Quick question about current transformers

[fieldwrangler]

Folks -

I remember seeing some excellent information supplied here (in the thread started by Joseph Rogowski, as I recall) on ultra-low-Z designs, and find one thing somewhat confusing.

Many commercially available high-ratio current transformers have the low side feed realized as a conductor passing through the center of what I assume to be a toroidal core for the high side windings. Since these units are generally sealed, I'm guessing at the internal orientation. Are these windings wound concentric to the pass-through or to the core center? My gross understanding of induction tells me that the core-centric version should be more efficient due to flux coupling but I'm not sure; also I'm not sure how this sort of winding would be done in production.

Bob Palmieri

[Mike Sulzer]

Hi Bob,

Think of the conductor that passes through and completes its circuit somewhere as a single turn around the core. It actually is just that, and if you wanted to make that more apparent, you could bend the conductor so that it seems more like a turn. Then it is more obvious that the secondary is just like that of any other toroidal xformer, wound around the core. The difference between a current transformer and a regular one is in the application rather than how it functions. It generally is inserted in series with a circuit and drops insignificant voltage. When used to step up the voltage of a very low impedance pickup, you can think of it (if you want) as a voltage transformer, one that works with very low voltage and impedance at the primary and has a very large step up ratio.

Folks -

I remember seeing some excellent information supplied here (in the thread started by Joseph Rogowski, as I recall) on ultra-low-Z designs, and find one thing somewhat confusing.

Many commercially available high-ratio current transformers have the low side feed realized as a conductor passing through the center of what I assume to be a toroidal core for the high side windings. Since these units are generally sealed, I'm guessing at the internal orientation. Are these windings wound concentric to the pass-through or to the core center? My gross understanding of induction tells me that the core-centric version should be more efficient due to flux coupling but I'm not sure; also I'm not sure how this sort of winding would be done in production.

Bob Palmieri

[fieldwrangler]

Thanks for this; not sure what I'd do without you guys that have a much deeper understanding of this stuff than I.

So, viewing the transformer in this way am I correct in thinking that the internal windings are wound "core-centric" (that is, around the core of a cylinder that is then bent into a toroidal configuration with the "doughnut hole" open to the outside world?)

[Mike Sulzer]

Thanks for this; not sure what I'd do without you guys that have a much deeper understanding of this stuff than I.

So, viewing the transformer in this way am I correct in thinking that the internal windings are wound "core-centric" (that is, around the core of a cylinder that is then bent into a toroidal configuration with the "doughnut hole" open to the outside world?)

Exactly. That is a good way to think about it.

[bbsailor]

Thanks for this; not sure what I'd do without you guys that have a much deeper understanding of this stuff than I.

So, viewing the transformer in this way am I correct in thinking that the internal windings are wound "core-centric" (that is, around the core of a cylinder that is then bent into a toroidal configuration with the "doughnut hole" open to the outside world?)

Bob, Mike is correct in viewing the current transfomer like a voltage transformer, however on the primary you need to think very low resistance for the primary wire loop going through the toroid center hole. The final output impedance of the toroid secondary will be determined by the primary loop resistance. AWG 11 wire is about 100 microohms per inch and with a primary loop about 6 inches long is 600 microohms. If the toroid has 1000 turns, then the output will be in the 600 ohm range. To calculate: multiply the primay loop resistance by the turns ratio of the current transformer squared. If this is too high, then use a heavier wire like AWG 6 which is about 35 microohms per inch or about 210 ohms to better match a mic input impedance or the input to a Shure A95U mic matching transformer.

The key to working with this current transformer concept in making guitar pickups is to consider the on board pickup design that produces an output in the 150 to 300 ohm range to feed a transmission line to match available low impedance input impedances on the mixer or matching transformer located close to the guitar amp. From my experience, using thicker primary loop wire tends to favor the lower frequencies better.

This is fun stuff to play with because making wire gauge changes is fast and easy to do. Remember, that in order to produce more current on the primary requires the lowest resistance primary winding. Obtain some thin wall copper tubing that matches your wire diameter to join the primay loop together for a good, low resistance joint. Try silver solder for final assemblies.

Joseph Rogowski

[fieldwrangler]

Joseph -

Thanks for real. I think I'm gonna need to expand my soldering skills (and possible gear) in order to feel confident about not throwing away primary potential.

I recall that you gave a good explanation of inferring the primary characteristics from the more realistically readable values seen in the secondary circuit in these applications; I intend to make good use of this information.

Bob Palmieri

[bbsailor]

Joseph -

Thanks for real. I think I'm gonna need to expand my soldering skills (and possible gear) in order to feel confident about not throwing away primary potential.

I recall that you gave a good explanation of inferring the primary characteristics from the more realistically readable values seen in the secondary circuit in these applications; I intend to make good use of this information.

Bob Palmieri

Bob,

The Extech LCR meter is very good for seeing the effect of making the low impedance string loop on the CT output impedance. Make sure that the CT output impedance, with the primary string loop installed, is between one tenth to one fifth of the input impedance of either the mic mixer input impedance (typically between 1500 to 2000 ohms) or the mic matching transformer (Shure A95U, or equivalent) which is near 3000 ohms at 1000 Hz. This will limit the turns ratio to between 500 turns to 1000 turns to keep the final output impedance of the CT between 150 to 300 ohms. If the source impedance is too much higher than either one fifth to one tenth of the matching transformer source impdeance, the transformer will start to act as a tonal filter with the bass and treble being attenuated and the transformer acting like a band pass filter.

I hope this helps?

Post your results.

Joseph Rogowski

[fieldwrangler]

Good to see this. I use the 1:10 impedance guide when interfacing with active circuitry but tend to think in terms of impedance matching when handing off audio signals between transformers.

I do intend to pursue my general M.O. of "guided empiricism" in this process, and will report back in a few weeks when I whip up some physical arrangements of magnets, copper, coils & cores.

Bob Palmieri