Isolation transformer

** So you nave no idea what an electrical circuit is ?? ** Not until YOU go figure what an electrical circuit is and how come a broken one is not one.

Draw yourself some diagrams.

Then figure out the CIRCUIT by which how folk get electric shocks from the AC power.

...... Phil

The classic All-American Five radio did not have a transformer. The tube filaments were in series, and they used tubes with differing filament voltages that added up to the line voltage. The ground of the radio was connected directly to one side of the AC line.

The plugs were generally not polarized, so there was a good chance that the "ground" of the radio was connected to the hot side of the AC line. Touch that, and touch ground, and you've got a big shock.

They generally fixed the problem by putting them in plastic cases, and not having any bits of metal on the inside touchabel on the outside.

But if you were servicing them, you'd risk that shock. At the very least, you'd risk connecting the ground lead of your VTVM (vacumm tube voltmeter) to the "chassis ground" of the radio, and then watch as that lead gets really hot as it passes the current, causing the insulation to melt. It happens.

But I'd say it's also dubious about how well grounded the "neutral" line is. After all, three-prong outlets were brought in, and the third prong is most definitely ground. There may at the very least be some small potential between the "neutral" side of the AC line and actual ground.

A unit with a transformer in it is never dangerous when you touch the "chassis". You know for certain it is at ground. If it's not, something is failing elsewhere. An isolation transformer just moves that transformer out of the box, for the times when you really need the isolation.

Nowadays, switching supplies are common. A bridge rectifier across the AC line. You can't look at the primary of that supply without connecting a ground probe, which then shorts out the AC line to ground.

Michael

If you put your fingers between live and neutral you get zapped. But neutral is locally connected to ground, and your body has at least some capacitance to ground, so you are connected to neutral via ground. If you touch live, it completes the circuit. Isoltation transformer neutral is not connected to ground.

Thanks Michael That classic radio sounds like a deathtrap primed for the kill.

AFAICR, in Australia, we have always had three-wire outlets, the green being usually connected to your water pipe or a copper spear, and the black going back to the power company's street transformer where it is earthed very well with a ginormous copper spear. (MEN)

I believe any live chassis in Australia will last at that potential less than 30 ms or whatever time the earth leakage breaker can disconnect you in. Failing that, the blow time of the fuse (breaker)

So, if I put a 1:1 transformer in the power cord of my TV set, I will be safer delving about in its innards with it turned on? I still don't quite see how. You will have the outlet black and red wires (alright, black and brown) connected to the primary winding and the TV set connected to the secondary. If I grab the black and brown wires from this tranny heading towards the TV I'm sure I will get deaded, or is there something I'm missing about induced power? I'm not trying to be a smart arse, just trying to understand. I've just invested a bunch in Horowitz and Hill's Art of Electronics, and finding it very mind stretching. Wish I'd started this 51 years ago when I was flinging rocks into trees for my crystal set at boarding school :) Thanks again, jack

Thought exercise: Fifty metres of copper water pipe through the wet sand. EVERY non-double-insulated piece of electrical equipment having it's metal frame/chassis firmly bonded to this water pipe. Earth leakage breakers on all pieces.

Yes, I know what an electrical circuit is. If you can't be civil and explain to me why an isolating transformer might be of advantage here, perhaps you might be better to leave it to others and remain silent. jack

ps Is an open circuit not a circuit? Enquiring minds want to know.

Thanks Andrew. Sorry, I still don't get it. How does the floating neutral of an isolation transformer help in one's safety? You still get a shock if you touch live and are in any way connected to earth. Surely, however you place yourself between a high potential and a low one is rather immaterial. If you allow electrons to be pushed through your body with more than about 80V, you are flirting with death? I understand the worst case (where an earth leakage won't protect you) is to hold the live/active wire in one hand and the neutral in the other. That is almost certain death in Australia (240V single phase). Even worse to do it with two phases of a three phase supply (415V). Does an isolating transformer perhaps only ground a live chassis with a non-polarised, non-earthed supply? jack

ps could you explain that "capacitance to ground"? I would have expected "electrical connection to ground". But then I'm on the steep learning curve :)

It's SEB Phil, remember? If you can't be bothered with BASIC questions, then don't read SEB.

In circuits like TV's, manufacturers don't include transformers and often have circuit ground tied to one side of the AC mains. You can Google on "hot chassis" for information on this. This is cheaper, but it can also be quite dangerous. For example, if you connect an oscilloscope ground to the chassis of a TV, you will likely see sparks fly. It stands to reason then that touching the chasis while also touching an earth ground will result in sever shock.

An isolation transformer breaks the DC path to ground. Don't get me wrong, there will still be plenty of power available in the TV to kill yourself, you just won't get shocked by touching one hand to the chasis. You will also be able to ground your scope to the chasis safeley. The output of a

1:1 isolation transformer is just as deadly as the input, it's just not referenced against an earth ground.

