Safe Oscilloscope Probing? - Page 1
Forum section: electronics / beginners

author: grantbob
author_rank: Unknown
timestamp: February 04, 2015, 11:40:22 pm
content: I've done a lot of scouring of the forum and I've watched the quintessential "How NOT to Blow Up Your Oscilloscope" video a few times now and am pretty sure I understand what NOT to do and what will cause bad things to happen. So now I have the fear of Dave instilled me when considering probing ANYTHING connected to mains earth ground (This is a good thing.)However, If I know the device I am testing is connected to mains earth ground and so is my scope, is it safe to just go ahead and probe with the ground clip of the scope's test lead attached to that ground? Or better with no ground lead at all?I'm wanting to look at the PSU output from a malfunctioning Tivo DVR (low voltage DC side). It's an isolated supply UNTIL you connect the HDMI cable to the television or the Cable TV Input to cable. As soon as you do either of those ground from the supply and ground on the Tivo cabinet is the same as the ground on the BNC connector on the scope. I have verified this with a DMM. Would it be safe to connect my scope probe to the 3.3, 5, 12V outputs? With or without a ground lead to ground? I kind of want to have it hooked up to the TV to observe when the Tivo is resetting itself and see what is happening with the PSU output at the same time.Just wanted to throw this example out there to make sure I'm not totally missing something.I'll be looking for a differential probe soon. Sure wish there were some less-expensive options out there.Thanks for any feedback,-grantbob

author: Paul Price
author_rank: Super Contributor
timestamp: February 04, 2015, 11:50:52 pm
content: Sure, go ahead and make these measurements of the DC outputs. There shouldn't be a problem and you don't need a differential probe to do this.

author: grantbob
author_rank: Unknown
timestamp: February 04, 2015, 11:56:10 pm
content: Quote from: Paul Price on February 04, 2015, 11:50:52 pmThanks for the prompt reply. Do you think I would be better off with or without the ground lead of the probe connected to the DUT? My scope is a Rigol 1074Z with 4 channels so I was going to look at all 3 at once.. with only ONE ground lead connected (if at all).-grantbob

author: Paul Price
author_rank: Super Contributor
timestamp: February 05, 2015, 12:00:11 am
content: You could try connecting the probe tips and the probe ground clips together at the ground point of the DVR, and see whether floating the scope ground or not gives the cleanest signal. Either way shouldn't harm the scope.

author: AG6QR
author_rank: Frequent Contributor
timestamp: February 05, 2015, 12:14:45 am
content: Quote from: grantbob on February 04, 2015, 11:40:22 pmIf you leave off the ground lead entirely, you'll effectively have a very long ground lead that at least goes through the power cords of your oscilloscope and the device under test. If they're plugged in to different outlets or different branch circuits, the ground path could be very long indeed. But even if it's only a few meters, it will pick up a lot of noise, and have enough inductance to reduce your bandwidth a lot. This is NOT recommended if you want to make accurate measurements, especially of anything that's going fast.So use that ground lead. Or use a spring clip that's even shorter, if you're working with fast signals.If you want to be super careful, then before you connect the ground lead, you can use a DMM between the ground lead and the point you were going to connect it to, to verify that there's no voltage there.