Oscilloscope ground loop problem ...advice wanted. - Page 1
Forum section: electronics / beginners

author: Robert Smith Eco Warrior
author_rank: Regular Contributor
timestamp: May 10, 2021, 02:51:15 pm
content: Tomorrow I am collecting a Hameg 303-6 oscilloscope.It is long drive and the guy maybe has other items that I would regret not asking about while there, hence this post to figure out what I might need, and he may have.The problem I can foresee is...I run a diesel generator that produces about 55v ac which is then rectified to dc and this is connected directly to my off grid battery bank to give the batteries a bit more charge during prolonged gloomy periods in the winter. This puts ripple on the dc battery cabling and input to our inverters.We also have a solar charge controller that has a pwm output that also puts ripple on the dc battery cabling and input to our inverters.I would like to measure this ripple....but...The oscilloscope would be running from the power coming from one of our inverters. The negative side of the battery bank is linked inside the inverter to the neutral side of the ac output and also earth. I am thinking this may cause me a grounding loop or some weirdness arriving from the negative battery connection to inverter neutral and earth to oscilloscope earth to the ground side of the oscilloscope probe. Maybe this is ok if all these 'low level' items are all sitting at 0v?The positive side of the battery cabling sits around 50 - 58v so I may need to get a higher voltage probe for that ? unless I probe a potential divider I could make up?I would like to go armed with a bit of a shopping list as the guy selling me the oscilloscope may have useful items tucked in drawers I have not yet learnt that I need. I don't want to drive hundreds of miles and then think 'I wish I had asked....'

author: CaptDon
author_rank: Super Contributor
timestamp: May 10, 2021, 03:08:09 pm
content: A low 'inter-winding capacitance' isolation transformer would be your best bet. Easily obtainable.

author: Robert Smith Eco Warrior
author_rank: Regular Contributor
timestamp: May 10, 2021, 03:18:06 pm
content: I forgot to add... I have another generator I could use to power the oscilloscope in an isolated manner, ie plonk the generator on a car tyre and plug in the oscilloscope to that. But that may produce it's own interference.I could also lash together a few other separate batteries, connect up a small inverter we have but not ground it and then run the oscilloscope from that.A few options.... all involving lifting heavy things

author: tggzzz
author_rank: Super Contributor
timestamp: May 10, 2021, 03:20:44 pm
content: One general purpose tool is a high voltage differential probe.Maybe your bloke has other classes of scope probe that would be useful to you? FFI, see the "ABC" references athttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/Whatever you do, don't isolate the scope's earth connection from the protective mains earth. "Floating the scope" has beenand will belethal to equipment and people.

author: fordem
author_rank: Regular Contributor
timestamp: May 10, 2021, 03:29:50 pm
content: Quote from: Robert Smith Eco Warrior on May 10, 2021, 02:51:15 pmYou'll be fine - the scope ground is connected to the negative side of the battery bank, via the earth & neutral of the AC supply.QuoteThe Hameg I have sitting here (HM203) has a 20V/div setting and an input spec of 400V(DC + peak AC) - the 303 will be similar, but you can check it if you want to be on the safe side - a standard X1 probe is all you need - since what you're interested in is the ripple, set the input for AC coupling.

author: Robert Smith Eco Warrior
author_rank: Regular Contributor
timestamp: May 10, 2021, 04:40:45 pm
content: Quote from: fordem on May 10, 2021, 03:29:50 pmThanks for the reply. I shall take a cautious approach and do a few multimeter measurements before I commit to using the oscilloscope just to check things.I want to get the ripple down as the ripple is close to being unacceptable for the inverters. Occasionally when the generator is running our newest inverter will flash up an error saying the dc input ripple is too high. You can audibly hear it too. The inverter buzzes at the pwm frequency, or maybe a resonant frequency, of the charge controller when the charge controller is reducing the charge going into the batteries on sunny days. The inverters may also be buzzing when the diesel generator is running but a plane could crash outside the shed and you wouldn't hear it, let alone a buzz from the inverter. That generator is the noisiest thing Petter ever built. It is awful. I have eight 12000uF 400v capacitors to try across the battery charging lines to see if that helps with the ripple. I know a single 3300uF capacitor I have tried reduces the inverter buzz quite a lot. It will be interesting to see if I can get the ripple down further but at the moment I don't have a method of seeing the ripple.I will fuse each individual capacitor if anyone is thinking that.