The term "neutral" indicates which side of the mains supply is earthed. There is no "live" and "neutral" on the secondary of an isolation transformer: it is symmetrical (balanced) with respect to ground.

Very little current can flow if you touch one end of the secondary when the other end is open-circuit. If you connected the open-circuit end to ground, that would be a different story.

If you stand out of doors, in your bare feet, you have an electrical connection to ground; but you can still get a shock in an upstairs room, wearing shoes because there is capacitance between you and ground. Capacitance is a property of capacitors. A capacitor is a gap in a circuit. Alternating current can flow through a capacitor. The amount of current that flows partly depends on the capacitance. Big gaps have low capacitance and little current can flow.

The wikipedia explanation is excellent, what part are you having trouble understanding?

An isolation transformer makes electrocution less likely since there are grounds all around and you are more likely to contact ground with one part of the body while touching a live circuit in whatever you are working on. You can still be electrocuted but it helps your chances of staying alive.

A ground fault interrupter may be a better (and cheaper) bet for staying alive - but don't use it in conjunction with an isolation transformer since that could negate the ground error leakage signal it needs to interrupt power.

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And if you grabbed one side of the secondary in each hand, that would be the end of the story.

** So you nave no idea what an electrical circuit is ??

** Not until YOU go figure what an electrical circuit is and how come a broken one is not one.

Draw yourself some diagrams.

Then figure out the CIRCUIT by which how folk get electric shocks from the AC power.

** Go get f***ed - cunt head.

** No you do **** f****ng *** not.

YOU baby f****ng pile of ASD f***ed SHIT !!

....... Phil

** So you nave no idea what an electrical circuit is ??

** Not until YOU go figure what an electrical circuit is and how come a broken one is not one.

Draw yourself some diagrams - CUNT HEAD !!!!!!!!!!!

Then figure out the CIRCUIT by which how folk get electric shocks from the AC power.

** Go get f***ed - you asinine cunt head !!

** No you do **** f****ng *** not.

YOU baby f****ng pile of ASD f***ed SHIT !!

....... Phil

--
View in Courier:

Here\'s a circuit of the "safe" way to use an un-isolated mains
supply:
                                    HV!
                                    /
HOT-----< ]--+--------+
                          |+       | 
                        [BFC]    [LOAD]
                          |        |
NEUT--+-<

Sorry, I still don't get it. How does the floating neutral of an

** My god - how can ANYONE be that f****ng STUPID !!

Better this brain dead CUNT were electrocuted right NOW.

What a pile of vile sub human SHIT !!!!!!!!!!!!!!!!!!

Must still vote for the long deceased Bob Menzies ......

Maggot are feasting on his putrid brain.

Eeeeeeyuuuccccckkkk

....... Phil

Thanks Anthony. Most helpful. I'm just a bit curious about this DC path to ground. If you put _yourself_ in the way of a path to ground, surely a DC voltage of 80 or more will be hazardous? I honestly can't see what benefit there is in breaking (filtering out) the DC path via the power cable/outlet.

What's the characteristic of our planet that is in infinite supply? An infinite ability to absorb charge and not change much? So if you are connected to it (wet feet?) any source of high potential current will try to use you as conduit to earth if you give it half a chance?

With the isolation transformer installed in the power-in lines of a hot chassis TV, how is the requisite voltage achieved between the chassis and the other end of the daisy chain of heaters/plates/etc ?

Is it that instead of having the chassis at mains potential, and the neutral at earth potential, it ensures the chassis is at earth and the other end is at mains? Or are these things fed by DC and it is merely this that you are "filtering out" with the isolation transformer? I think I need to sleep on this, my head hurts. I will catch up in the morning after a well-deserved Sunday sleep in :) jack

But you haven't actually said HOW it helps, have you? I'm trying to understand how an IT helps with grounded chassis equipment and polarised three pin plugs to the mains. jack

Aha. Now I see a glimmer of light. I understood that an IT was merely a 1:1 transformer. That is, mains voltage in: mains voltage out. So for the equipment to work, it still must have two wires entering that are mains potential apart, I would have thought. These would now be arranged to be + half mains potential and - half mains potential?

So you switch the equipment off after the IT? I understood the aim was to work on the equipment while running. So without the IT, the live wire can damage you, even when the neutral is open circuit, whereas with an IT installed under the same circumstances, the "live wire" out of the IT is almost impotent? Thanks for that, I think I'm getting it.

I understand that. The fully insulated body will allow current to flow into it until it is charged to it's "capacitance". On a totally dry wooden floor in low humidity atmosphere with dry rubber soles, touching 240V give one a nasty jolt -- DAMHIKT :) jack

Which is where I simplistically came in wondering why this was not the case with an IT. Thanks, jack