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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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Oscilloscope 'floating' measurements
Many advocate the use of an isolating transformer. The oscilloscope
probe and ground are then used like multimeter leads. Tektronix and others maintain this is a dangerous practice and may result in cumulative damage to the oscilloscope. Differential probes are one answer, though quite expensive. Comments please. Thanks BobH |
#2
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Oscilloscope 'floating' measurements
On Nov 30, 7:48*am, pfm wrote:
Many advocate the use of an isolating transformer. The oscilloscope probe and ground are then used like multimeter leads. Tektronix and others maintain this is a dangerous practice and may result in cumulative damage to the oscilloscope. Differential probes are one answer, though quite expensive. Comments please. Thanks BobH I can see no reason that using an isolation transformer properly can have any more detrimental effects on a probe or the 'scope itself than testing equipment that is self isolated [transformer operated]. When using an isolation transformer, the UUT [Unit Under Test] must have the circuit 'common' bonded to the 'scope common [and ground] usually by the probe 'pig tail' or even a separate test lead joining them. Indeed, if the probe tip is touched to an unbonded UUT, it can damage the 'scope or probe, regardless of wether it is powered through an external isolation transformer or through an internal transformer or SMPS. On the other hand, connecting an 'scope that has been 'floated' from ground by disconnecting the power cord ground pin CAN be severely damaged by connecting it to a non-isolated line operated UUT. The usual reason is excessive voltage applied between the 'scope common and the 'scope supply neutral or line. These usually turn out to be transient pulses that are unexpected. Neil S. |
#3
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Oscilloscope 'floating' measurements
pfm wrote:
Many advocate the use of an isolating transformer. The oscilloscope probe and ground are then used like multimeter leads. Tektronix and others maintain this is a dangerous practice and may result in cumulative damage to the oscilloscope. Differential probes are one answer, though quite expensive. Comments please. Thanks BobH What are you trying to measure? |
#4
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Oscilloscope 'floating' measurements
pfm wrote in message
... Many advocate the use of an isolating transformer. The oscilloscope probe and ground are then used like multimeter leads. Tektronix and others maintain this is a dangerous practice and may result in cumulative damage to the oscilloscope. Differential probes are one answer, though quite expensive. Comments please. Thanks BobH Damage maybe if some voltage rating of a cap or something is exceeded but "cumulative " damage, to what? |
#5
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Oscilloscope 'floating' measurements
It's not clear what you were trying to say.. "THIS? is a dangerous
practice". Floating a scope plugged into a utility line power source (and omitting the earth ground connection) is hazardous because the chassis (possibly the case and other exposed hardware) of the scope will be at the same potential as the probe ground clip while testing. Also, it's possible to exceed the scope input's safe limits. When using a proper isolation transformer, the equipment/device being tested is commonly plugged into the I-T, not the scope (some exceptions apply, in-place industrial equipment which can't be powered by a servicing I-T, for example). Some I-Ts have a pass-thru earth ground connection, and others may bond the line Neutral or one side of the line power (which isn't any type I've ever used, just read about here). When servicing gear, I want the DUT device under test to have no paths at all, to the utility service/line power Hot(s) or Neutral connections. The DUT is then as safe as if it were powered by battery (except for only a miniscule uA leakage at the I-T transformer). FWIW.. In the U.S. the AC line Neutral is often bonded to earth ground at the service panel). Common Variacs or autotransformers do not provide line isolation, and shouldn't be considered safety equipment. A 2 channel/input scope with a Differential Mode selection and proper isolation of the channel circuits can generally be used for safe testing, but only if this procedure is included in the scope's operating manual. -- Cheers, WB .............. "pfm" wrote in message ... Many advocate the use of an isolating transformer. The oscilloscope probe and ground are then used like multimeter leads. Tektronix and others maintain this is a dangerous practice and may result in cumulative damage to the oscilloscope. Differential probes are one answer, though quite expensive. Comments please. Thanks BobH |
