Galvanic disconnection with transformer
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From: John Fields on 28 Mar 2010 17:04 On Sun, 28 Mar 2010 10:39:42 -0500, "krw(a)att.bizzzzzzzzzzzz" <krw(a)att.bizzzzzzzzzzzz> wrote: >On Sun, 28 Mar 2010 04:57:07 -0500, John Fields ><jfields(a)austininstruments.com> wrote: > >>On Sun, 28 Mar 2010 12:00:41 +0300, Kari Laine <klaine8(a)gmail.com> >>wrote: >> >>>John Fields wrote: >>>> --- >>>> I'm not sure what you mean by "I connect probe (in x10) to mains". >>>> >>>> Can you be a little more specific or post a schematic of what you're >>>> trying to do? >>> >>>Thanks John! >>> >>>I think I understand it now. >>>Mains connector on the wall has three poles. >>>Active1, Active2 and ground >> >>--- >>Oops... looks like I confused you. >> >>Sorry about that. >> >>In actuality, the system looks more like this: >> >> >> 120V RECEPT >> METER BREAKER BOX O O >> +-----+ +----------+ | O | >>HV>----+ +-----//-----|L1 L1|--120---|---+-[CB]-|---+ | | >> | | | | | | | | | >> | | | +--|--GND---|---|--+---|-----+ | >> P||S | | | | | | | | >> R||E-+---//---+-|N-+-N|--NEUT--|-+-|--+---|-------+ >> I||C | | | | | | | | >> | | | | | | | | +-[CB]-|-------+ >> | | | | | | | +--------|-----+ | >>HV>----+ +-|---//---|-|L2 L2|--120---|---+-[CB]-|---+ | | >> | | +-----+ +----------+ | | | >> EARTH EARTH | O | >> O O >> 240V RECEPT >> >>Notice that for 120V, the receptacle is connected to one end of the >>transformer secondary (L1), the center tap,(N) and GND, which is >>connected to neutral in the breaker box and the meter. >> >>Neutral is also connected to rods driven into the Earth at the pole and >>the meter. >> >>What you described, however, is the 240V receptacle, which is connected >>across the entire secondary (across which is 240V) and the center tap, >>which is Neutral, so both "hot" terminals in the receptacle are at 120V >>with respect to Neutral and at 240V with respect to each other. > >You forgot the ground on the 240V receptacle. There may or may not be a >neutral (it must be there on new circuits with loads with components requiring >120V) but there *must* be a ground. --- Right you are... Thanks! METER BREAKER BOX +-----+ +-----------------+ HV>----+ +-----//-----|L1 L1|--120---|-----+-[CB]------|----O Line | | | +--|--GND---|-+---|-------+---|--O Ground | | | | | | | | | +-|----O Neutral | | | | | | | | +-|-|--------+ P||S | | | | | +-[CB]--|-|-|----OL | R||E-+---//---+-|N-+-N|--NEUT--|-+---|-------+-+-|--ON GO-+ I||C | | | | | +-|-[CB]--|---|----OL | | | | | | | | | | | | | | | | | | | +-[CB]--|---|----OL | | | | | | | | +---|--OG HV>----+ +-|---//---|-|L2 L2|--120---|---+---[CB]------|----OL | | +-----+ +-----------------+ EARTH EARTH JF
From: George Herold on 28 Mar 2010 22:16 On Mar 27, 11:27 am, John Fields <jfie...(a)austininstruments.com> wrote: > On Sat, 27 Mar 2010 13:40:53 +0200, Kari Laine <klai...