Why does so much current pass through my scope? - Page 1
Forum section: electronics / general

author: king.oslo
author_rank: Frequent Contributor
timestamp: October 28, 2011, 06:33:28 pm
content: Hello there,I've got a Rigol scope, just like Dave.Recently, I think the internals of the scope shorted out my circuit, because the probe leads, transformer, and rectifier got very hot.I connected CH1 to transformer output (18.8VAC) and CH2 and to the rectifier output (16.6VDC). I wanted to compare the voltage from before and after rectification. I could tell something was wrong because the fuse on transformer input blew. And all the leads and gear were current had been flowing was very hot. The waveforms on the scope didn't look like they were supposed to. When disconnecting CH2 and measuring one place at a time I had no problems, and the waveforms looked great.What happened you think?Thank you for your time.Kind regards,Marius

author: jahonen
author_rank: Super Contributor
timestamp: October 28, 2011, 06:37:15 pm
content: You did the error what most people do once in a lifetime, assumed that scope channels have separate grounds. In reality, they share common ground and are tightly connected together and to AC power cord earth terminal, so you short circuited your rectifierThat explains why the transformer got so hot.Thank your luck that you didn't do this on mains connected circuit, the experience would have been much more electrifying.Regards,Janne

author: king.oslo
author_rank: Frequent Contributor
timestamp: October 28, 2011, 06:41:57 pm
content: Sounds serious.1. The transformer was connected to the mains, but I suppose you are saying it would have been worse if the currect was straight from the mains without a transformer?2. Do you think I have damaged the transformer insulation? Or is it safe to continue?3. What would have happened if the circuit was connected straight to mains, not through transformer? Just curious. Would I have been hurt?Thanks!M

author: jahonen
author_rank: Super Contributor
timestamp: October 28, 2011, 06:47:32 pm
content: 1. Yes2. If there were no smoke from the transformer and it gives normal output voltage without abnormal heating after you replace the fuse, then I guess it is ok.3. It depends, but usually there would have been a huge spark and the fuse would have been blown (or circuit breaker trip). I guess that there is also a chance that scope and/or the probes would have been damaged.Also, if the scope is correctly connected to earthed mains outlet, then the fuse would have been blown just by connecting the scope ground clip to live wire. One should never ever connect normal scope probe anywhere directly to mains and scope should be always at earthed mains outlet, always use proper CAT-rated differential probe if measuring mains voltages. There are some exceptions to this rule, some scopes have isolated channels and can be directly connected to mains, but I guess that if one has such, then this issue is probably non-existing.Regards,Janne

author: king.oslo
author_rank: Frequent Contributor
timestamp: October 28, 2011, 06:51:29 pm
content: Thanks Janne. You legend!M

author: ejeffrey
author_rank: Super Contributor
timestamp: October 28, 2011, 06:51:55 pm
content: Welcome to the club

author: McMonster
author_rank: Frequent Contributor
timestamp: October 28, 2011, 07:32:59 pm
content: Is there a place where I can read more about "don't do this when using oscilloscope" (or any other lab hardware)? I've only destroyed a cheap Chinese DMM in my life and already know some things not to do with a meter, but I'd like do avoid buying a new oscilloscope (or burning the device under test).

author: Unknown
author_rank: Unknown
timestamp: October 28, 2011, 07:42:37 pm
content: Tektronix' "the XYZ's of oscilloscopes" and "the ABC of probes" might be good to read.

author: Zero999
author_rank: Super Contributor
timestamp: October 28, 2011, 09:18:24 pm
content: Yes most people make this mistake once.TO answer the question, the maximum current should through the 'scope's earth theoretically depend on the 'scope's fuse. In reality this may not be the case as the input might be connected to earth via thin wire or PCB traces so you shouldn't rely on it beeen able to take more than a few mA. Luckilly this time no damage seems to have been done - you're lucky.

author: Unknown
author_rank: Unknown
timestamp: October 28, 2011, 09:24:41 pm
content: Quote from: Hero999 on October 28, 2011, 09:18:24 pmThe scope's fuse? The earth connection should be able to handle transients from outside (eg. CAT II 300V), not just a fault inside the scope.Quote from: Hero999 on October 28, 2011, 09:18:24 pmTektronix recalled a series of scopes because the ground trace could vaporize under high current. I would expect a well designed scope to have a ground connection designed to be able to handle a fair amount of current, since a broken ground trace is a major safety risk.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 29, 2011, 12:56:16 am
content: Well,you learnt something!Re the mains:It seems from other postings on this forum, that the moment somebody gets an new Oscilloscope,the first thing they want to do is go & hang the thing across the mains!On the few occasions you may need to look at the mains,(just to see if it is present),you don't need a probe earth clip at all,as the fairly convoluted return path via the earthed chassis of the scope,& the earth/neutral connection is quite adequate for 50/60Hz.Even with a DMM,I usually just measure to earth ,(If you do this with an analog VOM,the RCD will trip!)If you are fussy,use a DMM for this test,connected between Active & NeutralIf you have a particular reason to look at the mains directly,you need an unearthed Oscilloscope,or a differential probe.---------DON"T remove the mains earth connection from your 'scope!VK6ZGO

