Fuse on reference clip lead, to reduce risk of scope damage? - Page 1
Forum section: electronics / beginners

author: RedOctobyr
author_rank: Contributor
timestamp: January 02, 2015, 11:31:26 pm
content: I just got my first scope. I've watched Dave's video on how to not blow up your scope. Excellent video, thank you Dave!It did get me thinking, though. Despite the video, there is still a risk of accidentally connecting the reference clip incorrectly, especially for us newbiesIs there a reason that a small fuse (maybe 1A or similar) couldn't be placed inline with the reference lead? I realize this is definitely a case where avoiding a problem in the first place (by making sure to connect things properly) is a better idea. But, well, stuff happens sometimes. If a simple precaution could help avoid damage, maybe there would be some value to that. Then, as the user gains experience, the "training wheels" could come off, and go back to a normal reference lead.I'm not proposing being sloppy with where things are connected, just wondering aloud if this could be done. Or, more likely, wondering why it's a bad/dumb idea.

author: DanielS
author_rank: Frequent Contributor
timestamp: January 03, 2015, 02:42:27 am
content: Depending on what you are attempting to measure, a fuse might not be an option at all: for high-speed signals (few MHz and up) and fast edges, the grounding clip adds too much inductance and you need to bypass it using a short springs that clips directly on the probe tip's shield to get accurate waveforms. No room for a fuse in-line with that.

author: wiss
author_rank: Frequent Contributor
timestamp: January 03, 2015, 12:21:49 pm
content: When the fuse blows, the voltage vill peak, maybe enough to damage the scope.Place a big, fat short between the 3 ground-points so the current passes through the scope.

author: RedOctobyr
author_rank: Contributor
timestamp: January 03, 2015, 02:40:07 pm
content: Good to know, thank you.

author: VK5RC
author_rank: Unknown
timestamp: January 03, 2015, 09:28:14 pm
content: I think it is not a bad idea, sure there are problems if you are pushing the envelope in terms of frequency but I have made a few 'earth' mistakes in my time and let out the smoke (not of the scope fortunately). Perhaps a very small fuse both physically and in amps on the clip itself.

author: tautech
author_rank: Super Contributor
timestamp: January 03, 2015, 09:47:59 pm
content: As all of us that have scopes also have a DMM, it is so quick to check for a Reference lead GND loop before connecting.

author: RedOctobyr
author_rank: Contributor
timestamp: January 03, 2015, 10:55:34 pm
content: Very true, of course.I have a few things I'm curious to check. Such as the output of my generator, and a grounded AC->DC power supply. And I'd really like to avoid doing something stupid.If I use the multimeter to check between the reference lead, and where I'd like to connect it, for both AC and DC voltage, I'm assuming that should be sufficient, if they both show close to 0V? If the device I was checking was definitely floating (like something battery powered and handheld), I wouldn't need to do this check, as I understood Dave's video.(Just to understand, could the scope also show me this? If I put the probe tip, rather than the reference clip, to the "questionable" location?)Alternately, I believe I could use the scope's Math functions. And rather than use the reference lead to go to ground, I could put channel 2 to ground, then subtract it from channel 1, to effectively accomplish the same thing. In this case I would not use the reference lead at all. It sounds like this method is generally safer, as nothing is risking a direct short to ground. I hope I have this right.The drawbacks, as they appear to me (not necessarily accurate), are mainly that the scope's display updates are slower, as it has more work to do (perhaps analog scopes aren't affected by this, but mine is an older DSO). And, of course, I use up an additional channel. But I can live with that, in exchange for things being safer.I apologize for asking basic questions (but there's a reason I'm in the Beginners forum). As wiser people have said, the really dumb questions are the ones you don't ask.

author: Jeroen3
author_rank: Super Contributor
timestamp: January 04, 2015, 01:14:49 pm
content: Quote from: tautech on January 03, 2015, 09:47:59 pmThat won't always work. Sometimes you'll measure a part of the phase voltage because the isolation isn't the same high quality as you lab power supply.For example, wall wart here gives me 30 Volts AC slowly decreasing to 20, between secondary negative and earth.