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| Nead an insulated probe for oscilloscope |
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| capt bullshot:
--- Quote from: FriedMule on June 06, 2019, 08:41:24 am --- --- Quote from: David Hess on June 06, 2019, 05:39:44 am ---The A6902 service manual at that page has a detailed description and full schematic. It works like you suggested with a parallel path using transformer isolation at high frequencies up to 20 MHz and a linear optocoupler for low frequencies down to DC. It would not be difficult to duplicate but it would be a lot of work. --- End quote --- Wary interesting! If we ignore the part of 3000V and instead say maybe 100V, could that not be made with SMD 1206 in a much smaller box with maybe some standard probes attached? You write "not difficult to duplicate but a lot of work" ehm I am not sure I understand? To me it look like a wary difficult task to duplicate the schematic, since I have about no idea on what does what, so I have to duplicate every line and comma. :-) --- End quote --- Since this is quite an old design, using parts that you cannot buy anymore, you'd have to duplicate the working principle, not the schematic. With some luck, an A6902 might appear at the evil bay within the next ten years ;) Edit: not within ten years, but today (you see me surprised): https://www.ebay.de/itm/TEKTRONIX-A6902B-Isolator/233187964339 |
| David Hess:
--- Quote from: FriedMule on June 06, 2019, 08:41:24 am ---If we ignore the part of 3000V and instead say maybe 100V, could that not be made with SMD 1206 in a much smaller box with maybe some standard probes attached? --- End quote --- The A6902 actually works with standard 1 megohm passive oscilloscope probes. It included special probes which had reinforced insulation for safety. Modern oscilloscopes which have isolated inputs use the same type of probes with reinforced insulation for the same reason. Honestly if you are measuring after the line isolation transformer but before the rectifiers, lifting the ground connection to your oscilloscope is relatively safe. However this only gives you one isolated domain; all of the oscilloscope probes share the same common connection to each other. Also the common mode capacitance from the probe commons to Earth ground still exists and would be high enough to disturb some measurements. The A6902 specifications say less than 100 picofarads of common mode capacitance which is only that low because the A6902 was designed to minimize it. --- Quote ---You write "not difficult to duplicate but a lot of work" ehm I am not sure I understand? To me it look like a wary difficult task to duplicate the schematic, since I have about no idea on what does what, so I have to duplicate every line and comma. :-) --- End quote --- I mean the concept and design is straightforward for someone skilled in the art. A modern implementation would be considerably simplified because integrated operational amplifiers are now commonly available with sufficient performance to replace most if not all of the discrete circuitry. (1) I think a transformer would still be required for the high frequency path. Modern isolation amplifiers are only good up to 500 kHz or so. But an isolation amplifier could replace the low frequency linear optocoupler path and yield better performance. (1) I am not a fan of using shunt feedback amplifiers in oscilloscope applications because of poor overload recovery time but at only 20 MHz this would not be a problem. |
| David Hess:
--- Quote from: David Hess on June 06, 2019, 05:08:12 pm ---Honestly if you are measuring after the line isolation transformer but before the rectifiers, lifting the ground connection to your oscilloscope is relatively safe. --- End quote --- Incidentally, Tektronix also made the A6901 Ground Isolation Monitor for exactly this application. http://w140.com/tekwiki/wiki/A6901 |
| FriedMule:
LOL WOW sometime luck is smiling to you! :-) I hope that it is still there when I have all the money. I can see that handheld scopes have galvanic isolated channels, would it be possible to build one, I mean a unit where you plug the standard passive probe in one end and a coax out in the other end to the scope, maybe with external power source? https://hackaday.com/2016/03/31/diy-active-sub-ghz-differential-scope-probe/ |
| ggchab:
This also seems interesting: http://www.paulvdiyblogs.net/2017/10/differential-amplifier-probe-buy-or-diy.html |
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