Oscilloscope shows FG wave without black lead connection - Page 1
Forum section: electronics / beginners

author: nadona
author_rank: Regular Contributor
timestamp: December 23, 2012, 02:26:11 am
content: The oscilloscope shows the wave from my FG without both black leads connected. Just red leads are connected.

author: c4757p
author_rank: Super Contributor
timestamp: December 23, 2012, 02:38:30 am
content: Your function generator is probably grounded, like mine is. (Pain in the ass, really)

author: c4757p
author_rank: Super Contributor
timestamp: December 23, 2012, 02:39:57 am
content: Anyway, you'll normally getsomesignal pickup without a ground connection, although it is very, very noisy. That on your screen isnotit. I'm pretty sure it's a grounded function generator. Probe the resistance between the gen's output BNCs and the scope's input BNCs.

author: nadona
author_rank: Regular Contributor
timestamp: December 23, 2012, 02:50:46 am
content: @c4757p;Yes, they both grounded. Resistance from the FG's ground terminal to the Scope's every GND Terminals is less than One Ohm!Is it safe to connect FG ground to the the Fluke 8840's PCB ground as long as FG output voltage is low?p.s.; Now I see the ground mark next to the terminal on both Scope and FG!I remember a Dave's video about the scope's ground. Now I have to be cautious about the FG's ground, too!

author: c4757p
author_rank: Super Contributor
timestamp: December 23, 2012, 03:29:44 am
content: Absolutely. Any bench multimeter is floating - they'd be pretty useless otherwise. The 8840 specifies up to 1000V DC/pkAC and 700V RMS between its common terminal and earth ground.Edit: Wait, what do you mean by the Fluke 8840's "PCB ground"?

author: nadona
author_rank: Regular Contributor
timestamp: December 23, 2012, 04:47:35 am
content: The 8840's circuit's ground. I called it as "PCB ground" to differentiate from the meter's case ground which is house AC ground.Question! The 8840's circuit ground is separated from the case. The case is grounded through my 120V AC. When I connect my scopes ground clip[ to the 8840's ground pin on the PCB, then in effect I connect the case and PCB which is against the engineers' intention to separate the two. But they new this; peoples going to connect two different ground together!p.s.; Sorry to say, "Thank you!"

author: c4757p
author_rank: Super Contributor
timestamp: December 23, 2012, 05:05:14 am
content: Ah, OK. I was worried you were going to mess with the PCB!The two grounds don't need to be separated - the only things that cannot be safely connected to the multimeter's common terminal are A) anything exceeding the 1000V/700V max, and B) anything inside the meter itself (common terminal is the internal ground, so you're shorting things out). The case is earth grounded to keep it electrically safe for you - you can connect your meter's common to a point 400V above ground, and you wouldn't want to touch that when you reach up to press a button! This way, even if there is a stray wire somewhere, the meter case will never have a potential above earth.

author: nadona
author_rank: Regular Contributor
timestamp: December 23, 2012, 06:01:21 am
content: This time I want to clarify this Ground concept:1) The oscilloscope and FG Gound is not an actually Ground. It is a common ground which both signal circuit and AC power supply share. It is actually connected to household AC'sNeutralline not Ground line. The narrow wider pin in the States.2) The bench type DMM's case is actually connected to household AC's Ground line, ie center top pin of the plug.3) So The scope's and FG's ground should not be connected other than the measuring equipment's ground while DMM's circuit ground is floating so that any two points value can be probed and measured.Thank you.

author: Monkeh
author_rank: Super Contributor
timestamp: December 23, 2012, 06:44:24 am
content: Quote from: nadona on December 23, 2012, 06:01:21 am.. That's not only wrong, but dangerous.

author: nadona
author_rank: Regular Contributor
timestamp: December 23, 2012, 07:11:42 am
content: Can you elaborate?

author: peter.mitchell
author_rank: Super Contributor
timestamp: December 23, 2012, 08:15:39 am
content: Uhhh. . Dodgy 'lectrician accidentally swaps active and neutral in socket, doesn't check, everything seems fine. . You now have mains on your bnc.

author: BravoV
author_rank: Super Contributor
timestamp: December 23, 2012, 08:19:54 am
content: Wow ... you must understand and get the basic thing straight 1st before even playing with this measurement stuffs, it might cost you your own life.

author: nadona
author_rank: Regular Contributor
timestamp: December 23, 2012, 08:37:55 am
content: I watched the blog video #279. At the beginning Dave explained that all the terminal rings are shorted to Earth Ground. By looking at his schematics on the dry board, I noticed the signal ground is also connected to earth ground. I think that explains all.Thanks Dave!