The causes of DC appear to be many and varied. A few of the easier to appreciate causes that can occur within an LV distribution network (ie. electric utility HV to LV distribution transformer out to all the residences/buildings/facilities served by the supply) are:
- half-wave rectifiers.
- imbalanced rectification (ie. a full-wave rectifier with different winding resistances, or diode resistances).
- in-rush waveform to inductive equipment (ie. peaks occur on one 'side' resulting in an imbalance).
- half-wave rectifiers.
- imbalanced rectification (ie. a full-wave rectifier with different winding resistances, or diode resistances).
- in-rush waveform to inductive equipment (ie. peaks occur on one 'side' resulting in an imbalance).
trobbins: those load-related causes are theoretical. For any of those things to affect the waveform at your device, they would almost certainly have to occur within the same circuit or at the very least your house. The cable resistances would make it almost impossible for you to be affected by any of those events happening on the next street.
And inrush is a momentary event, the best it can do is cause a condition for a few seconds.
Yes, a meteorite can destroy Earth within our lifetime. No, I'm not building a shelter 🙂
And inrush is a momentary event, the best it can do is cause a condition for a few seconds.
Yes, a meteorite can destroy Earth within our lifetime. No, I'm not building a shelter 🙂
Maybe I misunderstood, but there seemed to be some association made between audible signal hum and a hypothetical presence of DC.
I used to use a NAIM NAIT that had an integral 3-wire AC cord. In my first house, about 22 years ago, I initially used a 'ground-lifting' cord adapter because the house had ungrounded outlets. It sounded fine. I installed a 3-conductor AC outlet & rigged up some kind of ground...and had horrible hum. I eventually put an 8-foot copper-clad steel ground rod outside and connected the power company's puny token ground rod to that.
Couldn't get rid of the hum. I went back to the ground-lifting AC cord adapter.
Eventually, we upgraded our electrical service from the old 60A service with screw-ib Edison-base fuses, to 150A with breakers.
The electrician told me the 'drop' from the power pole also had to be replaced, and the ground conductor was almost completely corroded somewhere...after 22 years I don't remember or we didn't even discuss it.
The hum was gone and I could use the grounded AC cord. I never would have suspected that.
The isolation transformer humming, I'm sure you realize, is humming for a different reason...magnetostriction. The laminations vibrate at the frequency of the AC supply because they are subject to mechanical forces from the AC flux.
Some engineering textbooks include transformers in a chapter called AC machines, along with motors.
Obviously, motion is not desirable but the physics produces a tendency to respond to the forces induced...and some effort is put into reducing magnetostriction noise.
But not all transformers have to be quiet as audio gear transformers. That would just make them cost more for applications that don't require it.
As far as toroids humming in some applications, I noted on a datasheet for some toroidal power transformers that half-wave rectification was not to be used.
That imposes a DC component onto the power transformer secondary, and thus some DC flux in the core.
This happens with EI and other stacked-flat-lamination transformers. The VA rating depends on the type of rectification, when used.
Rectifiers in combination with filter capacitors (and probably inductors, but I have not personally read anything related to power factor and conducted emissions back into the power line) alter the power factor cosine of phase angle between AC voltage and current). Associated with that are other effects like harmonic distortion components subject to regulation (in commercially-manufactured equipment).
Power factor correction circuitry (active and passive) aims to reduce those undesirable effects...usually only to the extent required to get the product through any requirements.
In plain language, sometimes there there is a lot of crap on the power line.
DC is not at the top of the list. I don't think DC even appears in restrictions on distortion limits. It may effectively be considered non-existent.
I used to use a NAIM NAIT that had an integral 3-wire AC cord. In my first house, about 22 years ago, I initially used a 'ground-lifting' cord adapter because the house had ungrounded outlets. It sounded fine. I installed a 3-conductor AC outlet & rigged up some kind of ground...and had horrible hum. I eventually put an 8-foot copper-clad steel ground rod outside and connected the power company's puny token ground rod to that.
Couldn't get rid of the hum. I went back to the ground-lifting AC cord adapter.
Eventually, we upgraded our electrical service from the old 60A service with screw-ib Edison-base fuses, to 150A with breakers.
The electrician told me the 'drop' from the power pole also had to be replaced, and the ground conductor was almost completely corroded somewhere...after 22 years I don't remember or we didn't even discuss it.
The hum was gone and I could use the grounded AC cord. I never would have suspected that.
The isolation transformer humming, I'm sure you realize, is humming for a different reason...magnetostriction. The laminations vibrate at the frequency of the AC supply because they are subject to mechanical forces from the AC flux.
Some engineering textbooks include transformers in a chapter called AC machines, along with motors.
Obviously, motion is not desirable but the physics produces a tendency to respond to the forces induced...and some effort is put into reducing magnetostriction noise.
But not all transformers have to be quiet as audio gear transformers. That would just make them cost more for applications that don't require it.
As far as toroids humming in some applications, I noted on a datasheet for some toroidal power transformers that half-wave rectification was not to be used.