#6
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Oscilloscope 'floating' measurements
Thanks all for replies. Please google for 'Fundamentals of Floating
Measurements and Isolated Input Oscilloscopes'. This is a Tektronix application note. The situation to which I refer is is at the bottom of page 4. I want to use an oscilloscope to check the PWM switching chip on the primary side of an SMPS. BobH |
#7
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Oscilloscope 'floating' measurements
pfm wrote:
Many advocate the use of an isolating transformer. The oscilloscope probe and ground are then used like multimeter leads. ....which will get you exactly where you want to be. Tektronix and others maintain this is a dangerous practice ....to the bottom line of Tektronix, Inc. and may result in cumulative damage to the oscilloscope. ....of which they couldn't provide a convincing example. Differential probes are one answer, though quite expensive. See "bottom line", above. The claim is bull****, just like MICROS~1 telling you that software other than theirs will cause you problems. |
#8
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Oscilloscope 'floating' measurements
pfm wrote:
Many advocate the use of an isolating transformer. The oscilloscope probe and ground are then used like multimeter leads. Tektronix and others maintain this is a dangerous practice and may result in cumulative damage to the oscilloscope. Differential probes are one answer, though quite expensive. Comments please. Thanks BobH you can do it, but there is a potential shock hazard if your scope chassis is at some strange voltage- but all depends on what you're measuring. Floating your scope and probing around in a microwave oven so you don't don't smoke your ground leads is probably a bad idea for example. It's pretty hard to completely float stuff anyways. I once got a memorable shock from touching one terminal of 16 kV capacitor that was just sitting on a bench. I'm still not sure why it was able to conduct to the ground I was standing on as effectively as it did, but it sure did. |
#9
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Oscilloscope 'floating' measurements
On Wed, 30 Nov 2011 09:10:06 -0800, mike wrote:
What are you trying to measure? heh... Gullibility, perhaps? |
#10
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Oscilloscope 'floating' measurements
The claim is bull****, just like MICROS~1 telling you that software other than theirs will cause you problems. Has little or nothing to do with MS. We are talking safety here and a potential lethal shock. Has to do with the breakdown voltage of the scope case, the probes, internal components, leads, and so on. Tektronix needs to hire you, as you know more than they do. |
#11
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Oscilloscope 'floating' measurements
pfm wrote:
Many advocate the use of an isolating transformer. The oscilloscope probe and ground are then used like multimeter leads. Tektronix and others maintain this is a dangerous practice and may result in cumulative damage to the oscilloscope. Differential probes are one answer, though quite expensive. Comments please. Thanks BobH I have used differential probes and battery operated scopes. That's the only thing I would recommend. Unless it's a specific service mans isolation transformer, most every commercial isolation transformer is NOT isolated from ground. Ground isolation is often what you after because neutral is tied to ground. Using complete isolation through a service mans isolation transformer still passes a low level capacitive ac line connection to the circuit, and measuring high impedance circuits can throw you off. Using a battery operated scope still connected to the charger will also pass capacitive ac line coupling. Greg |
#12
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Oscilloscope 'floating' measurements
pfm wrote:
Thanks all for replies. Please google for 'Fundamentals of Floating Measurements and Isolated Input Oscilloscopes'. This is a Tektronix application note. The situation to which I refer is is at the bottom of page 4. I want to use an oscilloscope to check the PWM switching chip on the primary side of an SMPS. BobH You do NOT want to do this. Leave the scope grounded and put an isolation transformer on the device under test. But you still DON'T want to hook the ground of the probe onto the common node of the DUT. there's just too much common mode stuff floating around. You can get some information by using two probes and a dual-channel scope with one channel inverted and summed. Hook the probe grounds together, but to NOTHING else. You have to turn down the scope gain until EACH channel can handle all the common mode signals on the inputs. Often, that results in a sensitivity too low to get much info from the difference trace. And you'll see junk because the ground path is ill-defined. The TEK A6902A isolated probe works well in this application. You can hook the ground lead of the probe almost anywhere you want. In a perfect world, you can get by doing dangerous things. But it takes only one slip-up to smoke your probe, your scope, your DUT and YOURSELF!!! |
#13
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Oscilloscope 'floating' measurements
"pfm" wrote in message ... Thanks all for replies. Please google for 'Fundamentals of Floating Measurements and Isolated Input Oscilloscopes'. This is a Tektronix application note. The situation to which I refer is is at the bottom of page 4. I want to use an oscilloscope to check the PWM switching chip on the primary side of an SMPS. BobH I do this all the time - virtually every working day in fact. My current scope - a 4 channel 100MHz Hitachi that I've had from new - is probably 25 years old now, so if it has been suffering "cumulative damage", it's bloody slow to reach a point where it's a problem ... My scope is normally grounded to the incoming line power ground connection. The SMPS that I'm working on is connected via a variac, that is plugged into a full 1 : 1 isolation transformer. To make measurements on the primary-side drive waveforms etc, I first solder a stiff wire to the "-" tag of the main filter cap. That is then used as the scope probe ground point. This arrangement has never caused me a problem on any type of switcher from simple single stage ones, right through to the multi-stage types that have a PFC supply at the front end. Arfa |
#14
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Oscilloscope 'floating' measurements
On Nov 30, 8:14*pm, "Arfa Daily" wrote:
"pfm" wrote in message ... Thanks all for replies. Please google for 'Fundamentals of Floating Measurements and Isolated Input Oscilloscopes'. This is a Tektronix application note. The situation to which I refer is is at the bottom of page 4. *I want to use an oscilloscope to check the PWM switching chip on the primary side of an SMPS. BobH I do this all the time - virtually every working day in fact. My current scope - a 4 channel 100MHz Hitachi that I've had from new - is probably 25 years old now, so if it has been suffering "cumulative damage", it's bloody slow to reach a point where it's a problem ... My scope is normally grounded to the incoming line power ground connection. |
#15
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Oscilloscope 'floating' measurements
Differential probes are one answer, though quite expensive.
Comments please. Thanks BobH BobH We used our Tek scopes in the A-B mode to do this all the time. If you don't have the use of a dual channel scope then an isolation transformer is in order. You do need to let us know what signals or what you are measuring. Bob AZ |
#16
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Oscilloscope 'floating' measurements
"pfm" Many advocate the use of an isolating transformer. The oscilloscope probe and ground are then used like multimeter leads. Tektronix and others maintain this is a dangerous practice and may result in cumulative damage to the oscilloscope. ** Tek say no such thing - you bull**** artist. They do point out it can KILL the operator. .... Phil |
#17
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Oscilloscope 'floating' measurements
"pfm" Thanks all for replies. Please google for 'Fundamentals of Floating Measurements and Isolated Input Oscilloscopes'. This is a Tektronix application note. The situation to which I refer is is at the bottom of page 4. ** The warning is about the potential for a FATAL ELECTRIC SHOCK ! http://www2.tek.com/cmsreplive/tirep...18312_EN.p df I want to use an oscilloscope to check the PWM switching chip on the primary side of an SMPS. ** The Iso Tranny goes with the SMPS - you fool. .... Phil |
#18
