Hey Guys,
I've been poking around with my o'scope, and I now have a question. When I scope the "hot" mains, I have a nice (clean) sine wave. When I scope the neutral line, I have a very strange wave. I can not get a good photo of the wave, as it is blurry w/o a flash, but it looks like there is switching noise on the wave where it passes from positive to negative.
Does anyone know what this wave "should" look like?
Thanks,
Bryan
I've been poking around with my o'scope, and I now have a question. When I scope the "hot" mains, I have a nice (clean) sine wave. When I scope the neutral line, I have a very strange wave. I can not get a good photo of the wave, as it is blurry w/o a flash, but it looks like there is switching noise on the wave where it passes from positive to negative.
Does anyone know what this wave "should" look like?
Thanks,
Bryan
It's probably just a bunch of inducted noise, both high frequency, and 60Hz junk from the hot wire. I assume what you're talking about is a very low voltage signal. If not, you probably have a problem. If you set your scope's voltage range on about .1V and touch the probe to the tip of your finger, you should probably see a similar waveform.
The scope is set at .5V, with a 10X probe. When I touch the tip, I get a wave of similar amplitude, but tmuch more like a true sine wave compared to the neutral wave. The neutral wave has spikes at the crests of the wave, and where it switches polarity.....
My wires (neutral and hot) are twisted, so maybe it is inductive noise from the hot wire???
My wires (neutral and hot) are twisted, so maybe it is inductive noise from the hot wire???
Neutral "should" look like a nice straight line. And cheques should always arrive in the post next day...
In practice, the neutral is always messy. 5-10Vpk-pk of nasty, distorted, noisy, mess is not unusual. The amplitude you see is very dependent on measurement technique, and that's complex. I really wouldn't advise probing the mains at all. It's really dangerous. I had to do it once, and I thought long and hard about how I was going to do it. I had another qualified engineer watching closely as I explained what I intended to do, and I was quite ready to stop if he didn't agree with my proposed actions.
To sum up: It's dangerous and it doesn't tell you much. Don't do it. diyAudio gets 0.0001p (or is it 0.001p?) per member, so it can't afford members to kill themselves.
In practice, the neutral is always messy. 5-10Vpk-pk of nasty, distorted, noisy, mess is not unusual. The amplitude you see is very dependent on measurement technique, and that's complex. I really wouldn't advise probing the mains at all. It's really dangerous. I had to do it once, and I thought long and hard about how I was going to do it. I had another qualified engineer watching closely as I explained what I intended to do, and I was quite ready to stop if he didn't agree with my proposed actions.
To sum up: It's dangerous and it doesn't tell you much. Don't do it. diyAudio gets 0.0001p (or is it 0.001p?) per member, so it can't afford members to kill themselves.
Well, fortunately I am still alive!!!!
That being said, does this "junk" have any way of getting into the audio path?
Thanks,
Bryan
That being said, does this "junk" have any way of getting into the audio path?
Thanks,
Bryan
Yes. Unless your mains transformer has an electrostatic screen, there is a capacitor that couples it directly from the mains into your rectifiers. It's then luck as whether or not your filtering prevents it from getting into the audio.
I could be completely wrong, but I was under the impression that in the US the 'neutral' or 'cold' line is not grounded.
In the UK the neutral line is grounded, at the star point of the power station TX and also possibly at the consumers incomer (PME)
Even so, with lots of inductive loads (motors) etc, the neutral will still not be 'clean'
In the US I was led to believe that the 'pair' of hot and cold wires is floating, with a CT earth.
If this is the case, you would get all sorts of crap on the 'neutral' or 'cold' line
In the UK the neutral line is grounded, at the star point of the power station TX and also possibly at the consumers incomer (PME)
Even so, with lots of inductive loads (motors) etc, the neutral will still not be 'clean'
In the US I was led to believe that the 'pair' of hot and cold wires is floating, with a CT earth.