(a)gmail.com> > wrote: > > >Thank You for your time and effort. > > >I am now convinced that one cannot measure mains with an oscscope > >connected to same mains. > > --- > Sure you can! > > If you're talking 240V US residential mains, here's what you get from > the power company: (View in Courier) > > METER > +-----+ > HV>----+ +-----//-------|L1 L1|-->120V > | | | | > P||S | | > R||E-+---//-----+-|N N|-->0V > I||C | | | | > | | | | | | > HV>----+ +-|---//-----|-|L2 L2|-->120V > | | +-----+ > EARTH EARTH > > Your scope gets power from the mains like this: > > METER > +-----+ +----------+ > HV>----+ +-----//-------|L1 L1|--<120V<---|AC VERT| > | | | | | | > P||S | | | | > R||E-+---//-----+-|N N|--<0V<-----+AC--+--GND| | > I||C | | | | | | | > | | | | | | +----+-----+ > HV>----+ +-|---//-----|-|L2 L2|-->120V > | | +-----+ > EARTH EARTH > Thanks for the picture JF, I'm fairly ignorant about power electronics. I have a question about where the third pin (AC) ground is connected into the system? thanks again, George H. <snip> > JF
From: George Herold on 28 Mar 2010 22:22 On Mar 28, 5:57 am, John Fields <jfie...(a)austininstruments.com> wrote: > On Sun, 28 Mar 2010 12:00:41 +0300, Kari Laine <klai...(a)gmail.com> > wrote: > > >John Fields wrote: > >> --- > >> I'm not sure what you mean by "I connect probe (in x10) to mains". > > >> Can you be a little more specific or post a schematic of what you're > >> trying to do? > > >Thanks John! > > >I think I understand it now. > >Mains connector on the wall has three poles. > >Active1, Active2 and ground > > --- > Oops... looks like I confused you. > > Sorry about that. > > In actuality, the system looks more like this: > > 120V RECEPT > METER BREAKER BOX O O > +-----+ +----------+ | O | > HV>----+ +-----//-----|L1 L1|--120---|---+-[CB]-|---+ | | > | | | | | | | | | > | | | +--|--GND---|---|--+---|-----+ | > P||S | | | | | | | | > R||E-+---//---+-|N-+-N|--NEUT--|-+-|--+---|-------+ > I||C | | | | | | | | > | | | | | | | | +-[CB]-|-------+ > | | | | | | | +--------|-----+ | > HV>----+ +-|---//---|-|L2 L2|--120---|---+-[CB]-|---+ | | > | | +-----+ +----------+ | | | > EARTH EARTH | O | > O O > 240V RECEPT > > Notice that for 120V, the receptacle is connected to one end of the > transformer secondary (L1), the center tap,(N) and GND, which is > connected to neutral in the breaker box and the meter. > > Neutral is also connected to rods driven into the Earth at the pole and > the meter. > > What you described, however, is the 240V receptacle, which is connected > across the entire secondary (across which is 240V) and the center tap, > which is Neutral, so both "hot" terminals in the receptacle are at 120V > with respect to Neutral and at 240V with respect to each other. > --- > > >I would connect probes ground lead to ground. > >I would connect probes tip to either of the Active1 or Active2. > > --- > For the 240V circuit, yes. > > For the 120V circuit you don't have active 1 and active 2, you have one > or the other, neutral, and ground, so your scope ground would go to > neutral and your probe tip to the hot terminal. > --- > > >I WON'T connect probes ground to active1 and probes tip to active2. > > --- > Good! :-) > --- > > >My confusion was this probes ground lead. > > --- > OK, just remember that AC outlets and equipment aren't always wired up > like they're supposed to be, so make sure that you know what's what > before you clip that ground lead onto _anything_. > --- > > >Anyway I don't actually need to measure it - I was just curious. > > --- > OK :-) > > JF Ahh Thanks, (Sorry guess I should read ahead before asking questions) The neutral and ground are connected together both inside the meter and the breaker box? George H.