That imposes a DC component onto the power transformer secondary, and thus some DC flux in the core.
This happens with EI and other stacked-flat-lamination transformers. The VA rating depends on the type of rectification, when used.
Rectifiers in combination with filter capacitors (and probably inductors, but I have not personally read anything related to power factor and conducted emissions back into the power line) alter the power factor cosine of phase angle between AC voltage and current). Associated with that are other effects like harmonic distortion components subject to regulation (in commercially-manufactured equipment).
Power factor correction circuitry (active and passive) aims to reduce those undesirable effects...usually only to the extent required to get the product through any requirements.
In plain language, sometimes there there is a lot of crap on the power line.
DC is not at the top of the list. I don't think DC even appears in restrictions on distortion limits. It may effectively be considered non-existent.
dgta, it may well depend on the LV network. I have 3ph distribution here, where the neutral is connected at the HV-LV distribution transformer, and the neutral is distributed as a separate conductor with the 3 phase conductors. Nearly all domestic user premises just take in one phase and neutral. The neutral is therefore not clean, and handles all imbalanced phase currents including their harmonics. At the users equipment point of reference, the neutral includes voltage drop contributions from both within the premises and along the common neutral back to the distribution transformer. Yes all the neutral link impedances are very low, but obviously it depends on how far from the HV/LV transformer you are and how significant the imbalance contributors between your equipment and the transformer.
Everyone has a different set of circumstances, and a different set of neighbours, and probably a different LV distribution scheme. Utility regulations and most industrial/commercial sites are much better now than in decades past, but still some people with toroidal transformers do seem to be plagued - whether that is from dc imbalance or other subtleties with their equipment would come down to individual assessments like for the OP in this thread.
Yes in-rush is a transient occurrence - but can contribute to dc imbalance, albeit a 'varying' dc.
PS. one of the hassles with appreciation is that there is no 'dc battery' causing the 'dc' that generically gets thrown around in discussion forums, but rather an unbalance in all the currents flowing along the neutral segments, and that can be difficult to visualise.
Everyone has a different set of circumstances, and a different set of neighbours, and probably a different LV distribution scheme. Utility regulations and most industrial/commercial sites are much better now than in decades past, but still some people with toroidal transformers do seem to be plagued - whether that is from dc imbalance or other subtleties with their equipment would come down to individual assessments like for the OP in this thread.
Yes in-rush is a transient occurrence - but can contribute to dc imbalance, albeit a 'varying' dc.
PS. one of the hassles with appreciation is that there is no 'dc battery' causing the 'dc' that generically gets thrown around in discussion forums, but rather an unbalance in all the currents flowing along the neutral segments, and that can be difficult to visualise.
Last edited:
The ground rods are your responsibility, not DTE's. They need to conform to the electrical code, which currently requires 2 rods. But code is never retroactive, unless it was a violation at the time of original installation. So the small rod may have been installed by an electrician and may or may not have been required by code at the time.
any EI traffo will have three air gaps, the toroid just one....
so the effects of dc on toroid is more of a concern that with EI...
current distortions in the secondary as a result of rectification is unavoidable...
noise can be coupled to the primary and thence to the power lines, so that the EMI/RFI filters worked both ways....
the EU mandates power factor corrections for appliances, even passives...
so the effects of dc on toroid is more of a concern that with EI...
current distortions in the secondary as a result of rectification is unavoidable...
noise can be coupled to the primary and thence to the power lines, so that the EMI/RFI filters worked both ways....
the EU mandates power factor corrections for appliances, even passives...
Do you have 3-wire outlets in the house? Line, neutral and ground? Are neutral and ground connected together at your house panel? Is that then connected to a ground rod? Is neutral grounded to Earth at the power company's transformer?
To the OP.
A two diode drop and a couple of caps in parallel.
I find the circuit wanting. It will not block DC and does not address the problem of third harmonic power line distortion from CFLs, Computers, wall warts, LED bulbs, and almost all modern electronics.
Get a ferroresonant transformer and be done with it.
A two diode drop and a couple of caps in parallel.
I find the circuit wanting. It will not block DC and does not address the problem of third harmonic power line distortion from CFLs, Computers, wall warts, LED bulbs, and almost all modern electronics.
Get a ferroresonant transformer and be done with it.
dgta - yes it is TN-C-S here, so we are luckier than some, such that the impedance from our domestic residence switchboard neutral MEN link back to the LV transformer neutral is lowered by all the earth bonding.
I don't operate toroids so its never been an issue for me, although for one item of vintage test equipment (HV breakdown tester) that uses 'earth' as the breakdown path, the noise on the neutral is a hassle and requires the use of a local isolation transformer to provide a local MEN link - but that is off-topic here.
I don't operate toroids so its never been an issue for me, although for one item of vintage test equipment (HV breakdown tester) that uses 'earth' as the breakdown path, the noise on the neutral is a hassle and requires the use of a local isolation transformer to provide a local MEN link - but that is off-topic here.