Re: Isolation transformer
- From: spamfree@xxxxxxxxxxx
- Date: Sun, 06 May 2007 00:31:09 +0900
On Sat, 5 May 2007 08:14:18 -0500, "Anthony Fremont"
<spam-not@xxxxxxxxxxx> wrote:
spamfree@xxxxxxxxxxx wrote:
I'm trying to find out why these are used as safety precautions in
servicing mains connected equipment. Seems to me that not being
electrically connected to the mains is a moot point. Surely the pole
transformer isolates me from the generating dynamo at the power
station, but my outlet can surely still kill me. Induced currents can
surely be as dangerous as directly conducted currents?
Wikipedia states under this heading:
"In electronics testing, troubleshooting and servicing, an isolation
transformer is a 1:1 power transformer which is used as a safety
precaution. Since the neutral wire of an outlet is directly connected
to ground, grounded objects near the device under test (desk, lamp,
concrete floor, oscilloscope ground lead, etc.) may be at a hazardous
potential difference with respect to that device. By using an
isolation transformer, the bonding is eliminated, and the shock hazard
is entirely contained within the device."
Why would things connected to the same ground have dangerous potential
differences from that ground?
Could some kind soul 'splain this to me, please?
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.
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
.
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