Forums

Current transformer

Started by Tim Williams November 30, 2011
Actually Jim, that's exactly the problem.

A high impedance load will work perfectly.

Note the comment about series resonance...

You've been spending too much time on ICs where the transmission lines are 
approximately the resistivity of nichrome and the inductivity of mud ;-)

Tim

P.S. Tonight I also enjoy "cooking" with wine ;-)

-- 
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

"Jim Thompson" <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote 
in message news:fs5fd7lhejl841nvs1q8h1rsg92ppq743i@4ax.com...
> 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 12/1/2011 6:55 PM, Tim Williams wrote:
> "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 >
Or you could just measure it.
On Thu, 01 Dec 2011 16:53:52 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>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,....
mounting holes... John
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
Some people make high-frequency CTs Julian Bergoz makes CTs up to 500 MHz. Metglas cores, secret recipes. http://www.bergoz.com/ Pearson makes amazing CTs, some up to a couple of hundred MHz, and some down to 0.15 Hz http://www.pearsonelectronics.com/ There are also Rogowski coils, with no core at all. The induced voltage is the derivative of the current being measured. They are generally not used with a burden resistor. We've made home-made CTs with ferrite cores, risetimes in the 10 ns sort of range. We used them for measuring the currents into drift step-recovery diodes. There are DC current transformers, too, interesting animals. http://www.gmw.com/electric_current/Danfysik/866_867/867.html John
On a sunny day (Thu, 01 Dec 2011 21:14:38 -0800) it happened John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in
<vcmgd7117hiv4sjm27tlks6ofat0f648en@4ax.com>:

>We've made home-made CTs with ferrite cores, risetimes in the 10 ns >sort of range. We used them for measuring the currents into drift >step-recovery diodes. > >There are DC current transformers, too, interesting animals. > >http://www.gmw.com/electric_current/Danfysik/866_867/867.html > >John
Don't forget my old trick to use tape recorder playback heads to get a very good linearity and waveform, as well as a good separation between primary and secondary. I have published pictures and diagram of that setup here in the past. If anyone wants to see it I can upload it again. From a few Hz to 20 kHz should be possible with that, and a LOT of output (many secundary turns). And cheap, not much work, glue the primary against the head gap, in the right direction tha tis. Works up to 10000 kA, just keep some distance. A winner!
On 2 Dec., 06:14, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> On Thu, 01 Dec 2011 14:19:18 -0600, Jon Elson <jmel...@wustl.edu> > wrote: > > > > > > > > > > >On 12/01/2011 09:05 AM, Jim Thompson wrote: > >> On Wed, 30 Nov 2011 22:11:45 -0800, Robert Baer > >> <robertb...@localnet.com> =A0wrote: > > >>> 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. =A0Ah=
well,
> >>>> such is basic research... > > >>>> Tim > > >>> =A0 =A0Huh??? > > >> Some people don't understand that the "burden" resistor value has a > >> maximum value before a current transformer goes crappy on you. > > >> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =
=A0 =A0 =A0 =A0...Jim Thompson
> >Also, typical current transformers are designed for low-frequency > >performance. =A0If the burden resistor is high, and there is a lot > >of HF voltage on the primary conductor, it will capacitively couple to > >the secondary. =A0I have heard of people making the secondary winding wi=
th
> >coaxial cable, with the shield connected only at the one end, of course, > >to get rid of this capacitive effect. =A0Tim mentions MHz in his origina=
l
> >post, so that could be the problem. > > >Jon > > Some people make high-frequency CTs > > Julian Bergoz makes CTs up to 500 MHz. Metglas cores, secret recipes. > > http://www.bergoz.com/ > > Pearson makes amazing CTs, some up to a couple of hundred MHz, and > some down to 0.15 Hz > > http://www.pearsonelectronics.com/ > > There are also Rogowski coils, with no core at all. The induced > voltage is the derivative of the current being measured. They are > generally not used with a burden resistor. > > We've made home-made CTs with ferrite cores, risetimes in the 10 ns > sort of range. We used them for measuring the currents into drift > step-recovery diodes. > > There are DC current transformers, too, interesting animals. > > http://www.gmw.com/electric_current/Danfysik/866_867/867.html > > John
I guess you could say a directional coupler is just a current and voltage transformer cleverly arranged -Lasse
"Martin Riddle" <martin_rid@verizon.net> wrote in message 
news: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...
Consider yourself bitch slapped ( ;-P
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. >
There are a lot of parts that can transform themselves from multi-terminal parts into 1-terminal parts. Usually after some smoke has wafted off. In really bad cases they can turn into a 0-terminal part. Usually after a lot of smoke has wafted away, or after a loud bang. -- Regards, Joerg http://www.analogconsultants.com/
On Wed, 30 Nov 2011 18:23:18 -0600, "Tim Williams" =
<tmoranwms@charter.net>
wrote:

>Would've been nice if I had known a current transformer isn't a very =
good=20
>transformer. Instead of two terminals, it acts more like a single=20 >terminal, with your poor burden resistor caught in the middle. Ah well,=
=20
>such is basic research... > >Tim
Say what? ?-)
"John Larkin"  wrote in message=20
news:vcmgd7117hiv4sjm27tlks6ofat0f648en@4ax.com...

> Some people make high-frequency CTs
> Julian Bergoz makes CTs up to 500 MHz. Metglas cores, secret recipes.
> http://www.bergoz.com/
I wonder if these are Hall effect, but there does not seem to be a=20 requirement for excitation voltage. Or maybe a Rogowski coil with = built-in=20 integrator? But I don't think that would work all the way from 0.5 Hz to = 500=20 MHz. And up to 20 kA.
> Pearson makes amazing CTs, some up to a couple of hundred MHz, > and some down to 0.15 Hz
> http://www.pearsonelectronics.com/
Those appear to be Hall Effect. They have a limited current range, and=20 accuracy depends on the excitation voltage. But they have a wide = frequency=20 range, which includes DC.
> There are also Rogowski coils, with no core at all. The induced > voltage is the derivative of the current being measured. They > are generally not used with a burden resistor.
These were used for a long time in the welding industry: http://www.iernc.com/manufactures/D/Duffers%20Scientific%20Inc/ and were adapted for use in circuit breaker testing: http://www.pstech-inc.com/cbtest.htm We still use the air-core CTs at www.etiinc.com, for test sets up to = 6000=20 amps nominal (60kA peak). We make our own, using phenolic core inductors = placed around the high current bus, and the instrumentation uses an RC=20 integrator to reproduce the current. There seems to be no theoretical = limit=20 to the frequency response or current range, but what we use has an = output of=20 about 100 mV at 1000 A. Since we need to measure current as low as 50 = amps,=20 and the integrator drops the voltage by a factor of about 20:1 at 60 Hz, = the=20 instrumentation needs to measure as low as 250 uV. At the upper end, 60 = kA=20 produces 6 volts. Here is more on Rogowski coils: http://www.dynamp.com/dynamp/LDADocum.nsf/c2270fbdd892ac3e86256e75000ad88= a/e710af6d3e0f6255862565d7004b19db/$FILE/Report.pdf http://www.dynamp.com (in case that link doesn't work) http://www.latest-science-articles.com/Technology_Science/Study-of-Sensor= -Theory-Centered-on-Rogowski-Coil-for-Heavy-Current-Measurement-A-23405.h= tml http://www.pacw.org/no-cache/issue/autumn_2007_issue/protection_rogowski/= rogowski_coil_designs/complete_article/1/print.html http://customcurrentprobes.com/ http://www.highcurrenttech.com/products.htm
> We've made home-made CTs with ferrite cores, risetimes in the > 10 ns sort of range. We used them for measuring the currents > into drift step-recovery diodes.
Here are high frequency CTs for PCB mounting, 20 kHz to 250 kHz. But not = in=20 the 10 nS range. http://www.triadmagnetics.com/catalog_template.php?productCategoryId=3D7 I have also used some of these for 60 Hz current measurement. They have = a=20 rather good range of current with accuracy if you use the correct burden = resistor: http://www.talema.net/en/products/ct.shtml The 100 amp version worked well from less than 1 amp to more than 1000 = amps.
> There are DC current transformers, too, interesting animals.
> http://www.gmw.com/electric_current/Danfysik/866_867/867.html
These are definitely Hall Effect. Paul www.pstech-inc.com=20