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Current transformer

Started by Tim Williams November 30, 2011
On a sunny day (Wed, 30 Nov 2011 20:40:20 -0600) it happened "Tim Williams"
<tmoranwms@charter.net> wrote in <jb6pg4$931$1@dont-email.me>:

>"admformeto" <admformeto@onet.eu> wrote in message >news:jb6n5i$u9u$1@dont-email.me... >> All transformers are current transformers, see the Faraday's lows of >> mutual induction. >> In practice it can be configured in current, voltage or power mode >> depending on application. > >News to Faraday, my transformer doesn't transform. At least, not very >well. At least, not within the first, about a microsecond. > >Simple example: go buy a bog standard Triad CT206, it rings like a bell at >a rather low frequency (a MHz or so)! > >In fact, I'm pretty sure it's acting as a toroidial resonator, which isn't >much heard of, helical resonators are more common. Same idea, but with >four quadrants. > >Tim
I appreciate your deep knowledge of the subject, however maybe I should not read about it, as it makes something that I always perceived as extremely simple, sooooo complicated. Current transformer: current_in / turns_ratio = current_out. (for a low load resistor or short). current_out x load_resistor = voltage_out. Of course there are some details, but those can usually be disregarded as the errors caused by those are very small. NORMALLY. One lesson: Do Not Leave Out The Load Resistor, as then Ze Foltage Kan Beacome Ferry High
On a sunny day (Wed, 30 Nov 2011 20:26:34 -0500) it happened "Martin Riddle"
<martin_rid@verizon.net> wrote in <jb6l4h$kfg$1@dont-email.me>:

>Let me help you.... >Here's the Jive Translation: > >Would've been supa' fine if ah' had knode some current transfo'ma' isn't >some very baaaad transfo'mer. Ah be baaad... Instead uh two terminals, >it acts mo'e likes some sin'le terminal, wid yo' poo' burden resisto' >caught in de middle. What it is, Mama! Ah sheeit, such be basic >research... > >Cheers
LOL
On Dec 1, 4:36=A0am, "Tim Williams" <tmoran...@charter.net> wrote:
> "Martin Riddle" <martin_...@verizon.net> wrote in message > > news:jb6qcc$cq8$1@dont-email.me... > > > Your using it as an air core transformer. Didn't Telsa invent something > > similar ;) > > It sure feels like it, but I'm pretty sure the core is still mu_r >> 100 > at whatever resonant frequencies these things are doing it at. =A0I'd cal=
l
> it a ferrite-loaded helico-toroidial resonator, or something ungainly lik=
e
> that. > > Since L is large, that means F is small (~MHz), and the impedance is > high -- sadly, the Q is also high, so the impedance (ESR, since it's a > series resonant equivalent) at the feedpoint (i.e. burden R) is very low. > > It's very interesting to rotate the "primary" around the core and watch > the nodes in the standing wave. =A0I should see about making an air-cored > version, driving it with RF and putting it in a low pressure neon > environment or something.
You seem to have failed to take into acount the parallel capacitances of the windings. No electronic component is pure - in the sense of presenting only resistive, capacitative or inductive impedance - and inductors/transformers are more imperfect than most. -- Bill Sloman, Nijmegen
On Dec 1, 6:11=A0am, Robert Baer <robertb...@localnet.com> wrote:
> Tim Williams wrote: > > Would've been nice if I had known a current transformer isn't a very go=
od
> > transformer. Instead of two terminals, it acts more like a single > > terminal, with your poor burden resistor caught in the middle. =A0Ah we=
ll,
> > such is basic research... > > > Tim > > =A0 =A0Huh???
maybe he connected the wrong kind of 0 terminal capacitor? NT
On Wed, 30 Nov 2011 22:11:45 -0800, Robert Baer
<robertbaer@localnet.com> wrote:

>Tim Williams wrote: >> Would've been nice if I had known a current transformer isn't a very good >> transformer. Instead of two terminals, it acts more like a single >> terminal, with your poor burden resistor caught in the middle. Ah well, >> such is basic research... >> >> Tim >> > Huh???
Some people don't understand that the "burden" resistor value has a maximum value before a current transformer goes crappy on you. ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
On Thu, 1 Dec 2011 03:56:42 -0800 (PST), NT <meow2222@care2.com>
wrote:

>On Dec 1, 6:11&#2013266080;am, Robert Baer <robertb...@localnet.com> wrote: >> Tim Williams wrote: >> > Would've been nice if I had known a current transformer isn't a very good >> > transformer. Instead of two terminals, it acts more like a single >> > terminal, with your poor burden resistor caught in the middle. &#2013266080;Ah well, >> > such is basic research... >> >> > Tim >> >> &#2013266080; &#2013266080;Huh??? > >maybe he connected the wrong kind of 0 terminal capacitor? > > >NT
We don't have a lot of 1-terminal parts on our schematics. A few, usually. John
On 12/01/2011 09:05 AM, Jim Thompson wrote:
> On Wed, 30 Nov 2011 22:11:45 -0800, Robert Baer > <robertbaer@localnet.com> wrote: > >> Tim Williams wrote: >>> Would've been nice if I had known a current transformer isn't a very good >>> transformer. Instead of two terminals, it acts more like a single >>> terminal, with your poor burden resistor caught in the middle. Ah well, >>> such is basic research... >>> >>> Tim >>> >> Huh??? > > Some people don't understand that the "burden" resistor value has a > maximum value before a current transformer goes crappy on you. > > ...Jim Thompson
Also, typical current transformers are designed for low-frequency performance. If the burden resistor is high, and there is a lot of HF voltage on the primary conductor, it will capacitively couple to the secondary. I have heard of people making the secondary winding with coaxial cable, with the shield connected only at the one end, of course, to get rid of this capacitive effect. Tim mentions MHz in his original post, so that could be the problem. Jon
On Thu, 01 Dec 2011 14:19:18 -0600, Jon Elson <jmelson@wustl.edu>
wrote:

>On 12/01/2011 09:05 AM, Jim Thompson wrote: >> On Wed, 30 Nov 2011 22:11:45 -0800, Robert Baer >> <robertbaer@localnet.com> wrote: >> >>> Tim Williams wrote: >>>> Would've been nice if I had known a current transformer isn't a very good >>>> transformer. Instead of two terminals, it acts more like a single >>>> terminal, with your poor burden resistor caught in the middle. Ah well, >>>> such is basic research... >>>> >>>> Tim >>>> >>> Huh??? >> >> Some people don't understand that the "burden" resistor value has a >> maximum value before a current transformer goes crappy on you. >> >> ...Jim Thompson >Also, typical current transformers are designed for low-frequency >performance. If the burden resistor is high, and there is a lot >of HF voltage on the primary conductor, it will capacitively couple to >the secondary. I have heard of people making the secondary winding with >coaxial cable, with the shield connected only at the one end, of course, >to get rid of this capacitive effect. Tim mentions MHz in his original >post, so that could be the problem. > >Jon
Indeed! Back in my GenRad days, when I was designing switchers (before there were switcher-specific chips :-), I was fond on running my current transformers into essentially a virtual ground. ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
On 12/01/2011 10:23 AM, John Larkin wrote:
> On Thu, 1 Dec 2011 03:56:42 -0800 (PST), NT<meow2222@care2.com> > wrote: > >> On Dec 1, 6:11 am, Robert Baer<robertb...@localnet.com> wrote: >>> Tim Williams wrote: >>>> Would've been nice if I had known a current transformer isn't a very good >>>> transformer. Instead of two terminals, it acts more like a single >>>> terminal, with your poor burden resistor caught in the middle. Ah well, >>>> such is basic research... >>> >>>> Tim >>> >>> Huh??? >> >> maybe he connected the wrong kind of 0 terminal capacitor? >> >> >> NT > > We don't have a lot of 1-terminal parts on our schematics. A few, > usually. > > John >
Test pins, antennas, Van de Graaf generators, cattle prods,.... Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
"Bill Sloman" <bill.sloman@ieee.org> wrote in message 
news:d7f1d4b0-c8ee-48d8-840d-d1276c73d4b3@p2g2000vbj.googlegroups.com...
> You seem to have failed to take into acount the parallel capacitances > of the windings. No electronic component is pure - in the sense of > presenting only resistive, capacitative or inductive impedance - and > inductors/transformers are more imperfect than most.
Indeed, effective parallel parasitic capacitance is a valuable concept. Sadly, it's just that, a concept -- the *actual* capacitance from end to end of, say, a solenoidal coil (i.e., as more advanced modelers call it, a helical resonator) is dramatically smaller than the turn-to-turn capacitance. Consider, if instead of a helix, you had a stack of rings. It's the same basic structure, except skewed by a turn, so the turns aren't turns, they're loops. Now ground ALL the rings, except for just the two ends. What is the capacitance between those two rings? The capacitance will not only be small due to distance, but almost entirely shielded by the turns inbetween them. When you unground them, all the intervening turns have their own capacitance, but it still doesn't even act as an ideal capacitive divider, because there is finite propagation delay along the structure (i.e., the speed of light) and because the interspersed turns have a comparable loading all their own (self capacitance to free space as well as "mutual capacitance" to adjecent turns). The same is true of the toroid, with the added boundary condition that the magnetic field must be equal at both ends -- in the helical resonator, they can be equal or opposite, allowing (N + 1) / 2 wave resonances; toroids only allow integer N. Speaking of which, it stands to reason that the bandwidth of this resonance should correspond to the evenness of the winding; if the leads are not at exactly 0 and 360 degrees (give or take the external reactance between them), the wave can be skewed by that many degrees, across the unwound portion of the core. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms