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| Old analog oscilloscope questions |
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| tggzzz:
--- Quote from: james_s on August 08, 2018, 11:29:11 pm ---I wouldn't worry too much about whether the scope is grounded or not. If it originally had a grounded plug which has had the ground cut off or been replaced with a 2-prong plug then it should be replaced with a grounded plug. If it was never grounded to begin with then don't worry about it, it's ok that it's floating. When people speak of "floating a scope" they often mean deliberately disconnecting the ground in order to use the scope to measure a high voltage non-isolated device, this is dangerous, not the fact alone that the scope is floating. So long as you are staying away from the mains side of whatever gear you're working on then there should be no problem. A good rule to follow is if you wouldn't touch something with your bare finger, don't connect a scope to it. --- End quote --- No, that is dangerous advice: it presumes an ideal world with complete knowledge and complete understanding. If there is a mistake, or a misunderstanding, or a fault, then a floated scope can be lethal. Example mistake: a probe slips. Example misunderstanding: you misread a diagram, e.g. not recognising whether a protective mains earth connection is specified. Example fault: mains has jumped to an unexpected place in the UUT - and that's why you are probing it in the first place. And with a little imagination or knowledge, many many other examples can be found. |
| tggzzz:
--- Quote from: Gyro on August 09, 2018, 08:23:50 am ---tggzzz used the wrong word - it is not probe "calibration" it is probe compensation. --- End quote --- Sigh. Quite right. |
| james_s:
--- Quote from: tggzzz on August 09, 2018, 03:27:28 pm --- No, that is dangerous advice: it presumes an ideal world with complete knowledge and complete understanding. If there is a mistake, or a misunderstanding, or a fault, then a floated scope can be lethal. Example mistake: a probe slips. Example misunderstanding: you misread a diagram, e.g. not recognising whether a protective mains earth connection is specified. Example fault: mains has jumped to an unexpected place in the UUT - and that's why you are probing it in the first place. And with a little imagination or knowledge, many many other examples can be found. --- End quote --- No, it presumes a very basic understanding that poking around in mains powered equipment is potentially dangerous. Lots of old scopes never had grounded power cords even when they were new, no need to modify the equipment to add one. If a person can't figure out not to poke around in dangerous stuff when they don't know what they're doing then they should not be using a scope. There is too much safety hysteria these days, can't protect everyone from themselves. Your example fault of mains jumping to somewhere it shouldn't would be dangerous even with no test equipment involved. Loads of consumer equipment has no earth ground. |
| james_s:
--- Quote from: Gyro on August 09, 2018, 08:23:50 am ---tggzzz used the wrong word - it is not probe "calibration" it is probe compensation. The probe automatically has a DC divide ratio of 10 (and 1 in the other switch position), the compensation trimmer in the probe is there to match the probe to the input capacitance of your particular scope. When compensation is correct, you will see a nice square wave with square leading corners, the tops of the squares should be nice and flat (not tilted). A bit of noise shouldn't stop you being able to do this. --- End quote --- That's true, however on quite a few scopes the terminal used to adjust the probe compensation is called "Calibrator" so I don't think the term is entirely inappropriate. If Tektronix thought it was reasonable to call it a calibrator that's good enough for me. |
| tggzzz:
--- Quote from: james_s on August 09, 2018, 03:39:37 pm --- --- Quote from: tggzzz on August 09, 2018, 03:27:28 pm --- No, that is dangerous advice: it presumes an ideal world with complete knowledge and complete understanding. If there is a mistake, or a misunderstanding, or a fault, then a floated scope can be lethal. Example mistake: a probe slips. Example misunderstanding: you misread a diagram, e.g. not recognising whether a protective mains earth connection is specified. Example fault: mains has jumped to an unexpected place in the UUT - and that's why you are probing it in the first place. And with a little imagination or knowledge, many many other examples can be found. --- End quote --- No, it presumes a very basic understanding that poking around in mains powered equipment is potentially dangerous. Lots of old scopes never had grounded power cords even when they were new, no need to modify the equipment to add one. If a person can't figure out not to poke around in dangerous stuff when they don't know what they're doing then they should not be using a scope. --- End quote --- The OP is a beginner. Responsible responses should be tailored to that. --- Quote ---There is too much safety hysteria these days, can't protect everyone from themselves. --- End quote --- I've never over-protected my family; I backpacked round India with my 13yo, and before she could start to learn to drive a car, she was doing aerobatics that could have killed her within 10s, making forced landings, travelling at 90kt. However, it is necessary to protect people from other people, and from other people's bad advice. --- Quote ---Your example fault of mains jumping to somewhere it shouldn't would be dangerous even with no test equipment involved. Loads of consumer equipment has no earth ground. --- End quote --- That's false, and a failure of your imagination. |
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