Using Ch1 + (inv)Ch2 on hot side vs unhooking mains ground - Page 1
Forum section: electronics / beginners

author: lordvader88
author_rank: Frequent Contributor
timestamp: August 11, 2018, 10:09:07 pm
content: I want to probe the primary side of computer SMPS's, my DSO138 is too slow for some stuff.I don't have a working isolation transformer yet, and I just found out that u can use JUST the tips, of Ch1 + (inv)Ch2 as a diff. probe ? I have a Tek2340AIs that considered fine ? Does it change the wave form much compared to the proper way (I mean 1 Ch w/ isolation transformer). Is there anything/anywhere I should NOT probe using that method ?And if it is just for a quick check, what about just unhooking the Tek's main's GND from the plug/wall. I have cords without a GND prong. I don't want to run it like that all the time, just for a few minutes. And just be aware that the chasis could be hot if something fails at that time.

author: james_s
author_rank: Super Contributor
timestamp: August 11, 2018, 10:14:49 pm
content: No, do not do this, I don't know what the max common mode voltage is for that scope but it's really not safe. If you don't have an isolation transformer then just keep out of the mains side of the power supply. I have only very rarely had any need to look at waveforms there anyway.

author: lordvader88
author_rank: Frequent Contributor
timestamp: August 11, 2018, 10:19:28 pm
content: I will be getting a proper iso.trans. soon. If I had diff. probes, is there anything on the hot side they can't do ? I'm guessing they are a lot cheaper than an iso.trans. for +150 where I am

author: james_s
author_rank: Super Contributor
timestamp: August 11, 2018, 10:21:23 pm
content: You should be able to get an isolation transformer for little or nothing. You can make one yourself out of any two suitably large transformers connected back to back.

author: GregDunn
author_rank: Frequent Contributor
timestamp: August 11, 2018, 10:33:50 pm
content: If the scope has a math function, subtract the two channels with that. You won't be exceeding any common mode specs that way.

author: james_s
author_rank: Super Contributor
timestamp: August 11, 2018, 11:01:05 pm
content: I still don't think it's a good idea for someone who doesn't know exactly what they're doing to attempt this. It's one of those cases where if you have to ask, you probably shouldn't try.

author: innkeeper
author_rank: Unknown
timestamp: August 11, 2018, 11:16:28 pm
content: I HATE WORKING ON SMPS's ... I've done this a lot of unsafe, and semi unsafe ways in the past. ... and if you know what you are doing and understand your equipment limits you won't blow up your scope or kill yourself.... if you don't know what you are doing, or get it wrong, you can blow up your scope and/or end up dead.Invest in an isolation transformer.isolating your high voltage equipment from a return path that can kill you us just basic safety, regardless of what ground isolation you think you can leverage to probe. Safety first!The advice id go back and give my younger self would be toget a proper high voltage differential probe... and that is the same advice I will give to you.they may seem expensive.. but.. consider the alternative costs.cost of a new scope and hospital or funeral expenses... seems like a bargainthough to be fair you probably would not shell out for a new scope if your dead...its likely and either or deal.just so happens eevblog shop sells one at a reasonable cost.https://www.eevblog.com/product/hvp70/460authe minimum setup I use today is a differential probe, a variac, an isolation transformer, a load limiting setup (typically an incandescent light bulb that can be switched in series between the power source and the device under test - also referred to as the dim bulb test) if I don't have that full setup available, id rather not work on it.watch these videos if you are unfamiliar with the potential issues and methods.last edit: misspellings

author: David Hess
author_rank: Super Contributor
timestamp: August 12, 2018, 09:19:23 pm
content: The add and invert method with two channels works and is safe but has a limited common mode range so the sensitivity is poor.Probe and gain tolerances yield a poor common mode rejection unless the channels are adjusted to match using the variable function. Since the 2340A (2430A?) has analog variable gain control, it is better than many DSOs at this.

author: schmitt trigger
author_rank: Super Contributor
timestamp: August 12, 2018, 09:29:30 pm
content: Quote from: james_s on August 11, 2018, 11:01:05 pmI FULLY AGREE with this statement.A diff probe and/or an isolation transformer are cheaper than a ruined scope, probes and the device under test. Plus you will avoid a nasty shock.

author: Jwillis
author_rank: Super Contributor
timestamp: August 13, 2018, 01:07:18 am
content: The DSO138 Maximum Input voltage: 50Vpk .If you insist on seeing the wave form of mains you can use a step down transformer provided the out put voltage doesn't exceed 50 volts pk which likely means Peak to peak according to JYE specifications.So a safe step down would be around 20volts tops. The DSO138 is not designed for high voltages.I'm not sure why you need to see mains other than detecting external noise from some other source.But that still doesn't make sense because your PSU for your computer will have filtering on board for external noise.As long as your getting the correct voltage on a multi meter the mains side should be fine .

author: Brumby
author_rank: Unknown
timestamp: August 13, 2018, 04:05:05 am
content: Quote from: Jwillis on August 13, 2018, 01:07:18 amThis is my question also.Mains is typically rather low impedance, capable of delivering significant energy. This low impedance means it is going to take a lot to cause any significant impact on the delivered mains - aside from higher frequency EMI.I simply must ask "Do youreallyneedto see the mains waveform?"

author: lordvader88
author_rank: Frequent Contributor
timestamp: August 14, 2018, 12:59:25 am
content: Never mind all that, wow differential probes are super expensive.Back to plan A for now, get another 30-40 feet of wire and finish the MOT isolation transformer using 2 primaries on the same core.

author: bitseeker
author_rank: Super Contributor
timestamp: August 14, 2018, 06:55:54 am
content: Yes, they can be super expensive, especially from the A-brands. TheEEVblog HVP-70is made by Sapphire, the same company that makes them for Keysight, LeCroy, etc. So, it's a good value as compared to what you'd pay with one of those other brands on the cover.However, MicSig came out with some that are even more affordable. TheirDP10013is 100MHz and up to 1300V. However, it's a x50/x500 probe (unlike the x10/x100 of the HVP-70). So, the attenuation of the MicSig is higher, but also lighter on the budget. I wish they'd come out with a lower-voltage (500-700V is plenty for SMPS) probe that was also x10/x100.https://www.ebay.com/sch/i.html?_nkw=micsig+dp10013&_sacat=0&_sop=15https://www.ebay.ca/sch/i.html?_nkw=micsig+dp10013&_sacat=0&_sop=15Note that the voltages produced inside an SMPS can be well over 300VDC, even when connected to 120VAC in North America. Be careful when probing, even with a DMM, as a slip in the wrong place could make for a very bad day.

author: james_s
author_rank: Super Contributor
timestamp: August 14, 2018, 03:38:43 pm
content: Yes the DC bus is typically 330V whether the input is 120V or 240V, except in very small power supplies.I'm still not sure why you want to probe the mains side, I have the tools to do so safely, but I can't think of many reasons to try.