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Oscilloscope 'floating' measurements
gregz wrote in
rg: pfm wrote: Many advocate the use of an isolating transformer. The oscilloscope probe and ground are then used like multimeter leads. Tektronix and others maintain this is a dangerous practice and may result in cumulative damage to the oscilloscope. Differential probes are one answer, though quite expensive. Comments please. Thanks BobH I have used differential probes and battery operated scopes. That's the only thing I would recommend. Proper practice is to put the Device Under Test on the isolation transformer,not the scope. leave the scope grounded. Unless it's a specific service mans isolation transformer, most every commercial isolation transformer is NOT isolated from ground. Ground isolation is often what you after because neutral is tied to ground. Using complete isolation through a service mans isolation transformer still passes a low level capacitive ac line connection to the circuit, and measuring high impedance circuits can throw you off. Using a battery operated scope still connected to the charger will also pass capacitive ac line coupling. Greg TEK makes (or used to make) special probes/amps that isolate the scope from the device under test(DUT). they had one that used fiber optics,and another that actually disconnected the ground for up to 40V,and monitored the ground potential to reconnect the ground if the potential went above 40V. A6902,IIRC. TEK also makes differential amps that can be used with single ended scopes,but you need a separate probe power supply if it's not a TDS model scope. -- Jim Yanik jyanik at localnet dot com |
#19
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Oscilloscope 'floating' measurements
On 12/1/2011 9:43 AM, Jim Yanik wrote:
Proper practice is to put the Device Under Test on the isolation transformer,not the scope. leave the scope grounded. I don't know why this is so hard for people to understand. Test equipment is grounded. Period. If your probe can't handle the offset voltage, you're using the wrong probe. If you're trying to measure something normally connected directly across the utility power line, then use an isolation transformer and you can ground any point in the device being tested. Use a solid ground jumper. Do NOT rely on the ground path back through the scope probe. Jeff-1.0 wa6fwi -- "Everything from Crackers to Coffins" |
#20
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Oscilloscope 'floating' measurements
On Dec 1, 11:38*am, Jeffrey Angus wrote:
On 12/1/2011 9:43 AM, Jim Yanik wrote: Proper practice is to put the Device Under Test on the isolation transformer,not the scope. leave the scope grounded. I don't know why this is so hard for people to understand. Test equipment is grounded. Period. If your probe can't handle the offset voltage, you're using the wrong probe. If you're trying to measure something normally connected directly across the utility power line, then use an isolation transformer and you can ground any point in the device being tested. Use a solid ground jumper. Do NOT rely on the ground path back through the scope probe. Jeff-1.0 wa6fwi -- "Everything from Crackers to Coffins" Thanks. No question about the scope being grounded, no one suggested otherwise. Tektronix maintain that floating measurements, taken in a DUT powered by an isolating transformer, is bad practice. |
#21
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Oscilloscope 'floating' measurements
On Wed, 30 Nov 2011 07:48:46 -0800 (PST), pfm wrote:
Many advocate the use of an isolating transformer. The oscilloscope probe and ground are then used like multimeter leads. Tektronix and others maintain this is a dangerous practice and may result in cumulative damage to the oscilloscope. Differential probes are one answer, though quite expensive. Comments please. Thanks BobH Don't isolate the scope; instead power the unit under test through a isolation transformer. PlainBill |
#22
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Oscilloscope 'floating' measurements
"pfm = bull**** artist " Thanks. No question about the scope being grounded, no one suggested otherwise. Tektronix maintain that floating measurements, taken in a DUT powered by an isolating transformer, is bad practice. ** Nothing like that is in the app note YOU cited. In fact there is no mention of the idea at all making the app note seem deliberately misleading. .... Phil |
#23
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Oscilloscope 'floating' measurements
It's not clear what you're after (echo).
