It's not safe to float the scope. For TV work, you used to have to float the TV but it has never been safe to float a scope. Tektronix warns about this repeatedly in published documents.
If you float a scope and ground it to 300v, the entire body of the scope is at that 300v.
If you float a scope and ground it to 300v, the entire body of the scope is at that 300v.
As I stated but it is being done all the time in labs around the world. Working on your house breaker panel is dangerous too. If you cannot follow the rules of working on "hot" circuits, do not attempt any work. At least keep one hand in your pocket.
You can also use two isolation transformers to float both the DUT and test equipment. I have multiple isolation transformers in my lab just for this scenario.
From someone who knows more about scopes than anyone here:
Floating An Oscilloscope: A Definition “Floating” a ground referenced oscilloscope is the technique of defeating the oscilloscope’s protective grounding system – disconnecting “signal common” from earth, either by defeating the grounding system or using an isolation transformer. This allows accessible parts of the instrument such as chassis, cabinet, and connectors to assume the potential of the probe ground lead connection point. This is dangerous, not only from the standpoint of elevated voltages present on the oscilloscope (a shock hazard to the operator), but also due to cumulative stresses on the oscilloscope’s power transformer insulation. This stress may not cause immediate failure, but may lead to future dangerous failures (a shock and fire hazard), even after returning the oscilloscope to properly grounded operation! Not only is floating a ground-referenced oscilloscope dangerous, but the measurements are often inaccurate. This results from the total capacitance of the oscilloscope chassis being directly connected to the circuit under test at the point where the common lead is connected. TEKTRONIX RECOMMENDS ONLY THOSE MEASUREMENT TECHNIQUES THAT COMPLY WITH SAFETY ENGINEERING PRINCIPLES AND ENSURE ACCURATE MEASUREMENTS.
From the following document:
https://download.tek.com/document/51W_10640_1.pdf
Floating An Oscilloscope: A Definition “Floating” a ground referenced oscilloscope is the technique of defeating the oscilloscope’s protective grounding system – disconnecting “signal common” from earth, either by defeating the grounding system or using an isolation transformer. This allows accessible parts of the instrument such as chassis, cabinet, and connectors to assume the potential of the probe ground lead connection point. This is dangerous, not only from the standpoint of elevated voltages present on the oscilloscope (a shock hazard to the operator), but also due to cumulative stresses on the oscilloscope’s power transformer insulation. This stress may not cause immediate failure, but may lead to future dangerous failures (a shock and fire hazard), even after returning the oscilloscope to properly grounded operation! Not only is floating a ground-referenced oscilloscope dangerous, but the measurements are often inaccurate. This results from the total capacitance of the oscilloscope chassis being directly connected to the circuit under test at the point where the common lead is connected. TEKTRONIX RECOMMENDS ONLY THOSE MEASUREMENT TECHNIQUES THAT COMPLY WITH SAFETY ENGINEERING PRINCIPLES AND ENSURE ACCURATE MEASUREMENTS.
From the following document:
https://download.tek.com/document/51W_10640_1.pdf
Hi busaboy,
Perry is absolutely correct!
What you are suggesting is extremely dangerous in any part of the world. You may chose to be a lemming, but do not encourage others to follow suit.
Never float test equipment! It is deadly dangerous and I have more than 45 years in the repair business. When I was working on Carver, which used an AC mains regulator, we would have loved to just hang a scope on the various points of interest. But we never did this and I made sure none of my employees ever did this.
You can float the equipment you are working on, and I generally do just that. But floating the 'scope?!! Never, or any other equipment.
I didn’t mentioned at any given time I was repairing TV sets, worse CRT TV sets.
My working mains voltages are 110-130vac and devices across cap filter stages not exceeding 400vdc.
My main reasons is just to check for various waveform in pwm circuits in switching psu, AC-DC converters, office rated UPS (non commercial) and non industrial equipments (military nor medical).
