Galvanic disconnection with transformer

It's called an "isolation transformer" or "isolating transformer". It isolates anything connected to the secondary from the public electricity supply lines. This makes it less dangerous to work with, but you still have to take care while doing anything at mains voltages.

The basic idea is this: You connect the primary (input side) of the isolation transformer to the mains outlet, and the primary of the PSU transformer to the secondary (output) of the isolation transformer. You can now measure voltages at various points on your PSU. But there are some complicating factors, especially since you want to measure the input to your PSU transformer, and that is still at mains voltage (and dangerous if you don't know what you're doing) even though it is isolated from the public mains.

It's a bit complex and it's already close to 3:00 AM here now. So I'll have to sign off now and continue tomorrow unless someone else has given provided the answers by then.

Isolating transformer

On the PSU.

Can be cheap second-hand, but pay a few bucks to get them tested if you do get a used one.

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You could get one of these isolation transformers:

I have used this particular one:

I can handle loads up to 80 VA.

Depending on how much power your PSU is going to be drawing while you work on it, you could select a smaller or larger one to meet your needs.

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.

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.

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?

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?

Best Regards Kari

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On the DUT (Device Under Test)

Connect all test equipment direct to supply, and supply grounding. IOW, just plug 'em in. Test equipment grounds should all be just that - grounds.

Your first thought was right.

--
"For a successful technology, reality must take precedence 
over public relations, for nature cannot be fooled."
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--- 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|----------|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.

---

--- 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'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

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

--- 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.

---

--- 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.

---

--- Good! :-)

---

--- 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_.

---

--- OK :-)

JF

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

=A0 =A0

=A0 =A0 +----------+

=A0 =A0 | =A0 =A0 =A0 =A0 =A0|

=A0 +----+-----+

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.

=A0 =A0 =A0 =A0 =A0 =A0 =A0 120V RECEPT

AKER BOX =A0 =A0O =A0 O =A0

--------+ =A0 | O | =A0

=A0 | =A0 =A0 =A0| =A0 =A0 | | =A0 =A0

|-----+ | =A0 =A0

=A0 | =A0| =A0 | =A0 =A0 =A0 | =A0 =A0

=A0

=A0 =A0 =A0|

-[CB]-|-------+

------|-----+ |

----+ =A0 | | |

=A0 =A0 =A0 =A0 =A0 =A0 | O |

=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0O =A0 O

=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0240V RECEPT

V

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.

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.

Thanks also. I think I should get a book about these things.

any good one?

Kari

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The NEC:

http://en.wikipedia.org/wiki/National_Electrical_Code#Public_access_to_the_NEC

JF

Kari:

Note that these responses have been assuming US-type wiring standards. I don't know anything about the standards where you are, but I suspect you have 240 only, no 120 ground. I assume that one side of the 240 is always grounded, but make sure that whatever book or reference you consult is for the system in your own area.

Best regards,

Bob Masta DAQARTA v5.10 Data AcQuisition And Real-Time Analysis

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