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James Nash
 
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Default Building a circuit with no power transformer ?

Please forgive a newbie question...

I've read in a number of places that a circuit should always have
transformer isolation from the wall AC, even if the circuit needs only
120V AC input. Then again, a number of people have told me that it
really doesn't matter. That the only reason to use a power transformer
is to step voltage up or down... if the circuit needs 120V AC, there's
no reason to use a transformer.

I'd really like to understand this issue, and I haven't thought of any
compelling advantages to having a transformer in the power supply path.
With audio transformers, I see how removing DC continuity can prevent
stray ground current, but I don't understand what isolation is gained by
using a 1:1 *power* transformer.

I've heard it said that if a power transformer is not used, that the
chassis of the device will be "hot." Does that make sense to anyone ?
If I'm using a 3-prong AC connection, I'm wiring my chassis to safety
ground, so as long as the wall ground connection is properly wired,
isn't it impossible for my chassis to have any voltage on it relative to
ground ? (And if the wall safety ground is miswired, doesn't any
circuit become potentially unsafe, regardless of its design ?)

If I use a 1:1 power transformer, and thus have no galvanic connection
to the wall hot and neutral, I still have high DC and AC voltages in my
circuit, and my chassis is still connected to the wall safety ground...
there are plenty of ways a user could get a serious shock by touching
things inside the device. And the chassis is only safe to touch if the
house AC ground is wired properly (otherwise a circuit fault or AC
polarity problem could result in voltage on the chassis) and if you
aren't simultaneously touching some other high voltage source (at which
point, the chassis just becomes a path to ground). This is true
regardless of whether or not there's a power transformer, right ?

The only safety advantage I can immediately see from the power
transformer is that it makes it impossible for DC currents to flow to
the neutral wall connection... is that a significant advantage, or am I
missing something here ?

Thanks for any explanation! There's probably a good explanation for the
convention of using 1:1 power supply transformers, and I'd really like
to understand it!

James

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Kurt Albershardt
 
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Default Building a circuit with no power transformer ?

James Nash wrote:

I've read in a number of places that a circuit should always have
transformer isolation from the wall AC, even if the circuit needs only
120V AC input. Then again, a number of people have told me that it
really doesn't matter. That the only reason to use a power transformer
is to step voltage up or down... if the circuit needs 120V AC, there's
no reason to use a transformer.


Electrical and fire codes, for one. Your life for another.



With audio transformers, I see how removing DC continuity can prevent
stray ground current, but I don't understand what isolation is gained by
using a 1:1 *power* transformer.


It isolates the chassis and knob shafts from the AC line. Reversing or
miswiring a power cord or pluggin into a miswired outlet or extension
cord, or plugging into an outlet with a ground fault is likely to kill
someone.


Power Solution used to sell a 2 kW class D bass amplifier, and it had a
GFI on the back panel to prevent stupidity. If you absolutely insist on
building something without a power transformer, at least do this...




  #3   Report Post  
Edvard Puskaric
 
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Default Building a circuit with no power transformer ?

On Wed, 22 Oct 2003 21:47:40 GMT, James Nash
wrote:


Thanks for any explanation! There's probably a good explanation for the
convention of using 1:1 power supply transformers, and I'd really like
to understand it!

James


Yes; it prevents a medical condition known as death.

Ed
  #4   Report Post  
Scott Dorsey
 
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Default Building a circuit with no power transformer ?

James Nash wrote:

I've read in a number of places that a circuit should always have
transformer isolation from the wall AC, even if the circuit needs only
120V AC input. Then again, a number of people have told me that it
really doesn't matter. That the only reason to use a power transformer
is to step voltage up or down... if the circuit needs 120V AC, there's
no reason to use a transformer.


If you don't care about noise or isolation from the line, and you can
sheild everything so that there is no way anybody can ever touch the
chassis, you can do hot chassis work. A lot of TV and radio gear was
this way.

I'd really like to understand this issue, and I haven't thought of any
compelling advantages to having a transformer in the power supply path.
With audio transformers, I see how removing DC continuity can prevent
stray ground current, but I don't understand what isolation is gained by
using a 1:1 *power* transformer.


