Ac mains voltage - Page 1
Forum section: electronics / beginners

author: Daniel_Reyes
author_rank: Regular Contributor
timestamp: October 11, 2015, 04:24:09 pm
content: I think I'm getting ac mains through some incorrect voltage double circuit setup and i can't figure out why. Using a feeltech fy3200s function generator and a rigol ds1054z.I think it may have to do with the unkeyed connector on the function generator.Sent from my SM-G900P using Tapatalk

author: Daniel_Reyes
author_rank: Regular Contributor
timestamp: October 11, 2015, 04:27:01 pm
content: For the record, I've watched the "How not to blow up your oscope" video twice before, about to watch it again. Just thought I had my based covered but apparently, I did not.

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 11, 2015, 04:28:26 pm
content: When you can get to a proper keyboard, your post could benefit from some better description of how things are supposed to be connected, and what you are trying to do.

author: Daniel_Reyes
author_rank: Regular Contributor
timestamp: October 11, 2015, 04:31:06 pm
content: Quote from: Richard Crowley on October 11, 2015, 04:28:26 pmin the bottom picture, signal generator is connected to the surge protector along with the oscope. I had turned the signal generator off and had the negative lead of the signal generator channel 1 probe connected to the circuit still. At the end of the circuit, I have the oscope's probe tip connected and somehow I am getting ac mains.

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 11, 2015, 04:36:00 pm
content: Must assume that the oscilloscope has the proper 3-pin safety-ground mains cord.What signal generator? We know nothing about it.

author: Daniel_Reyes
author_rank: Regular Contributor
timestamp: October 11, 2015, 04:36:59 pm
content: Quote from: Richard Crowley on October 11, 2015, 04:36:00 pmYou ASSume correctly and if you reference my first post, I state the name of the signal generator used.

author: Monkeh
author_rank: Super Contributor
timestamp: October 11, 2015, 04:42:36 pm
content: Your input is floating and you're surrounded by wiring carrying AC.What did you expect, a dead flat line to the nanovolt level?

author: Daniel_Reyes
author_rank: Regular Contributor
timestamp: October 11, 2015, 04:43:42 pm
content: Quote from: Monkeh on October 11, 2015, 04:42:36 pmSo I should get 300 volts peak to peak measured on the scope as well?

author: Monkeh
author_rank: Super Contributor
timestamp: October 11, 2015, 04:44:38 pm
content: Quote from: Daniel_Reyes on October 11, 2015, 04:43:42 pmWell now, you never, ever mentioned what voltage you were seeing andwe can't see it. Nor can we see the scope settings, the probe... You ASSume too much.We can't even see what you're actually connected to.

author: Daniel_Reyes
author_rank: Regular Contributor
timestamp: October 11, 2015, 04:46:05 pm
content: Quote from: Monkeh on October 11, 2015, 04:44:38 pmWell I guess that's expected. The beginner forum is where the sharks hang out to "help" people.

author: Monkeh
author_rank: Super Contributor
timestamp: October 11, 2015, 04:46:33 pm
content: Quote from: Daniel_Reyes on October 11, 2015, 04:46:05 pmWell perhaps if you'd actually show us and explain what you have connected and why, we can help..

author: Daniel_Reyes
author_rank: Regular Contributor
timestamp: October 11, 2015, 04:47:25 pm
content: I did. If you cannot use my description and a picture, then maybe you shouldn't be trying to help.

author: Monkeh
author_rank: Super Contributor
timestamp: October 11, 2015, 04:48:58 pm
content: Quote from: Daniel_Reyes on October 11, 2015, 04:47:25 pmWe cannot see your scope settings. We cannot actually see what it's connected to. We don't know why you're connecting it to whatever you've connected it to. We do not know what 'the circuit' is.The picture is at an angle which makes observing connections impossible, is out of focus, and is too far away.

author: SteveLy
author_rank: Regular Contributor
timestamp: October 11, 2015, 04:49:21 pm
content: I haven't been here long but salud to OP for bizarrest post I've seen so far on the EEVBlog forums. Like RC I have no idea what you're doing or asking, if anything.

