Oscilloscope Grounding - Page 1
Forum section: electronics / beginners

author: SewingYard
author_rank: Contributor
timestamp: March 22, 2016, 11:16:14 pm
content: Hi, Hopefully this something silly I'm missing with how things are done.I have a 12v AC Wall Wart that I have removed the bridge rectifier from, I have rebuilt the bridge rectifier on a breadboard, it all works fine and I get 12v DC out of the other end.Now comes the interesting thing.I want to analyse the circuit with my scope and put one probe on the AC side and one probe on the DC side so I can see the difference.I connect my first probe to the AC and my ground to the other side of the AC which is fine.I then connect my second probe to the DC side +12v and the ground to the DC ground.The current on the circuit immediately rises to 1.2 Amps and the Wall Wart starts getting very hot and buzzing.Why would this be, I'm assuming its something to do with the ground of the AC and the ground of the DC being shorted in some way but I cant explain exactly what im seeing.If I only use the DC Ground everything is fine and the current drops right down again so its definitely the scope probe interacting with the circuit.I hope this makes sense.

author: MK14
author_rank: Super Contributor
timestamp: March 22, 2016, 11:24:48 pm
content: The grounds coming out of your scope are connected together, inside the scope.So you could easily short-circuit your transformer, depending on exactly how you connect it up. It sounds like that is what happened.I.e. if you had a 9V battery, and connected up one ground to the -, and the other ground (from the second probe), to the +. The battery would be short-circuited, and run down very quickly.Also most test equipment is grounded (to the electricity ground), as is the scopes, ground. So great care is needed, to avoid shorting out equipment, and causing problems.

author: Zero999
author_rank: Super Contributor
timestamp: March 22, 2016, 11:25:20 pm
content: You short circuited it so all of the current flows through one of the diodes. Look at the schematic and trace the current path when the AC and DC sides are connected together.

author: danadak
author_rank: Super Contributor
timestamp: March 22, 2016, 11:29:13 pm
content: Most scopes each channel ground is a common ground.To measure differential V set up two channels, one inverted,hit "add" button, and you will see the voltage between thetwo channels. No grounds are used.Regards, Dana.

author: tggzzz
author_rank: Super Contributor
timestamp: March 23, 2016, 12:06:18 am
content: Learn how to do this safely by reading the relevant references athttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/Get it wrong and there is the possibility of loud noises followed by unpleasant smells anddeathlysilence.

author: SewingYard
author_rank: Contributor
timestamp: March 23, 2016, 12:59:23 am
content: Thanks everyone for the responses.and Danadak, thanks for the video link, it makes perfect sense now and also made me happy that I went for a 4 channel scope as I now realise that I can end up using 2 channels just for one measurement in some circumstances.I had to look through the Rigol manual for how to do it exactly but it is part of the math functions to combine the two channels, I also wondered what exactly the invert function was on the scope, that makes sense now too.Thanks again.Ill also be sure to look through the link that tggzzz posted.Thanks again all, this forum is making my learning curve much easier-Wayne

author: jh15
author_rank: Frequent Contributor
timestamp: March 25, 2016, 03:00:28 pm
content: Be really careful around line voltage connections. Build or buy an isolation transformer for best safety.Dave did a video "how not to blow up your oscilloscope" too.