MASSIVE isolation from line noise. Also, your secondary is now isolated
from the power line which prevents the chassis from floating at high voltages.

I've heard it said that if a power transformer is not used, that the
chassis of the device will be "hot." Does that make sense to anyone ?
If I'm using a 3-prong AC connection, I'm wiring my chassis to safety
ground, so as long as the wall ground connection is properly wired,
isn't it impossible for my chassis to have any voltage on it relative to
ground ? (And if the wall safety ground is miswired, doesn't any
circuit become potentially unsafe, regardless of its design ?)


There is a difference between being potentially unsafe (if both the plug
is miswired AND the transformer fails AND the ground is open, you get hurt),
and just plain unsafe (if any one of those three takes place, you get hurt).

The only safety advantage I can immediately see from the power
transformer is that it makes it impossible for DC currents to flow to
the neutral wall connection... is that a significant advantage, or am I
missing something here ?


That's another issue, but frankly just the RFI rejection from the transformer
acting as a line filter is worth the price of admission.

Thanks for any explanation! There's probably a good explanation for the
convention of using 1:1 power supply transformers, and I'd really like
to understand it!


Talk to any of the number of musicians who have been killed due to lifting
grounds on hot-chassis guitar amps.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."
  #5   Report Post  
henryf
 
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Default Building a circuit with no power transformer ?

James Nash wrote:
Please forgive a newbie question...

I've read in a number of places that a circuit should always have
transformer isolation from the wall AC, even if the circuit needs only
120V AC input. Then again, a number of people have told me that it
really doesn't matter. That the only reason to use a power transformer
is to step voltage up or down... if the circuit needs 120V AC, there's
no reason to use a transformer.


ATTENTION! Using an isolation transformer is a serious safety
matter -- a matter of life and death! If you are a hobbyist
building an electronic circuit using wall AC, you could kill
yourself or someone else by not using an isolation
transformer. DON'T TRY THIS AT HOME!

Pros can save money by designing circuits that can be
manufactured without an isolation transformer, but it would be
very dangerous for an amateur to try it, and even pros would
be wise to plug such equipment into an external isolation
transformer before working on it.

Here's the secret of why an isolation transformer gives you
some protection: because electrical current only flows if a
circuit is completed. Consider the paths that the electricity
follows with and without an isolation transformer.

If you just used the wall AC without an isolation transformer,
one lead is "neutral" and is connected to a ground near the
service entrance, such as a cold water pipe. Another lead is
"hot" and has 120 VAC relative to the neutral lead. (There
may also be a safety ground wire.) Since the neutral wire is
grounded somewhere, anything (such as you) connecting the hot
lead to a ground will complete the circuit, and current will
flow through it. If the current passing through you goes
through your heart, you could die.

A 1:1 isolation transformer still gives you two leads with 120
VAC relative to each other. The difference is that neither of
these is leads is electrically grounded. As long as your
circuitry doesn't ground a lead, neither lead is "hot"
relative to ground; they are only "hot" relative to each
other. So, accidentally touching either lead while some other
part of your body is grounded will not complete the circuit,
current doesn't flow through your heart, and you remain alive.

If you touch both leads, however, you'll still get shocked. A
good safety practice attributed to Tesla is to always keep one
hand in a pocket while working around dangerous voltages, to
reduce the risk of a fatal shock.



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S O'Neill
 
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Default Building a circuit with no power transformer ?



James Nash wrote:
If I use a 1:1 power transformer, and thus have no galvanic connection
to the wall hot and neutral, I still have high DC and AC voltages in my
circuit, and my chassis is still connected to the wall safety ground...
there are plenty of ways a user could get a serious shock by touching
things inside the device. And the chassis is only safe to touch if the
house AC ground is wired properly (otherwise a circuit fault or AC
polarity problem could result in voltage on the chassis) and if you
aren't simultaneously touching some other high voltage source (at which
point, the chassis just becomes a path to ground). This is true
regardless of whether or not there's a power transformer, right ?



If you have no power transformer, and everything is wired correctly (that's
putting a LOT of faith in electricians, from the wall outlet to the incoming
service panel over how many years?) you get what you're asking for.