I have an isolation transformer that I can use for my DUT’s but I want to be able to probe hot circuits (with added personal safety) but not the fear of breaking the instrument or worsen the job.
Hi Formas,
Then you should be using isolation probes.
Note that high voltage probes have voltage limits below what you might expect. X100 probes may break down at as little as 300 VDC, which I found very surprising. Probably designed for little USB scopes.
Another probe that might be useful to you would be a current probe. They just clamp around a wire, but they have voltage limits for their insulation you need to pay attention to. What you are doing is specialized, and that means specialized equipment that can be costly. We would rather you remain a member than join the list of Darwin awardees.
-Chris
Then you should be using isolation probes.
Note that high voltage probes have voltage limits below what you might expect. X100 probes may break down at as little as 300 VDC, which I found very surprising. Probably designed for little USB scopes.
Another probe that might be useful to you would be a current probe. They just clamp around a wire, but they have voltage limits for their insulation you need to pay attention to. What you are doing is specialized, and that means specialized equipment that can be costly. We would rather you remain a member than join the list of Darwin awardees.
-Chris
400v is within the capabilities of your scope. With 110v. you won't have over 200v.
What's the highest voltage measured with a multimeter across the caps in this device?
What's the highest voltage measured with a multimeter across the caps in this device?
I will repeat post #12 and cease participating here.
Absolutely not, you need an isolated differential probe for that.
All scopes are ground referenced, unless they run on batteries.
You cannot use any combination of unbalanced probes with a grounded scope for your purposes.
You can't fool around with mains voltage, this is extremely dangerous.
Absolutely not, you need an isolated differential probe for that.
All scopes are ground referenced, unless they run on batteries.
You cannot use any combination of unbalanced probes with a grounded scope for your purposes.
You can't fool around with mains voltage, this is extremely dangerous.
You can use the probes for anything within the voltage ratings for the scope with those probes. He's not measuring 20kv. It's 200v max on the primary side. Since he's not participating any longer, maybe someone can tell me why you can't probe a 200v point with standard scope probes.
So the mere fact that Mr Babin suggested that using the dual function on the scope disregard the ground and invert “CH2”, center lining both channels on the graticule and use the probe positives to test for measurements on the HOT circuit (energized not exceeding the rating of scope) is that safe? Or what could be the shortfall excluding slipping or personal errors?
The scope I uses has a 20v max on ch1, 200v (yes 200 when I rotate it to maximum) max on ch2 and two 1x-10x probes
The scope I uses has a 20v max on ch1, 200v (yes 200 when I rotate it to maximum) max on ch2 and two 1x-10x probes
Hi Perry,
I think Rayma is concerned with the ground reference on the probes. If you need more detail, you cannot float the scope and connect your ground reference to anything. Doing so places the entire oscilloscope at that reference and you are in an excellent position to electrocute yourself, or make holes in circuit boards when something that is ground referenced comes in contact with your oscilloscope. In order to do what you might want to do, you would have to disconnect the ground on the oscilloscope. That is what causes the most danger. No matter what you are trying to do, you must never disconnect the ground on the power cord of your 'scope!
I understand why Rayma won't participate further. His message was not coming across to other participants here. Rayma is correct! The second you disconnect the ground on your oscilloscope, you have placed yourself and others in harms way. It is possible to kill yourself and anyone that cones to your aid if you do this.
Any further suggestions to lift the ground on the oscilloscope and the moderating team will have no choice but to close the thread. Read the rules we have posted.
-Chris
I think Rayma is concerned with the ground reference on the probes. If you need more detail, you cannot float the scope and connect your ground reference to anything. Doing so places the entire oscilloscope at that reference and you are in an excellent position to electrocute yourself, or make holes in circuit boards when something that is ground referenced comes in contact with your oscilloscope. In order to do what you might want to do, you would have to disconnect the ground on the oscilloscope. That is what causes the most danger. No matter what you are trying to do, you must never disconnect the ground on the power cord of your 'scope!
I understand why Rayma won't participate further. His message was not coming across to other participants here. Rayma is correct! The second you disconnect the ground on your oscilloscope, you have placed yourself and others in harms way. It is possible to kill yourself and anyone that cones to your aid if you do this.
Any further suggestions to lift the ground on the oscilloscope and the moderating team will have no choice but to close the thread. Read the rules we have posted.
-Chris
Hi Formas,
If you hang a probe on each point, then switch to Y1 - Y2, you should be safe as long as the 'scope ground remains connected to the earth ground. However, if you need to see details smaller than the common mode voltage at your test points, you cannot reduce the range to something below the common mode voltage. So while the subtraction might work for you on the proper range, you cannot make the difference trace any larger. In order to do that you would need an isolated differential probe. That is exactly why they are made.
-Chris
If you hang a probe on each point, then switch to Y1 - Y2, you should be safe as long as the 'scope ground remains connected to the earth ground. However, if you need to see details smaller than the common mode voltage at your test points, you cannot reduce the range to something below the common mode voltage. So while the subtraction might work for you on the proper range, you cannot make the difference trace any larger. In order to do that you would need an isolated differential probe. That is exactly why they are made.
-Chris
Do you realize that I suggested using the scope in differential mode with 2 channels and no ground connection to the device?
The scope needs a reference. Generally, that is ground. In differential mode, the reference is the other probe. Anything that within the range of the scope's input is safe.
Again, the ground of the scope is NOT used. The second channel input (center conductor, not ground) is the reference.
The scope needs a reference. Generally, that is ground. In differential mode, the reference is the other probe. Anything that within the range of the scope's input is safe.
Again, the ground of the scope is NOT used. The second channel input (center conductor, not ground) is the reference.
Hi Perry,
Yes, I know exactly what you are suggesting, and I don't have a problem with it as long as the 'scope remains connected to the ground. If the common mode voltage is too large, there is no other way to get detail except to use a differential probe that is isolated.
-Chris
Yes, I know exactly what you are suggesting, and I don't have a problem with it as long as the 'scope remains connected to the ground. If the common mode voltage is too large, there is no other way to get detail except to use a differential probe that is isolated.
-Chris
What's the highest voltage in the device? Across the main filter cap, across the rectified mains, I'd guess.
I haven’t energized the device it’s a RV charger 120vac-12vdc 80amp. The filter cap is 200v 2200uf, (I’m guessing about 160+ across).
Is it that if connected the scope and I try to inspect the or a waveform by decreasing my volts/div the scope would break?
Is it that if connected the scope and I try to inspect the or a waveform by decreasing my volts/div the scope would break?
Attachments
You will be using the probe at 10x. In that position, the probe inserts a 9.09M ohm resistor in series with the scope input. The input has protection against overload. That protection and that 9M resistor will protect the scope.
How large would be too large, and will it break the scope if I try to inspect a large volt or waveform?
Hi Formas,
Well, the wrong range wouldn't do the inputs any good, depending on how serious the overload is. But you would find the circuit would saturate and you wouldn't get a usable waveform. So it is a pointless thing to try (don't).
In a nutshell, if you can use the probes (X10) on a range where there isn't any clipping, you can subtract one for the other (on the same range). If you need to see the difference amplified, you will then have to use an isolated, differential probe. Just remember that you can't remove the safety ground, and you can't overload the input amplifiers and get any useful information.
-Chris
Well, the wrong range wouldn't do the inputs any good, depending on how serious the overload is. But you would find the circuit would saturate and you wouldn't get a usable waveform. So it is a pointless thing to try (don't).
In a nutshell, if you can use the probes (X10) on a range where there isn't any clipping, you can subtract one for the other (on the same range). If you need to see the difference amplified, you will then have to use an isolated, differential probe. Just remember that you can't remove the safety ground, and you can't overload the input amplifiers and get any useful information.
-Chris