If this is the case, you would get all sorts of crap on the 'neutral' or 'cold' line
In Canada, the neutral is grounded at the terminal box. Probably the same for the US. A neutral line, grounded or not, carries currents that create voltages across the resistance of the neutral line; the higher the resistance (ie, the longer the neutral line), the higher the voltage. Those voltages unquestionably find their way into any device connected to it (read: every device).
In the U.S. neutral is bonded to ground at exactly one point. (in theory) That being said, I have a book on electrical wiring that says you can ground to your copper pipes if you have old wiring that lacks a ground wire.
X-10
Could be X-10 control signals, these use the neutral side, at zero cross, to transmit data for home automation, and the power supply companies also use a similar system for controlling sub-stations etc.
Between cycles, the whole national grid is just a network of dead copper, what better way to transmit data to substations!!
Could be X-10 control signals, these use the neutral side, at zero cross, to transmit data for home automation, and the power supply companies also use a similar system for controlling sub-stations etc.
Between cycles, the whole national grid is just a network of dead copper, what better way to transmit data to substations!!
(UK) I was under the impression that neutral was not connected to anything other than itself. AFAIK, the main supply usually originates from the sub-station transformer as a 3 phase supply, each phase supplying current to a load with the otherside of the (balanced) loads all connected togeather (neutral). I am pretty sure neutral and earth should not be connected in a domestic environment and the residual noise seen on neutral (which you should not be probing
) is due to load imbalance with the transients often resulting from the presence of switching power supplies.Dave (who really should know, having once worked for the national power company albeit at the other end of the wire
)The neutral in my home in the US goes back to the panel, and from the main panel both main cables along with IIRC, a bare neutral cable go underground from the main entry back to the transformer.
Where it leaves the main outside panel there's a four foot cooper rod driven into the ground that connects to the neutral in the main panel. It's purpose in life is to provide a good path to ground in the event of a lightning strike or in the event that the neutral fails.
In the US, three phase is only used in commercial or industrial environments.
Sure does.
How do you spell _isolation transformer_ ?
Regards
Ken L
Where it leaves the main outside panel there's a four foot cooper rod driven into the ground that connects to the neutral in the main panel. It's purpose in life is to provide a good path to ground in the event of a lightning strike or in the event that the neutral fails.
In the US, three phase is only used in commercial or industrial environments.
Bryan said:
Sure does.
How do you spell _isolation transformer_ ?
Regards
Ken L
Absolutely. Many touring rigs for concert and theater sound carry along 200A 3phase iso transformers and some derive thier own ground to prevent noice from local sources.
DRC said:
(UK) I was under the impression that neutral was not connected to anything other than itself. AFAIK, the main supply usually originates from the sub-station transformer as a 3 phase supply, each phase supplying current to a load with the otherside of the (balanced) loads all connected togeather (neutral). I am pretty sure neutral and earth should not be connected in a domestic environment and the residual noise seen on neutral (which you should not be probing
Dave (who really should know, having once worked for the national power company albeit at the other end of the wire
In the UK the system PME (Protective Multiple Earthing) is often used.
The designation for this IIRC (its been a long time since I went to college) is TN-C-S
Whereby the household earth (i.e. from householers dis-board) is connected to the neutral in the electricity board's incomer.
The electricity board's incomer in this instance does not have a seperate earth conductor (i.e. two wire//phase and neutral only)
The system of two wire with an earth rod (designated TT) is often used in rural areas.
On a three phase supply, the point to which all of the (hopefully) balanced loads are connected is the 'star point' this is sometimes 'floating' sometimes earthed.
At the substation the neutral and earth are connected to the 'star point' of the TX.
Forgive the novice question here, but now I'm confused and am hesitant when not 100% sur of myself.
If ground and neutral are at the same potential relative to each other (in theory), and we measure other potentials relative to that. What is the amplitude of the sine wave on the hot side? Ignoring the difference between rms and peak for a moment, if e say 120V AC, does that mean 120 * sin(wt + phi)? So the peak to peak difference is 240V (plus120 and minus120)?