For most technicians, the term "floating" in connection with scope use, refers to 'floating the scope' which means: Read This Part separation of the scope power cord's 3rd pin from earth ground. I didn't look at Tek's cautions/recommendations, but I know scope manufacturers are familiar with the (nearly always) ill-advised practice of Floating A Scope. Are you unfamiliar with proper and safe practices, or unfamiliar with using a scope, or unable to express what it is you want ask about? All of the above? I'm fairly certain that all of Tek's recommended procedures are safe.. maybe you're just not understanding what's presented in the literature.. or maybe you're trying to apply the wrong procedure for your intended purpose. As far as various types of electronic gear/equipment go, it's very common that there are floating voltages present inside them. No one was placed here to try to understand what you meant, even though you didn't say it. Maybe you were thinking of sci.telepathic.electronics. -- Cheers, WB .............. "pfm" wrote in message ... Thanks. No question about the scope being grounded, no one suggested otherwise. Tektronix maintain that floating measurements, taken in a DUT powered by an isolating transformer, is bad practice. |
#24
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Oscilloscope 'floating' measurements
On Nov 30, 9:51*am, "Wild_Bill" wrote:
It's not clear what you were trying to say.. "THIS? is a dangerous practice". Floating a scope plugged into a utility line power source (and omitting the earth ground connection) is hazardous because the chassis (possibly the case and other exposed hardware) of the scope will be at the same potential as the probe ground clip while testing. Also, it's possible to exceed the scope input's safe limits. When using a proper isolation transformer, the equipment/device being tested is commonly plugged into the I-T, not the scope (some exceptions apply, in-place industrial equipment which can't be powered by a servicing I-T, for example). Some I-Ts have a pass-thru earth ground connection, and others may bond the line Neutral or one side of the line power (which isn't any type I've ever used, just read about here). When servicing gear, I want the DUT device under test to have no paths at all, to the utility service/line power Hot(s) or Neutral connections. The DUT is then as safe as if it were powered by battery (except for only a miniscule uA leakage at the I-T transformer). FWIW.. In the U.S. the AC line Neutral is often bonded to earth ground at the service panel). Common Variacs or autotransformers do not provide line isolation, and shouldn't be considered safety equipment. A 2 channel/input scope with a Differential Mode selection and proper isolation of the channel circuits can generally be used for safe testing, but only if this procedure is included in the scope's operating manual. -- Cheers, WB ............. "pfm" wrote in message ... Many advocate the use of an isolating transformer. The oscilloscope probe and ground are then used like multimeter leads. Tektronix and others maintain this is a dangerous practice and may result in cumulative damage to the oscilloscope. Differential probes are one answer, though quite expensive. Comments please. Thanks BobH Agreed on the dangerous aspect but I have had issues with ground loops between an audio generator and the scope when testing audio amplifiers. I carry a ground lifter in the scope probe bag and use it when needed - not very often. I NEVER connect the scope ground lead to any elevated supplies when using a ground lifter. There are other better and safe ways to measure high Voltage circuits. G² |
#25
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Oscilloscope 'floating' measurements
Agreed on the dangerous aspect but I have had issues with ground loops between an audio generator and the scope when testing audio amplifiers. I carry a ground lifter in the scope probe bag and use it when needed - not very often. ** Typical ground loops can be eliminated by fitting a resistor between the scope chassis ground and safety ground of about 10 to 22 ohms. For obvious safety reasons this resistor must be protected by shunt diodes in each polarity. A simple way is to use a 25 amp rated bridge rectifier and connect the resistor across the AC input terminals while shorting the " + " and " - " terminals - giving two diode drops in each direction with current capacity enough to trip a 20 amp supply breaker if necessary without damage. The bridge and resistor can go inside the scope. .... Phil |
#26
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Oscilloscope 'floating' measurements
On Nov 30, 2:24*pm, JeffM wrote:
pfm wrote: Many advocate the use of an isolating transformer. The oscilloscope probe and ground are then used like multimeter leads. ...which will get you exactly where you want to be. Tektronix and others maintain this is a dangerous practice ...to the bottom line of Tektronix, Inc. and may result in cumulative damage to the oscilloscope. ...of which they couldn't provide a convincing example. Differential probes are one answer, though quite expensive. See "bottom line", above. The claim is bull****, just like MICROS~1 telling you that software other than theirs will cause you problems. Thanks, I think you are right. |
#27
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Oscilloscope 'floating' measurements
On Nov 30, 5:34*pm, Allodoxaphobia
wrote: On Wed, 30 Nov 2011 09:10:06 -0800, mike wrote: What are you trying to measure? heh... *Gullibility, perhaps? You could be right |
#28
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Oscilloscope 'floating' measurements
On Dec 1, 6:25*pm, "Phil Allison" wrote:
"pfm = *bull**** artist " Thanks. No question about the scope being grounded, no one suggested otherwise. Tektronix maintain that floating measurements, taken in a DUT powered by an isolating transformer, is bad practice. ** Nothing like that is in the app note YOU cited. *In fact there is no mention of the idea at all making the app note seem deliberately misleading. ... *Phil Try this: http://www.tek.com/Measurement/App_N...ng/safety.html |
#29
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Oscilloscope 'floating' measurements
On 12/2/2011 7:10 AM, pfm wrote:
On Dec 1, 6:25 pm, "Phil wrote: "pfm = bull**** artist" Thanks. No question about the scope being grounded, no one suggested otherwise. Tektronix maintain that floating measurements, taken in a DUT powered by an isolating transformer, is bad practice. ** Nothing like that is in the app note YOU cited. In fact there is no mention of the idea at all making the app note seem deliberately misleading. ... Phil Try this: http://www.tek.com/Measurement/App_N...ng/safety.html Phil's still right. Yes they show an isolation transformer. But they show it isolating the scope, NOT the device being tested. There's no mention of using an isolation transformer to float the device being tested. Tek's app note is based on NOT being able to isolate the device being tested. Jeff-1.0 -- "Everything from Crackers to Coffins" |
#30
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out of logic
Phil Allison wrote:
snipped profanity and name calling = what you do when you have no logical argument, but still need to hear yourself speak. |
#31
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Oscilloscope 'floating' measurements
On Dec 2, 9:41*am, Jeffrey Angus wrote:
On 12/2/2011 7:10 AM, pfm wrote: On Dec 1, 6:25 pm, "Phil *wrote: "pfm = *bull**** artist" Thanks. No question about the scope being grounded, no one suggested otherwise. Tektronix maintain that floating measurements, taken in a DUT powered by an isolating transformer, is bad practice. ** Nothing like that is in the app note YOU cited. * In fact there is no mention of the idea at all making the app note seem deliberately misleading. ... *Phil Try this:http://www.tek.com/Measurement/App_N...fs/tds3000-flo... Phil's still right. Yes they show an isolation transformer. But they show it isolating the scope, NOT the device being tested. There's no mention of using an isolation transformer to float the device being tested. Tek's app note is based on NOT being able to isolate the device being tested. Jeff-1.0 -- "Everything from Crackers to Coffins" You are right, thank you. |
#32
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out of logic
"mike" ** You snipped all the facts out and then say there are none. What sort of ****ING TENTH WIT MORON does that ?? One like YOU of course. FOAD. |
#33
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Oscilloscope 'floating' measurements
"Jeffrey Angus" Phil's still right. ** Course he is.... Yes they show an isolation transformer. But they show it isolating the scope, NOT the device being tested. There's no mention of using an isolation transformer to float the device being tested. Tek's app note is based on NOT being able to isolate the device being tested. ** Tek are very remiss in not mentioning the MOST COMMON method of using a scope on non mains isolated circuitry - which is to install a 1:1 mains isolation tranny and then ground any part of the circuit you need to. What this achieves is ELIMINATING the need to make floating measurements. Similarly, even with equipment that has mains isolation by virtue of a 50/60 Hz transformer - one can temporarily disconnect the secondary side from supply ground and float the circuitry. This again allows you to ground any part of the circuit you desire to facilitate scope measurements. .... Phil |
#34
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Oscilloscope 'floating' measurements
"Wild_Bill" wrote in message ... It's not clear what you're after (echo). For most technicians, the term "floating" in connection with scope use, refers to 'floating the scope' which means: Read This Part separation of the scope power cord's 3rd pin from earth ground. I didn't look at Tek's cautions/recommendations, but I know scope manufacturers are familiar with the (nearly always) ill-advised practice of Floating A Scope. Are you unfamiliar with proper and safe practices, or unfamiliar with using a scope, or unable to express what it is you want ask about? All of the above? I'm fairly certain that all of Tek's recommended procedures are safe.. maybe you're just not understanding what's presented in the literature.. or maybe you're trying to apply the wrong procedure for your intended purpose. As far as various types of electronic gear/equipment go, it's very common that there are floating voltages present inside them. No one was placed here to try to understand what you meant, even though you didn't say it. Maybe you were thinking of sci.telepathic.electronics. -- Cheers, WB ............. The OP was very clear in what he wanted to do i.e. make scope measurements on the primary side of a non isolated switcher. The only 'problem' was that he misinterpreted what had been said in a Tek application note, which he now understands correctly for what it was. I have seen no indication that he is incompetent at using a scope or understanding safety procedures anywhere in his post or various replies. He has merely asked for what people on here who are involved in this type of work, recommend in terms of safe practice from both a personal injury and equipment damage point of view. There has been a consensus expressed, and as far as I can see, he accepts and fully understands what he has been advised. Arfa |
#35
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Oscilloscope 'floating' measurements
"Arfa Daily" The OP was very clear in what he wanted to do i.e. make scope measurements on the primary side of a non isolated switcher. The only 'problem' was that he misinterpreted what had been said in a Tek application note, which he now understands correctly for what it was. I have seen no indication that he is incompetent at using a scope or understanding safety procedures anywhere in his post or various replies. He has merely asked for what people on here who are involved in this type of work, recommend in terms of safe practice from both a personal injury and equipment damage point of view. There has been a consensus expressed, and as far as I can see, he accepts and fully understands what he has been advised. ** The OP reminds me of the very old joke about the difference between a computer and a drummer. Nothing. Both require new information to be punched into them ... .... Phil |
#36
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Oscilloscope 'floating' measurements
"Phil Allison" ** Typical ground loops can be eliminated by fitting a resistor between the scope chassis ground and safety ground of about 10 to 22 ohms. For obvious safety reasons this resistor must be protected by shunt diodes in each polarity. A simple way is to use a 25 amp rated bridge rectifier and connect the resistor across the AC input terminals while shorting the " + " and " - " terminals - giving two diode drops in each direction with current capacity enough to trip a 20 amp supply breaker if necessary without damage. The bridge and resistor can go inside the scope. ** Just to spell it out, the surge current rating of a 25 amp bridge rectifier is several hundred amps - adequate to survive repeated dead shorts across the AC supply with no damage. So, if the earth clip of your probe contacts mains active - the only result will be a tripped breaker, just as it would be without the above modification. Also, diode bridges fail short when overloaded so in this case would fail safe. .... Phil |
#37
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Oscilloscope 'floating' measurements
"Arfa Daily" wrote in
: "Wild_Bill" wrote in message ... It's not clear what you're after (echo). For most technicians, the term "floating" in connection with scope use, refers to 'floating the scope' which means: Read This Part separation of the scope power cord's 3rd pin from earth ground. I didn't look at Tek's cautions/recommendations, but I know scope manufacturers are familiar with the (nearly always) ill-advised practice of Floating A Scope. Are you unfamiliar with proper and safe practices, or unfamiliar with using a scope, or unable to express what it is you want ask about? All of the above? I'm fairly certain that all of Tek's recommended procedures are safe.. maybe you're just not understanding what's presented in the literature.. or maybe you're trying to apply the wrong procedure for your intended purpose. As far as various types of electronic gear/equipment go, it's very common that there are floating voltages present inside them. No one was placed here to try to understand what you meant, even though you didn't say it. Maybe you were thinking of sci.telepathic.electronics. -- Cheers, WB ............. The OP was very clear in what he wanted to do i.e. make scope measurements on the primary side of a non isolated switcher. The only 'problem' was that he misinterpreted what had been said in a Tek application note, which he now understands correctly for what it was. I have seen no indication that he is incompetent at using a scope or understanding safety procedures anywhere in his post or various replies. He has merely asked for what people on here who are involved in this type of work, recommend in terms of safe practice from both a personal injury and equipment damage point of view. There has been a consensus expressed, and as far as I can see, he accepts and fully understands what he has been advised. Arfa Heh,I've seen the results of people connecting scope grounds to what they - thought- was an OK place to connect. line V can really do a lot of destruction. I've also replaced a LOT of scope power cords that had the ground pin cut off. I used to include a copy of TEK's publication on safe isolated scope measurements with each of those scopes;my own personal policy,not TEKs. I don't know if it ever did any good.... -- Jim Yanik jyanik at localnet dot com |
#38
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Oscilloscope 'floating' measurements
On Wed, 30 Nov 2011 08:53:36 -0800 (PST), nesesu
wrote: On Nov 30, 7:48*am, pfm wrote: Many advocate the use of an isolating transformer. The oscilloscope probe and ground are then used like multimeter leads. Tektronix and others maintain this is a dangerous practice and may result in cumulative damage to the oscilloscope. Differential probes are one answer, though quite expensive. Comments please. Thanks BobH I can see no reason that using an isolation transformer properly can have any more detrimental effects on a probe or the 'scope itself than testing equipment that is self isolated [transformer operated]. When using an isolation transformer, the UUT [Unit Under Test] must have the circuit 'common' bonded to the 'scope common [and ground] usually by the probe 'pig tail' or even a separate test lead joining them. Indeed, if the probe tip is touched to an unbonded UUT, it can damage the 'scope or probe, regardless of wether it is powered through an external isolation transformer or through an internal transformer or SMPS. On the other hand, connecting an 'scope that has been 'floated' from ground by disconnecting the power cord ground pin CAN be severely damaged by connecting it to a non-isolated line operated UUT. The usual reason is excessive voltage applied between the 'scope common and the 'scope supply neutral or line. These usually turn out to be transient pulses that are unexpected. Neil S. Tek has historical basis for its warnings, the cases of 'scopes used to be metal and connected to probe ground. If every exposed (metal) part of the scope becomes live there is a definite shock hazard involved. ?-) |
#39
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Oscilloscope 'floating' measurements
On Fri, 2 Dec 2011 04:21:43 -0800 (PST), pfm wrote:
On Nov 30, 2:24*pm, JeffM wrote: pfm wrote: Many advocate the use of an isolating transformer. The oscilloscope probe and ground are then used like multimeter leads. ...which will get you exactly where you want to be. Tektronix and others maintain this is a dangerous practice ...to the bottom line of Tektronix, Inc. and may result in cumulative damage to the oscilloscope. ...of which they couldn't provide a convincing example. Differential probes are one answer, though quite expensive. See "bottom line", above. The claim is bull****, just like MICROS~1 telling you that software other than theirs will cause you problems. Thanks, I think you are right. Just the same, i always prefer floating the UUT. Sometimes this is not possible though, commonly with higher power equipment (10 kVA). ?-) |
#40
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Oscilloscope 'floating' measurements
On Thu, 1 Dec 2011 11:16:20 -0800 (PST), pfm wrote:
On Dec 1, 11:38*am, Jeffrey Angus wrote: On 12/1/2011 9:43 AM, Jim Yanik wrote: Proper practice is to put the Device Under Test on the isolation transformer,not the scope. leave the scope grounded. I don't know why this is so hard for people to understand. Test equipment is grounded. Period. If your probe can't handle the offset voltage, you're using the wrong probe. If you're trying to measure something normally connected directly across the utility power line, then use an isolation transformer and you can ground any point in the device being tested. Use a solid ground jumper. Do NOT rely on the ground path back through the scope probe. Jeff-1.0 wa6fwi -- "Everything from Crackers to Coffins" Thanks. No question about the scope being grounded, no one suggested otherwise. Tektronix maintain that floating measurements, taken in a DUT powered by an isolating transformer, is bad practice. Well they did want to sell expensive differential probes after all. ?-) |
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