Galvanic disconnection with transformer
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From: pimpom on 27 Mar 2010 07:03 Kari Laine wrote: > Thank You for your time and effort. > > I am now convinced that one cannot measure mains with an > oscscope > connected to same mains. And it is recommended to use battery > powered > oscilloscope - why is that? It's not an absolute "cannot". One can, and I've done it on occasion. BUT it requires caution and a clear knowledge of what's involved. This is why: 1) If the house wiring has a proper ground (earth) line and the scope is powered with a three-wire mains cord via a three-prong mains plug, the body (chassis) of the scope is now connected to ground. 2) The ground lead of the scope is often connected to the chassis. Some scopes have provision to isolate, but not all scopes do. If the probe is NOT isolated from the chassis and you connect the ground lead to the live side of the mains supply, you are shorting the mains live and neutral lines together via the probe ground plus mains ground wire. PHUT!!! Something will blow, burn or trip. Another possible scenario: Let's say your scope is not properly grounded through the mains cord. In that case, you may not blow a fuse or trip a circuit breaker, but as soon as you touch the probe to the mains, all parts of the scope that's not isolated from the probe will now be "live" and dangerous. > > I actually do not need to measure mains. I am sure the > electrical > company is good enough. I only have to measure ripple after the > bridge > before regulator and ripple after regulator. I am just curious > what > would happen if I do following. > > I have a PC-scope connected to machine, which is grounded > through the > mains cable. I connect probe (in x10) to mains? Will I generate > smoke? You may not actually produce smoke as the series resistor in the 10x probe will limit the current, but it's still not safe - for you or for the PC. Some important visitors have just come in (clients for a project I'm doing). If it's still not clear enough, just ask.
From: John Fields on 27 Mar 2010 11:27 On Sat, 27 Mar 2010 13:40:53 +0200, Kari Laine <klaine8(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 so all that's required to measure the mains voltage is to connect the scope's vertical input to L1 or L2 through a suitable voltage divider: +----------+ +---|VERT--+ | | | | | [9M] | | | | | [1M] | 120AC>---+---|AC | | | | | NEUT>--------|GND---+ | +----------+ The 9 megohm resistor being part of a X10 probe, with 1 megohm being the usual input resistance of the scope's vertical amplifiers. --- >And it is recommended to use battery powered >oscilloscope - why is that? --- Because a battery powered oscilloscope carries its own power supply and is _totally_ isolated from the mains, making it somewhat safer to use than a conventional scope with its ground hardwired to neutral. --- >I actually do not need to measure mains. I am sure the electrical >company is good enough. I only have to measure ripple after the bridge >before regulator and ripple after regulator. I am just curious what >would happen if I do following. > >I have a PC-scope connected to machine, which is grounded through the >mains cable. I connect probe (in x10) to mains? Will I generate smoke? --- 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? JF
From: Kari Laine on 28 Mar 2010 05:00 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 I would connect probes ground lead to ground. I would connect probes tip to either of the Active1 or Active2. I WON'T connect probes ground to active1 and probes tip to active2. My confusion was this probes ground lead. Anyway I don't actually need to measure it - I was just curious. Best Regards Kari
From: John Fields on 28 Mar 2010 05:57 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. --- >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
From: krw on 28 Mar 2010 11:39
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. <...> |