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 11, 2015, 04:51:31 pm
content: We can't see most of the important information from your photos.Are you asking us to assume that black 2-blade mains plug is the power cord for the signal generator?Are you implying that there is a problem with the signal generator power supply?We are pretty much shooting in the dark here with very little pertinent information.

author: Daniel_Reyes
author_rank: Regular Contributor
timestamp: October 11, 2015, 04:52:05 pm
content: Quote from: Monkeh on October 11, 2015, 04:48:58 pm"The Circuit" isn't of issue. Identify the key players here. Negative lead of the function generator and positive probe tip on the scope. Somehow AC mains is getting through.

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 11, 2015, 04:53:36 pm
content: Does the function generator have a 3-pin mains power plug?If it has that 2-blade black power plug you showed in the first photo, then what you are seeing is pretty normal.

author: Daniel_Reyes
author_rank: Regular Contributor
timestamp: October 11, 2015, 04:54:02 pm
content: Quote from: Richard Crowley on October 11, 2015, 04:51:31 pmYes the 2 blade mains plug is for the signal generator. I tried to upload those photos in order but they were reversed which probably added some confusion to the story. I think there is an issue with the function generator's power supply but I did not want to bias anyone.

author: SteveLy
author_rank: Regular Contributor
timestamp: October 11, 2015, 05:03:41 pm
content: Confusion may be right. At first glance given the topic of the thread and the first picture I thought it might be a jackass style stunt of plugging your scope input into the mains.

author: Daniel_Reyes
author_rank: Regular Contributor
timestamp: October 11, 2015, 05:09:17 pm
content: Well, Thank you for the condescending warm welcome everyone. I'm reminded of my older mentors who used to say they would bend over backwards before asking people for help and I can see why. If I don't come with an exactly formatted question, I get this.

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 11, 2015, 05:22:48 pm
content: Format doesn't matter. But details are crucial. Without proper details, at best we are all wasting our time, at worst, we give improper responses based on guesswork and assumptions. As a "Regular Contributor" with 80 postings, we didn't take you for a newbie unfamiliar with using online forums.

author: SeanB
author_rank: Super Contributor
timestamp: October 11, 2015, 05:26:57 pm
content: Function generator is an ungrounded device, so the internal circuitry will float to around half mains potential through the parasitic capacitances in the power supply. This will generate 300V peak to peak into a 10M input scope probe, just from the few pf of capacitance.Solutions are to connect the FG output ground to the scope ground, either using a coax BNC lead to connect to an unused channel on the scope to an unused BNC connector on the function generator ( trigger, square wave output it does not matter as all you want is the shield of the cable to connect ground together on both devices with a low resistance), or to connect the FG ground to the scope ground by some other means, like a lead with crocodile clips both ends to metal of the spare BNC socket on FG and to scope ground connection.

author: Daniel_Reyes
author_rank: Regular Contributor
timestamp: October 11, 2015, 05:29:01 pm
content: Quote from: SeanB on October 11, 2015, 05:26:57 pmThanks! I left everything setup so I will try that later this evening. I apologize for the misunderstanding everyone. Hope you all have a great day!Sent from my SM-G900P using Tapatalk

author: SeanB
author_rank: Super Contributor
timestamp: October 11, 2015, 05:37:43 pm
content: Been bitten ( literally in some cases) by those ungrounded supplies, even on equipment that had a 3 pin mains lead. Turns out the manufacturer decided that the 3 pin socket was cheap, so used it, and then made the mains lead cheaper, even though the wire inside was barely conductive, and could possibly once have contained copper, by omitting the PE conductor, leaving the plated steel pins in and the plated steel contacts in the moulded socket as well.

author: Daniel_Reyes
author_rank: Regular Contributor
timestamp: October 11, 2015, 05:52:23 pm
content: Quote from: SeanB on October 11, 2015, 05:37:43 pmReally appreciate the info. I'll take it to heart and try to keep my wits about me.Sent from my SM-G900P using Tapatalk

author: Daniel_Reyes
author_rank: Regular Contributor
timestamp: October 11, 2015, 08:05:30 pm
content: Quote from: SeanB on October 11, 2015, 05:26:57 pmConnecting fg ground to scope ground worked for me. All my readings are making sense now. Thanks again! Glad this riddle was solved!Sent from my SM-G900P using Tapatalk