I came across this datasheet, and I am unsure what the CT portion means.

The datasheet does say that CT means center tap, but specifically in the image below what does the "10k CT" betweeen primary and secondary mean ?

enter image description here


It means what it says. A transformer winding that is center-tapped has an extra connection (the center-tap) in the center of the winding.

Sounds circular, doesn't it.

This allows several options when using the winding. You can get half the voltage, or do impedance matching (you get a 4:1 ratio), or treat the center-tap as AC ground and get half-voltage signals that are 180 degrees out of phase.


simulate this circuit – Schematic created using CircuitLab

| improve this answer | |
  • 1
    \$\begingroup\$ What does the 10k signify ? \$\endgroup\$ – efox29 Feb 13 '15 at 4:22
  • \$\begingroup\$ For bonus points: why is the DC resistance unequal between primary and secondary (I would have expected them to be equal on a 1:1 transformer)? \$\endgroup\$ – KTM Feb 13 '15 at 4:23
  • 1
    \$\begingroup\$ @KTM - Most likely because different sized wire was used on one winding versus the other. A 1::1 turns ratio transformer does not need to use the same size wire in both windings. \$\endgroup\$ – Michael Karas Feb 13 '15 at 4:27
  • 2
    \$\begingroup\$ @KTM If the secondary is wound on the outside, then the average length per turn will be longer. \$\endgroup\$ – Spehro Pefhany Feb 13 '15 at 4:30
  • 1
    \$\begingroup\$ You load the whole secondary with 10kohm, or half the secondary with 2.5k. Primary should be lower impedance because it has to supply both the magnetising current and the secondary current. (Otherwise there would be no need to distinguish between pri and sec). That makes it convenient to wind the primary innermost, as Spehro says, but there are ways to control the relative impedances if that doesn't work. Like winding part of the primary, then the secondary, then the rest of the primary. This also reduces leakage inductance but can increase interwinding capacitance. \$\endgroup\$ – Brian Drummond Feb 13 '15 at 12:49

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.