But if the hot side of the AC ever leaks, in any way (that includes stray
capacitance, corrosion, who knows), to the outside world, the return path of the
circuit is ANY grounded point nearby. This is because the mains return is ground.

If you have a transformer, no current can flow that isn't going back inside the
unit to the transformer.

  #7   Report Post  
James Nash
 
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Default Building a circuit with no power transformer ?

In article ,
(Scott Dorsey) wrote:

I'd really like to understand this issue, and I haven't thought of any
compelling advantages to having a transformer in the power supply path.
With audio transformers, I see how removing DC continuity can prevent
stray ground current, but I don't understand what isolation is gained by
using a 1:1 *power* transformer.


MASSIVE isolation from line noise. Also, your secondary is now isolated
from the power line which prevents the chassis from floating at high voltages.


Ah... now that makes sense to me. I didn't think of that advantage.
Thanks for the thoughtful response, Scott!


I've heard it said that if a power transformer is not used, that the
chassis of the device will be "hot." Does that make sense to anyone ?
If I'm using a 3-prong AC connection, I'm wiring my chassis to safety
ground, so as long as the wall ground connection is properly wired,
isn't it impossible for my chassis to have any voltage on it relative to
ground ? (And if the wall safety ground is miswired, doesn't any
circuit become potentially unsafe, regardless of its design ?)


There is a difference between being potentially unsafe (if both the plug
is miswired AND the transformer fails AND the ground is open, you get hurt),
and just plain unsafe (if any one of those three takes place, you get hurt).


This I still don't understand, though... sorry if I'm being dense here.
This does make more sense to me with regards to older equipment with
only a 2-prong AC line... in that case, the chassis is referenced to the
wall neutral, right, so if the polarity is wrong, the chassis is now at
117 volts... very dangerous. And this situation is even worse if the
circuit is unbalanced, because then chassis ground is also audio
ground... that's why it's so easy to get a massive shock from an old
guitar amp, right ? If the AC polarity is wrong, you'll get high
voltage not only on the chassis but also right on the strings of your
guitar, so when your lips get too close to that conductive wind screen
on the SM58... yeah, I most certainly DO know what you're talking about!

But in most modern gear, the chassis is wired directly to safety ground,
so as long as the house ground wire has a decent reference, any voltage
on the chassis will flow directly to ground, possibly blowing a fuse or
tripping a breaker in the process, right ? Now, of course, there's
always the chance that the house ground is bad, or not even connected,
or lifted by unwise 3-to-2-prong AC adapter use. But in that situation,
I guess I don't see how the power transformer would help prevent
electrocution. As long as there's a path to ground somewhere, isn't
high voltage equally dangerous, even on the isolated side of a
transformer ? Maybe the point is that the power transformer limits the
current that can flow through it, even at high voltage, while the wall
voltage has virtually unlimited current capacity ? If that's the point,
then maybe having a low-current fuse in the transformer's stead could
accomplish a similar task ?

I hate to belabor this point, but if someone wouldn't mind explaining an
example situation in which electrocution could be prevented by the
insertion of a power isolation transformer, I'd really appreciate it.

(And to clarify why I'm asking these questions, two reasons. It's not
that I'm too stubborn or cheap to buy a power transformer :-) First,
it's because I've been unsuccessful in finding a 110V transformer that
will fit in the chassis of the power supply I'm working on. And second,
I like to understand the underlying concepts as best as I can, instead
of just blindly following the advice of people who clearly DO know more
than I do about electronics.)

Thanks again for taking the time to explain this stuff!

James

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Mike Rivers
 
Posts: n/a
Default Building a circuit with no power transformer ?


In article writes:

I've read in a number of places that a circuit should always have
transformer isolation from the wall AC, even if the circuit needs only
120V AC input. Then again, a number of people have told me that it
really doesn't matter. That the only reason to use a power transformer
is to step voltage up or down... if the circuit needs 120V AC, there's
no reason to use a transformer.


That's what manufacturers of inexpensive radios in the '50's thought
too. There were a lot of "AC-DC" radios that didn't have a power
transformer. I don't know if any records were kept about users being
electrocuted from these radios, but I'll bet there were a few.