From: krw on 28 Mar 2010 23:38 On Sun, 28 Mar 2010 19:22:31 -0700 (PDT), George Herold <ggherold(a)gmail.com> wrote: >On Mar 28, 5:57�am, John Fields <jfie...(a)austininstruments.com> wrote: >> On Sun, 28 Mar 2010 12:00:41 +0300, Kari Laine <klai...(a)gmail.com> >> wrote: >> >> >John Fields wrote: >> >> --- >> >> I'm not sure what you mean by "I connect probe (in x10) to mains". >> >> >> Can you be a little more specific or post a schematic of what you're >> >> trying to do? >> >> >Thanks John! >> >> >I think I understand it now. >> >Mains connector on the wall has three poles. >> >Active1, Active2 and ground >> >> --- >> Oops... looks like I confused you. >> >> Sorry about that. >> >> In actuality, the system looks more like this: >> >> � � � � � � � � � � � � � � � � � � � � � � � � � 120V RECEPT >> � � � � � � � � � � � � METER � � � � BREAKER BOX � �O � O � >> � � � � � � � � � � � �+-----+ � � � �+----------+ � | O | � >> HV>----+ �+-----//-----|L1 L1|--120---|---+-[CB]-|---+ | | � � >> � � � �| �| � � � � � �| � � | � � � �| � | � � �| � � | | � � >> � � � �| �| � � � � � �| �+--|--GND---|---|--+---|-----+ | � � >> � � � �P||S � � � � � �| �| �| � � � �| � | �| � | � � � | � � >> � � � �R||E-+---//---+-|N-+-N|--NEUT--|-+-|--+---|-------+ � � >> � � � �I||C | � � � �| | � � | � � � �| | | � � �| >> � � � �| �| | � � � �| | � � | � � � �| | +-[CB]-|-------+ >> � � � �| �| | � � � �| | � � | � � � �| +--------|-----+ | >> HV>----+ �+-|---//---|-|L2 L2|--120---|---+-[CB]-|---+ | | >> � � � � � � | � � � �| +-----+ � � � �+----------+ � | | | >> � � � � � EARTH � �EARTH � � � � � � � � � � � � � � | O | >> � � � � � � � � � � � � � � � � � � � � � � � � � � �O � O >> � � � � � � � � � � � � � � � � � � � � � � � � � �240V RECEPT >> >> Notice that for 120V, the receptacle is connected to one end of the >> transformer secondary (L1), the center tap,(N) and GND, which is >> connected to neutral in the breaker box and the meter. >> >> Neutral is also connected to rods driven into the Earth at the pole and >> the meter. >> >> What you described, however, is the 240V receptacle, which is connected >> across the entire secondary (across which is 240V) and the center tap, >> which is �Neutral, so both "hot" terminals in the receptacle are at 120V >> with respect to Neutral and at 240V with respect to each other. >> --- >> >> >I would connect probes ground lead to ground. >> >I would connect probes tip to either of the Active1 or Active2. >> >> --- >> For the 240V circuit, yes. >> >> For the 120V circuit you don't have active 1 and active 2, you have one >> or the other, neutral, and ground, so your scope ground would go to >> neutral and your probe tip to the hot terminal. >> --- >> >> >I WON'T connect probes ground to active1 and probes tip to active2. >> >> --- >> Good! :-) >> --- >> >> >My confusion was this probes ground lead. >> >> --- >> OK, just remember that AC outlets and equipment aren't always wired up >> like they're supposed to be, so make sure that you know what's what >> before you clip that ground lead onto _anything_. >> --- >> >> >Anyway I don't actually need to measure it - I was just curious. >> >> --- >> OK :-) >> >> JF > >Ahh Thanks, (Sorry guess I should read ahead before asking questions) >The neutral and ground are connected together both inside the meter >and the breaker box? Yes, the last place they're connected is in the entrance panel. They're separated in any boxes and sub-panels downstream of the entrance panel.
From: Kari Laine on 29 Mar 2010 03:14
George Herold wrote: > Thanks for the picture JF, I'm fairly ignorant about power > electronics. I have a question about where the third pin (AC) ground > is connected into the system? > > thanks again, > > George H. Thanks also. I think I should get a book about these things. any good one? Kari |