Oscilloscope probe grounded to mains ground measuring an isolated power supply? - Page 1
Forum section: electronics / projects

author: okw
author_rank: Frequent Contributor
timestamp: July 26, 2025, 09:33:40 am
content: I have a simple project with a comparator and relay, which is supplied from an isolated 5V power supply module (thx208+transformer/etc).The circuit is grounded together with mains ground (3rd party equipment I'm measuring was designed like this).Is it safe to clip my oscilloscope probe ground to this same mains ground when measuring the 5V (transformer isolated)?I've heard measuring live mains blows up your oscilloscope, but that wouldn't be the case when it's isolated?

author: mtwieg
author_rank: Frequent Contributor
timestamp: July 26, 2025, 01:33:35 pm
content: QuoteWhat blows up your scope is connecting the ground clip of the probe to something which is not isolated from the mains (i.e. hot and neutral).There are some quick checks you can perform with a cheap DMM:1. Set the DMM to measure voltage. With the equipment under test (EUT) powered off, connect one lead to the earth in your electrical outlet (the one you power your scope from). Connect the other lead to the place on the EUT you want to connect the probe clip to.2. Power up the EUT and check the measured voltage. Check both AC and DC readings, just to be safe. If the measured voltage is small (like a volt or less), then it's probably safe. If the measured voltage is large, then proceed to step 3.3. Set the DMM to measure current. Make sure the DMM has a fuse on its ammeter (preferably with a rating of 1A or less).4. Power up the EUT again and check the AC/DC current readings. If the measured current is small (like <10mA), then it's safe to connect the probe clip there.But make sure the fuse hasn't blown after this measurement!

author: okw
author_rank: Frequent Contributor
timestamp: July 26, 2025, 02:40:47 pm
content: Quote from: mtwieg on July 26, 2025, 01:33:35 pmI see 5VDC and 0VAC.Quote from: mtwieg on July 26, 2025, 01:33:35 pmNot sure how I can do this, except shorting out my power supply. Circuit (power supply, comparator, relay, etc) GND and mains ground are tied together (same potential), so measuring current between mains ground and the signals I want to measure (including the 5V otuput), will basically short everything out.

author: moffy
author_rank: Super Contributor
timestamp: July 26, 2025, 11:09:09 pm
content: You could try a small low wattage resistor in series with the ground clip, 10R/0.125W, it can act as a fuse also if it gets warm current is flowing which is not desireable.

author: mtwieg
author_rank: Frequent Contributor
timestamp: July 27, 2025, 11:49:20 am
content: Quote from: okw on July 26, 2025, 02:40:47 pmThis doesn't make sense based on the description in your first post.To clarify, the second DMM lead should connect to where you intend to clip the probe'sgroundlead to (probably the negative output of your power supply). It sounds like you are instead connecting to the positive output.

author: okw
author_rank: Frequent Contributor
timestamp: July 27, 2025, 12:40:18 pm
content: Quote from: mtwieg on July 27, 2025, 11:49:20 amWhy doesn't it make sense? The power supply is grounded ("negative output") to mains (230V) ground. And I connected second (black) DMM lead to mains ground, and read 5VDC and 0VAC (or rather 4.5mVAC) on the 5V power supply output (red DMM lead).If I clipped the probe's ground lead to the positive output, I would read (with the probe tip) a negative (-)5VDC from the power supply.

author: bostonman
author_rank: Super Contributor
timestamp: July 27, 2025, 01:18:10 pm
content: QuoteI think the safest advice is: when in doubt, use a differential probe.Until a few years ago, I never dealt with using my scope for AC power. Most stuff I worked on had secondaries and/or DC. It wasn't until the advice of others on here that I invested in one. Now whenever in doubt, I use it.