Today, in the commercial world, we have Underwriters Laboratories here
and CE in Europe that tell us that you can't sell something
commercially that has any chance of having the hot side of the AC line
connected to anything conductive that a user might touch. This is why
we have "double insulated" tools and wall warts.

With audio transformers, I see how removing DC continuity can prevent
stray ground current, but I don't understand what isolation is gained by
using a 1:1 *power* transformer.


I've heard it said that if a power transformer is not used, that the
chassis of the device will be "hot." Does that make sense to anyone ?



One side (the neutral) of the AC power line is connected to ground at
the point where the power comes into the building. If you build a
transformerless piece of audio equipment, the neutral (hopefully) side
of the line will be connected to the equipment ground since this is
the reference point for the power supply. That's usually the chassis,
and even if it's in a plastic case, you have the shield of connectors
that's connected to the same point. If some boob manages to put the
plug in the outlet backwards, the hot side of the line will be
connected to the chassis. If you touch the chassis while you're
grounded (like standing in your bare feet on the basement floor),
you'll complete the path from the AC line to ground through your body
and most likely will have a shocking experience that you may or may
not live to tell about.

The secondary of the transformer is isolated from both sides of the
the power line, so it's safe to touch either side and ground and there
will be negligible current flowing through you.

If I'm using a 3-prong AC connection, I'm wiring my chassis to safety
ground, so as long as the wall ground connection is properly wired,
isn't it impossible for my chassis to have any voltage on it relative to
ground ? (And if the wall safety ground is miswired, doesn't any
circuit become potentially unsafe, regardless of its design ?)


If you connect the chassis to ground through the third wire, a short
between the hot side of the line and the chassis should result in a
blown fuse or circuit breaker. This is why it's called a "safety"
ground. But people (not you, of course) are inclined to do things like
lift the safety ground because it reduces hum.

If I use a 1:1 power transformer, and thus have no galvanic connection
to the wall hot and neutral, I still have high DC and AC voltages in my
circuit, and my chassis is still connected to the wall safety ground...
there are plenty of ways a user could get a serious shock by touching
things inside the device.


But the "hot" side of your transformer would no longer be connected to
the wire that's hot relative to the chassis ground.



--
I'm really Mike Rivers - )
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John LeBlanc
 
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Default Building a circuit with no power transformer ?


"James Nash" wrote in message
...

I hate to belabor this point, but if someone wouldn't mind explaining an
example situation in which electrocution could be prevented by the
insertion of a power isolation transformer, I'd really appreciate it.


James, if you make yourself knowledgeable about the power system in the USA it
might help you understand the value of isolating power from the phase source.

You obviously have an understanding of phase current, neutral and ground. If you
grab a 120v phase current source in one hand and neutral in the other, you
complete the circuit and kill yourself. If you grab 120v phase current and
ground, you complete the circuit and kill yourself.

If you go back in your post and read the number of times you used "but" and "if"
you can count up the number of times you may accidentally grab the phase current
and either neutral or ground (according to code, they should return to the same
spot in your electrical service panel) and kill yourself.

The use of an isolation transformer decouples the phase current from earth
grounding so that you cannot as easily complete the circuit accidentally and
kill yourself.

The second time I lifted the ground on a guitar amplifier was the last time for
me. (I'm a slow learner. Fortunately I had a second chance to make the same
stupid mistake.)


(And to clarify why I'm asking these questions, two reasons. It's not
that I'm too stubborn or cheap to buy a power transformer :-) First,
it's because I've been unsuccessful in finding a 110V transformer that
will fit in the chassis of the power supply I'm working on. And second,
I like to understand the underlying concepts as best as I can, instead
of just blindly following the advice of people who clearly DO know more
than I do about electronics.)


Others may not agree, but I'll separate electricity from electronics for the
simple reason that electricity is basically understood as votage potential,
load, and current flow. Electronics, while far more interesting to me, is less
likely to kill me than a poorly wired piece of gear plugged into a poorly wired
outlet.

I'm sure if you Googled the subject, you could spend the next month on it. Or
head to your local library and read. Of head to a Lowes of Home Depot and buy a
book on basic electricity. It's not a difficult subject to understand, but if
you're going to muck around with it, you're better off in the know.


Thanks again for taking the time to explain this stuff!


Don't know if I helped, but I know *I* feel better.