Is there anyadvantage to balancing the setup -- have ground constant, and sinusoidally vary neutral and hot sides, but 180 degrees out of phase?
If ground and neutral are at the same potential relative to each other (in theory), and we measure other potentials relative to that. What is the amplitude of the sine wave on the hot side? Ignoring the difference between rms and peak for a moment, if e say 120V AC, does that mean 120 * sin(wt + phi)? So the peak to peak difference is 240V (plus120 and minus120)?
Is there anyadvantage to balancing the setup -- have ground constant, and sinusoidally vary neutral and hot sides, but 180 degrees out of phase?
I understand that -- that's why I said something like ignoring the difference btwn rms and peak -- guess I wasn';t clear. I understand the relationship between rms and amplitude for sinusoidal (or any kind) of distribution.
thank you for clarifying it though -- neutral is constant and hot is relative to neutral. so hot goes from +120*sqrt(2) to -120*sqrt(2) if the 120V mains is a perfect sine wave.
thank you for clarifying it though -- neutral is constant and hot is relative to neutral. so hot goes from +120*sqrt(2) to -120*sqrt(2) if the 120V mains is a perfect sine wave.
AC Current Measurment -- Fun With Units
Hi RIF,
An excellent attitude when dealing with lethal
power levels, and one that might help keep
you alive.
To answer your questions:
1) AC voltage is measured in units such that its
power is equivalent to the same value in DC volts.
This is nice, because it means
that in areas where AC or DC voltage can be
used (like tube filaments), one can use the same
value for either, and have the same amount
of power dissipated across resistors, filaments,
etc.
Thats why you hear folks can talk about running
filaments at 6.3VDC or 6.3VAC.
The units that allow this magic is RMS, which
is the PP (Peak-to-Peak) value divided by the
square root of two, which is about 1.4
As Bricolo pointed out, this means that the
V-PP value is about 1.4*VAC.
To answer your original question, the hot
line carries 120VAC = 12OV-RMS =
Sqrt(2)*120V-PP.
2) To answer your second question, there
can be an advantage to balancing the
setup, for it rejects noise--and that is
what power isolation transformers do.
For an article on the whys and how's of
this, check out:
http://www.plitron.com/Pages/news.htm
The article is the August 2003 one, currently
at the top of the page.
The nice thing is that, reconfigured
correctly, this power-line noise filter for
audio equipment can do double duty as
a ground isolation transformer--floating the
ground of the power on your
construction/test bench, and thus providing
another line of defense against shocks!
Anyway, I hope this helps!
Best,
George "Sparky" Ferguson
Hi RIF,
An excellent attitude when dealing with lethal
power levels, and one that might help keep
you alive.
To answer your questions:
1) AC voltage is measured in units such that its
power is equivalent to the same value in DC volts.
This is nice, because it means
that in areas where AC or DC voltage can be
used (like tube filaments), one can use the same
value for either, and have the same amount
of power dissipated across resistors, filaments,
etc.
Thats why you hear folks can talk about running
filaments at 6.3VDC or 6.3VAC.
The units that allow this magic is RMS, which
is the PP (Peak-to-Peak) value divided by the
square root of two, which is about 1.4
As Bricolo pointed out, this means that the
V-PP value is about 1.4*VAC.
To answer your original question, the hot
line carries 120VAC = 12OV-RMS =
Sqrt(2)*120V-PP.
2) To answer your second question, there
can be an advantage to balancing the
setup, for it rejects noise--and that is
what power isolation transformers do.
For an article on the whys and how's of
this, check out:
http://www.plitron.com/Pages/news.htm
The article is the August 2003 one, currently
at the top of the page.
The nice thing is that, reconfigured
correctly, this power-line noise filter for
audio equipment can do double duty as
a ground isolation transformer--floating the
ground of the power on your
construction/test bench, and thus providing
another line of defense against shocks!
Anyway, I hope this helps!
Best,
George "Sparky" Ferguson
What You describe here is known as "Balanced Line Power".
You can find lots of infos about that here:
http://www.equitech.com/
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