Current transformer

Or you could just measure it.

mounting holes...

John

Some people make high-frequency CTs

Julian Bergoz makes CTs up to 500 MHz. Metglas cores, secret recipes.

Pearson makes amazing CTs, some up to a couple of hundred MHz, and some down to 0.15 Hz

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.

John

On a sunny day (Thu, 01 Dec 2011 21:14:38 -0800) it happened John Larkin wrote in :

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!

good

well,

=A0 =A0 =A0 =A0...Jim Thompson

th

l

I guess you could say a directional coupler is just a current and voltage transformer cleverly arranged

-Lasse

Consider yourself bitch slapped ( ;-P

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/

good=20

=20

Say what?

?-)

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.

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.

These were used for a long time in the welding industry:

and were adapted for use in circuit breaker testing:

We still use the air-core CTs at

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:

$FILE/Report.pdf (in case that link doesn't work) tml

Here are high frequency CTs for PCB mounting, 20 kHz to 250 kHz. But not = in=20 the 10 nS range.

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:

The 100 amp version worked well from less than 1 amp to more than 1000 = amps.

These are definitely Hall Effect.

Paul

I met Julian once, big burly funny not-typically-French guy. He likes to allude to his tricks without giving things away entirely. He did say that some of his core materials have u around a million. He mostly makes real CTs, not Hall things. Halls are slow, I think, and not very stable.

I have a Pearson current sensor that works from below 1 Hz to many KHz. It's a CT with an internal burden resistor, and doesn't need power.

There are Hall effect CTs, open-loop and zero-flux, but the Danfysic units aren't Halls. They are compound devices using two toroidal cores and both AC and second-harmonic DC feedback. I think they have app notes that explain the theory. Or google dcct .

The Danfysik units are amazing... stunning linearity and PPM zero offsets.

John

Interesting. The link on the GMW site for more information is broken:

Probably an iron core of some type?

I think this describes them, known as "integrated parametric current=20 transformers":

27&Itemid=3D34 .pdf

Interesting range of devices here:

There were lots of simplified explanations, but this seems to be rather=20 detailed (and apparently a rather old scan of a typewritten document):

And this gives details of the instrumentation:

Thanks for the information.

Paul John=20

Yes, I meant to say CST- earlier. These things ring strongly at two nearby frequencies, making a sort of AM-DSB-SC waveform at a MHz or so. Impossible to filter sharply with just RCs, it's not merely HF trash. When they say 250kHz, they *mean* 250kHz, because you aren't getting any useful output beyond that.

When you're building a power supply that *starts* at 250kHz, you find you have to spend like gangbusters, or wind custom.

Tim

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

As I noted, we just did a few turns around a ferrite toroid. We were driving a DSRD, just a power diode that happens to accidentally have the right diffusion profile. We slammed 48 volts across it in the forward direction until the current built up to 50 amps or so, then reverse biased it from a 400 volt source in series with a bit of inductance. It conducts in the reverse direction for 50 ns or so, then snaps open. Big voltage spike. The CT really helped us see what was going on.

Sometimes it's better to use a current shunt, and couple that back down to ground with a transmission-line transformer. Or just a shunt and a TPS2024.

John

"John Larkin" wrote in message news: snipped-for-privacy@4ax.com...

See, that'll work -- I've seen tens-of-turns toroids used in RF equipment for things like VSWR meters, matching transformers, plain old inductors and RFCs, etc. Most times, the resonance is sufficiently out of band, or has the wrong impedance or Q to cause any trouble.

The trouble comes when you're doing a high ratio, like if you want over

100 turns, evenly distributed around the core, so you aren't burning a hundred watts in the poor burden resistor under whatever load it is you're testing. That needs a progressive wind at least, if not more creative measures.

Even so, a couple turns probably had a little ringing when that thing snapped off. Though that'll be as true with the rest of the circuit. Without a perfectly smooth RF signal path, you get reflections off everything. As I recall, the waveform on your website still has some trash on it, but only a few volts bounce, not at all bad when you're making a 20kW pulse into 50 ohms.

There are, of course, coaxial resistors for that sort of thing. Or whatever those fancy swirls you're etching into manganin are doing.

Tim

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

Tim Williams a écrit :

The problem is that if your primary side conductor is quite small WRT your core diameter, the small leakage inductance that you have on the primary side is multiplied by N^2 on the secondary side, which can be quite a lot for high turns ratios. It's that inductance that resonate with the secondary parasitics. The first thing is to reduce as much as possible that empty space.

--
Thanks,
Fred.

a

me

, t

I once amused myself by splitting the notional 1pF parallel capacitance of a single layer winding into two 2pF capacitors across two - linked - mutual inductances in series, fudging the two series inductances and the coupling to come out at roughly the original single inductance.

It was all very rough and I can't remember the numbers. What I can remember is that I might as well not have bothered.

Once you've got deep enough into high frequencies that the propagation delay along the solenoid matters, you are obviously into a different ball-park, but I was a bit surprised how useful the simple lumped component model was in my application.

-- Bill Sloman, Nijmegen

Oh, if you've got the right model and the system is reasonable (possibly, it must be non-dispersive), it even works all the way up to the first resonance. This is true of 1/4 and 1/2 wave resonators, in coax / stripline form, and still reasonable for microstrip (which is dispersive). Probably because the infinnitessimal circuit is a scaled down model of the same layout. Take a chunk of coax and it looks like series L and parallel C on almost any scale (limited by diameter, when higher modes take over); Z_o = sqrt(L/C) and F_res = 1 / (2*pi*sqrt(L*C)) are always true.

I don't think it applies for helical and thin toroidial resonators, because the calculated effective parallel capacitance and inductance vary with frequency (going to extreme values at resonance, of course); and this is going to be a consequence of the infinnitessimal model, which involves mutual inductances and chains of capacitors, producing dispersive modes that are difficult to model in the same way.

Tim

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

short).

very small.

Why? I dont understand why one cant simply use the open ckt voltage on the secondary as a measure of the primary current. Heres my argument:

B = prim. ct. * I(geometry-dependent stuff) (Biot Law); (I() => integral) flux = B * A; A => area Vsec = d(coupled-flux)/dt

For sinusoidals therefore, the open circuit is proportional to the prim. current. If you consider the ct. transformer as a coupled inductor, then the actual calculations are quite simple.

Right?

vkj

--------------------------------------- Posted through

Because a current transformer is normally in the 1000:1 range. Do you really want to try to measure 120,000 volts?

(also: this is why it's important to hook up the load resistor *before* you put the current transformer on the primary wire)