John


  #11   Report Post  
James Nash
 
Posts: n/a
Default Building a circuit with no power transformer ?

In article ,
"John LeBlanc" wrote:

If you grab a 120v phase current source in one hand and neutral in the other, you
complete the circuit and kill yourself. If you grab 120v phase current and
ground, you complete the circuit and kill yourself.


Makes sense to me.


The use of an isolation transformer decouples the phase current from earth

grounding so that you cannot as easily complete the circuit accidentally and
kill yourself.


I guess I don't fully understand the implications of decoupling here.
Are you saying that if I connect 120v phase current to one side of a 1:1
transformer primary and neutral or ground to the other side of the
primary, that the two sides of the secondary are somehow less lethal
than the original wall current that's connected to the primary ?

There's still 120v between the secondaries, so I guess I don't
understand why I wouldn't still get the same shock if I grabbed the two
sides of the secondary. And wouldn't there still be roughly 120v
between the higher voltage side of the secondary and either the wall's
neutral or earth ground ? Logically, as a general principle, wouldn't
there *have* to be at least 60v minimum potential difference between one
side of the secondary and *any* neutral or ground reference ?

I understand that there's no physical connection between the secondary
and the house current, but AC current is still induced through the
transformer, right ? There *must* be something here that I
fundamentally don't understand... if there's a 120v swing inside the
circuit, doesn't there exist a lethal potential between *some* places in
the circuit and *any* earth reference you choose ? Does the safety lie
in the fact that the transformer limits the amount of current that can
flow from the wall to the secondary ?

James

  #12   Report Post  
James Nash
 
Posts: n/a
Default Building a circuit with no power transformer ?

In article znr1066869692k@trad, (Mike Rivers)
wrote:

If I use a 1:1 power transformer, and thus have no galvanic connection
to the wall hot and neutral, I still have high DC and AC voltages in my
circuit, and my chassis is still connected to the wall safety ground...
there are plenty of ways a user could get a serious shock by touching
things inside the device.


But the "hot" side of your transformer would no longer be connected to
the wire that's hot relative to the chassis ground.


I guess this is what I fundamentally don't understand... what's the
difference ? If there's a 120v difference between the two sides of the
secondary, doesn't the chassis ground have some quantifiable voltage
potential in relation to the two sides of that secondary in spite of the
fact that there's no galvanic connection (at least, until someone starts
touching things inside the circuit unwisely, of course) ? And doesn't
at least one side of the secondary *have* to be at least 60v different
from chassis ground, no matter *what* the chassis is connected to ?

AC current flows through the transformer even though there's no galvanic
connection, so I guess I don't understand how a voltage on one side of
the transformer is any safer than a voltage on the other side. If
there's a potential difference between a tap on the secondary and earth
ground, and you touch both spots, won't current flow from the wall right
through the transformer, then through you to ground ?

Thanks again for taking the time to tutor me in electrical safety!

James

  #13   Report Post  
Justin Ulysses Morse
 
Posts: n/a
Default Building a circuit with no power transformer ?

James Nash wrote:

I guess I don't fully understand the implications of decoupling here.
Are you saying that if I connect 120v phase current to one side of a 1:1
transformer primary and neutral or ground to the other side of the
primary, that the two sides of the secondary are somehow less lethal
than the original wall current that's connected to the primary ?

There's still 120v between the secondaries, so I guess I don't
understand why I wouldn't still get the same shock if I grabbed the two
sides of the secondary. And wouldn't there still be roughly 120v
between the higher voltage side of the secondary and either the wall's
neutral or earth ground ? Logically, as a general principle, wouldn't
there *have* to be at least 60v minimum potential difference between one
side of the secondary and *any* neutral or ground reference ?

I understand that there's no physical connection between the secondary
and the house current, but AC current is still induced through the
transformer, right ? There *must* be something here that I
fundamentally don't understand... if there's a 120v swing inside the
circuit, doesn't there exist a lethal potential between *some* places in
the circuit and *any* earth reference you choose ? Does the safety lie
in the fact that the transformer limits the amount of current that can
flow from the wall to the secondary ?


