oscilloscope 120/240 VAC Measurment - Page 1
Forum section: electronics / testgear

author: tmandell
author_rank: Newbie
timestamp: February 05, 2015, 06:36:26 pm
content: I have recently ordered a Rigol ds1054z scope. One of the things I had intended to use it for is measuring the AC output of items such as generators and power inverters. From my understanding this should not be an issue as both generators and power inverters are isolated from the mains ground.I am however not positive on if or how I should go about measuring such a high voltage. As the ds1054z has a max vertical scale of 10V/division and 8 divisions it would only be able to show a wave for with 80 volts peak to peak. This is well short of the 340 Volts peak to peak of 120 volt RMS. If I understand things correctly I should be able to use a x10 attenuation probe to reduce the signal to level that the Rigol can handle. Is the correct?ThanksTerry

author: SeanB
author_rank: Super Contributor
timestamp: February 05, 2015, 06:49:45 pm
content: Isolated 100x probe will be a better bet, or a small transformer with a 240V primary and a 12V secondary will provide a safer method for measuring voltage and looking at gross distortion. Power inverters and generators are also generally intended for connection to an earth point, generally an earth rod or water pipe depending on your local regulations. This means that they are mains earth referenced.

author: tautech
author_rank: Super Contributor
timestamp: February 05, 2015, 09:07:42 pm
content: Quote from: tmandell on February 05, 2015, 06:36:26 pmWelcome to the forum Terry.The one thing that is not clear is your experience with scope usage and HV?Yes the Rigols only have 300V inputs and ANY error could blow then.10x probes will THEORETICALLY do the job, but the safety margin and CAT ratings for both you and the scope are not large.Get a 100:1 probe, not just for the increased attenuation/safety but the increased "voltage de-rating with frequency" spec of 100x probes.

author: tmandell
author_rank: Newbie
timestamp: February 05, 2015, 09:37:45 pm
content: Quote from: tautech on February 05, 2015, 09:07:42 pmI have used scopes countless times for both school and work. I am familiar and comfortable with the use and operation of an oscilloscope. However every scope I have used has been a hand held portable type with CAT III ratings. As these scopes were purchased by well funded tech schools or by companies where the cost of the equipment is of little concern, they have all been very high end Fluke meters.As I am in the process control and automation industry nearly everything I work on is 24VDC and is galvanically isolated or intrinsically safe. I am however quite familiar with and comfortable working on the 120 VAC and 240VAC circuits we have here in Canada.From my understanding when used with the correct probe the Rigol is a CAT II device and is therefore safe to use on 240VAC power as long as care is taken to ensure no grounding issues exist. I would prefer to have a CAT III rated device or a battery powered hand held scope, however the cost of these is significantly more than the cost of the Rigol. That being said if the Rigol is not safe to use on AC power, or if I need to purchase a differential probe, I might as well Cancel my order for the Rigol and save up for a hand held scope. This might be the better option anyway as one of the intended uses is for diagnostics of automotive circuits, so the hand held would be much more convenient than a bench top scope.ThanksTerry

author: DanielS
author_rank: Frequent Contributor
timestamp: February 06, 2015, 12:40:54 am
content: If it was for me on my workbench, I would simply put together some sort of jig with a 10M resistor on top of my 10X probe (or soldered on the test point) to make a 20X ~600V probe. This inexpensive trick works well for low frequency stuff.As for your generator being isolated from mains ground, that is no longer true once you attach the oscilloscope's ground leads to your machine-under-test's local ground to provide a reference voltage before you can start making actual measurements - unless you use an isolated differential probe.