author: robrenz
author_rank: Super Contributor
timestamp: October 29, 2011, 02:35:30 am
content: Quote from: McMonster on October 28, 2011, 07:32:59 pmThe toddfun video inhttps://www.eevblog.com/forum/index.php?topic=5306.msg69255#msg69255is slow but very good. It actually is about proper isolation transformer usage. but covers scope grounding toward the end also since it is a related situation. I learned a lot.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 29, 2011, 05:35:00 am
content: Yes,I would suggest you watch that video carefully---Todd covers the subject extremely well.An isolation transformer is virtually essential if you are troubleshooting a mains type switchmode supply.As I mentioned before,if you are just checking for the presence of mains, as you would in a transformer type supply,you can usually place the probe on the mains active or neutral,& just rely on the earthing of the 'scope to provide the return path.(Remove the earth leads so they don't dangle down & short on something.)The active side should show a waveform of some 120/240 volts RMS (depending on your system) & the neutral some low value close to zero.If both sides are hot w.r.t earth,either your neutral circuit is open,or you are looking at an North American 240volt system.VK6ZGO

author: Zero999
author_rank: Super Contributor
timestamp: October 29, 2011, 09:17:51 am
content: Quote from: alm on October 28, 2011, 09:24:41 pmWhat I meant was the scope's current handling capacity depends on the cross-sectional area of the earth conductor. The fuse is often selected depending on the cable size, rather than the current rating of the 'scope.

author: Unknown
author_rank: Unknown
timestamp: October 29, 2011, 02:52:53 pm
content: The fuse is selected depending on the cable size? Fuses are usually sized based on the power supply, to prevent the transformer from catching fire. Of course the cable's current rating should exceed that of the fuse. Most scopes have standard IEC power connectors that are rated for 10A, but the first scope I checked has a 1A fuse. Are you talking about the fuses in the plugs which are only used in the UK?

author: FreeThinker
author_rank: Frequent Contributor
timestamp: October 29, 2011, 03:40:56 pm
content: Quote from: Hero999 on October 28, 2011, 09:18:24 pmIt was the TDS 220 I think. I had one and recieved a recall notice but had sold it on by then. Passed the recall on but don't know if it was acted on

author: Unknown
author_rank: Unknown
timestamp: October 29, 2011, 03:46:48 pm
content: Indeed, the fix was to directly connect a grounding wire from the power connector to the acquisition board.

author: Zero999
author_rank: Super Contributor
timestamp: October 29, 2011, 04:00:52 pm
content: Quote from: alm on October 29, 2011, 02:52:53 pmYes, you're right, I was talking about the plug top fuse, the internal fuse often has a lower rating. I forgot other countries don't have fused plugs.

author: Chet T16
author_rank: Frequent Contributor
timestamp: October 29, 2011, 05:10:35 pm
content: Ok maybe my brain isn't working but why can't you measure either side of the rectifier using the scope? Of is this a case of the ground clips being put in the wrong place?

author: Zero999
author_rank: Super Contributor
timestamp: October 29, 2011, 06:02:31 pm
content: It's not possible to measure any earth bonded power supply (e.g. the mains or a rectifier connected directly to the mains) with a 'scope who's chassis is also connected to earth, without using an isolation probe or two probes in differential mode.

author: Unknown
author_rank: Unknown
timestamp: October 29, 2011, 07:50:12 pm
content: If neutral is connected to earth somewhere, just measuring the live wire and clipping the ground clip to earth (not neutral!) should give you a fair idea of what the mains voltage looks like, since the voltage between neutral and earth tends to be fairly low in most systems. I agree that for best results you should use a differential probe or a poor man's differential probe (two probes in subtract mode).

author: vk6zgo
author_rank: Super Contributor
timestamp: October 30, 2011, 03:06:36 am
content: Quote from: alm on October 29, 2011, 07:50:12 pmThis is the point I was trying to make!VK6ZGO