I think I understand what you're asking, and I think I know the answer.
You hook up the 120VAC mains to the primary of a 1:1 power transformer.
The secondary has 120V across it. But no, there is no 120V circuit
that you can complete with your body between a secondary lead and a
primary lead. Well, there is, but it's only due to the inter-winding
capacitance of the transformer, which is small, and will not let very
much 60Hz current through at all. But the 120VAC *secondary* circuit
is ISOLATED from the primary circuit. They're not connected. So,
suppose your body is grounded (as it usually is). Suppose you touch
one of the leads on the transformer secondary by accident. Will you
get a shock? Probably. Will it kill you? Probably not. Now, suppose
you touch the hot lead on the primary. Much bigger shock. Because the
120VAC mains supply is referenced to "earth" and that's where you are.
But the transformer secondary is referenced to the other end of the
transformer secondary, which you are hopefully not connected to. Of
course, if the circuit ground is connected to the chassis also, then
you ARE connected to it.

On a fairly separate point from one of your other posts that I think
you had a grasp of but that warrants further discussion...
You intuitively suppose that with the chassis bonded to safety ground,
you're pretty much safe with or without a power transformer, and if the
safety ground is disconnected then you're pretty unsafe either way.
But the thing you need to remember is that the safety ground is NEVER
really ground. It's sitting on the other end of a long resistor (a
piece of copper wire) from "real" ground. So there's more than simple
"connected" and "disconnected" safety ground. It's always somewhere in
between. Hopefully it's sitting at a few tens of millivolts or less,
but it's not zero. In very many cases, the safety ground is not carried
by copper wire, but by glavanized steel conduit that goes through
perhaps dozens of junctions between the panel and the outlet. The
impedance of that ground connection is determined in large part by how
clean those junctions are and how tightly fastened they are. Some
appliances don't connect to the safety ground at all. But if you do,
you have to make sure you're protected from the possibility of that
ground having voltage on it.


ulysses
  #14   Report Post  
Justin Ulysses Morse
 
Posts: n/a
Default Building a circuit with no power transformer ?

James Nash wrote:


AC current flows through the transformer even though there's no galvanic
connection, so I guess I don't understand how a voltage on one side of
the transformer is any safer than a voltage on the other side. If
there's a potential difference between a tap on the secondary and earth
ground, and you touch both spots, won't current flow from the wall right
through the transformer, then through you to ground ?



No. AC current does not flow from the primary to the secondary, except
through parasitic capacitance. It's only a magnetic field that
connects the two sides. Magnetic fields don't electrocute you.

Try this: Take an ungrounded 12VAC wall wart and plug it into an
outlet. Now take an AC voltmeter and measure the voltage between one
of the wallwart output terminals and the powerline safety ground. How
many volts would you expect to see? 120 or 12? Or somewhere in
between? I just tried it, and I got about 0.8 volts. That small
voltage is only there because of interwinding capacitance, and even
that would disappear if the voltmeter had a lower impedance. If you
measured between the wallwart output and the powerline hot, you'd get
about the same thing (I got about 2.3V). Even though one is at 120V
and one is at 12V, there is no real voltage between them.

ulysses
  #15   Report Post  
Scott Dorsey
 
Posts: n/a
Default Building a circuit with no power transformer ?

James Nash wrote:

I guess I don't fully understand the implications of decoupling here.
Are you saying that if I connect 120v phase current to one side of a 1:1
transformer primary and neutral or ground to the other side of the
primary, that the two sides of the secondary are somehow less lethal
than the original wall current that's connected to the primary ?


Yes, because they are not referenced to the building ground.

There's still 120v between the secondaries, so I guess I don't
understand why I wouldn't still get the same shock if I grabbed the two
sides of the secondary. And wouldn't there still be roughly 120v
between the higher voltage side of the secondary and either the wall's
neutral or earth ground ? Logically, as a general principle, wouldn't
there *have* to be at least 60v minimum potential difference between one
side of the secondary and *any* neutral or ground reference ?


You are always in some contact with the building ground reference.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."


  #16   Report Post  
Mike Rivers
 
Posts: n/a
Default Building a circuit with no power transformer ?


