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Registered Member #1062
Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529
Hello, A flyback driver stopped working, so I tried to figure out the problem. When I hooked up my scope to the output , while on the primary, The scope drew enough current to cause the ground clip insulation to melt. The power was a revound MOT (~40v) switched through a IGBT at 1000hz. Is this my fault or is something wrong with the scope? It is rated for 400v p-p, and has a 10mohm input impendance, with a 50ohm probe. Thanks.
EDIT: aah.... I think this is what I did : problem 11 at
Registered Member #103
Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845
it's a classic, I've destroyed a few things like this
The normal way is to simply use an isolating transformer between mains and your scope, or make one with 2 back to back mains transformers capable of running your scope. Another way that can solve the problem in some cases is to just disconnect the earth connection to the scope. At work I run my scope from an old UPS - it stays plugged in all day charging the battery, then whenever I need to float it I just unplug it. The week before last I tripped a 3-phase breaker by forgetting to unplug it before swiching on
Registered Member #1062
Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529
My scope is fine, the ground clip is just bare. I have one of those things that 3l posted, I am using it to power my MOT from a comp PSU cable. Those won't do anything, the little metal tab is supposed to touch the screw of the wall outlet. Mine broke, so I guess that will work. I saw a UPS at my dad's work, Ile see if they have any extra.
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
Note-- be EXTREMELY cautious when 'floating' your scope on a ups or ungrounded plug--the scope chassis, buttons, 'ground' clip etc will be floating at whatever voltage the circuit is at! A much better solution is to leave the scope securely grounded, and float whatever it is that you are measuring on the isolation transformer/ungrounded circuit (just watch out for 'hot' parts on the device being measured) or getting a scope that can handle ungrounded measurements (the all plastic battery powered ones are good for that, just be careful that anything 'grounded' on the scope will be hot, and if you plug it into the wall to charge while connected things will go caboom).
Also, the ground 'cheaters' don't necessarily isolate things, they just let you remove the ground pin--the neutral is still connected to ground in your service panel and possibly several places in the device. IF it only goes to a mains transformer to power it you can usually get away with floating it on a cheater (watching out for the case that is now at possibly high voltages) but if the device isn't isolated you are still stuck.
Registered Member #1497
Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801
A safer method to make measurements at non ground references is to use 2 probes with the ground leads isolated and protected. Set them in add mode with the 'ground' trace inverted.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Tektronix made several DSOs that had independently isolated floating channels, allowing you to clip the ground clip to almost anything you wanted. The amplifiers and A/D converters inside all float, and the digital data gets transferred to the main CPU (which draws it on the screen) through a bunch of tiny ferrite transformers. The floating stuff is all powered by DC-DC converters with high isolation.
The method suggested by aonomus doesn't always work 100%, since the two channels and two probes are never quite identical. It can help to clip both of your probes to the calibration output on your scope, and adjust the trimmer cap on one probe until the spikes are nulled out.. It also helps to have your two probes the same make and model, and if you're working around power electronics, twist the two probe cords together to minimize magnetic pickup.
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scope drawing too much current
