would like SY to tell me what fault should make the class II equipment's secondary side dangerous to touch.
(If that's at all possible. So far he has refused to answer most of my other questions... a very odd way to moderate.)
With the secondary floating, you are free to have the scope grounded, and still measure that voltage drop on the EUT supply resistor, regardless of it's potential relative to EUT supply zero. also, this is the other way of keeping SY's probes at earth potential. All that happens is that
That is also the case if you touch the rails of a (not rarely found) +/- 50 V supply, which has the mid (zero) connected to ground.
I have never claimed that one should not be careful when working with elecronics, but unless you don't DIY at all, there is always a risc. You can never avoid it, and there's no excuse for not taking care.
A system ground, which for class II equipment is "grounded" for shielding on the secondary side. Thus the chassis floats relative to mains ground.
Not if the EUT is floating. One should never play with class I PSU's unless the person really knows what he/she is doing.
Jennice
Jennice said:[snip]Not if the EUT is floating. One should never play with class I PSU's unless the person really knows what he/she is doing.
Jennice
Jennice,
I was just joking about SY, about his title, not about his attitude. If I had to absolutely live under a dictator, SY it would be! Loosen up, Jennice!
On the grounding stuff, yes, IF the eut is floating. You'll be surprised how easy it is to have the eut connected to a power supply, a signal generator, maybe a dummy load, and then try to use the scope ground lead on a non-ground test point...
I agree with you, you absolutely have to know what you are doing, unfortunately, as a newby reading these forums you can easily be fooled into thinking audio is all a piece of cake. Better advice on the safe side.
Jan Didden
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.