In article writes:

I guess this is what I fundamentally don't understand... what's the
difference ? If there's a 120v difference between the two sides of the
secondary, doesn't the chassis ground have some quantifiable voltage
potential in relation to the two sides of that secondary in spite of the
fact that there's no galvanic connection (at least, until someone starts
touching things inside the circuit unwisely, of course) ? And doesn't
at least one side of the secondary *have* to be at least 60v different
from chassis ground, no matter *what* the chassis is connected to ?


Grounding the chassis can't protect idiots and those really careless
from getting shocked. You're correct that with one side of the power
supply going to earth through the safety ground, you can indeed get
shocked if you stand in your bare feet on a reasonably well grounded
surface and touch the hot lead of the power supply.

What grounding the chassis does is assures that if something goes
wrong inside the box, like a component shorts out and connects the HOT
side of the power supply to the chassis, this will short out the power
supply and hopefully blow a fuse before doing any other damage. If it
was possible for the hot side to get connected to the chassis
inadvertently, by touching the chassis, you could become the wire that
completes the circuit from the hot side (which is now connected to the
chasis) to earth. This is why it's not a good idea to lift a safety
ground.



--
I'm really Mike Rivers - )
  #17   Report Post  
Mike Rivers
 
Posts: n/a
Default Building a circuit with no power transformer ?


In article writes:

I think I understand what you're asking, and I think I know the answer.
You hook up the 120VAC mains to the primary of a 1:1 power transformer.
The secondary has 120V across it. But no, there is no 120V circuit
that you can complete with your body between a secondary lead and a
primary lead. Well, there is, but it's only due to the inter-winding
capacitance of the transformer, which is small, and will not let very
much 60Hz current through at all. But the 120VAC *secondary* circuit
is ISOLATED from the primary circuit. They're not connected.


James has a legitimate questions. Since the low side of the power
supply is normally signal ground, which in most cases is also chassis
ground, and with a safety ground, therefore earth ground, there IS a
possible path between the hot side of the power supply and earth
ground. Haven't you ever measured voltages around a tube socket by
clipping the negative lead of the voltmeter to the chassis? No
difference (assuming a good safety ground) between that and clipping
it to a ground rod outside the window.

The safety ground is there for one reason, and that's to assure that
if for some reason the hot side of the power supply (or the power
line, in the case of a transformerless unit) does manage to get
connected to the chassis, it will find its way back to ground through
a lower impedance path than through someone who touches the chassis
while it's in that state. Presumably a fuse will blow before long (or
the power cord will melt, or if it's bigger than the wiring in the
house, the house will catch fire).



--
I'm really Mike Rivers - )
  #18   Report Post  
S O'Neill
 
Posts: n/a
Default Building a circuit with no power transformer ?

One additional point, when discussing "hot chassis" circuits (without a power
transformer) is that no assumptions can be made about which side of the line IS
grounded. The code is specific (White wire, wide blade, silver screw, etc), but
this relies on everyone (electrician or not) who ever worked on the wiring, from
the outlet to the power pole, to do meticulous work. It's probably not a good
idea to bet your life on this.

Mike Rivers wrote:
In article writes:


I guess this is what I fundamentally don't understand... what's the
difference ? If there's a 120v difference between the two sides of the
secondary, doesn't the chassis ground have some quantifiable voltage
potential in relation to the two sides of that secondary in spite of the
fact that there's no galvanic connection (at least, until someone starts
touching things inside the circuit unwisely, of course) ? And doesn't
at least one side of the secondary *have* to be at least 60v different
from chassis ground, no matter *what* the chassis is connected to ?



Grounding the chassis can't protect idiots and those really careless
from getting shocked. You're correct that with one side of the power
supply going to earth through the safety ground, you can indeed get
shocked if you stand in your bare feet on a reasonably well grounded
surface and touch the hot lead of the power supply.

What grounding the chassis does is assures that if something goes
wrong inside the box, like a component shorts out and connects the HOT
side of the power supply to the chassis, this will short out the power
supply and hopefully blow a fuse before doing any other damage. If it
was possible for the hot side to get connected to the chassis
inadvertently, by touching the chassis, you could become the wire that
completes the circuit from the hot side (which is now connected to the
chasis) to earth. This is why it's not a good idea to lift a safety
ground.



--
I'm really Mike Rivers - )


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