Transformer Inductance Ratio (datasheet)

I am looking at a current sense transformer, PE-51687NL (Digikey). I must be missing something in understanding the datasheet.

Digikey says the inductance is 20mH. I assume that this is the "single-turn" primary inductance of the magnetic core.

The datasheet says the secondary has 100 turns and the minimum secondary inductance is 2mH.

I know that the turns ratio is related to the inductance ratio: L1/L2 = (n1/n2)^2.

No matter how I twist the numbers I can't get the datasheet to make sense! Would someone help me make sense of this datasheet?

• 20 mH, or even 2 mH, is an enormous inductance to be the primary of a current transformer. A CT's primary inductance is usually very low. I'm guessing whoever put the info into digikey's site made a mistake. Nov 13 '21 at 19:42
• The schematic for PE-51687NL is the figure 2A. So, what is needed is only the value of the termination resistor which is generally a low value as 20 Ohm ... notes 4 & 5 : RT=100 Ohm max which give -> 1V / A (20 A max -> 20V ! ) Just verify the bandwidth of this wiring. You just have to pass a wire through the CT and you have then a 100:1 "transformer". When using the CT for lower current, pass more turns through the CT. Sensitivity will then be increased ... but max current must be lower ! Nov 13 '21 at 21:23
• For comparison see this D1870L or CS1100L and note 2 : coilcraft.com/getmedia/5cabc2b0-54ea-4f3d-a76a-5681314bc042/… Nov 13 '21 at 21:29

It's a 100:1, through-hole current transformer (CT).

There are errors in the Digikey description. Here's the datasheet. The 100 turn secondary,with an inductance of 2.0 mH, is to be terminated with a 100 Ω burden.

With 20A through the 1 turn primary, the secondary current would be (1/100) * 20 = 0.2 A and the secondary voltage 0.2 * 100 = 20 V. In other words, the CT scale factor would be 1 secondary Volt per 1 primary Ampere.

• I appreciate this response because it helped to confirm my understanding of how transformers work. I also realized that the datasheet is misprinted! PE-51687NL is in fact a 20mH secondary. Nov 14 '21 at 21:32
• Hi aosborne, Many thanks for the information. I am pleased to know that my answer was useful. Nov 15 '21 at 3:21