AC Power At the Plug - Page 1
Forum section: electronics / beginners

author: spirulerlocus
author_rank: Contributor
timestamp: November 19, 2015, 03:37:25 pm
content: In regard to a three wire plug in the US. If I measure the voltage across the two flat blades then I get 120Vrms. When I measure the voltage between either flat blade and the round (neutral) pin, I measure zero volts. I did some reading on Wikipediahttps://en.wikipedia.org/wiki/Split-phase_electric_power, and from Figure 1 it would seem that I should measure 240 Vrms as the line-to-line voltage and 120Vrms as the line-to-neutral. What is going on here?

author: IanB
author_rank: Super Contributor
timestamp: November 19, 2015, 03:45:37 pm
content: If you are in the USA then the round pin is not the neutral pin, it is the safety ground pin.You should measure 120 V RMS between the narrower flat terminal (the hot terminal) and the ground terminal, and you should measure almost zero volts between the wider flat terminal (the neutral terminal) and the ground terminal.If you do not measure any voltage to the ground terminal then your safety ground is not connected and you should call an electrician to fix your dangerous wiring.Also, by strict terminology, you should never measure any voltage on the blades or pins of a plug. Since the blades are exposed this would be dangerous. The voltage should only be present at the terminals enclosed inside a socket.(Note: Figure 1 in that Wikipedia article does not apply to the wiring of a 120 V domestic household socket.)

author: smbaker
author_rank: Regular Contributor
timestamp: November 19, 2015, 04:25:52 pm
content: I would recommend something like thishttp://www.amazon.com/Sperry-Instruments-GFI6302-Outlet-Tester/dp/B000RUL2UU-- it'll quickly tell you whether the ground is connected and whether the polarity is correct. While you can do it with a multimeter, one of these is ~ $7, usually available at your local hardware store, and you can do the whole house in 10 minutes.If you have one outlet that's missing a ground, then you may have more.

author: spirulerlocus
author_rank: Contributor
timestamp: November 19, 2015, 05:30:34 pm
content: Thanks! My question is based on trying to understand a PFC. For example, where is ground in Figure 1 of this document:http://pe.org.pl/articles/2013/2a/39.pdf?

author: Monkeh
author_rank: Super Contributor
timestamp: November 19, 2015, 05:51:52 pm
content: Quote from: spirulerlocus on November 19, 2015, 05:30:34 pmProtective ground is not in those figures because it is not part of the current-carrying circuit.

author: spirulerlocus
author_rank: Contributor
timestamp: November 19, 2015, 06:03:50 pm
content: So I can arbitrarily choose either side of the AC source as ground? The bottom of CB, RB, S1, and S2 can also be chosen as ground?

author: helius
author_rank: Super Contributor
timestamp: November 19, 2015, 06:31:33 pm
content: If you don't already know the answer, then I do not believe you can safely build a nonisolated (transformerless) circuit such as that one. I suggest you come back to it after you have more experience with power supplies that use transformers to isolate the input from the AC mains, such as simple linear regulators.

author: spirulerlocus
author_rank: Contributor
timestamp: November 19, 2015, 07:07:19 pm
content: My interest is in understanding the circuit, not building it.

author: IanB
author_rank: Super Contributor
timestamp: November 19, 2015, 08:12:31 pm
content: Quote from: spirulerlocus on November 19, 2015, 06:03:50 pmNo, there is no ground shown in those circuits. You cannot choose any location as ground because ground isn't there.

author: edavid
author_rank: Super Contributor
timestamp: November 19, 2015, 08:27:48 pm
content: Quote from: IanB on November 19, 2015, 03:45:37 pmI thought this met code if there's a GFCI/RCD in the circuit. It's certainly very common in older homes.

author: spirulerlocus
author_rank: Contributor
timestamp: November 19, 2015, 08:31:57 pm
content: Ok, what about the schematic on p.8 of this .pdf (Figure 11):http://www.onsemi.com/pub_link/Collateral/AND8392-D.PDF?This figure shows ground at the output and earth at the plug. How can one sufficiently suppress common mode noise if LN & N are independent of Earth? How may the AC line be modeled so that I can model the common mode response of the EMI filter?

author: IanB
author_rank: Super Contributor
timestamp: November 19, 2015, 09:47:23 pm
content: Quote from: edavid on November 19, 2015, 08:27:48 pmI don't know about meeting code, but I do think that if there is a three pin socket then the ground should be connected to ground and not floating. Does code really allow the ground connection to be present but unconnected?

author: IanB
author_rank: Super Contributor
timestamp: November 19, 2015, 10:08:17 pm
content: Quote from: spirulerlocus on November 19, 2015, 08:31:57 pmI think that the ground symbol shown in those circuits is the internal circuit ground for voltage reference purposes when displaying wave forms. It is not the same thing as the mains protective ground, and it is not connected to the mains protective ground.One always has to remember that ground has two separate meanings in circuits. One is the protective ground that is used as a safety system in mains wiring. This ground is literally connected to the ground beneath your feet.The other meaning is as a convenient shorthand in electronic circuits to indicate the "zero volt" reference point when quoting expected voltages at other points of the circuit, or when graphing voltage waves on charts. This "electronic" ground has no implied connection to safety ground and often is not connected to earth at all.You started out by asking about the protective ground in your mains socket and then suddenly switched to electronic circuits. By doing so you caused some confusion, certainly to me.

author: edavid
author_rank: Super Contributor
timestamp: November 20, 2015, 03:41:29 am
content: Quote from: IanB on November 19, 2015, 09:47:23 pmHere's an NEC excerpt I found on another site:QuoteMost people don't bother with the labels.

author: spirulerlocus
author_rank: Contributor
timestamp: November 20, 2015, 01:42:22 pm
content: Sorry, my intent was to understand what the equivalent circuit is at the plug. Can the grid simply be modeled as an AC source between "L" and "N", and "N" is connected to earth?If the grounds are independent then it seems there would be an issue taking measurements as a scope ground would be the same as the grid ground.

author: vk6zgo
author_rank: Super Contributor
timestamp: November 20, 2015, 04:01:09 pm
content: Quote from: spirulerlocus on November 20, 2015, 01:42:22 pmYes!This is something that comes up in this forum a lot.If you have to connect between two points in such a circuit where neither are at the mains protective earth potential,you need to use either an Oscilloscope with inputs which are not referenced to mains earth,two channels in a "quasi differential connection" or use a differential probe.If you are basically just "troubleshooting" in a circuit like this,you can remove your ground clip from the probe entirely,& just probe the various points,using the 'scope's earthing via its power cord as the return path.Obviously,if you need to see high frequency components,this is not satisfactory,as you have tens of metres of wire in the round trip via the Neutral/Earth connection at the power entry point.

author: spirulerlocus
author_rank: Contributor
timestamp: November 20, 2015, 09:12:12 pm
content: Great. So is it safe to say that the input is an AC source with one side grounded (earth/protective ground) and that ground is independent of the somewhat arbitrary ground of the PFC?One can also say that the only real common mode suppression is through the filtering action of CM1, CM2, C18, and C19 in the On Semi note?